L-histidine → histamine via histidine decarboxylase (HDC), restricted to mast cells, basophils, enterochromaffin-like (ECL) cells of the stomach, and histaminergic neurons of the tuberomammillary nucleus. Two parallel degradation paths: peripheral diamine oxidase (DAO; intestinal, renal) → imidazole acetaldehyde; central histamine-N-methyltransferase (HNMT; cytosolic) → N-methylhistamine. DAO deficiency (genetic or post-enteritis) is the proposed mechanism of "histamine intolerance" — symptoms from dietary histamine + DAO-substrate foods (aged cheese, wine, fermented foods).
Organ Systems
immune-hematologic
digestive
nervous
Pathway Steps
l-histidine → histamine — via histidine decarboxylase (HDC) — restricted to mast/basophil/ECL/TMN. Histidine decarboxylase (PLP/B6-dependent) makes histamine in mast cells, basophils, enterochromaffin-like (ECL) cells, and histaminergic neurons. ECL histamine drives gastric acid (the H2-blocker axis); mast-cell histamine drives allergy.
histamine → imidazole-acetaldehyde — via diamine oxidase (DAO) — peripheral path. Diamine oxidase is the main peripheral/gut route for clearing dietary and luminal histamine; low DAO (genetic, or inhibited by alcohol/drugs) is the proposed basis of “histamine intolerance” to fermented/aged foods.
histamine → n-methylhistamine — via histamine-N-methyltransferase (HNMT) — central path. HNMT, using SAM, is the principal route for inactivating histamine inside cells — especially in the CNS and bronchi, which lack DAO. The two routes (DAO peripheral, HNMT central) split histamine clearance by compartment.