Multiple catalytic sites of rat brain mitochondrial monoamine oxidase |
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Authors: | Toru Egashira Yukio Kuroiwa Kazuya Kamijo |
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Institution: | 1. Department of Biochemical Toxicology, School of Pharmaceutical Sciences, Tokyo 142 Japan;3. Department of Pharmacology, School of Medicine, Showa University, Tokyo 142 Japan |
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Abstract: | We compared the inhibitory and catalytic effects of various monoamines on forms A and B of monoamine oxidase (MAO) on mitochondrial preparations from rat brain in mixed substrate experiments. MAO activity was determined by a radioisotopic assay. MAO showed lower Km values for tryptamine and β-phenylethylamine than for tyramine and serotonin. The Km values of the untreated preparation for tyramine, tryptamine, and β-phenylethylamine obtained were the same as those of the form B enzyme and the Km value for serotonin was the same as that of the form A enzyme. Tyramine and tryptamine were competitive inhibitors of serotonin oxidation and β-phenylethylamine did not bind with form A enzyme or inhibit the oxidation of serotonin, while tyramine and tryptamine were competitive inhibitors of β-phenylethylamine oxidation. Although serotonin was not oxidized by form B enzyme, serotonin was a competitive inhibitor of β-phenylethylamine oxidation. It is suggested that rat brain mitochondrial MAO is characterized by two kinds of binding sites. |
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