On the mechanism of inactivation of muscle glycogen phosphorylase by insulin.

Glucose 6-phosphate, an allosteric inhibitor of skeletal muscle phosphorylase b, inhibits at physiological concentrations and conditions the phosphorylation and activation of the enzyme by phosphorylase b kinase. AMP inhibits the dephosphorylation of phosphorylase a, but is without effect on the phosphorylation of phosphorylase b. Glucose 6-phosphate has no effect on the activity of phosphorylase a and does not affect its dephosphorylation by phosphatases 1 or 2A. The inhibition of the phosphorylation of phosphorylase b by glucose 6-phosphate may explain the reported decreased phosphorylation of phosphorylase in muscle following insulin treatment, which elevates intracellular levels of glucose 6-phosphate.