Category: biosynthesis
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).