Abstract
The substrate specificity of MAO in dog stomach was investigated manometrically. The MAO activities in total stomach homogenate with tyramine, serotonin, 3-phenylethylamine and benzylamine as substrates were found to be 111, 98, 32 and 21μl O2/3 hr/2.0 ml homogenate, respectively. Almost the same results in specific activity were obtained in the three parts of stomach, such as lesser curvature, greater curvature and pylorus. However, in cardia the specific activity with tyramine as substrate was clearly decreased to 71 compared with 111 in total stomach, and the specific activity with serotonin as substrate in cardia was decreased to 36 compared with 98 in total stomach. Moreover, different MAO activities with serotonin as substrate were found in these four parts of stomach. According to the method of Cox and Perhach, the contents of serotonin in these four parts of dog stomach were studied. It was found that the greater curvature contained about twice of serotonin (12.74 μg/g tissue) as much as that in the other three parts. Clorgyline and harmine, which are specific inhibitors of type A MAO, inhibited MAO activity in stomach with serotonin as substrates. On the other hand, these two inhibitors showed a slight inhibition of MAO activity with β-phenylethylamine as substrates which are specific substrates for type B MAO. Deprenyl and pargyline which are the specific inhibitors for type B MAO did not showed any marked inhibition of MAO activity with any of the four substrates. The inhibitors of hydrazine type, such as semicarbazide, iproniazid and pheniprazine showed a slight inhibition of MAO activity with serotonin and tyramine as substrates. On the other hand, marked inhibition of MAO activity with β-phenylethylamine and benzylamine as substrates were observed. Any marked inhibition of MAO activity was not obtained in the case of isoniazid, cuprizone and KCN were used as inhibitor.
From these results, it was suggested that there might be three types of MAO in dog stomach and among them, type A MAO would have a special role in serotonin metabolism.
