{"id":"39a664fe-9996-52be-b4d3-8416b43893e6","stable_key":"911fb3c7-8cc3-5667-b677-5682fab67648:histidine-hnmt-methylation","predicate":"forms","statement":"Human HNMT methylates histamine using SAM; structural complexes locate histamine and the reaction product SAH at the enzyme.","claim_class":"mechanistic","status":"source_derived_draft","evidence_grade":"ungraded","direction":"context_dependent","is_public":true,"mechanism_event_id":"50cd73fa-a746-5552-9592-243ea2c378a9","mechanism_event_label":"Histamine disposal connects to the cellular methyl-donor system.","subject":{"id":"470c158d-c9b9-5b79-b743-771272e353e8","slug":"hnmt","display_name":"Human histamine N-methyltransferase / HNMT","entity_type_key":"protein"},"object":{"id":"a76f3be3-75ce-5dcd-9a70-3769a9034492","slug":"n-tau-methylhistamine","display_name":"N-tau-Methylhistamine","entity_type_key":"small_molecule"},"evidence_count":1,"mechanism_event":{"id":"50cd73fa-a746-5552-9592-243ea2c378a9","stable_key":"911fb3c7-8cc3-5667-b677-5682fab67648:histidine-hnmt-methylation-event","event_type":"observed_relationship","label":"Histamine disposal connects to the cellular methyl-donor system.","description":"Human HNMT methylates histamine using SAM; structural complexes locate histamine and the reaction product SAH at the enzyme.","status":"provisional","compartment":null,"participants":[{"entity":{"id":"470c158d-c9b9-5b79-b743-771272e353e8","slug":"hnmt","display_name":"Human histamine N-methyltransferase / HNMT","entity_type_key":"protein"},"role":"subject","stoichiometry":null,"state_label":"","sequence_order":0,"notes":""},{"entity":{"id":"a76f3be3-75ce-5dcd-9a70-3769a9034492","slug":"n-tau-methylhistamine","display_name":"N-tau-Methylhistamine","entity_type_key":"small_molecule"},"role":"target","stoichiometry":null,"state_label":"","sequence_order":1,"notes":""},{"entity":{"id":"44374451-3436-5136-a8ae-5dcc6d8c353b","slug":"histidine","display_name":"L-Histidine","entity_type_key":"small_molecule"},"role":"context_participant","stoichiometry":null,"state_label":"","sequence_order":2,"notes":""},{"entity":{"id":"847daefd-ce13-541a-b426-919975168a3d","slug":"histamine","display_name":"Histamine","entity_type_key":"small_molecule"},"role":"context_participant","stoichiometry":null,"state_label":"","sequence_order":3,"notes":""},{"entity":{"id":"825f2da2-01bc-5874-a3c6-64f5ac867db5","slug":"s-adenosylmethionine","display_name":"S-Adenosyl-L-methionine","entity_type_key":"small_molecule"},"role":"context_participant","stoichiometry":null,"state_label":"","sequence_order":4,"notes":""},{"entity":{"id":"e84829b1-0607-558e-8add-aaa3af4747e7","slug":"s-adenosylhomocysteine","display_name":"S-Adenosyl-L-homocysteine","entity_type_key":"small_molecule"},"role":"context_participant","stoichiometry":null,"state_label":"","sequence_order":5,"notes":""}]},"contexts":[{"dimension":"evidence_access","value_text":"Primary abstract","comparator":null,"unit":null,"notes":"","entity":null},{"dimension":"experimental_model","value_text":"Purified recombinant human enzyme; ternary structures and steady-state kinetics.","comparator":null,"unit":null,"notes":"","entity":null},{"dimension":"limitations","value_text":"A shared SAM requirement does not prove clinically important methyl depletion from histamine turnover.","comparator":null,"unit":null,"notes":"","entity":null},{"dimension":"nutrient_topic","value_text":"L-Histidine collection; species, compartment, exposure, co-substrates and manipulation remain explicit.","comparator":null,"unit":null,"notes":"","entity":{"slug":"histidine","display_name":"L-Histidine","entity_type_key":"small_molecule"}},{"dimension":"plain_language","value_text":"Histamine disposal connects to the cellular methyl-donor system.","comparator":null,"unit":null,"notes":"","entity":null},{"dimension":"primary_references","value_text":"Two polymorphic forms of human histamine methyltransferase: structural, thermal, and kinetic comparisons. · 2001 · https://pubmed.ncbi.nlm.nih.gov/11566133/ · DOI 10.1016/s0969-2126(01)00643-8","comparator":null,"unit":null,"notes":"","entity":null}],"evidence":[{"id":"b02eab64-8f3d-5f6f-b481-e8e2d0cdd59b","evidence_kind":"source_excerpt","locator":"Lines 210-216","start_line":210,"end_line":216,"excerpt":"## histidine-hnmt-methylation\nHistamine disposal connects to the cellular methyl-donor system.\nHuman HNMT methylates histamine using SAM; structural complexes locate histamine and the reaction product SAH at the enzyme.\nModel: Purified recombinant human enzyme; ternary structures and steady-state kinetics.\nLimitations: A shared SAM requirement does not prove clinically important methyl depletion from histamine turnover.\nEvidence access: Primary abstract\nTwo polymorphic forms of human histamine methyltransferase: structural, thermal, and kinetic comparisons. · 2001 · https://pubmed.ncbi.nlm.nih.gov/11566133/ · DOI 10.1016/s0969-2126(01)00643-8","model_system":"Purified recombinant human enzyme; ternary structures and steady-state kinetics.","directness":"reported_statement","verification_status":"source_derived_draft","notes":"Original curation paraphrase; evidence access and experimental limitations specified.","relationship":"supports","weight":1.0,"link_notes":"","source":{"id":"ce59e6c9-1213-523b-9d0b-400b7a81cd1c","stable_key":"import-911fb3c7-8cc3-5667-b677-5682fab67648","title":"L-Histidine: supply, catabolism, histamine, receptors and cross-nutrient mechanisms (2026-09-19)","document_type":"imported_text","citation_label":"AI-assisted research curation; primary references, access levels and experimental limitations individually identified. 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