METABOLISM OF ANGIOTENSION-(1-7) BY ANGIOTENSIN-CONVERTING ENZYME

Angiotensin converting enzyme (ACE) inhibitors augment circulating lev els of the vasodilator peptide angiotensin-(1-7) [Ang-(1-7)] in man an d animals. Increased concentrations of the peptide may contribute to t he antihypertensive effects associated with ACE inhibitors. The rise i n Ang-(1-7) following ACE inhibition may result from increased product ion of the peptide or inhibition of the metabolism of Ang-(1-7)-simila r to that observed for bradykinin. To address the latter possibility, we determined whether Ang-(1-7) is a substrate for ACE in vitro. In a pulmonary membrane preparation, the ACE inhibitor lisinopril attenuate d the metabolism of low concentrations of I-125-Ang-(1-7). The primary product of I-125-Ang-(1-7) metabolism was identified as Ang-(1-5). Us ing affinity-purified ACE from canine lung, HPLC separation and amino acid analysis revealed that ACE functioned as a dipeptidyl carboxypept idase cleaving Ang-(1-7) to the pentapeptide Ang-(1-5). The ACE inhibi tors lisinopril and enalaprilat (1 mu mol/L), as well as the chelating agents EDTA, o-phenanthroline, and DTT (0.1-1 mmol/L) abolished the g eneration of Ang-(1-5) and did not yield other metabolic products. Ang -(1-5) was not further hydrolyzed by ACE, Kinetic analysis of the hydr olysis of Ang-(1-7) by ACE revealed a substrate affinity of 0.81 mu mo l/L and maximal velocity of 0.65 mu mols min(-1) mg(-1). The calculate d turnover constant for the peptide was 1.8 sec(-1) with a catalytic e fficiency (Kcat/Km) of 2200 sec(-1) mmol/L-1. These findings suggest t hat increased levels of Ang-(1-7) following ACE inhibition may be due, in part, to decreased metabolism of the peptide.