Diverse effects of two allosteric inhibitors on the phosphorylation state of glycogen phosphorylase in hepatocytes

Two distinct allosteric inhibitors of glycogen phosphorylase, 1,4-dideoxy-1,4-imino-D-arabinitol (DAB) and CP-91149 (an indole-2-carboxamide), were investigated for their effects on the phosphorylation state of the enzyme in hepatocytes in vitro. CP-91149 induced inactivation (dephosphorylation) of phosphorylase in the absence of hormones and partially counteracted the phosphorylation caused by glucagon. Inhibition of glycogenolysis by CP-91149 can be explained by dephosphorylation of phosphorylase a. This was associated with activation of glycogen synthase and stimulation of glycogen synthesis. DAB, in contrast, induced a small degree of phosphorylation of phosphorylase. This was associated with inactivation of glycogen synthase and inhibition of glycogen synthesis. Despite causing phosphorylation (activation) of phosphorylase, DAB is a very potent inhibitor of glycogenolysis in both the absence and presence of glucagon. This is explained by allosteric inhibition of phosphorylase a, which overrides the increase in-activation state. In conclusion, two potent phosphorylase inhibitors exert different effects on glycogen metabolism in intact hepatocytes as a result of opposite effects on the phosphorylation state of both phosphorylase and glycogen synthase.