Identification of aminopeptidase M as an enkephalin-inactivating enzyme in rat cerebral membranes

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Abstract
Two membrane-bound enkephalin-hydrolyzing aminopeptidase activities were partially purified from rat brain membranes. The first, which represents 90% of the total activity, was highly sensitive to both puromycin (Ki = 1 .mu.M) and bestatin (Ki = 0.5 .mu.M). The second was inhibited much more by bestatin (Ki = 4 .mu.M) than by puromycin (Ki = 100 .mu.M). The latter puromycin-insensitive aminopeptidase was found to resemble aminopeptidase M purified from rat kidney brush border membranes. Both displayed the same purification pattern and the same kinetic constants of substrates and inhibitors, and both were similarly inactivated by metal chelating agents. Antibodies raised in rabbits against rat kidney aminopeptidase M inhibited the aminopeptidase activities of both kidney and brain puromycin-insensitive enzymes at similar dilutions, while the brain puromycin-sensitive aminopeptidase activity was not affected. Aminopeptidase M (EC 3.4.11.2) was found to occur in brain, and the role of this enzyme in inactivating endogenous enkephalins released from their neuronal stores is suggested.