Studies on human placental monoamine oxidase using mixed substrates.

The activity of mitochondrial monoamine oxidase (MAO) from human placenta was measured with mixtures of labelled and unlabelled tyramine, serotonin (5-HT), benzylamine and beta-phenylethylamine (PEA). Tyramine deamination was inhibited by benzylamine and PEA but not by 5-HT, while benzylamine deamination was inhibited by tyramine and PEA, but not by 5-HT, 5-HT deamination was inhibited by tyramine, benzylamine and PEA and PEA deamination was inhibited by tyramine, benzylamine and 5-HT. These results suggest that MAO in human placenta has multiple catalytic sites or consists of different enzymes. Probably, tyramine, benzylamine and PEA are deaminated oxidatively at a common catalytic site while 5-HT is deaminated at another catalytic site. Benzylamine deamination was inhibited in a mixed noncompetitive fashion by tyramine and PEA in air, but benzylamine deamination was competitively inhibited by PEA at higher concentrations of oxygen. The deaminations of other substrates were inhibited competitively by other substrates, in air. Reciprocal plots of PEA deamination with benzylamine, 5-HT and tyramine gave hyperbolic curves.