Tetrathionate-blocked creatine kinase as a substrate for human plasma 'creatine kinase conversion factor'

'Creatine kinase conversion factor' has been partially purified from whole plasma. The preparation has the ability to convert slowly migrating, native CK-MM into a faster electrophoretic form through a form of intermediate mobility. These changes are accompanied by a loss in CK activity. The rates of both electrophoretic conversion and loss of enzyme activity are reduced or prevented by the presence of 2-mercaptoethanol, EDTA, or by formation of the creatine-MgADP-nitrate dead-end complex. When the thiol group essential for enzyme activity found at the CK active site is first blocked by treatment with potassium tetrathionate, full activity may be retained under conditions that fully convert CK to the faster electrophoretic forms. The conversion factor activity was not inhibited by a pre-incubation with the protease inhibitor, PMSF and the converted form of CK showed no evidence of proteolytic cleavage. The molecular basis of the chemical alteration caused by the conversion is discussed.