Differences in the hydrolysis of enkephalin congeners by the two domains of angiotensin converting enzyme

The hydrolysis of enkephalin (Enk) congeners by the isolated N- (N-ACE) and C-domain of angiotensin I converting enzyme (ACE) and by the two-domain somatic ACE was investigated. Both Leu(5)- and Met(5)-Enk were cleaved faster by the C-domain than by N-ACE; rates with somatic ACE were 1600 and 2500 nmol/min/nmol enzyme with both active sites being involved. Substitution of Gly(2) by D-Ala(2) reduced the rate to 1/3rd to 1/7th of that of the Enks. N-ACE cleaved Met(5)-Enk-Arg(6)-Phe(7) faster than the C-domain, probably with the highest turnover number of any naturally occurring ACE substrate (7600 min(-1)). This heptapeptide is also hydrolyzed in the absence of Cl-, but the activation by Cl- is unique; Cl- enhances the hydrolysis of the heptapeptide by N-ACE but inhibits it by the C-domain, yielding about a 5 fold difference in the turnover number at physiological pH. This difference may result in the predominant role of the N-domain in converting Met(5)-Enk-Arg(6)-Phe(7) to Enk in vivo. (C) 1997 Elsevier Science Inc.