Tele-methylhistamine is a specific MAO B substrate in man

Tele-methylhistamine, the first metabolite of histamine in tissues which lack diamine oxidase, is shown to be a substrate for human MAO B. Human liver homogenates were incubated with ³H-tele-methylhistamine and the products separated using thinlayer chromatography. The major product was 3-methylimidazoleacetic acid, the oxidatively deaminated metabolite of tele-methylhistamine. The reaction was inhibited by low concentrations of (-)deprenyl, the specific MAO B inhibitor. Tele-methylhistamine was also found to inhibit competitively the oxidation of phenylethylamine, but not that of 5-hydroxytryptamine, providing further evidence that it is oxidized by MAO B itself and not a related enzyme. This finding implies that (-)deprenyl and other MAO inhibitors used clinically may interfere with histamine metabolism.