The hepatic defect in glycogen synthesis in chronic diabetes involves the G-component of synthase phosphatase

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Abstract
Hepatocytes from normal fed rats and from chronically (90 h) alloxan-diabetic rats were compared. The rate and the extent of activation of glycogen synthase in response to 60 mM-glucose were greatly decreased in diabetes. During incubation of gel-filtered extracts from broken hepatocytes, diabetes only decreased the rate of the activation, which became ultimately complete in either preparation. Synthase phosphatase activity, as measured by the activation of purified hepatic synthase b, was decreased in chronic diabetes. The decrease was proportional to the severity of the diabetes, and reached 90% when the plasma glucose concentration was greater than or equal to 55 mM. In contrast, phosphorylase phosphatase activity was not decreased. Synthase phosphatase activity was progressively restored by treatment with insulin for 20-68 h. During the induction of diabetes and during insulin treatment there was a good correlation between the activity of synthase phosphatase and the maximal activation of synthase in glucose-stimulated hepatocytes from the same livers. The decreased activity of synthase phosphatase in diabetes cannot be explained by an inhibitor. The decrease was much less marked when synthase phosphatase was assayed by the dephosphorylation of 32P-labelled synthase from muscle. This observation suggested a loss of only one component of synthase phosphatase. Cross-combination of subcellular fractions from control rats and from diabetic rats showed a preferential loss of G-component, with little or no loss of S-component. No G-component could be detected in severe diabetes. The concentration of G-component is therefore of critical importance in the glucose-induced activation of glycogen synthase in the liver.