[Effects of oxygen and drugs on monoamine oxidase in the dog liver].

The substrate specificity and effects of oxygen concentration and inhibitors on dog liver mitochondrial monoamine oxidase were investigated. MAO activity was determined by measuring oxygen uptake by Warburg's manometer. Dog liver mitochondrial MAO oxidized tyramine most strongly, but oxidation of serotonin, benzylamine and beta-phenylethylamine by this enzyme was found to be very weak, showing less than 30% that of tyramine oxidation. Clorgyline and harmine showed slight inhibition of oxidation of all substrates used in this experiment; however, deprenyl and pargyline strongly inhibited the oxidation of tyramine. The affinity of MAO towards oxygen was found to be strongest with benzylamine as substrate and weakest with hexylamine, and it was intermediate with serotonin. The differences in affinities towards oxygen of this enzyme with various substrates were not changed with any MAO inhibitors added to the reaction mixture. These results suggest that MAO in dog liver mitochondria belongs to type B MAO because of the characteristic behavior of this enzyme to inhibitors. It is suggested that the multiplicity of MAO, which is characterized by substrate and inhibitor specificities, is different from the multiplicity, which is characterized by the affinities towards oxygen.