By using sodium dodecyl sulphage/polyacrylamide-gel electrophoresis it was shown that rabbit muscle creatine kinase, both in a homogenate and purified, appears to be composed of a mixture of two peptides (mol.wts. 42100 and 40300) differing in length by about 15 amino acids. It is found that low concentrations of proteinase K from the fungus Tritirachium album can cleave about 38 amino acids from each chain of creatine kinase, leaving two large fragments (mol.wts 37700 and 35500). Scission of the whole enzyme was found to be concomitant with complete loss of enzyme activity. MgADP in the presence of absence of creatine slowed the rate of proteolysis by about 50%, but the transition-state analogue complex creatine-NO3—MgADP appeared to protect completely. The time course for the proteolytic inactivation in the presence of this complex, but not in its absence, was biphasic.

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