Some interrelated properties of A and B form monoamine oxidase in monkey brain mitochondria.

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Abstract
The multiplicity of monoamine oxidase (MAO) in monkey brain was studied by comparing the relationship between the selective substrates of MAO and the pH-activity curves obtained using these substrates. When mitochondrial and A form MAO were used as the enzyme preparations with serotonin [5-hydroxytryptamine] (5-HT) and norepinephrine (NE), preferential substrates for A form MAO, the pH optima were 8.8 and 7.8 with 5-HT and 8.5 and 7.2 with NE. These substrates were also oxidized by B form MAO after changing the pH of the incubation medium (shift to alkaline); these pH optima were 9.0 and 8.2, respectively. When common substrates of MAO were used (tyramine, octopamine, dopamine and tryptamine), the pH activity curves obtained were all broad and bell-shaped with pH optima for the 3 species of enzyme (mitochondria, A form and B form MAO) at 8.0, 7.8 and 8.0 with tyramine; 8.3, 7.5 and 8.5 with octopamine; 7.8, 7.5 and 8.5 with dopamine; and 8.0, 8.3 and 6.9 with tryptamine, respectively. The pH optima were 6.6 with .beta.-phelethylamine (.beta.-PEA) and 9.0 with benzylamine, preferential substrates for B form MAO, for either mitochondria or B-form MAO. The Km values obtained for tryptamine and .beta.-PEA were lower than those for the other substrates of MAO, regardless of the enzyme preparations. The Km and Vmax values of both forms MAO for 5-HT and NE were similar to those of the A form MAO. The differences in the Km and Vmax values of the A form MAO and B form MAO for common substrates were comparable. Tyramine, octopamine and dopamine were substrates for both forms MAO, with only a slight preference for B form MAO over A form MAO. Tryptamine may be deaminated predominantly by A form MAO.