Localization of immunoreactive glutamyl aminopeptidase in rat brain. I. Association with cerebral microvessels

Glutamyl aminopeptidase (aminopeptidase-A, EC 3.4.11.7) is an ectoenzyme that selectively hydrolyses N-terminal glutamyl and aspartyl residues from oligopeptides, including (Asp1) angiotensin II. Here we sought to determine the distribution of glutamyl aminopeptidase (EAP) in rat brain. EAP was purified to homogeneity from rat kidney and polyclonal antiserum raised in rabbits. Immune serum inhibited EAP enzyme activity in kidney homogenates and labeled two major protein bands of M(r) = 136,000 and M(r) = 101,000 in immunoblots of kidney protein. EAP-like immunoreactivity was concentrated on kidney proximal tubule brush borders. Immunocytochemical staining of rat brain indicated that EAP-like immunoreactivity was primarily associated with cerebral microvessels. Positive staining was detected in microvessels ranging in size from capillaries up to vessels approximately 50 microns in diameter. Isolated cerebral microvessels had a 23-fold enrichment in EAP enzyme activity (193.1 +/- 40.4 nmol/mg protein/h) compared to brain homogenates. Finally, immunoblots of isolated cerebral microvessels resulted in a pattern of labeling similar to that seen with kidney homogenates. These results indicate that EAP activity in brain is primarily associated with cerebral microvessels, and suggest that EAP may be involved in the metabolism of circulating or locally formed peptides.