Effect of oxyanions of the early transition metals on rabbit skeletal muscle phosphorylase

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Abstract
The differential effects of the oxyanions of the early transition metals ions V(V), W(VI) and Mo(VI) on the catalytic activity and coenzyme binding of rabbit skeletal muscle phosphorylase are studied. The oligoanions of V(V), W(VI) and Mo(VI) are potent inhibitors of phosphorylase. Kinetic studies revealed that oligovanadates inhibit pyridoxal-reconstituted phosphorylase b by competing with both the substrate, glucose-1-phosphate, and the activator, phosphite, with Ki values of 4 .mu.M and 6 .mu.M, respectively. Oligovanadates in the millimolar concentration range inhibit phosphorylases a and b and competing with glucose 1-phosphate binding. The polymeric decavanadate and paratungstates caused time-dependent inactivation of phosphorylase. Spectral studies with tungstate and phosphorylase b revealed that the inactivation is due to deformation of the coenzyme site. Kinetic studies and the protective effects of substrate and effectors on inactivation and deformation by tungstate or vanadate suggest that deformation and inactivation is caused by a primary binding of the oligoanions at the glucose 1-phosphate site. NMR studies of vanadate-phosphorylase complexes and vanadate solutions under different conditions were carried out to ascertain the nature of vanadate ions interacting with functional groups in phosphorylase. Decavanadate is probably the major protein-bound species. NMR studies also showed that guanidino groups react with decavanadate and suggest that arginine residues in phosphorylase are potential functional groups that can interact with decavanadate. The effects of the oligoanions on the catalytic activity and coenzyme binding of phosphorylase are explained in terms of preferential binding of these ions at the catalytic site encompassing the glucose 1-phosphate and pyridoxal 5''-phosphate binding sites.