Aminopeptidase M from human liver. II. Kinetic analysis of inhibition of the enzyme by bile acids

The mechanism of inhibition of aminopeptidase M by bile acids was analyzed by application of the specific velocity plot that was introduced by Baici Eur. J. Biochem. 119, 9-14 (1981)]. Kinetic studies with three bile acids (cholic acid, deoxycholic acid, and chenodeoxycholic acid) and three substrates (Leu-Met, Leu-Gly, and Leu-pNA) showed that the inhibition constants Ki for the bile acids were appreciably different from each other, but that the Ki for each was not affected by the substrates used, being 0.89-1.03 mM for cholic acid, 0.42-0.66 mM for deoxycholic acid, and 0.24-0.31 mM for chenodeoxycholic acid. The values of the kinetic coefficient alpha (apparent Ks in the presence of inhibitor)/Ks] for cholic acid with Leu-Met and Leu-Gly were 9.0 and 2.5, respectively. These values were very similar to those for chenodeoxycholic acid (7.0 and 2.7) but smaller than those for deoxycholic acid (21 and 11). The values of the other kinetic coefficient beta (apparent kp in the presence of inhibitor)/kp] were 0 except in the case of the combinations of Leu-Gly with cholic acid (0.33) and Leu-Gly with chenodeoxycholic acid (0.13). On the basis of these kinetic parameters, the inhibitions by bile acids were classified into 4 types: competitive-noncompetitive linear mixed type (1 less than alpha less than infinity, beta = 0), noncompetitive-uncompetitive linear mixed type (0 less than alpha less than 1, beta = 0), pure noncompetitive type (alpha = 1, beta = 0), and hyperbolic mixed type (1 less than alpha less than infinity, 0 less than beta less than 1).(ABSTRACT TRUNCATED AT 250 WORDS)