Tetrahydrobiopterin / BH4

evidence_span

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experimental_model

Recombinant enzyme purification and catalysis

exposure

L-tryptophan with tetrahydrobiopterin; purified tetrameric enzyme

limitations

TPH1 and neuronal TPH2 are distinct. A cofactor-dependent reaction does not prove supplemental cofactor increases melatonin in a replete person.

nutrient_topic

Melatonin research collection; topical membership is not evidence of a direct dietary effect. · Melatonin

organism

Human pineal TPH1 expressed in E. coli

plain_language

The pterin cofactor participates chemically; it is not interchangeable with folate.

primary_references

[melatonin-p10525150] Cloning and expression of recombinant human pineal tryptophan hydroxylase in Escherichia coli: purification and characterization of the cloned enzyme. (1999). https://pubmed.ncbi.nlm.nih.gov/10525150/ DOI: 10.1016/s0167-4838(99)00184-3

tissue_or_cell_type

Tryptophan hydroxylation

Melatonin: synthesis, receptors, circadian timing and nutrient interactions (2026-09-17) · lines 149–160

AI-assisted literature curation; primary study URLs and scope retained in the document and extraction. Not publisher full text. · supports · Recombinant enzyme purification and catalysis · source_derived_draft · unverified_draft

### melatonin-tph1-bh4 Human pineal TPH activity was measured with BH4, and a transient 4alpha-hydroxytetrahydrobiopterin intermediate was detected. Condition category: normal nutrient_topic: Melatonin research collection; topical membership is not evidence of a direct dietary effect. plain_language: The pterin cofactor participates chemically; it is not interchangeable with folate. organism: Human pineal TPH1 expressed in E. coli tissue_or_cell_type: Tryptophan hydroxylation experimental_model: Recombinant enzyme purification and catalysis limitations: TPH1 and neuronal TPH2 are distinct. A cofactor-dependent reaction does not prove supplemental cofactor increases melatonin in a replete person. exposure: L-tryptophan with tetrahydrobiopterin; purified tetrameric enzyme evidence_span: {"source_cache": "artifacts/melatonin-research/10525150.abstract.txt", "locator": "Primary indexed abstract; zero-based, end-exclusive Unicode character offsets", "file_sha256": "032acfb7b1818f5282f5457e093a7e371895bbdc95f058c8958766fc57f9118f", "start_char": 0, "end_char": 1489, "text_sha256": "032acfb7b1818f5282f5457e093a7e371895bbdc95f058c8958766fc57f9118f"} [melatonin-p10525150] Cloning and expression of recombinant human pineal tryptophan hydroxylase in Escherichia coli: purification and characterization of the cloned enzyme. (1999). https://pubmed.ncbi.nlm.nih.gov/10525150/ DOI: 10.1016/s0167-4838(99)00184-3

evidence_access

Primary abstract

experimental_model

Human purified proteins, crystallography and binding/kinetic studies.

limitations

Cofactor synthesis has additional steps; structural regulation does not establish a clinical depletion or supplementation effect.

nutrient_topic

L-Phenylalanine collection; species, compartment, exposure, co-substrates and manipulation remain explicit. · L-Phenylalanine

plain_language

Cofactor production has both a demand signal and a feedback brake.

primary_references

Biophysical and structural investigation of the regulation of human GTP cyclohydrolase I by its regulatory protein GFRP. · 2021 · https://pubmed.ncbi.nlm.nih.gov/33387654/ · DOI 10.1016/j.jsb.2020.107691

L-Phenylalanine: transport, protein synthesis, cofactor recycling and cross-nutrient mechanisms (2026-09-19) · lines 46–52

AI-assisted research curation; primary references, access levels and experimental limitations individually identified. Not publisher full text. · supports · Human purified proteins, crystallography and binding/kinetic studies. · source_derived_draft · unverified_draft

## l-phenylalanine-gch1-feedback Cofactor production has both a demand signal and a feedback brake. BH4 and phenylalanine favored inhibited and activated human GCH1-GFRP complexes, respectively; ligand-dependent conformational changes stabilized binding. Model: Human purified proteins, crystallography and binding/kinetic studies. Limitations: Cofactor synthesis has additional steps; structural regulation does not establish a clinical depletion or supplementation effect. Evidence access: Primary abstract Biophysical and structural investigation of the regulation of human GTP cyclohydrolase I by its regulatory protein GFRP. · 2021 · https://pubmed.ncbi.nlm.nih.gov/33387654/ · DOI 10.1016/j.jsb.2020.107691

evidence_access

Primary abstract

experimental_model

Human TH1 kinetics and surface plasmon resonance; PKA phosphorylation.

limitations

These kinetic values are not blood targets or instructions to supplement BH4.

nutrient_topic

L-Tyrosine collection; species, compartment, exposure, co-substrates and manipulation remain explicit. · L-Tyrosine

plain_language

The cofactor response changes with enzyme regulation.

primary_references

Tyrosine hydroxylase binds tetrahydrobiopterin cofactor with negative cooperativity, as shown by kinetic analyses and surface plasmon resonance detection. · 1999 · https://pubmed.ncbi.nlm.nih.gov/10411647/ · DOI 10.1046/j.1432-1327.1999.00445.x

L-Tyrosine: catecholamines, thyroid chemistry, pigment, metabolism and cross-nutrient mechanisms (2026-09-19) · lines 28–34

AI-assisted research curation; primary references, access levels and experimental limitations individually identified. Not publisher full text. · supports · Human TH1 kinetics and surface plasmon resonance; PKA phosphorylation. · source_derived_draft · unverified_draft

## l-tyrosine-th-cofactor-regulation The cofactor response changes with enzyme regulation. Recombinant human TH1 bound natural BH4 with negative cooperativity; Ser40 phosphorylation increased apparent BH4 affinity. Model: Human TH1 kinetics and surface plasmon resonance; PKA phosphorylation. Limitations: These kinetic values are not blood targets or instructions to supplement BH4. Evidence access: Primary abstract Tyrosine hydroxylase binds tetrahydrobiopterin cofactor with negative cooperativity, as shown by kinetic analyses and surface plasmon resonance detection. · 1999 · https://pubmed.ncbi.nlm.nih.gov/10411647/ · DOI 10.1046/j.1432-1327.1999.00445.x

evidence_span

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experimental_model

Purified recombinant enzyme activity and cofactor analysis

exposure

Arginine substrate; BH4, FAD, FMN, heme, iron and zinc measurements

limitations

Biochemical cofactor findings do not show that extra dietary cofactors increase NO in healthy people.

nutrient_topic

Citrulline research collection; topical membership is not evidence of a direct dietary effect. · L-Citrulline

organism

Human NOS3 expressed in yeast

plain_language

Supplying substrate does not replace the required pterin cofactor.

primary_references

[citrulline-p10608822] Characterization of recombinant human endothelial nitric-oxide synthase purified from the yeast Pichia pastoris. (1999). https://pubmed.ncbi.nlm.nih.gov/10608822/ DOI: 10.1074/jbc.274.53.37658

tissue_or_cell_type

Endothelial NOS enzyme preparation

Citrulline: arginine recycling, nitrogen disposal and nutrient connections (2026-09-17) · lines 515–526

AI-assisted literature curation; primary study URLs and scope retained in the document and extraction. Not publisher full text. · supports · Purified recombinant enzyme activity and cofactor analysis · source_derived_draft · unverified_draft

### citrulline-nos-bh4 Purified human eNOS showed an absolute BH4 requirement for the measured catalytic reaction. Condition category: normal nutrient_topic: Citrulline research collection; topical membership is not evidence of a direct dietary effect. plain_language: Supplying substrate does not replace the required pterin cofactor. organism: Human NOS3 expressed in yeast tissue_or_cell_type: Endothelial NOS enzyme preparation experimental_model: Purified recombinant enzyme activity and cofactor analysis limitations: Biochemical cofactor findings do not show that extra dietary cofactors increase NO in healthy people. exposure: Arginine substrate; BH4, FAD, FMN, heme, iron and zinc measurements evidence_span: {"source_cache": "artifacts/citrulline-research/10608822.abstract.txt", "locator": "Primary indexed abstract; zero-based, end-exclusive Unicode character offsets", "file_sha256": "1600b193cc63f30b204285729710bb81f34bc2918a8ca7f97aa97bc4acd998fd", "start_char": 0, "end_char": 2100, "text_sha256": "1600b193cc63f30b204285729710bb81f34bc2918a8ca7f97aa97bc4acd998fd"} [citrulline-p10608822] Characterization of recombinant human endothelial nitric-oxide synthase purified from the yeast Pichia pastoris. (1999). https://pubmed.ncbi.nlm.nih.gov/10608822/ DOI: 10.1074/jbc.274.53.37658

evidence_span

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experimental_model

Purified recombinant enzyme activity and cofactor analysis

exposure

Arginine substrate; BH4, FAD, FMN, heme, iron and zinc measurements

limitations

Biochemical cofactor findings do not show that extra dietary cofactors increase NO in healthy people.

nutrient_topic

Citrulline research collection; topical membership is not evidence of a direct dietary effect. · L-Citrulline

organism

Human NOS3 expressed in yeast

plain_language

Cofactor and substrate helped stabilize the paired enzyme structure.

primary_references

[citrulline-p10608822] Characterization of recombinant human endothelial nitric-oxide synthase purified from the yeast Pichia pastoris. (1999). https://pubmed.ncbi.nlm.nih.gov/10608822/ DOI: 10.1074/jbc.274.53.37658

tissue_or_cell_type

Endothelial NOS enzyme preparation

Citrulline: arginine recycling, nitrogen disposal and nutrient connections (2026-09-17) · lines 580–591

AI-assisted literature curation; primary study URLs and scope retained in the document and extraction. Not publisher full text. · supports · Purified recombinant enzyme activity and cofactor analysis · source_derived_draft · unverified_draft

### citrulline-nos-dimer BH4 plus arginine shifted human eNOS toward dimers during low-temperature electrophoresis. Condition category: normal nutrient_topic: Citrulline research collection; topical membership is not evidence of a direct dietary effect. plain_language: Cofactor and substrate helped stabilize the paired enzyme structure. organism: Human NOS3 expressed in yeast tissue_or_cell_type: Endothelial NOS enzyme preparation experimental_model: Purified recombinant enzyme activity and cofactor analysis limitations: Biochemical cofactor findings do not show that extra dietary cofactors increase NO in healthy people. exposure: Arginine substrate; BH4, FAD, FMN, heme, iron and zinc measurements evidence_span: {"source_cache": "artifacts/citrulline-research/10608822.abstract.txt", "locator": "Primary indexed abstract; zero-based, end-exclusive Unicode character offsets", "file_sha256": "1600b193cc63f30b204285729710bb81f34bc2918a8ca7f97aa97bc4acd998fd", "start_char": 0, "end_char": 2100, "text_sha256": "1600b193cc63f30b204285729710bb81f34bc2918a8ca7f97aa97bc4acd998fd"} [citrulline-p10608822] Characterization of recombinant human endothelial nitric-oxide synthase purified from the yeast Pichia pastoris. (1999). https://pubmed.ncbi.nlm.nih.gov/10608822/ DOI: 10.1074/jbc.274.53.37658

evidence_span

{"source_cache": "artifacts/citrulline-research/10749876.abstract.txt", "locator": "Primary indexed abstract; zero-based, end-exclusive Unicode character offsets", "file_sha256": "10605941fa542bd62ed4ee0fb042ea7569f3ab619f1cb893b54d7e62386712ed", "start_char": 0, "end_char": 1519, "text_sha256": "10605941fa542bd62ed4ee0fb042ea7569f3ab619f1cb893b54d7e62386712ed"}

experimental_model

Cell exposure and recombinant-enzyme assays

exposure

Ascorbate loading; BH4 or sepiapterin comparisons

limitations

Preclinical experiments; not proof that vitamin C is needed as a co-supplement with citrulline.

nutrient_topic

Citrulline research collection; topical membership is not evidence of a direct dietary effect. · L-Citrulline

organism

Porcine aortic endothelial cells and bovine eNOS

plain_language

The benefit depended on the starting cofactor conditions.

primary_references

[citrulline-p10749876] Ascorbic acid enhances endothelial nitric-oxide synthase activity by increasing intracellular tetrahydrobiopterin. (2000). https://pubmed.ncbi.nlm.nih.gov/10749876/ DOI: 10.1074/jbc.m002248200

tissue_or_cell_type

BH4 availability and NO bioactivity

Citrulline: arginine recycling, nitrogen disposal and nutrient connections (2026-09-17) · lines 723–734

AI-assisted literature curation; primary study URLs and scope retained in the document and extraction. Not publisher full text. · supports · Cell exposure and recombinant-enzyme assays · source_derived_draft · unverified_draft

### citrulline-vitaminc-bh4-saturation Ascorbate had no additional effect at high BH4 or after sepiapterin raised cellular BH4. Condition category: normal nutrient_topic: Citrulline research collection; topical membership is not evidence of a direct dietary effect. plain_language: The benefit depended on the starting cofactor conditions. organism: Porcine aortic endothelial cells and bovine eNOS tissue_or_cell_type: BH4 availability and NO bioactivity experimental_model: Cell exposure and recombinant-enzyme assays limitations: Preclinical experiments; not proof that vitamin C is needed as a co-supplement with citrulline. exposure: Ascorbate loading; BH4 or sepiapterin comparisons evidence_span: {"source_cache": "artifacts/citrulline-research/10749876.abstract.txt", "locator": "Primary indexed abstract; zero-based, end-exclusive Unicode character offsets", "file_sha256": "10605941fa542bd62ed4ee0fb042ea7569f3ab619f1cb893b54d7e62386712ed", "start_char": 0, "end_char": 1519, "text_sha256": "10605941fa542bd62ed4ee0fb042ea7569f3ab619f1cb893b54d7e62386712ed"} [citrulline-p10749876] Ascorbic acid enhances endothelial nitric-oxide synthase activity by increasing intracellular tetrahydrobiopterin. (2000). https://pubmed.ncbi.nlm.nih.gov/10749876/ DOI: 10.1074/jbc.m002248200

evidence_span

{"source_cache": "artifacts/cold-research/9753429.abstract.txt", "locator": "Indexed abstract; zero-based, end-exclusive Unicode character offsets", "file_sha256": "3c76ec6d5dadb463c0a044175d5a78fbd02df4da7c339029c8c232e8878175b9", "start_char": 0, "end_char": 1695, "text_sha256": "3c76ec6d5dadb463c0a044175d5a78fbd02df4da7c339029c8c232e8878175b9"}

experimental_model

Crystal structure of rat tyrosine hydroxylase with a cofactor analogue and iron at 2.3 angstrom

exposure

Bound 7,8-dihydrobiopterin and iron

limitations

Structural chemistry of the rate-limiting step. It is a rat enzyme structure with a cofactor analogue, not a measurement of iron status in an animal.

nutrient_topic

Cold water immersion research collection; topical membership is not evidence of a direct clinical effect, and a therapeutic exposure is not a dietary intake. · Cold water immersion

organism

Rat enzyme

plain_language

The oxygen molecule slots into the gap between the cofactor and the iron.

primary_references

[cold-p9753429] Crystal structure of tyrosine hydroxylase with bound cofactor analogue and iron at 2.3 A resolution: self-hydroxylation of Phe300 and the pterin-binding site. (1998). https://pubmed.ncbi.nlm.nih.gov/9753429/ DOI: 10.1021/bi981462g

tissue_or_cell_type

Purified catalytic and tetramerization domains

Cold water immersion: cold sensing, heat production, the catecholamine axis and what repeated exposure changes (2026-09-19) · lines 546–557

AI-assisted literature curation; primary study URLs and scope retained in the document and extraction. Not publisher full text. · supports · Crystal structure of rat tyrosine hydroxylase with a cofactor analogue and iron at 2.3 angstrom · source_derived_draft · unverified_draft

### cold-th-pterin-geometry The pterin binds in the active-site cleft forming an aromatic stacking interaction with Phe300, which is itself hydroxylated in the meta position by an autocatalytic process and anchored by a hydrogen bond to the carbonyl of Gln310; the iron sits 5.6 angstrom from the pterin 4a carbon, and molecular oxygen could bind in a bridging position between the pterin and the iron before substrate hydroxylation. Condition category: normal nutrient_topic: Cold water immersion research collection; topical membership is not evidence of a direct clinical effect, and a therapeutic exposure is not a dietary intake. plain_language: The oxygen molecule slots into the gap between the cofactor and the iron. organism: Rat enzyme tissue_or_cell_type: Purified catalytic and tetramerization domains experimental_model: Crystal structure of rat tyrosine hydroxylase with a cofactor analogue and iron at 2.3 angstrom limitations: Structural chemistry of the rate-limiting step. It is a rat enzyme structure with a cofactor analogue, not a measurement of iron status in an animal. exposure: Bound 7,8-dihydrobiopterin and iron evidence_span: {"source_cache": "artifacts/cold-research/9753429.abstract.txt", "locator": "Indexed abstract; zero-based, end-exclusive Unicode character offsets", "file_sha256": "3c76ec6d5dadb463c0a044175d5a78fbd02df4da7c339029c8c232e8878175b9", "start_char": 0, "end_char": 1695, "text_sha256": "3c76ec6d5dadb463c0a044175d5a78fbd02df4da7c339029c8c232e8878175b9"} [cold-p9753429] Crystal structure of tyrosine hydroxylase with bound cofactor analogue and iron at 2.3 A resolution: self-hydroxylation of Phe300 and the pterin-binding site. (1998). https://pubmed.ncbi.nlm.nih.gov/9753429/ DOI: 10.1021/bi981462g

evidence_access

Primary abstract

experimental_model

Rat cerebellar NOS preparation; separated partial reactions.

limitations

Assay-specific stimulation, not a dietary BH4 or arginine treatment rule.

nutrient_topic

L-Arginine collection; tissue, species, dose and formulation distinctions retained. · L-Arginine

plain_language

Substrate availability and cofactor availability are separate requirements.

primary_references

Characteristics of the nitric oxide synthase-catalyzed conversion of arginine to N-hydroxyarginine, the first oxygenation step in the enzymic synthesis of nitric oxide. · 1995 · https://pubmed.ncbi.nlm.nih.gov/7530247/ · DOI 10.1074/jbc.270.4.1721

L-Arginine: transport, metabolic branches, nutrient interactions, availability and discovery questions (2026-09-18) · lines 86–92

AI-assisted research curation; primary-abstract references and experimental limitations individually identified. Not publisher full text. · supports · Rat cerebellar NOS preparation; separated partial reactions. · source_derived_draft · unverified_draft

## arg-nos-bh4 Substrate availability and cofactor availability are separate requirements. BH4 stimulated both partial reactions; the first increased approximately threefold. Model: Rat cerebellar NOS preparation; separated partial reactions. Limitations: Assay-specific stimulation, not a dietary BH4 or arginine treatment rule. Evidence access: Primary abstract Characteristics of the nitric oxide synthase-catalyzed conversion of arginine to N-hydroxyarginine, the first oxygenation step in the enzymic synthesis of nitric oxide. · 1995 · https://pubmed.ncbi.nlm.nih.gov/7530247/ · DOI 10.1074/jbc.270.4.1721

evidence_access

Primary abstract

experimental_model

Human PCBD1 mutation study; established biochemical role and disease background.

limitations

The renal study does not directly measure whole-body BH4 recycling flux. The transcriptional role below is separately recorded.

nutrient_topic

L-Phenylalanine collection; species, compartment, exposure, co-substrates and manipulation remain explicit. · L-Phenylalanine

plain_language

One cofactor-recycling protein has another important job in the kidney.

primary_references

Mutations in PCBD1 cause hypomagnesemia and renal magnesium wasting. · 2014 · https://pubmed.ncbi.nlm.nih.gov/24204001/ · DOI 10.1681/ASN.2013040337

L-Phenylalanine: transport, protein synthesis, cofactor recycling and cross-nutrient mechanisms (2026-09-19) · lines 62–68

AI-assisted research curation; primary references, access levels and experimental limitations individually identified. Not publisher full text. · supports · Human PCBD1 mutation study; established biochemical role and disease background. · source_derived_draft · unverified_draft

## l-phenylalanine-pcbd-recycling-role One cofactor-recycling protein has another important job in the kidney. PCBD1 has a pterin-carbinolamine dehydratase role in BH4 regeneration; biallelic defects are associated with transient neonatal hyperphenylalaninemia and primapterinuria. Model: Human PCBD1 mutation study; established biochemical role and disease background. Limitations: The renal study does not directly measure whole-body BH4 recycling flux. The transcriptional role below is separately recorded. Evidence access: Primary abstract Mutations in PCBD1 cause hypomagnesemia and renal magnesium wasting. · 2014 · https://pubmed.ncbi.nlm.nih.gov/24204001/ · DOI 10.1681/ASN.2013040337

evidence_span

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experimental_model

Recombinant enzyme purification and catalysis

exposure

L-tryptophan with tetrahydrobiopterin; purified tetrameric enzyme

limitations

TPH1 and neuronal TPH2 are distinct. A cofactor-dependent reaction does not prove supplemental cofactor increases melatonin in a replete person.

nutrient_topic

Melatonin research collection; topical membership is not evidence of a direct dietary effect. · Melatonin

organism

Human pineal TPH1 expressed in E. coli

plain_language

The amino acid tryptophan must pass through an enzyme-controlled first step.

primary_references

[melatonin-p10525150] Cloning and expression of recombinant human pineal tryptophan hydroxylase in Escherichia coli: purification and characterization of the cloned enzyme. (1999). https://pubmed.ncbi.nlm.nih.gov/10525150/ DOI: 10.1016/s0167-4838(99)00184-3

tissue_or_cell_type

Tryptophan hydroxylation

Melatonin: synthesis, receptors, circadian timing and nutrient interactions (2026-09-17) · lines 136–147

AI-assisted literature curation; primary study URLs and scope retained in the document and extraction. Not publisher full text. · supports · Recombinant enzyme purification and catalysis · source_derived_draft · unverified_draft

### melatonin-tph1-hydroxylation Purified human pineal TPH catalyzed tryptophan hydroxylation to 5-hydroxytryptophan. Condition category: normal nutrient_topic: Melatonin research collection; topical membership is not evidence of a direct dietary effect. plain_language: The amino acid tryptophan must pass through an enzyme-controlled first step. organism: Human pineal TPH1 expressed in E. coli tissue_or_cell_type: Tryptophan hydroxylation experimental_model: Recombinant enzyme purification and catalysis limitations: TPH1 and neuronal TPH2 are distinct. A cofactor-dependent reaction does not prove supplemental cofactor increases melatonin in a replete person. exposure: L-tryptophan with tetrahydrobiopterin; purified tetrameric enzyme evidence_span: {"source_cache": "artifacts/melatonin-research/10525150.abstract.txt", "locator": "Primary indexed abstract; zero-based, end-exclusive Unicode character offsets", "file_sha256": "032acfb7b1818f5282f5457e093a7e371895bbdc95f058c8958766fc57f9118f", "start_char": 0, "end_char": 1489, "text_sha256": "032acfb7b1818f5282f5457e093a7e371895bbdc95f058c8958766fc57f9118f"} [melatonin-p10525150] Cloning and expression of recombinant human pineal tryptophan hydroxylase in Escherichia coli: purification and characterization of the cloned enzyme. (1999). https://pubmed.ncbi.nlm.nih.gov/10525150/ DOI: 10.1016/s0167-4838(99)00184-3

evidence_access

Primary abstract

experimental_model

Human DNAJC12 deficiency study with functional interaction experiments.

limitations

Interaction does not imply identical effects in all tissues or rescue by extra substrate.

nutrient_topic

L-Phenylalanine collection; species, compartment, exposure, co-substrates and manipulation remain explicit. · L-Phenylalanine

plain_language

A shared chaperone supports several amino-acid processing enzymes.

primary_references

Biallelic Mutations in DNAJC12 Cause Hyperphenylalaninemia, Dystonia, and Intellectual Disability. · 2017 · https://pubmed.ncbi.nlm.nih.gov/28132689/ · DOI 10.1016/j.ajhg.2017.01.002

L-Phenylalanine: transport, protein synthesis, cofactor recycling and cross-nutrient mechanisms (2026-09-19) · lines 102–108

AI-assisted research curation; primary references, access levels and experimental limitations individually identified. Not publisher full text. · supports · Human DNAJC12 deficiency study with functional interaction experiments. · source_derived_draft · unverified_draft

## l-phenylalanine-dnaj-chaperone A shared chaperone supports several amino-acid processing enzymes. DNAJC12 interacted with aromatic amino-acid hydroxylases, including PAH, tyrosine hydroxylase and tryptophan hydroxylases. Model: Human DNAJC12 deficiency study with functional interaction experiments. Limitations: Interaction does not imply identical effects in all tissues or rescue by extra substrate. Evidence access: Primary abstract Biallelic Mutations in DNAJC12 Cause Hyperphenylalaninemia, Dystonia, and Intellectual Disability. · 2017 · https://pubmed.ncbi.nlm.nih.gov/28132689/ · DOI 10.1016/j.ajhg.2017.01.002

availability_state

machinery_impairment Imported condition classification; unverified.

evidence_access

Primary abstract

experimental_model

Human genetic case series and functional experiments.

limitations

Early combined BH4/neurotransmitter-precursor treatment was not a controlled test of phenylalanine supplementation.

nutrient_topic

L-Phenylalanine collection; species, compartment, exposure, co-substrates and manipulation remain explicit. · L-Phenylalanine

plain_language

Phenylalanine can accumulate because its supporting machinery is defective.

primary_references

Biallelic Mutations in DNAJC12 Cause Hyperphenylalaninemia, Dystonia, and Intellectual Disability. · 2017 · https://pubmed.ncbi.nlm.nih.gov/28132689/ · DOI 10.1016/j.ajhg.2017.01.002

trigger_kind

machinery_impairment Imported condition classification; unverified.

L-Phenylalanine: transport, protein synthesis, cofactor recycling and cross-nutrient mechanisms (2026-09-19) · lines 110–116

AI-assisted research curation; primary references, access levels and experimental limitations individually identified. Not publisher full text. · supports · Human genetic case series and functional experiments. · source_derived_draft · unverified_draft

## l-phenylalanine-dnaj-failure Phenylalanine can accumulate because its supporting machinery is defective. Biallelic DNAJC12 variants in six patients from four families accompanied hyperphenylalaninemia and neurotransmitter abnormalities; functional studies found reduced PAH activity. Model: Human genetic case series and functional experiments. Limitations: Early combined BH4/neurotransmitter-precursor treatment was not a controlled test of phenylalanine supplementation. Evidence access: Primary abstract Biallelic Mutations in DNAJC12 Cause Hyperphenylalaninemia, Dystonia, and Intellectual Disability. · 2017 · https://pubmed.ncbi.nlm.nih.gov/28132689/ · DOI 10.1016/j.ajhg.2017.01.002

evidence_access

Primary abstract

experimental_model

Recombinant human proteins; primary abstract.

limitations

GCH1 controls an early BH4-biosynthesis step; this is not proof that supplemental phenylalanine raises BH4 in every tissue.

nutrient_topic

L-Phenylalanine collection; species, compartment, exposure, co-substrates and manipulation remain explicit. · L-Phenylalanine

plain_language

The substrate can signal for more of the cofactor-making machinery to work.

primary_references

Bacterial lipopolysaccharide down-regulates expression of GTP cyclohydrolase I feedback regulatory protein. · 2002 · https://pubmed.ncbi.nlm.nih.gov/11799107/ · DOI 10.1074/jbc.M107326200

L-Phenylalanine: transport, protein synthesis, cofactor recycling and cross-nutrient mechanisms (2026-09-19) · lines 38–44

AI-assisted research curation; primary references, access levels and experimental limitations individually identified. Not publisher full text. · supports · Recombinant human proteins; primary abstract. · source_derived_draft · unverified_draft

## l-phenylalanine-gch1-feedforward The substrate can signal for more of the cofactor-making machinery to work. Recombinant human GFRP enabled phenylalanine-dependent stimulation of recombinant human GCH1. Model: Recombinant human proteins; primary abstract. Limitations: GCH1 controls an early BH4-biosynthesis step; this is not proof that supplemental phenylalanine raises BH4 in every tissue. Evidence access: Primary abstract Bacterial lipopolysaccharide down-regulates expression of GTP cyclohydrolase I feedback regulatory protein. · 2002 · https://pubmed.ncbi.nlm.nih.gov/11799107/ · DOI 10.1074/jbc.M107326200

evidence_access

Primary abstract

experimental_model

Human THP-1 cells; interferon-gamma alone compared with interferon-gamma plus LPS.

limitations

Pteridine output is not synonymous with bioavailable BH4; no clinical inflammatory threshold is defined.

nutrient_topic

L-Phenylalanine collection; species, compartment, exposure, co-substrates and manipulation remain explicit. · L-Phenylalanine

plain_language

Inflammatory stimulation changed how this cell model responded to phenylalanine.

primary_references

Bacterial lipopolysaccharide down-regulates expression of GTP cyclohydrolase I feedback regulatory protein. · 2002 · https://pubmed.ncbi.nlm.nih.gov/11799107/ · DOI 10.1074/jbc.M107326200

L-Phenylalanine: transport, protein synthesis, cofactor recycling and cross-nutrient mechanisms (2026-09-19) · lines 54–60

AI-assisted research curation; primary references, access levels and experimental limitations individually identified. Not publisher full text. · supports · Human THP-1 cells; interferon-gamma alone compared with interferon-gamma plus LPS. · source_derived_draft · unverified_draft

## l-phenylalanine-inflammation-feedback Inflammatory stimulation changed how this cell model responded to phenylalanine. LPS at 1 microgram/mL reduced GFRP expression in human THP-1 cells; with interferon-gamma plus LPS, pteridine production became phenylalanine-independent. Model: Human THP-1 cells; interferon-gamma alone compared with interferon-gamma plus LPS. Limitations: Pteridine output is not synonymous with bioavailable BH4; no clinical inflammatory threshold is defined. Evidence access: Primary abstract Bacterial lipopolysaccharide down-regulates expression of GTP cyclohydrolase I feedback regulatory protein. · 2002 · https://pubmed.ncbi.nlm.nih.gov/11799107/ · DOI 10.1074/jbc.M107326200

evidence_access

Primary full text; reaction background distinguished from new structural experiments

experimental_model

Human PAH structural study; established reaction described in the full-text introduction.

limitations

This record describes the established reaction, not a new dietary intervention or a human iron-deficiency threshold.

nutrient_topic

L-Phenylalanine collection; species, compartment, exposure, co-substrates and manipulation remain explicit. · L-Phenylalanine

plain_language

Making tyrosine requires a working enzyme and its chemical helpers.

primary_references

Structural basis for ligand-dependent dimerization of phenylalanine hydroxylase regulatory domain. · 2016 · https://pubmed.ncbi.nlm.nih.gov/27049649/ · DOI 10.1038/srep23748

L-Phenylalanine: transport, protein synthesis, cofactor recycling and cross-nutrient mechanisms (2026-09-19) · lines 14–20

AI-assisted research curation; primary references, access levels and experimental limitations individually identified. Not publisher full text. · supports · Human PAH structural study; established reaction described in the full-text introduction. · source_derived_draft · unverified_draft

## l-phenylalanine-pah-conversion Making tyrosine requires a working enzyme and its chemical helpers. PAH catalyzes phenylalanine hydroxylation to tyrosine using BH4, molecular oxygen and non-heme ferrous iron. Model: Human PAH structural study; established reaction described in the full-text introduction. Limitations: This record describes the established reaction, not a new dietary intervention or a human iron-deficiency threshold. Evidence access: Primary full text; reaction background distinguished from new structural experiments Structural basis for ligand-dependent dimerization of phenylalanine hydroxylase regulatory domain. · 2016 · https://pubmed.ncbi.nlm.nih.gov/27049649/ · DOI 10.1038/srep23748

evidence_access

Primary abstract

experimental_model

Human QDPR expressed in E. coli; enzyme purification, kinetic characterization and structure.

limitations

This binary structure alone does not show that niacin supplementation changes phenylalanine clearance.

nutrient_topic

L-Phenylalanine collection; species, compartment, exposure, co-substrates and manipulation remain explicit. · L-Phenylalanine

plain_language

The cofactor-recycling network includes an NADH-binding enzyme.

primary_references

The crystallographic structure of a human dihydropteridine reductase NADH binary complex expressed in Escherichia coli by a cDNA constructed from its rat homologue. · 1993 · https://pubmed.ncbi.nlm.nih.gov/8262916/

L-Phenylalanine: transport, protein synthesis, cofactor recycling and cross-nutrient mechanisms (2026-09-19) · lines 70–76

AI-assisted research curation; primary references, access levels and experimental limitations individually identified. Not publisher full text. · supports · Human QDPR expressed in E. coli; enzyme purification, kinetic characterization and structure. · source_derived_draft · unverified_draft

## l-phenylalanine-qdpr-nadh The cofactor-recycling network includes an NADH-binding enzyme. A catalytically characterized recombinant human dihydropteridine reductase was crystallized in complex with NADH at 2.5-angstrom resolution. Model: Human QDPR expressed in E. coli; enzyme purification, kinetic characterization and structure. Limitations: This binary structure alone does not show that niacin supplementation changes phenylalanine clearance. Evidence access: Primary abstract The crystallographic structure of a human dihydropteridine reductase NADH binary complex expressed in Escherichia coli by a cDNA constructed from its rat homologue. · 1993 · https://pubmed.ncbi.nlm.nih.gov/8262916/

evidence_access

Primary abstract

experimental_model

Randomized human PKU study enriched through prior responsiveness assessment.

limitations

Not all PAH defects respond; trial dose is historical evidence, not individualized guidance.

nutrient_topic

L-Phenylalanine collection; species, compartment, exposure, co-substrates and manipulation remain explicit. · L-Phenylalanine

plain_language

Providing the pharmaceutical cofactor helped some patients, with substantial response variation.

primary_references

Efficacy of sapropterin dihydrochloride (tetrahydrobiopterin, 6R-BH4) for reduction of phenylalanine concentration in patients with phenylketonuria: a phase III randomised placebo-controlled study. · 2007 · https://pubmed.ncbi.nlm.nih.gov/17693179/ · DOI 10.1016/S0140-6736(07)61234-3

L-Phenylalanine: transport, protein synthesis, cofactor recycling and cross-nutrient mechanisms (2026-09-19) · lines 222–228

AI-assisted research curation; primary references, access levels and experimental limitations individually identified. Not publisher full text. · supports · Randomized human PKU study enriched through prior responsiveness assessment. · source_derived_draft · unverified_draft

## l-phenylalanine-sapropterin-response Providing the pharmaceutical cofactor helped some patients, with substantial response variation. In an 89-participant six-week PKU trial, sapropterin 10 mg/kg/day lowered mean phenylalanine by 236 micromol/L versus a 3 micromol/L rise with placebo; 44% versus 9% had at least a 30% reduction. Model: Randomized human PKU study enriched through prior responsiveness assessment. Limitations: Not all PAH defects respond; trial dose is historical evidence, not individualized guidance. Evidence access: Primary abstract Efficacy of sapropterin dihydrochloride (tetrahydrobiopterin, 6R-BH4) for reduction of phenylalanine concentration in patients with phenylketonuria: a phase III randomised placebo-controlled study. · 2007 · https://pubmed.ncbi.nlm.nih.gov/17693179/ · DOI 10.1016/S0140-6736(07)61234-3

evidence_access

Primary abstract

experimental_model

Human TH variant study; established enzyme reaction described in the abstract.

limitations

This is a downstream connection, not evidence that oral phenylalanine predictably raises brain dopamine.

nutrient_topic

L-Phenylalanine collection; species, compartment, exposure, co-substrates and manipulation remain explicit. · L-Phenylalanine

plain_language

Producing tyrosine is only the first part of the route toward dopamine.

primary_references

Tyrosine hydroxylase variants influence protein expression, cellular localization, stability, enzymatic activity and the physical interaction between tyrosine hydroxylase and GTP cyclohydrolase 1. · 2024 · https://pubmed.ncbi.nlm.nih.gov/38084654/ · DOI 10.1002/jimd.12690

L-Phenylalanine: transport, protein synthesis, cofactor recycling and cross-nutrient mechanisms (2026-09-19) · lines 118–124

AI-assisted research curation; primary references, access levels and experimental limitations individually identified. Not publisher full text. · supports · Human TH variant study; established enzyme reaction described in the abstract. · source_derived_draft · unverified_draft

## l-phenylalanine-tyrosine-next-step Producing tyrosine is only the first part of the route toward dopamine. Human tyrosine hydroxylase catalyzes BH4-dependent tyrosine hydroxylation to L-DOPA, a downstream step in dopamine biosynthesis. Model: Human TH variant study; established enzyme reaction described in the abstract. Limitations: This is a downstream connection, not evidence that oral phenylalanine predictably raises brain dopamine. Evidence access: Primary abstract Tyrosine hydroxylase variants influence protein expression, cellular localization, stability, enzymatic activity and the physical interaction between tyrosine hydroxylase and GTP cyclohydrolase 1. · 2024 · https://pubmed.ncbi.nlm.nih.gov/38084654/ · DOI 10.1002/jimd.12690

evidence_span

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experimental_model

Hypertension and knockout mouse experiments

exposure

NADPH-oxidase/NOS genetic comparisons and BH4 treatment

limitations

Disease model; the exact response cannot be assumed in people taking citrulline.

nutrient_topic

Citrulline research collection; topical membership is not evidence of a direct dietary effect. · L-Citrulline

organism

Mouse DOCA-salt hypertension model

plain_language

Adding precursor is different from fixing a damaged cofactor system.

primary_references

[citrulline-p12697739] Oxidation of tetrahydrobiopterin leads to uncoupling of endothelial cell nitric oxide synthase in hypertension. (2003). https://pubmed.ncbi.nlm.nih.gov/12697739/ DOI: 10.1172/jci14172

tissue_or_cell_type

Aortic redox state and NO signaling

Citrulline: arginine recycling, nitrogen disposal and nutrient connections (2026-09-17) · lines 801–812

AI-assisted literature curation; primary study URLs and scope retained in the document and extraction. Not publisher full text. · supports · Hypertension and knockout mouse experiments · source_derived_draft · unverified_draft

### citrulline-bh4-oxidation BH4 oxidation accompanied eNOS uncoupling with increased ROS and reduced NO in hypertensive mouse aortas. Condition category: normal nutrient_topic: Citrulline research collection; topical membership is not evidence of a direct dietary effect. plain_language: Adding precursor is different from fixing a damaged cofactor system. organism: Mouse DOCA-salt hypertension model tissue_or_cell_type: Aortic redox state and NO signaling experimental_model: Hypertension and knockout mouse experiments limitations: Disease model; the exact response cannot be assumed in people taking citrulline. exposure: NADPH-oxidase/NOS genetic comparisons and BH4 treatment evidence_span: {"source_cache": "artifacts/citrulline-research/12697739.abstract.txt", "locator": "Primary indexed abstract; zero-based, end-exclusive Unicode character offsets", "file_sha256": "d56604695c8c1bda167b76198a472d814d284a3c0c1e1e1eed8943cce31d0ad0", "start_char": 0, "end_char": 1474, "text_sha256": "d56604695c8c1bda167b76198a472d814d284a3c0c1e1e1eed8943cce31d0ad0"} [citrulline-p12697739] Oxidation of tetrahydrobiopterin leads to uncoupling of endothelial cell nitric oxide synthase in hypertension. (2003). https://pubmed.ncbi.nlm.nih.gov/12697739/ DOI: 10.1172/jci14172

availability_state

machinery_impairment Imported condition classification; unverified.

evidence_span

{"source_cache": "artifacts/citrulline-research/19666465.abstract.txt", "locator": "Primary indexed abstract; zero-based, end-exclusive Unicode character offsets", "file_sha256": "b1ef76ebbd312932011edd1b0f9f428026a4e4cdd53301bd66f99eaf543e77af", "start_char": 0, "end_char": 1716, "text_sha256": "b1ef76ebbd312932011edd1b0f9f428026a4e4cdd53301bd66f99eaf543e77af"}

experimental_model

DHFR knockdown/inhibition and controlled GTPCH-expression cell experiments

exposure

Methotrexate or RNA interference; comparison with GTPCH knockdown

limitations

Mouse models; DHFR also handles folate, but this study does not make folate and BH4 interchangeable.

nutrient_topic

Citrulline research collection; topical membership is not evidence of a direct dietary effect. · L-Citrulline

organism

Mouse endothelial and NIH-3T3-based experimental cell systems

plain_language

Recycling quality matters, not just the total amount of pterin.

primary_references

[citrulline-p19666465] Critical role for tetrahydrobiopterin recycling by dihydrofolate reductase in regulation of endothelial nitric-oxide synthase coupling: relative importance of the de novo biopterin synthesis versus salvage pathways. (2009). https://pubmed.ncbi.nlm.nih.gov/19666465/ DOI: 10.1074/jbc.m109.041483

tissue_or_cell_type

BH4/BH2 balance and NOS coupling

trigger_kind

machinery_impairment Imported condition classification; unverified.

Citrulline: arginine recycling, nitrogen disposal and nutrient connections (2026-09-17) · lines 775–786

AI-assisted literature curation; primary study URLs and scope retained in the document and extraction. Not publisher full text. · supports · DHFR knockdown/inhibition and controlled GTPCH-expression cell experiments · source_derived_draft · unverified_draft

### citrulline-dhfr-ratio DHFR knockdown decreased intracellular BH4 and increased BH2 in the cell systems. Condition category: machinery_impairment nutrient_topic: Citrulline research collection; topical membership is not evidence of a direct dietary effect. plain_language: Recycling quality matters, not just the total amount of pterin. organism: Mouse endothelial and NIH-3T3-based experimental cell systems tissue_or_cell_type: BH4/BH2 balance and NOS coupling experimental_model: DHFR knockdown/inhibition and controlled GTPCH-expression cell experiments limitations: Mouse models; DHFR also handles folate, but this study does not make folate and BH4 interchangeable. exposure: Methotrexate or RNA interference; comparison with GTPCH knockdown evidence_span: {"source_cache": "artifacts/citrulline-research/19666465.abstract.txt", "locator": "Primary indexed abstract; zero-based, end-exclusive Unicode character offsets", "file_sha256": "b1ef76ebbd312932011edd1b0f9f428026a4e4cdd53301bd66f99eaf543e77af", "start_char": 0, "end_char": 1716, "text_sha256": "b1ef76ebbd312932011edd1b0f9f428026a4e4cdd53301bd66f99eaf543e77af"} [citrulline-p19666465] Critical role for tetrahydrobiopterin recycling by dihydrofolate reductase in regulation of endothelial nitric-oxide synthase coupling: relative importance of the de novo biopterin synthesis versus salvage pathways. (2009). https://pubmed.ncbi.nlm.nih.gov/19666465/ DOI: 10.1074/jbc.m109.041483

availability_state

machinery_impairment Imported condition classification; unverified.

evidence_span

{"source_cache": "artifacts/citrulline-research/19666465.abstract.txt", "locator": "Primary indexed abstract; zero-based, end-exclusive Unicode character offsets", "file_sha256": "b1ef76ebbd312932011edd1b0f9f428026a4e4cdd53301bd66f99eaf543e77af", "start_char": 0, "end_char": 1716, "text_sha256": "b1ef76ebbd312932011edd1b0f9f428026a4e4cdd53301bd66f99eaf543e77af"}

experimental_model

DHFR knockdown/inhibition and controlled GTPCH-expression cell experiments

exposure

Methotrexate or RNA interference; comparison with GTPCH knockdown

limitations

Mouse models; DHFR also handles folate, but this study does not make folate and BH4 interchangeable.

nutrient_topic

Citrulline research collection; topical membership is not evidence of a direct dietary effect. · L-Citrulline

organism

Mouse endothelial and NIH-3T3-based experimental cell systems

plain_language

A damaged cofactor balance can change the enzyme output.

primary_references

[citrulline-p19666465] Critical role for tetrahydrobiopterin recycling by dihydrofolate reductase in regulation of endothelial nitric-oxide synthase coupling: relative importance of the de novo biopterin synthesis versus salvage pathways. (2009). https://pubmed.ncbi.nlm.nih.gov/19666465/ DOI: 10.1074/jbc.m109.041483

tissue_or_cell_type

BH4/BH2 balance and NOS coupling

trigger_kind

machinery_impairment Imported condition classification; unverified.

Citrulline: arginine recycling, nitrogen disposal and nutrient connections (2026-09-17) · lines 788–799

AI-assisted literature curation; primary study URLs and scope retained in the document and extraction. Not publisher full text. · supports · DHFR knockdown/inhibition and controlled GTPCH-expression cell experiments · source_derived_draft · unverified_draft

### citrulline-dhfr-uncoupling DHFR loss increased eNOS-dependent superoxide while reducing NO production. Condition category: machinery_impairment nutrient_topic: Citrulline research collection; topical membership is not evidence of a direct dietary effect. plain_language: A damaged cofactor balance can change the enzyme output. organism: Mouse endothelial and NIH-3T3-based experimental cell systems tissue_or_cell_type: BH4/BH2 balance and NOS coupling experimental_model: DHFR knockdown/inhibition and controlled GTPCH-expression cell experiments limitations: Mouse models; DHFR also handles folate, but this study does not make folate and BH4 interchangeable. exposure: Methotrexate or RNA interference; comparison with GTPCH knockdown evidence_span: {"source_cache": "artifacts/citrulline-research/19666465.abstract.txt", "locator": "Primary indexed abstract; zero-based, end-exclusive Unicode character offsets", "file_sha256": "b1ef76ebbd312932011edd1b0f9f428026a4e4cdd53301bd66f99eaf543e77af", "start_char": 0, "end_char": 1716, "text_sha256": "b1ef76ebbd312932011edd1b0f9f428026a4e4cdd53301bd66f99eaf543e77af"} [citrulline-p19666465] Critical role for tetrahydrobiopterin recycling by dihydrofolate reductase in regulation of endothelial nitric-oxide synthase coupling: relative importance of the de novo biopterin synthesis versus salvage pathways. (2009). https://pubmed.ncbi.nlm.nih.gov/19666465/ DOI: 10.1074/jbc.m109.041483

evidence_span

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experimental_model

Ex-vivo vessel exposure and randomized pre-harvest infusion

exposure

5-MTHF ex vivo 1-100 micromolar; intravenous 5-MTHF or placebo in a separate subset

limitations

Uses 5-MTHF and acute vessel endpoints; cannot be generalized to all folic-acid doses or combination supplements.

nutrient_topic

Citrulline research collection; topical membership is not evidence of a direct dietary effect. · L-Citrulline

organism

Human, vessels from 117 CABG patients

plain_language

A folate form affected availability of the NO-synthase cofactor.

primary_references

[citrulline-p16940192] 5-methyltetrahydrofolate rapidly improves endothelial function and decreases superoxide production in human vessels: effects on vascular tetrahydrobiopterin availability and endothelial nitric oxide synthase coupling. (2006). https://pubmed.ncbi.nlm.nih.gov/16940192/ DOI: 10.1161/circulationaha.106.612325

tissue_or_cell_type

Saphenous veins and internal mammary arteries

Citrulline: arginine recycling, nitrogen disposal and nutrient connections (2026-09-17) · lines 736–747

AI-assisted literature curation; primary study URLs and scope retained in the document and extraction. Not publisher full text. · supports · Ex-vivo vessel exposure and randomized pre-harvest infusion · source_derived_draft · unverified_draft

### citrulline-folate-bh4 5-MTHF increased vascular BH4 and its proportion of total biopterin in human vessel experiments. Condition category: normal nutrient_topic: Citrulline research collection; topical membership is not evidence of a direct dietary effect. plain_language: A folate form affected availability of the NO-synthase cofactor. organism: Human, vessels from 117 CABG patients tissue_or_cell_type: Saphenous veins and internal mammary arteries experimental_model: Ex-vivo vessel exposure and randomized pre-harvest infusion limitations: Uses 5-MTHF and acute vessel endpoints; cannot be generalized to all folic-acid doses or combination supplements. exposure: 5-MTHF ex vivo 1-100 micromolar; intravenous 5-MTHF or placebo in a separate subset evidence_span: {"source_cache": "artifacts/citrulline-research/16940192.abstract.txt", "locator": "Primary indexed abstract; zero-based, end-exclusive Unicode character offsets", "file_sha256": "42c07dd0bbacfc1ff44f78d6ce69810a58946e53c76bf5d3bf8385db5278ba8b", "start_char": 0, "end_char": 1942, "text_sha256": "42c07dd0bbacfc1ff44f78d6ce69810a58946e53c76bf5d3bf8385db5278ba8b"} [citrulline-p16940192] 5-methyltetrahydrofolate rapidly improves endothelial function and decreases superoxide production in human vessels: effects on vascular tetrahydrobiopterin availability and endothelial nitric oxide synthase coupling. (2006). https://pubmed.ncbi.nlm.nih.gov/16940192/ DOI: 10.1161/circulationaha.106.612325

evidence_span

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experimental_model

Ex-vivo vessel exposure and randomized pre-harvest infusion

exposure

5-MTHF ex vivo 1-100 micromolar; intravenous 5-MTHF or placebo in a separate subset

limitations

Uses 5-MTHF and acute vessel endpoints; cannot be generalized to all folic-acid doses or combination supplements.

nutrient_topic

Citrulline research collection; topical membership is not evidence of a direct dietary effect. · L-Citrulline

organism

Human, vessels from 117 CABG patients

plain_language

The enzyme used its electron supply more productively under the tested conditions.

primary_references

[citrulline-p16940192] 5-methyltetrahydrofolate rapidly improves endothelial function and decreases superoxide production in human vessels: effects on vascular tetrahydrobiopterin availability and endothelial nitric oxide synthase coupling. (2006). https://pubmed.ncbi.nlm.nih.gov/16940192/ DOI: 10.1161/circulationaha.106.612325

tissue_or_cell_type

Saphenous veins and internal mammary arteries

Citrulline: arginine recycling, nitrogen disposal and nutrient connections (2026-09-17) · lines 749–760

AI-assisted literature curation; primary study URLs and scope retained in the document and extraction. Not publisher full text. · supports · Ex-vivo vessel exposure and randomized pre-harvest infusion · source_derived_draft · unverified_draft

### citrulline-folate-coupling 5-MTHF improved eNOS coupling and increased its dimer-to-monomer ratio and activity in the human vessel study. Condition category: normal nutrient_topic: Citrulline research collection; topical membership is not evidence of a direct dietary effect. plain_language: The enzyme used its electron supply more productively under the tested conditions. organism: Human, vessels from 117 CABG patients tissue_or_cell_type: Saphenous veins and internal mammary arteries experimental_model: Ex-vivo vessel exposure and randomized pre-harvest infusion limitations: Uses 5-MTHF and acute vessel endpoints; cannot be generalized to all folic-acid doses or combination supplements. exposure: 5-MTHF ex vivo 1-100 micromolar; intravenous 5-MTHF or placebo in a separate subset evidence_span: {"source_cache": "artifacts/citrulline-research/16940192.abstract.txt", "locator": "Primary indexed abstract; zero-based, end-exclusive Unicode character offsets", "file_sha256": "42c07dd0bbacfc1ff44f78d6ce69810a58946e53c76bf5d3bf8385db5278ba8b", "start_char": 0, "end_char": 1942, "text_sha256": "42c07dd0bbacfc1ff44f78d6ce69810a58946e53c76bf5d3bf8385db5278ba8b"} [citrulline-p16940192] 5-methyltetrahydrofolate rapidly improves endothelial function and decreases superoxide production in human vessels: effects on vascular tetrahydrobiopterin availability and endothelial nitric oxide synthase coupling. (2006). https://pubmed.ncbi.nlm.nih.gov/16940192/ DOI: 10.1161/circulationaha.106.612325

evidence_span

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experimental_model

Ex-vivo vessel exposure and randomized pre-harvest infusion

exposure

5-MTHF ex vivo 1-100 micromolar; intravenous 5-MTHF or placebo in a separate subset

limitations

Uses 5-MTHF and acute vessel endpoints; cannot be generalized to all folic-acid doses or combination supplements.

nutrient_topic

Citrulline research collection; topical membership is not evidence of a direct dietary effect. · L-Citrulline

organism

Human, vessels from 117 CABG patients

plain_language

The experiment distinguished this effect from simply lowering homocysteine.

primary_references

[citrulline-p16940192] 5-methyltetrahydrofolate rapidly improves endothelial function and decreases superoxide production in human vessels: effects on vascular tetrahydrobiopterin availability and endothelial nitric oxide synthase coupling. (2006). https://pubmed.ncbi.nlm.nih.gov/16940192/ DOI: 10.1161/circulationaha.106.612325

tissue_or_cell_type

Saphenous veins and internal mammary arteries

Citrulline: arginine recycling, nitrogen disposal and nutrient connections (2026-09-17) · lines 762–773

AI-assisted literature curation; primary study URLs and scope retained in the document and extraction. Not publisher full text. · supports · Ex-vivo vessel exposure and randomized pre-harvest infusion · source_derived_draft · unverified_draft

### citrulline-folate-superoxide 5-MTHF reduced vascular superoxide; the effect was not explained by direct superoxide scavenging or changed plasma homocysteine. Condition category: normal nutrient_topic: Citrulline research collection; topical membership is not evidence of a direct dietary effect. plain_language: The experiment distinguished this effect from simply lowering homocysteine. organism: Human, vessels from 117 CABG patients tissue_or_cell_type: Saphenous veins and internal mammary arteries experimental_model: Ex-vivo vessel exposure and randomized pre-harvest infusion limitations: Uses 5-MTHF and acute vessel endpoints; cannot be generalized to all folic-acid doses or combination supplements. exposure: 5-MTHF ex vivo 1-100 micromolar; intravenous 5-MTHF or placebo in a separate subset evidence_span: {"source_cache": "artifacts/citrulline-research/16940192.abstract.txt", "locator": "Primary indexed abstract; zero-based, end-exclusive Unicode character offsets", "file_sha256": "42c07dd0bbacfc1ff44f78d6ce69810a58946e53c76bf5d3bf8385db5278ba8b", "start_char": 0, "end_char": 1942, "text_sha256": "42c07dd0bbacfc1ff44f78d6ce69810a58946e53c76bf5d3bf8385db5278ba8b"} [citrulline-p16940192] 5-methyltetrahydrofolate rapidly improves endothelial function and decreases superoxide production in human vessels: effects on vascular tetrahydrobiopterin availability and endothelial nitric oxide synthase coupling. (2006). https://pubmed.ncbi.nlm.nih.gov/16940192/ DOI: 10.1161/circulationaha.106.612325

evidence_span

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experimental_model

Crystal structures of zinc-free and zinc-bound heme domains

exposure

Zinc-free versus zinc-tetrathiolate-containing structures

limitations

Direct structure is NOS2; similarity to NOS3 does not justify silently treating the proteins as identical.

nutrient_topic

Citrulline research collection; topical membership is not evidence of a direct dietary effect. · L-Citrulline

organism

Human NOS2

plain_language

The structural metal and the pterin-binding region are connected.

primary_references

[citrulline-p10409685] Crystal structures of zinc-free and -bound heme domain of human inducible nitric-oxide synthase. Implications for dimer stability and comparison with endothelial nitric-oxide synthase. (1999). https://pubmed.ncbi.nlm.nih.gov/10409685/ DOI: 10.1074/jbc.274.30.21276

tissue_or_cell_type

NOS dimer interface and pterin-binding region

Citrulline: arginine recycling, nitrogen disposal and nutrient connections (2026-09-17) · lines 606–617

AI-assisted literature curation; primary study URLs and scope retained in the document and extraction. Not publisher full text. · supports · Crystal structures of zinc-free and zinc-bound heme domains · source_derived_draft · unverified_draft

### citrulline-inos-zinc-interface The NOS2 zinc-tetrathiolate center supported intersubunit contacts and integrity of the BH4-binding site. Condition category: normal nutrient_topic: Citrulline research collection; topical membership is not evidence of a direct dietary effect. plain_language: The structural metal and the pterin-binding region are connected. organism: Human NOS2 tissue_or_cell_type: NOS dimer interface and pterin-binding region experimental_model: Crystal structures of zinc-free and zinc-bound heme domains limitations: Direct structure is NOS2; similarity to NOS3 does not justify silently treating the proteins as identical. exposure: Zinc-free versus zinc-tetrathiolate-containing structures evidence_span: {"source_cache": "artifacts/citrulline-research/10409685.abstract.txt", "locator": "Primary indexed abstract; zero-based, end-exclusive Unicode character offsets", "file_sha256": "c320fb6f3fd659a2e7df4a8ddd3467a902a07b84380075d429040eaf244ccf6b", "start_char": 0, "end_char": 942, "text_sha256": "c320fb6f3fd659a2e7df4a8ddd3467a902a07b84380075d429040eaf244ccf6b"} [citrulline-p10409685] Crystal structures of zinc-free and -bound heme domain of human inducible nitric-oxide synthase. Implications for dimer stability and comparison with endothelial nitric-oxide synthase. (1999). https://pubmed.ncbi.nlm.nih.gov/10409685/ DOI: 10.1074/jbc.274.30.21276

evidence_span

{"source_cache": "artifacts/citrulline-research/39985883.abstract.txt", "locator": "Primary indexed abstract; zero-based, end-exclusive Unicode character offsets", "file_sha256": "0007fe7b3fcc746e77c450f3aca50ca2834c78b84aa83977b3078ae05051ab9c", "start_char": 0, "end_char": 1888, "text_sha256": "0007fe7b3fcc746e77c450f3aca50ca2834c78b84aa83977b3078ae05051ab9c"}

experimental_model

Phase II double-blind placebo-controlled crossover trial

exposure

3 g citrulline twice daily for 12 weeks; additional BH4 0.45 g/day for two weeks in one arm

limitations

Preliminary trial; subgroup-dependent BH4 result and study-specific ADMA cutoff require confirmation; not a general supplement prescription.

nutrient_topic

Citrulline research collection; topical membership is not evidence of a direct dietary effect. · L-Citrulline

organism

Human, 51 peripheral-artery-disease patients

plain_language

The cofactor combination result depended on the measured starting state.

primary_references

[citrulline-p39985883] Nutritional L-Citrulline and Tetrahydrobiopterin in Peripheral Artery Disease: A Phase II Randomized Trial (CIPER Study). (2025). https://pubmed.ncbi.nlm.nih.gov/39985883/ DOI: 10.1016/j.jacadv.2025.101590

tissue_or_cell_type

Absolute claudication distance and plasma biomarkers

Citrulline: arginine recycling, nitrogen disposal and nutrient connections (2026-09-17) · lines 1217–1228

AI-assisted literature curation; primary study URLs and scope retained in the document and extraction. Not publisher full text. · supports · Phase II double-blind placebo-controlled crossover trial · source_derived_draft · unverified_draft

### citrulline-pad-bh4-subgroup Adding BH4 increased the reported walking-distance change to 28.15% only in the subgroup with baseline ADMA above 0.4 micromolar. Condition category: normal nutrient_topic: Citrulline research collection; topical membership is not evidence of a direct dietary effect. plain_language: The cofactor combination result depended on the measured starting state. organism: Human, 51 peripheral-artery-disease patients tissue_or_cell_type: Absolute claudication distance and plasma biomarkers experimental_model: Phase II double-blind placebo-controlled crossover trial limitations: Preliminary trial; subgroup-dependent BH4 result and study-specific ADMA cutoff require confirmation; not a general supplement prescription. exposure: 3 g citrulline twice daily for 12 weeks; additional BH4 0.45 g/day for two weeks in one arm evidence_span: {"source_cache": "artifacts/citrulline-research/39985883.abstract.txt", "locator": "Primary indexed abstract; zero-based, end-exclusive Unicode character offsets", "file_sha256": "0007fe7b3fcc746e77c450f3aca50ca2834c78b84aa83977b3078ae05051ab9c", "start_char": 0, "end_char": 1888, "text_sha256": "0007fe7b3fcc746e77c450f3aca50ca2834c78b84aa83977b3078ae05051ab9c"} [citrulline-p39985883] Nutritional L-Citrulline and Tetrahydrobiopterin in Peripheral Artery Disease: A Phase II Randomized Trial (CIPER Study). (2025). https://pubmed.ncbi.nlm.nih.gov/39985883/ DOI: 10.1016/j.jacadv.2025.101590

evidence_span

{"source_cache": "artifacts/citrulline-research/39985883.abstract.txt", "locator": "Primary indexed abstract; zero-based, end-exclusive Unicode character offsets", "file_sha256": "0007fe7b3fcc746e77c450f3aca50ca2834c78b84aa83977b3078ae05051ab9c", "start_char": 0, "end_char": 1888, "text_sha256": "0007fe7b3fcc746e77c450f3aca50ca2834c78b84aa83977b3078ae05051ab9c"}

experimental_model

Phase II double-blind placebo-controlled crossover trial

exposure

3 g citrulline twice daily for 12 weeks; additional BH4 0.45 g/day for two weeks in one arm

limitations

Preliminary trial; subgroup-dependent BH4 result and study-specific ADMA cutoff require confirmation; not a general supplement prescription.

nutrient_topic

Citrulline research collection; topical membership is not evidence of a direct dietary effect. · L-Citrulline

organism

Human, 51 peripheral-artery-disease patients

plain_language

A patient-relevant walking endpoint improved in this preliminary PAD trial.

primary_references

[citrulline-p39985883] Nutritional L-Citrulline and Tetrahydrobiopterin in Peripheral Artery Disease: A Phase II Randomized Trial (CIPER Study). (2025). https://pubmed.ncbi.nlm.nih.gov/39985883/ DOI: 10.1016/j.jacadv.2025.101590

tissue_or_cell_type

Absolute claudication distance and plasma biomarkers

Citrulline: arginine recycling, nitrogen disposal and nutrient connections (2026-09-17) · lines 1204–1215

AI-assisted literature curation; primary study URLs and scope retained in the document and extraction. Not publisher full text. · supports · Phase II double-blind placebo-controlled crossover trial · source_derived_draft · unverified_draft

### citrulline-pad-walking Walking-distance change was 20.11% with citrulline versus 5.73% with placebo, P=0.011. Condition category: normal nutrient_topic: Citrulline research collection; topical membership is not evidence of a direct dietary effect. plain_language: A patient-relevant walking endpoint improved in this preliminary PAD trial. organism: Human, 51 peripheral-artery-disease patients tissue_or_cell_type: Absolute claudication distance and plasma biomarkers experimental_model: Phase II double-blind placebo-controlled crossover trial limitations: Preliminary trial; subgroup-dependent BH4 result and study-specific ADMA cutoff require confirmation; not a general supplement prescription. exposure: 3 g citrulline twice daily for 12 weeks; additional BH4 0.45 g/day for two weeks in one arm evidence_span: {"source_cache": "artifacts/citrulline-research/39985883.abstract.txt", "locator": "Primary indexed abstract; zero-based, end-exclusive Unicode character offsets", "file_sha256": "0007fe7b3fcc746e77c450f3aca50ca2834c78b84aa83977b3078ae05051ab9c", "start_char": 0, "end_char": 1888, "text_sha256": "0007fe7b3fcc746e77c450f3aca50ca2834c78b84aa83977b3078ae05051ab9c"} [citrulline-p39985883] Nutritional L-Citrulline and Tetrahydrobiopterin in Peripheral Artery Disease: A Phase II Randomized Trial (CIPER Study). (2025). https://pubmed.ncbi.nlm.nih.gov/39985883/ DOI: 10.1016/j.jacadv.2025.101590

evidence_span

{"source_cache": "artifacts/citrulline-research/32073878.abstract.txt", "locator": "Primary indexed abstract; zero-based, end-exclusive Unicode character offsets", "file_sha256": "3362b0aae303b9c2e54cfa8a291e4d993ab4496d26cf11a1dba96c27c4c3fb7f", "start_char": 0, "end_char": 1850, "text_sha256": "3362b0aae303b9c2e54cfa8a291e4d993ab4496d26cf11a1dba96c27c4c3fb7f"}

experimental_model

Controlled neonatal hypoxia and combination-treatment experiment

exposure

L-citrulline, sapropterin, or both during days 3-10 of hypoxia

limitations

Animal combination evidence; does not establish neonatal clinical dosing or treatment benefit in humans.

nutrient_topic

Citrulline research collection; topical membership is not evidence of a direct dietary effect. · L-Citrulline

organism

Newborn pig

plain_language

The measured signaling machinery responded alongside the vascular outcome.

primary_references

[citrulline-p32073878] Combined l-citrulline and tetrahydrobiopterin therapy improves NO signaling and ameliorates chronic hypoxia-induced pulmonary hypertension in newborn pigs. (2020). https://pubmed.ncbi.nlm.nih.gov/32073878/ DOI: 10.1152/ajplung.00280.2019

tissue_or_cell_type

Pulmonary arteries and hemodynamics

Citrulline: arginine recycling, nitrogen disposal and nutrient connections (2026-09-17) · lines 827–838

AI-assisted literature curation; primary study URLs and scope retained in the document and extraction. Not publisher full text. · supports · Controlled neonatal hypoxia and combination-treatment experiment · source_derived_draft · unverified_draft

### citrulline-pig-combination-coupling Combination treatment improved eNOS dimer ratios and NO production more than either single intervention. Condition category: normal nutrient_topic: Citrulline research collection; topical membership is not evidence of a direct dietary effect. plain_language: The measured signaling machinery responded alongside the vascular outcome. organism: Newborn pig tissue_or_cell_type: Pulmonary arteries and hemodynamics experimental_model: Controlled neonatal hypoxia and combination-treatment experiment limitations: Animal combination evidence; does not establish neonatal clinical dosing or treatment benefit in humans. exposure: L-citrulline, sapropterin, or both during days 3-10 of hypoxia evidence_span: {"source_cache": "artifacts/citrulline-research/32073878.abstract.txt", "locator": "Primary indexed abstract; zero-based, end-exclusive Unicode character offsets", "file_sha256": "3362b0aae303b9c2e54cfa8a291e4d993ab4496d26cf11a1dba96c27c4c3fb7f", "start_char": 0, "end_char": 1850, "text_sha256": "3362b0aae303b9c2e54cfa8a291e4d993ab4496d26cf11a1dba96c27c4c3fb7f"} [citrulline-p32073878] Combined l-citrulline and tetrahydrobiopterin therapy improves NO signaling and ameliorates chronic hypoxia-induced pulmonary hypertension in newborn pigs. (2020). https://pubmed.ncbi.nlm.nih.gov/32073878/ DOI: 10.1152/ajplung.00280.2019

evidence_span

{"source_cache": "artifacts/citrulline-research/32073878.abstract.txt", "locator": "Primary indexed abstract; zero-based, end-exclusive Unicode character offsets", "file_sha256": "3362b0aae303b9c2e54cfa8a291e4d993ab4496d26cf11a1dba96c27c4c3fb7f", "start_char": 0, "end_char": 1850, "text_sha256": "3362b0aae303b9c2e54cfa8a291e4d993ab4496d26cf11a1dba96c27c4c3fb7f"}

experimental_model

Controlled neonatal hypoxia and combination-treatment experiment

exposure

L-citrulline, sapropterin, or both during days 3-10 of hypoxia

limitations

Animal combination evidence; does not establish neonatal clinical dosing or treatment benefit in humans.

nutrient_topic

Citrulline research collection; topical membership is not evidence of a direct dietary effect. · L-Citrulline

organism

Newborn pig

plain_language

Both substrate supply and cofactor support mattered in this animal model.

primary_references

[citrulline-p32073878] Combined l-citrulline and tetrahydrobiopterin therapy improves NO signaling and ameliorates chronic hypoxia-induced pulmonary hypertension in newborn pigs. (2020). https://pubmed.ncbi.nlm.nih.gov/32073878/ DOI: 10.1152/ajplung.00280.2019

tissue_or_cell_type

Pulmonary arteries and hemodynamics

Citrulline: arginine recycling, nitrogen disposal and nutrient connections (2026-09-17) · lines 814–825

AI-assisted literature curation; primary study URLs and scope retained in the document and extraction. Not publisher full text. · supports · Controlled neonatal hypoxia and combination-treatment experiment · source_derived_draft · unverified_draft

### citrulline-pig-combination-pvr Combined citrulline and BH4 improved pulmonary vascular resistance more than either alone in hypoxic piglets. Condition category: normal nutrient_topic: Citrulline research collection; topical membership is not evidence of a direct dietary effect. plain_language: Both substrate supply and cofactor support mattered in this animal model. organism: Newborn pig tissue_or_cell_type: Pulmonary arteries and hemodynamics experimental_model: Controlled neonatal hypoxia and combination-treatment experiment limitations: Animal combination evidence; does not establish neonatal clinical dosing or treatment benefit in humans. exposure: L-citrulline, sapropterin, or both during days 3-10 of hypoxia evidence_span: {"source_cache": "artifacts/citrulline-research/32073878.abstract.txt", "locator": "Primary indexed abstract; zero-based, end-exclusive Unicode character offsets", "file_sha256": "3362b0aae303b9c2e54cfa8a291e4d993ab4496d26cf11a1dba96c27c4c3fb7f", "start_char": 0, "end_char": 1850, "text_sha256": "3362b0aae303b9c2e54cfa8a291e4d993ab4496d26cf11a1dba96c27c4c3fb7f"} [citrulline-p32073878] Combined l-citrulline and tetrahydrobiopterin therapy improves NO signaling and ameliorates chronic hypoxia-induced pulmonary hypertension in newborn pigs. (2020). https://pubmed.ncbi.nlm.nih.gov/32073878/ DOI: 10.1152/ajplung.00280.2019

evidence_span

{"source_cache": "artifacts/citrulline-research/10749876.abstract.txt", "locator": "Primary indexed abstract; zero-based, end-exclusive Unicode character offsets", "file_sha256": "10605941fa542bd62ed4ee0fb042ea7569f3ab619f1cb893b54d7e62386712ed", "start_char": 0, "end_char": 1519, "text_sha256": "10605941fa542bd62ed4ee0fb042ea7569f3ab619f1cb893b54d7e62386712ed"}

experimental_model

Cell exposure and recombinant-enzyme assays

exposure

Ascorbate loading; BH4 or sepiapterin comparisons

limitations

Preclinical experiments; not proof that vitamin C is needed as a co-supplement with citrulline.

nutrient_topic

Citrulline research collection; topical membership is not evidence of a direct dietary effect. · L-Citrulline

organism

Porcine aortic endothelial cells and bovine eNOS

plain_language

Vitamin C can affect cofactor availability in this cell model.

primary_references

[citrulline-p10749876] Ascorbic acid enhances endothelial nitric-oxide synthase activity by increasing intracellular tetrahydrobiopterin. (2000). https://pubmed.ncbi.nlm.nih.gov/10749876/ DOI: 10.1074/jbc.m002248200

tissue_or_cell_type

BH4 availability and NO bioactivity

Citrulline: arginine recycling, nitrogen disposal and nutrient connections (2026-09-17) · lines 697–708

AI-assisted literature curation; primary study URLs and scope retained in the document and extraction. Not publisher full text. · supports · Cell exposure and recombinant-enzyme assays · source_derived_draft · unverified_draft

### citrulline-vitaminc-bh4 Ascorbate increased intracellular BH4 in porcine aortic endothelial cells. Condition category: normal nutrient_topic: Citrulline research collection; topical membership is not evidence of a direct dietary effect. plain_language: Vitamin C can affect cofactor availability in this cell model. organism: Porcine aortic endothelial cells and bovine eNOS tissue_or_cell_type: BH4 availability and NO bioactivity experimental_model: Cell exposure and recombinant-enzyme assays limitations: Preclinical experiments; not proof that vitamin C is needed as a co-supplement with citrulline. exposure: Ascorbate loading; BH4 or sepiapterin comparisons evidence_span: {"source_cache": "artifacts/citrulline-research/10749876.abstract.txt", "locator": "Primary indexed abstract; zero-based, end-exclusive Unicode character offsets", "file_sha256": "10605941fa542bd62ed4ee0fb042ea7569f3ab619f1cb893b54d7e62386712ed", "start_char": 0, "end_char": 1519, "text_sha256": "10605941fa542bd62ed4ee0fb042ea7569f3ab619f1cb893b54d7e62386712ed"} [citrulline-p10749876] Ascorbic acid enhances endothelial nitric-oxide synthase activity by increasing intracellular tetrahydrobiopterin. (2000). https://pubmed.ncbi.nlm.nih.gov/10749876/ DOI: 10.1074/jbc.m002248200

evidence_span

{"source_cache": "artifacts/citrulline-research/10749876.abstract.txt", "locator": "Primary indexed abstract; zero-based, end-exclusive Unicode character offsets", "file_sha256": "10605941fa542bd62ed4ee0fb042ea7569f3ab619f1cb893b54d7e62386712ed", "start_char": 0, "end_char": 1519, "text_sha256": "10605941fa542bd62ed4ee0fb042ea7569f3ab619f1cb893b54d7e62386712ed"}

experimental_model

Cell exposure and recombinant-enzyme assays

exposure

Ascorbate loading; BH4 or sepiapterin comparisons

limitations

Preclinical experiments; not proof that vitamin C is needed as a co-supplement with citrulline.

nutrient_topic

Citrulline research collection; topical membership is not evidence of a direct dietary effect. · L-Citrulline

organism

Porcine aortic endothelial cells and bovine eNOS

plain_language

Protecting the machinery can matter as well as supplying arginine.

primary_references

[citrulline-p10749876] Ascorbic acid enhances endothelial nitric-oxide synthase activity by increasing intracellular tetrahydrobiopterin. (2000). https://pubmed.ncbi.nlm.nih.gov/10749876/ DOI: 10.1074/jbc.m002248200

tissue_or_cell_type

BH4 availability and NO bioactivity

Citrulline: arginine recycling, nitrogen disposal and nutrient connections (2026-09-17) · lines 710–721

AI-assisted literature curation; primary study URLs and scope retained in the document and extraction. Not publisher full text. · supports · Cell exposure and recombinant-enzyme assays · source_derived_draft · unverified_draft

### citrulline-vitaminc-no Ascorbate increased arginine-to-citrulline conversion and NO-related output in porcine endothelial cells. Condition category: normal nutrient_topic: Citrulline research collection; topical membership is not evidence of a direct dietary effect. plain_language: Protecting the machinery can matter as well as supplying arginine. organism: Porcine aortic endothelial cells and bovine eNOS tissue_or_cell_type: BH4 availability and NO bioactivity experimental_model: Cell exposure and recombinant-enzyme assays limitations: Preclinical experiments; not proof that vitamin C is needed as a co-supplement with citrulline. exposure: Ascorbate loading; BH4 or sepiapterin comparisons evidence_span: {"source_cache": "artifacts/citrulline-research/10749876.abstract.txt", "locator": "Primary indexed abstract; zero-based, end-exclusive Unicode character offsets", "file_sha256": "10605941fa542bd62ed4ee0fb042ea7569f3ab619f1cb893b54d7e62386712ed", "start_char": 0, "end_char": 1519, "text_sha256": "10605941fa542bd62ed4ee0fb042ea7569f3ab619f1cb893b54d7e62386712ed"} [citrulline-p10749876] Ascorbic acid enhances endothelial nitric-oxide synthase activity by increasing intracellular tetrahydrobiopterin. (2000). https://pubmed.ncbi.nlm.nih.gov/10749876/ DOI: 10.1074/jbc.m002248200

evidence_span

{"source_cache": "artifacts/cold-research/9753429.abstract.txt", "locator": "Indexed abstract; zero-based, end-exclusive Unicode character offsets", "file_sha256": "3c76ec6d5dadb463c0a044175d5a78fbd02df4da7c339029c8c232e8878175b9", "start_char": 0, "end_char": 1695, "text_sha256": "3c76ec6d5dadb463c0a044175d5a78fbd02df4da7c339029c8c232e8878175b9"}

experimental_model

Crystal structure of rat tyrosine hydroxylase with a cofactor analogue and iron at 2.3 angstrom

exposure

Bound 7,8-dihydrobiopterin and iron

limitations

Structural chemistry of the rate-limiting step. It is a rat enzyme structure with a cofactor analogue, not a measurement of iron status in an animal.

nutrient_topic

Cold water immersion research collection; topical membership is not evidence of a direct clinical effect, and a therapeutic exposure is not a dietary intake. · Cold water immersion

organism

Rat enzyme

plain_language

The first and slowest step of making noradrenaline needs an iron atom and a molecule of oxygen.

primary_references

[cold-p9753429] Crystal structure of tyrosine hydroxylase with bound cofactor analogue and iron at 2.3 A resolution: self-hydroxylation of Phe300 and the pterin-binding site. (1998). https://pubmed.ncbi.nlm.nih.gov/9753429/ DOI: 10.1021/bi981462g

tissue_or_cell_type

Purified catalytic and tetramerization domains

Cold water immersion: cold sensing, heat production, the catecholamine axis and what repeated exposure changes (2026-09-19) · lines 533–544

AI-assisted literature curation; primary study URLs and scope retained in the document and extraction. Not publisher full text. · supports · Crystal structure of rat tyrosine hydroxylase with a cofactor analogue and iron at 2.3 angstrom · source_derived_draft · unverified_draft

### cold-th-iron-oxygen-pterin Tyrosine hydroxylase is a non-heme iron enzyme that uses molecular oxygen to hydroxylate tyrosine to L-DOPA and tetrahydrobiopterin to 4a-hydroxybiopterin in the rate-limiting step of the catecholamine biosynthetic pathway. Condition category: normal nutrient_topic: Cold water immersion research collection; topical membership is not evidence of a direct clinical effect, and a therapeutic exposure is not a dietary intake. plain_language: The first and slowest step of making noradrenaline needs an iron atom and a molecule of oxygen. organism: Rat enzyme tissue_or_cell_type: Purified catalytic and tetramerization domains experimental_model: Crystal structure of rat tyrosine hydroxylase with a cofactor analogue and iron at 2.3 angstrom limitations: Structural chemistry of the rate-limiting step. It is a rat enzyme structure with a cofactor analogue, not a measurement of iron status in an animal. exposure: Bound 7,8-dihydrobiopterin and iron evidence_span: {"source_cache": "artifacts/cold-research/9753429.abstract.txt", "locator": "Indexed abstract; zero-based, end-exclusive Unicode character offsets", "file_sha256": "3c76ec6d5dadb463c0a044175d5a78fbd02df4da7c339029c8c232e8878175b9", "start_char": 0, "end_char": 1695, "text_sha256": "3c76ec6d5dadb463c0a044175d5a78fbd02df4da7c339029c8c232e8878175b9"} [cold-p9753429] Crystal structure of tyrosine hydroxylase with bound cofactor analogue and iron at 2.3 A resolution: self-hydroxylation of Phe300 and the pterin-binding site. (1998). https://pubmed.ncbi.nlm.nih.gov/9753429/ DOI: 10.1021/bi981462g