Some properties of human skeletal muscle creatine kinase

Creatine kinase has been purified from human skeletal muscle. The properties of the human enzyme are similar to those of the enzyme from rabbit muscle. The molecular weight was determined as approximately 80000 with a probable two reactive sulphydryl groups per molecule. Manganous (II) ion was almost as effective as magnesium as the activating metal ion, and calcium and cobalt could also act in this capacity. Under standardized conditions the nucleotide specificity was ADP greater than dADP greater than IDP greater than GDP greater than UDP greater than XDP in the reverse reaction. No hydrolytic activity was observed with ATP. Initial velocity and product inhibition studies were used to determine various kinetic constants for the substrates of the enzyme. It was concluded that, as for the rabbit muscle enzyme, the reaction probably followed a rapid equilibrium random mechanism. Anomalous kinetic behaviour, however, was observed for the forward reaction for MgATP2- but not for creatine, when measurements were extended over a much wider range than normally used. The reciprocal plot of velocity as a function of substrate concentration gave a curve, concave downwards, instead of a straight line.