The kinetics of carboxypeptidase B activity. II. Kinetic parameters of the cobalt and cadmium enzymes.

Purified carboxypeptidase B contains 1 nondialyzable g atom of zinc per molecule based on a molecular weight of 34,300 (1). Inhibition studies with metal chelating agents have indicated that zinc is a functional component of this enzyme (1). The zinc may be replaced by cobalt or by cadmium and the resulting cobalt and cadmium enzymes have pronouncedly different actions toward specific peptide and ester substrates (2). An increase in the rate of hydrolysis of peptide substrates is characteristic of the cobalt enzyme. With the cadmium enzyme, on the other hand, there is a complete loss of the ability to catalyze the hydrolysis of peptide substrates, accompanied by an enhanced esterase activity. The previous paper in this series (3) dealt with the methods employed for rate measurements and kinetic analyses of the native zinc carboxypeptidase B. The kinetic parameters toward several substrates were evaluated and the structural requirements of specific inhibitors for this enzyme were outlined. These studies have been extended, by employing the outlined procedures (3), to include determinations of the kinetic parameters for carboxypeptidase B in which the native zinc has been replaced by cobalt or by cadmium. These constants are tabulated in the present communication along with the enzymeinhibitor dissociation constants of certain competitive inhibitors for the zinc, cobalt, and cadmium enzymes.