Free adenosine diphosphate as an intermediary in the phosphorylation by creatine phosphate of adenosine diphosphate bound to actin.

Pretreatment of adenosine diphosphate-G-actin solutions with apyrase results in a marked decrease in their rate of polymerization at 0” induced by KCl, creatine phosphate, and creatine kinase. In contrast, apyrase has no effect on the rate of polymerization of ATP-G-actin. The retardation can be reversed by the addition of a low concentration of ADP or of ATP. By comparing the rate of polymerization of actin solutions at various concentrations with that of solutions in which the ADP content has been decreased by apyrase treatment, it appears that the effect of apyrase treatment cannot be accounted for by the decrease in polymerizable actin owing to the removal of the bound nucleotide. Studies on the time course of the action of apyrase on W-ADP bound to actin indicate that about 7% of the nucleotide initially present is free and that the free nucleotide can be rapidly hydrolyzed by apyrase with relatively little effect on the bound nucleotide. These observations suggest that free ADP is a necessary intermediary in the polymerization of ADP-G-actin by creatine kinase at O”, and appear to rule out direct phosphorylation of the bound nucleotide of ADP-G-actin by creatine kinase.