On the Mechanism of Activation of Phosphorylase b Kinase by Calcium

Abstract

Kinase-activating factor, a protein required for the activation of phosphorylase b kinase by Ca⁺⁺, was purified from brain tissue and from skeletal muscle and myocardium. All fractions contained calcium-activated proteinase activity as measured by the formation of acid-soluble, tyrosine-positive material with casein as substrate. The ratio of kinase-activating factor activity to proteinase activity remained constant throughout the purification, and both activities coincided on Sephadex G-100 filtration. Kinase-inhibitory factor, which prevents activation of kinase by Ca⁺⁺, inhibited proteinase activity. Kinase-inhibitory factor also prevented the activation of kinase by trypsin. Acid-soluble, ninhydrin-positive material was formed during the activation of phosphorylase kinase by kinase-activating factor in the presence of Ca⁺⁺. It is concluded that kinase-activating factor is a calcium-activated proteolytic enzyme, that kinase-inhibitory factor is a proteolytic inhibitor, and that activation of phosphorylase b kinase by Ca⁺⁺ involves proteolysis.