Studies on beta-phenylethylamine deamination by human placental monoamine oxidase

Kinetical properties of human placental monoamine oxidase (MAO) were investigated in studies on inhibitors and mixed substrates. MAO activity was determined by a radioisotopic assay. Lineweaver-Burk plots were linear at higher and lower concentrations of PEA, whereas at intermediate substrate concentrations, a downward curving plot was obtained. The Km values of the low- and high-affinity sites for PEA deamination were estimated. Studies with mixed substrates showed that 5-HT was a competitive inhibitor and tyramine a mixed-type inhibitor of deamination at high concentrations of PEA, whereas both were non-competitive inhibitors at lower concentrations of PEA. After pre-incubation of human placental mitochondrial preparations with deprenyl, Lineweaver-Burk plots were completely linear, and the Km value was the same as that obtained at low concentrations of PEA in the absence of deprenyl. Tyramine and 5-HT were competitive inhibitors of PEA deamination by deprenyl-treated MAO. From these results it is concluded that there are two kinds of MAO with high- and low-affinity sites for PEA in mitochondria of human placenta, corresponding to type B and A Mao, and that tyramine, 5-HT and PEA share a substrate-binding site on type A Mao, while tyramine and 5-HT bind to a site on type B MAO that is different from the PEA binding site.