When used at 10 mm concentrations in assays in vitro, seven of 37 chemicals strongly inhibited (> 75% inhibition) rat jejunal mucosa histamine-N-methyltransferase (HMT), while eight of 37 chemicals strongly inhibited rat jejunal mucosa diamine oxidase (DAO). The HMT inhibitors included tyramine, β-phenylethylamine, tryptamine, octopamine, agmatine, aminoguanidine and nicotine. The most potent DAO inhibitors were aminoguanidine, anserine, carnosine, histamine, agmatine, thiamine, cadaverine and tyramine. Since many of these inhibitors occur together with histamine in spoiled tuna, the inhibition of the two major intestinal histamine-catabolizing enzymes, HMT and DAO, may play a vital role in the chemical potentiation of histamine toxicity.
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