Human 'creatine kinase conversion factor' identified as a carboxypeptidase.

Area of Science:

• Biochemistry
• Enzymology

Background:

• Creatine kinase (CK) exists in multiple electrophoretic forms.
• The 'creatine kinase conversion factor' (CKCF) was previously identified but its enzymatic activity was unclear.

Purpose of the Study:

• To elucidate the enzymatic activity of the partially purified CKCF.
• To characterize the substrate specificity and properties of CKCF.
• To propose a name for the novel enzyme.

Main Methods:

• Enzymatic assays using rabbit muscle creatine kinase and hemoglobin A.
• Electrophoretic analysis of modified proteins.
• Characterization of enzyme kinetics, including substrate specificity, pH optimum, and metal ion effects.

Main Results:

• CKCF functions as a carboxypeptidase, cleaving the C-terminal lysine residue from both subunits of creatine kinase.
• These modifications fully account for the observed three-banded electrophoretic patterns of creatine kinase.
• CKCF also modifies hemoglobin A, removing C-terminal residues from both alpha- and beta-subunits.
• The enzyme exhibits distinct properties differentiating it from known carboxypeptidases A, B, and N.

Conclusions:

• The creatine kinase conversion factor is a carboxypeptidase.
• The enzyme's activity explains the heterogeneity observed in creatine kinase electrophoretic patterns.
• The novel enzyme is provisionally named carboxypeptidase K (CPK).