Phenylethanolamine (PEOA) and octopamine (OA) were characterized as substrates for type A and type B monoamine oxidase (MAO) at various substrate concentrations, using rat brain mitochondria. The experiments on sensitivity to clorgyline and deprenyl showed that the inhibition patterns with PEOA as substrate differed markedly at different substrate concentrations: at 12.5 μM, PEOA acted as a specific substrate for type B MAO, but at 125 and 1250 μM it became a common substrate for both types of MAO. However, when OA was used as substrate, there were only slight or no differences in the inhibition patterns among the various concentrations tested; OA was found to be a common substrate for both types of MAO. Benzylamine was also examined for comparison and confirmed to be highly specific for type B MAO over a wide concentration range of the substrate. Kinetic analyses were carried out for PEOA and OA. High and low affinities for MAO were identified for PEOA: K m values were 22.7 and 465 μM, and V max values were 6.90 and 19.2 nmoles/mg of protein/30 min respectively. Pretreatment of the enzyme with 10 −6 M clorgyline resulted in the disappearance of the low affinity component, and pretreatment with 10 −6 M deprenyl resulted in the disappearance of the high affinity component. Therefore, the high affinity corresponded to that for type B MAO and the low one to that for type A MAO. For OA, however, the double reciprocal plots were linear with a single affinity component showing K m and V max values of 455 μM and 90.9 nmoles/mg of protein/ 30 min respectively. From the present study, it can be concluded that, when sensitivity of MAO to clorgyline or deprenyl is studied, it is necessary to check the effect of substrate concentration for each substrate and enzyme preparation, suspecting the different affinities of the substrate for type A and type B MAO.
