Human ‘creatine kinase conversion factor’ identified as a carboxypeptidase

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Abstract
Effect of partially purified creatine kinase conversion factor on rabbit muscle creatine kinase is shown to be that of a carboxypeptidase, removing the C-termial lysine residue from both subunits. These changes fully explain the 3-banded electrophoretic patterns of the partially and the fully modified rabbit and human enzymes. The factor also produces a similar electrophoretic pattern with HbA; comparison with the effects of carboxypeptidases A and B implies that the C-terminal residue of both .alpha.- and .beta.-subunits are removed. Small synthetic peptides are poor or non-substrates. A low activity with hippuryl-L-lysine may be due to contamination of the preparation with carboxypeptidase N. The possibility has been excluded that the action of conversion factor on creatine kinase involves modification of the protein thiol groups. MW substrate-specificity, pH-activity profile and effects of metal ions distinguish creatine kinase conversion factor from carboxypeptidases A, B and N. The provisional name of carboxypeptidase K is prepared for the conversion factor.