Anion activation of angiotensin converting enzyme: dependence on nature of substrate

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Abstract
Anion activation of rabbit pulmonary angiotensin converting enzyme was examined by using 23 furanacryloyl- and 3 benzoyl-tripeptides as substrates. Cl- stimulates hydrolysis of all substrates at least 24-fold; however, the kinetic mechanism, the amount of Cl- required, and the efffect of pH on activation, plus the relative activating potencies of various anions, are all strongly dependent on the substrate employed. Three substrate classes were identified. Class I substrates appear to be hydrolyzed at pH 7.5 by an ordered bireactant mechanism in which anion must bind before substrate. The apparent activation constant (KA'') for Cl- ranges from 75-150 mM at pH 7.5, doubles at pH 9.0, and decreases to .apprx. 3 mM at pH 6.0. Class II substrates, in contrast, are hydrolyzed by a nonessential activator mechanism. The kinetically determined KA'' for Cl- at pH 7.5 ranges from 2.9-5.0 mM and changes only slightly with pH. Class III substrates are also hydrolyzed by a nonessential kinetic mechanism but one different from that followed by class II peptides. KA'' values for Cl- at pH 7.5 measured with class III substrates are 18-30 mM. Class II substrates have arginine or lysine at the ultimate or penultimate position. The features distinguishing clss I and III peptides are less clear, although all class III substrates identified have penultimate alanine residues. Possible explanations for the substrate dependence are offered.