Metabolism of Angiotensin-(1-7) by Angiotensin-Converting Enzyme

Abstract

Angiotensin converting enzyme (ACE) inhibitors augment circulating levels of the vasodilator peptide angiotensin-(1-7) [Ang-(1-7)] in man and animals. Increased concentrations of the peptide may contribute to the antihypertensive effects associated with ACE inhibitors. The rise in Ang-(1-7) following ACE inhibition may result from increased production of the peptide or inhibition of the metabolism of Ang-(1-7)-similar 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 attenuated the metabolism of low concentrations of I-Ang-(1-7). The primary product of I-Ang-(1-7) metabolism was identified as Ang-(1-5). Using affinity-purified ACE from canine lung, HPLC separation and amino acid analysis revealed that ACE functioned as a dipeptidyl carboxypeptidase cleaving Ang-(1-7) to the pentapeptide Ang-(1-5). The ACE inhibitors lisinopril and enalaprilat (1 micro mol/L), as well as the chelating agents EDTA, o-phenanthroline, and DTT (0.1-1 mmol/L) abolished the generation of Ang-(1-5) and did not yield other metabolic products. Ang-(1-5) was not further hydrolyzed by ACE. Kinetic analysis of the hydrolysis of Ang-(1-7) by ACE revealed a substrate affinity of 0.81 micro mol/L and maximal velocity of 0.65 micro mols min-1 mg-1. The calculated turnover constant for the peptide was 1.8 sec-1 with a catalytic efficiency (Kcat/Km) of 2200 sec-1 mmol/L-1. These findings suggest that increased levels of Ang-(1-7) following ACE inhibition may be due, in part, to decreased metabolism of the peptide. (Hypertension. 1998;31[part 2]:362-367.). © 1998 Lippincott Williams & Wilkins, Inc.