PROPERTIES OF AMINOPEPTIDASE ACTIVITY INVOLVED IN THE CONVERSION OF VASOPRESSIN BY RAT-BRAIN MEMBRANES

Previously it has been shown that vasopressin (VP) and oxytocin are co nverted by aminopeptidase activity in brain membranes into fragments w ith potent CNS activities. This report concerns the properties of this enzyme activity, addressed as VP-converting aminopeptidase (VP-AP) ac tivity, in membranes of the rat brain. The VP-AP activity had a pH opt imum at pH 7.0 and had a K(m) of 17 muM for its action on VP. Amastati n was the most potent aminopeptidase inhibitor. Enzyme activity was in hibited by relatively low concentrations of metal chelators. Treatment of brain membranes by EDTA resulted in loss of enzyme activity that w as completely reversed by 10 muM Zn2+, indicating that VP-AP activity is a metallopeptidase. Several VP analogues and fragments, in particul ar VP(1-8), inhibited the action of enzyme activity on VP. Among pepti des unrelated to VP, angiotension I, somatostatin, and porcine ACTH(1- 39) markedly inhibited enzyme activity. Solubilization of VP-AP activi ty from brain membranes and gel filtration on Sephadex G200 showed two peaks of activity, one eluting with an apparent mass of about 140 kDa , the other in the void volume. Gel filtration fractions were able to convert [H-3][Phe3]VP in a step-wise fashion. The VP-AP-like activity was found in many tissues outside the brain. Highest activity was pres ent in lung, kidney, parts of the gastrointestinal tract, ovary, and u terus. The results indicate that VP-AP activity is a widely distribute d enzyme with probably multiple functions, one of which involves the m etabolism of vasopressin in the brain.