In the short-term reaction (1 min) NH2OH decreased the MAO activity by either tyramine or benzylamine as substrate. In the long-term reaction (60 min), there was an increased activity in the MAO reaction by tyramine and a decrease by benzylamine under various concentrations of NH2OH. In the long-term reaction, Km-values for tyramine and benzylamine were 1.82 .times. 10-3.0 M and 2.50 .times. 10-2.0 M, respectively, which were consistent with those in the presence of NH2OH. Pursuing the changes of MAO activities by various concentrations of NH2OH using the tyramine substrate at intervals of 1, 10, 30 and 60 min, there was a decrease of MAO activity accompanying an increased concentration of NH2OH after 1 min period, a partial increase after 10 min, and an entire increase in extensive concentrations of NH2OH after 30 min. With the other substrates such as tryptamine, amylamine, hexylamine and .beta.-phenylethylamine, the effects of NH2OH on MAO were similar to those in tyramine. In the butylamine substrate, NH2OH caused inhibitory effects as shown in the benzylamine substrate. The existence of a complex of MAO enzymes corresponding to monoamine substrates is suggested. The multiplicity of MAO enzyme is discussed.
