Identification of the carboxypeptidase responsible for the post-synthetic modification of creatine kinase in human serum.

The enzyme responsible for the post-translational modification of creatine kinase-MM isoenzyme was purified from human plasma. The enzymatic activity of this enzyme (modifying protein) on the synthetic substrates hippuryl-L-arginine, hippuryl-L-lysine, 3-(2-furylacryloyl)-L-arginine and 3-(2-furylacryloyl)-L-alanyl-L-lysine and the ratio of activities on these substrates are in good agreement with the enzymatic activity of the human serum carboxypeptidase N. The effect of metal ions, chelating agents, proteolytic inhibitors and carboxypeptidase N inhibitor could not differentiate the modifying protein from human serum carboxypeptidase N. Affinity chromatography on Concanavalin-A-Sepharose demonstrated the glycoprotein nature of the modifying protein. The difference in molecular weight observed between modifying protein and carboxypeptidase N can be explained by known instability characteristics and the influence of proteolytic enzymes during purification. Double immunodiffusion analysis with purified antiserum to human carboxypeptidase N confirmed the identity of the modifying protein and carboxypeptidase N.