histaminase
Summary
Histaminase (diamine oxidase) is an enzyme that degrades histamine by oxidative deamination, helping terminate histamine's biological activity. It is notably produced in increased amounts by the placenta during pregnancy, which can reduce histamine-mediated allergic and asthmatic symptoms. It is also found in the kidney and intestinal mucosa.
Detail
Histaminase (diamine oxidase, DAO) catalyzes the oxidative deamination of histamine, converting it to imidazole acetaldehyde, which is then further metabolized. This is one of two major pathways for histamine degradation, the other being methylation by histamine-N-methyltransferase (HNMT). Histaminase is produced by the placenta, kidney, and intestinal mucosa. Clinically, elevated placental histaminase during pregnancy is classically cited as a reason why some women with asthma or allergic conditions experience symptomatic improvement during pregnancy, since increased histamine breakdown reduces bronchoconstriction and allergic responses. Conversely, deficiency or inhibition of DAO (e.g., by certain medications like isoniazid, or in conditions affecting gut mucosa) can lead to histamine intolerance, causing symptoms such as flushing, headache, GI upset, and urticaria due to accumulation of dietary or endogenous histamine. DAO activity can also be relevant in scombroid food poisoning, where bacterial histidine decarboxylase in spoiled fish produces excess histamine that overwhelms the body's histaminase capacity, leading to histamine toxicity (flushing, headache, GI symptoms, similar to allergic reaction but not IgE-mediated).
Sources
- First Aid for the USMLE Step 1
- Guyton and Hall Textbook of Medical Physiology
- Katzung's Basic and Clinical Pharmacology
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