Divalent cations but not other activators enhance phosphorylase kinase's affinity for glycogen phosphorylase

To better understand the physical interaction between glycogen phosphorylase-b (P-b) and its only known kinase, phosphorylase kinase (PbK) and the relationship of this interaction to the activation of PbK, direct binding studies are necessary. By utilizing an enzyme-linked immunosorbent assay, a method was developed for measuring the binding of PbK to immobilized P-b under a variety of experimental conditions. A monoclonal antibody specific for the alpha subunit of PbK that had no effect on the phosphorylation of P-b by PbK or on the interaction of PbK with known effectors was used to detect PbK bound to plated P-b. Hyperbolic binding curves were obtained regardless of whether the concentration of Pbk or P-b was varied, and the assay detected changes in relative affinity caused by certain effectors of the kinase. The allosteric effector ADP, alkaline pH, and phosphorylation by cAMP-dependent protein kinase, all activators of PbK, did not cause significant changes in its relative affinity for P-b; however, Ca2+ and Mg2+ ions, which also stimulate PbK, increased its affinity for P-b, with Mg2+ being more effective. Mn2+, which inhibits the P-b conversion activity of PbK, was found to be the most potent enhancer of its affinity for P-b, although divalent cations may enhance binding. Inclusion of ATP analogs in the binding assay with Ca2+ and Mg2+ to stimulate catalytic assay conditions did not further affect the apparent affinity for P-b, which is consistent with the previously reported rapid equilibrium random bi-bi kinetic mechanism for P-b conversion.