Abstract
Using beef brain and liver mitochondria, the effects of several inhibitors on the oxidation of serotonin and tyramine were studied. Mitochondrial MAO of beef brain showed pH optima of 8.8 with serotonin and 8.0 with tyramine. Pheniprazine had similar effects on the oxidations of serotonin and tyramine by brain MAO at pH 7.4 and pH 8.5. Amphetamine inhibited serotonin oxidation much more than tyramine oxidation by brain MAO both at pH 7.4 and pH 8.5. Harmine selectively inhibited serotonin oxidation by brain MAO at pH 7.4. It inhibited serotonin oxidation by brain MAO competitively and tyramine oxidation uncompetitively. However, after heat treatment of mitochondrial MAO of brain at 60°C for 10 min, its effects on the oxidations of serotonin and tyramine were similar. The pI-activity curve of inhibition of oxidation of serotonin coincided with that of tyramine. The effect of harmine on MAO of brain mitochondria differed markedly before and after heat treatment: the pI-activity curve was biphasic before heat treatment but it became sigmoidal after heat treatment. These results suggest that there are at least two types of MAO in beef brain mitochondria: one heat labile, with low affinity for harmine, and the other rather heat stable, with high atnity for harmine. Harmine inhibited liver MAO only very slightly with either serotonin or tyramine as substrate.
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