Influence of fructose on the glycogen synthase and phosphorylase systems in rat liver

Abstract

Fructose and glucose, when administered as a single, large i.v. dose (500 mg/kg) produced opposite effects on key regulatory enzymes of glycogen metabolism in intact normal fed animals. Glucose rapidly stimulated glycogen synthase phosphatase activity and increased the proportion of glycogen synthase in the active (I) form as expected; fructose reduced synthase phosphatase activity and the proportion of synthase in the 1 form. Glucose stimulated a reduction in the proportion of phosphorylase in the active (a) form, whereas fructose stimulated an increase in the proportion of phosphorylase in the a form. The effect of fructose was not mediated by an increase in cAMP concentration nor by a conversion of phosphorylase kinase b to phosphorylase kinase a. As expected the concentration of ATP decreased significantly. The increase in proportion of phosphorylase in the a form may be due to stimulation of phosphorylase kinase b activity by a decrease in the intracellular ATP:Mg2+ ratio or by an increase in intracellular Ca2+ concentration. The mechanism of the fructose-induced change in synthase phosphatase activity and in synthase I activity is unknown.