The Mg2+ requirements of nonactivated and activated rat liver phosphorylase kinase. Inhibition of the activated form by free Mg2+

Incubation of rat liver phosphorylase kinase in the presence of MgATP results in a time-dependent increase in activity, i.e., activation. Determination of the magnitude of activation depends, in large part, on the relative concentrations of Mg2+ and ATP used in the phosphorylase kinase activity assay, such that as the Mg2+ to ATP ratio increases less activation is detectable. Prior to activation, maximal activity of nonactivated phosphorylase kinase requires a 2–3 fold molar excess of Mg2+ (i.e., free Mg2+) over ATP. MgATP-dependent activation of the enzyme results in an alteration in the free Mg2+ requirement such that the activity of the activated enzyme is sharply inhibited by the free cation. Inhibition by free Mg2+ of the activated enzyme is rapidly reversed by removal of free Mg2+ but is not affected by addition of Ca2+. Both nonactivated and activated forms of enzyme appear to be inhibited by free ATP4–. The results show that the use of high concentrations of free Mg2+ in the phosphorylase kinase activity assay can blunt or completely obscure changes in enzyme activity following activation of the enzyme.