GLUCOSE-INDUCED GLYCOGENESIS IN THE LIVER INVOLVES THE GLUCOSE-6-PHOSPHATE-DEPENDENT DEPHOSPHORYLATION OF GLYCOGEN-SYNTHASE

Non-metabolized glucose derivatives may cause inactivation of phosphor ylase but, unlike glucose, they are unable to elicit activation of gly cogen synthase in isolated hepatocytes. We report here that, after the previous inactivation of phosphorylase by one of these glucose deriva tives (2-deoxy-2-fluoro-alpha-glucosyl fluoride), glycogen synthase wa s progressively activated by addition of increasing concentrations of glucose. Under these conditions, the degree of activation of glycogen synthase was linearly correlated with the intracellular glucose-6-phos phate (Glc-6-P) concentration. Addition of glucosamine, an inhibitor o f glucokinase, decreased both parameters in parallel. Further experime nts using an inhibitor of either protein kinases (5-iodotubercidin) or protein phosphatases (microcystin) in isolated hepatocytes indicated that Glc-6-P does not affect glycogen-synthase kinase activity but enh ances the glycogen-synthase phosphatase reaction. Experiments in vitro showed that the synthase phosphatase activity of glycogen-bound type- 1 protein phosphatase was increased by physiological concentrations of Glc-6-P (0.1-0.5 mM), but not by 2.5 mM fructose-6-P, fructose-1-P or glucose-1-P. At physiological ionic strength, the glycogen-associated synthase phosphatase activity was nearly entirely Glc-6-P-dependent, but Glc-6-P did not relieve the strong inhibitory effect of phosphoryl ase alpha. The large stimulatory effects of 2.5 mM Glc-6-P, with glyco gen synthase b and phosphorylase a as substrates, appeared to be mostl y substrate-directed, while the modest effects observed with casein an d histone IIA pointed to an additional stimulation of glycogen-bound p rotein phosphatase-1 by Glc-6-P. We conclude that glucose elicits hepa tic synthase phosphatase activity both by removal of the inhibitor, ph osphorylase alpha, and by generation of the stimulator, Glc-6-P.