Regional distribution of soluble and membrane-bound aminopeptidase activities in rat brain

Soluble and membrane-bound aminopeptidase activities in 11 regions of the rat brain were assayed using L-Leucine-2-naphthylamide as a substrate. In addition, two metabolic enzymatic activities were compared: lactate dehydrogenase and aspartate aminotransferase. All enzymatic activities showed significant regional differences when the data were analyzed statistically. Soluble aminopeptidase and aspartate aminotransferase activities were significantly lower in cortical than in subcortical areas. Membrane-bound aminopeptidase activity levels were higher in cortical areas. Lactate dehydrogenase activities did not differ between cortical areas and the rest of the zones studied. However, although no wide regional differences were found for the other enzymatic activities, membrane-bound aminopeptidase varied markedly across brain regions: a fivefold difference was observed between zones such as parietotemporal cortex and medulla. The differential distribution of this enzymatic activity is consistent with the hypothesis that it could be responsible for the enzymatic inactivation of some neuroactive peptides.