Site of action and biphasic effect of neutral salts in the phosphorylase kinase reaction

Abstract

The inhibition of [rabbit skeletal muscle] phosphorylase kinase [EC 2.7.1.38] catalytic activity by 0.1 M neutral salts was predicted by the Hofmeister series of anions. The site of action of the salts was determined to be on the phosphorylase kinase molecule directly, rather than on its protein substrate by the following evidence. Nonactivated kinase was more sensitive to salt inhibition than the activated form. Ca2+ partially overcame the inhibition of nonactivated kinase. Inhibition by Cl- occurred with either phosphorylase or a tetradecapeptide containing the convertible seryl residue as substrate. Phosphorylation of nonactivated phosphorylase kinase by protein kinase was markedly inhibited by NaNO3, but this salt had little effect on the phosphorylation of histone by protein kinase. The influence of neutral salts on phosphorylase kinase activity was biphasic. Although activity was inhibited at low salt concentrations, it actually was stimulated as the salt concentration was increased. A similar biphasic response to various salt concentrations was observed in the velocities of autophosphorylation of phosphorylase kinase. The lag in the rate of product formation seen during the activity assay was less pronounced at inhibitory salt concentrations and was abolished at stimulatory salt concentrations. How the influence of salts relates to autophosphorylation and the lag is considered.