Inhibition of human brain type B monoamine oxidase by tricyclic psychoactive drugs

The ability of a variety of tricylic psychoactive drugs to inhibit human brain mitochondrial type B monoamine oxidase as measured by phenylethylamine (PEA) deamination was examined in vitro. At 50 μM all drugs tested, with the exception of chlorpromazine sulfoxide and imipramine N-oxide, inhibited this reaction between 40.8% and 78.4%. Lineweaver-Burk plots for imipramine, chlorpromazine, and chlorprothixene inhibition of PEA deamination displayed a mixed inhibition pattern when incubations were performed at normal atmospheric oxygen tension. When the oxygen concentration was elevated, inhibition of this reaction by each of the three drugs became more competitive. These results suggest that these drugs inhibit the B form of monoamine oxidase by binding to both the oxidized and reduced forms of the enzyme. Inhibition of monoamine oxidase by imipramine and desmethylimipramine increased as the pH was raised from 7.0 to 9.0, but because the ratio of the increase remained constant for the two drugs, inhibition probably was independent of the degree of ionization of the side chain alipathic amine. It was also found that the optimal pH for human brain mitochondrial deamination of PEA shifted from 8.0 to 8.5 as the oxygen concentration was increased.