Characterization of N-methylphenylethylamine and N-methylphenylethanolamine as substrates for type A and type B monoamine oxidase

Abstract

N-Methylphenylethylamine (MPEA) and N-methylphenylethanolamine (MPEOA) were characterized as substrates for type A and type B monoamine oxidase (MAO) in rat brain mitochondria. The inhibition experiments with clorgyline and deprenyl showed that the inhibition patterns with MPEA as substrate were dependent on substrate concentrations but that this amine was a common substrate for both types of MAO at all substrate concentrations tested. When MPEOA was used as substrate, the inhibition patterns differed markedly at different substrate concentrations; at 10.0 μM, MPEOA acted as a specific substrate for type B MAO, but at 100 and 1000, μM it became a common substrate for both types. Kinetic analyses were carried out for MPEA and MPEOA with the uninhibited, the clorgyline-treated (type B MAO), and the deprenyl-treated enzyme (type A MAO). With the uninhibited enzyme, there were downward deflections in the curves of Lineweaver-Burk plots for both MPEA and MPEOA, suggesting the existence of different affinity components derived from type A and type B MAO. By means of the double-reciprocal plots, using the clorgyline- and the deprenyl-treated enzyme, it was confirmed that the high affinity corresponded to that for type B MAO and the low affinity to that for type A MAO for both MPEA and MPEOA. Therefore, the changes in the inhibition pattern at different substrate concentrations may be due to different affinities of the substrate for both types. By comparing the K m and V max values of both types observed for MPEA and MPEOA, it was pointed out that the β-hydroxylation of MPEA tended to increase the K m value for type A MAO and to decrease the V max values for both types.