Intra- and extra-dopamine-synaptosomal localization of monoamine oxidase in striatal homogenates from four species.

MAO-A and MAO-B activities within and outside dopaminergic synaptosomes in homogenates of striatal tissue from pig, cat, rat and human brains have been studied by using a specific "low substrate concentration technique" with dopamine. It was found that within the synaptosomes, MAO-A activity predominated in all species. Outside the synaptosomes there were more pronounced differences and only in the rat did MAO-A predominate, while in the other species MAO-B predominated. When estimating MAO-A and -B activities with a conventional method the activity of MAO-B predominated in man, cat and pig. Thus, also in species where the MAO-B activity (as estimated in a conventional way) was dominating, the intrasynaptosomal deamination of dopamine was brought about mainly by MAO-A. The "low substrate concentration technique", more adequately reflects physiological conditions by taking into account the higher concentration of monoamine transmitter substrates within the monoamine neurons. With this technique it was found that in all species (with the possible exception of man) the oxidation rate was higher within than that outside the DA-synaptosomes. In man the unavoidable longer time between death and estimation of the enzyme activity may be the cause of the deviating result.