Angiotensinase Activity of Dipeptidyl Aminopeptidase I (Cathepsin C) of Rat Liver

Abstract

Abstract Dipeptidyl aminopeptidase I (cathepsin C) purified from rat liver was shown to have marked angiotensinase activity arising from its ability to catalyze the rapid removal of two dipeptide fragments, in succession, from the NH2 terminus of a variety of angiotensin II analogues. The most rapid rates of degradation were observed on Asn1-angiotensin II (Hypertensin, Ciba), angiotensin II (bovine), and Ile5-angiotensin II (human). Kinetic studies with the first (Km = 0.44 mm) and the last (Km = 0.34 mm) showed essentially the same turnover number (5220 min-1) for the removal of the first dipeptide at pH 5.0 and 37°. The Km for α-Asp-Arg-β-naphthylamide (a model, fluorogenic substrate) was 0.31 mm. Unnatural, biologically active analogues such as β-Asp1-angiotensin II and α-d-Asp1-angiotensin II were also degraded, although at lower rates. No action was detected on Asn1, d-Arg2-angiotensin II. The degradation of Asn1-angiotensin II and Ile5-angiotensin II by dipeptidyl amino-peptidase I was pronounced over a wide range of pH (3 to 7.5) with a maximum between pH 5 and 6. About 20% of the (pH 7.3) activity of the purified enzyme was manifested when added to rat blood plasma that did not contain added Cl- and —SH activators. The predominant products and course of angiotensin II degradation at pH 5.5 by isolated rat liver lysosomes were identical with those produced by purified dipeptidyl aminopeptidase I, thereby demonstrating that this enzyme is probably the major contributor to the angiotensinase activity of liver lysosomes.