FIGURE 24-1 Pathways of histamine metabolism in humans. There are two major paths of histamine metabolism in humans. The more important of these involves ring methylation to form N-methylhistamine, catalyzed by histamine-N-methyltransferase, which is distributed widely. Most of the N-methylhistamine formed is then converted to N-methylimidazoleacetic acid by monoamine oxidase (MAO). Alternatively, histamine may undergo oxidative deami-nation catalyzed mainly by the nonspecific enzyme diamine oxidase (DAO), yielding imidazoleacetic acid, which is then converted to imidazoleacetic acid riboside. These metabolites have little or no activity and are excreted in the urine. Measurement of N-methylhistamine in urine affords a more reliable index of histamine production than assessment of hista-mine itself. Artifactually elevated levels of histamine in urine arise from genitourinary tract bacteria that can decarboxylate histidine.
histamine, thereby explaining some of their untoward effects. Histamine has a major role in the regulation of gastric acid secretion and also modulates neurotransmitter release.
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