The Action of Calcium Ion on Cardiac Phosphorylase b Kinase

Nonactivated phosphorylase b kinase has been purified from beef hearts. Kinase-activating factor which is required for calcium activation of kinase and an inhibitory factor which prevents calcium activation were also partially purified. Kinase, kinase-activating factor, and inhibitory factor were each obtained essentially free of the other. The action of kinase-activating factor was proportional to protein concentration but was not strictly time-dependent. The activating factor appeared to act on kinase in a stoichiometric manner rather than catalytically. The activated enzyme, however, was unstable under the experimental conditions so that the variation of activation with time could have been the result of competing activation and inactivation. The action of the inhibitory factor was antagonized by activating factor. The data suggested that the action of the inhibitor was noncatalytic and that it interacted stoichiometrically with kinase-activating factor.