Subcellular localization of types A and B monoamine oxidase in rat brain

The subcellular localization of the A and B forms of MAO in rat brain was examined. Although both enzymatic forms were found to be present in synaptosomal fractions, the B:A ratio in the synaptosomal fraction was one-half that in the extrasynaptosomal mitochondrial fraction obtained from whole brain. The synaptic mitochondria, isolated following lysis of the synaptosomes. had the same B:A ratio as the nonlysed synaptosomes, suggesting that (1) MAO-B activity is, indeed, associated with synaptosomes and (2) that this activity is located in the mitochondria. The relative activities of the A and B enzymes in synaptosomal fractions were found to be greatly different among discrete brain regions, even though only small variations are evident in the whole homogenates. We conclude from these data that the activity of MAO-B may be nonuniformly located among different populations of synaptic endings.