Role of chloride ion as an allosteric activator of angiotensin-converting enzyme

The nature of chloride ion as an activator of angiotensin-converting enzyme was studied by a series of kinetic experiments with hog plasma enzyme preparation. The enzyme required the presence of chloride ion for its full catalytic activity, but its requirement of monovalent anion was not absolute. The K_A value for the enzymechloride binding was estimated to be about 150 mm in all cases regardless of the peptide substrates employed. In the presence of chloride ion, the activity of the enzyme was increased, but its optimum pH was shifted gradually to the alkaline region up to pH 8.2 depending on the concentration of chloride ion. In addition, in the presence of chloride ion, the apparent K_m values were reduced markedly while the V_max values were not much altered; for example, for the hydrolysis of angiotensin I decapeptide, the K_m value decreased by a factor of 50 while only an 18% increase in V_max was observed when the enzyme was saturated with chloride ion. The result suggests that chloride ion acts as a conformational modifier inducing the affinity of synergistic binding of substrate.