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

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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 125 I-Ang-(1–7). The primary product of 125 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 μmol/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 μmol/L and maximal velocity of 0.65 μmols 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 due, in part, to decreased metabolism of the peptide.