Platelet phosphorylase kinase activity and its regulation by the calcium-dependent regulatory protein, calmodulin

Platelet phosphorylase kinase (ATP:phosphorylase phosphotransferase, EC 2.7.1.38) was found to be a Ca2+-sensitive enzyme. It was two Ka values for Ca2+ viz. 0.25 and 2.6 microM, respectively. The "calcium-dependent regulator" or calmodulin can enhance the activity of phosphorylase kinase, increasing its affinity for Ca2+. In the presence of calmodulin phosphorylase kinase has only one, high affinity binding site for Ca2+ (Ka = 0.27 microM). Platelet phosphorylase kinase can be phosphorylated by endogenous cyclic AMP-dependent protein kinase increasing its catalytic activity and this activation process is reversed by dephosphorylation. The changing level of intracellular Ca2+ and cyclic AMP may control the activity of phosphorylase kinase, regulating the mobilization of glycogen.