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

Anion activation of pulmonary angiotensin converting enzyme has been examined by using 23 furanacryloyl- and 3 benzoyl-tripeptides as substrates. Chloride stimulates hydrolysis of all substrates at least 24-fold. However, the kinetic mechanism, the amount of chloride required, and the effect of pH on activation, plus the relative activating potencies of various anions, are all strongly dependent on the substrate employed. Three substrate classes have been 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 to 150 mM at pH 7.5, doubles at pH 9.0, and decreases to about 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 to 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 Arg or Lys at the ultimate or penultimate position. The features distinguishing class I and III peptides are less clear, although all class III substrates identified have penultimate alanine residues. Possible explanations for this substrate dependence are offered.