Direct glucose stimulation of glycogen synthase phosphatase activity in a liver glycogen particle preparation

In glycogen particle suspensions prepared from fed rats given either glucagon or glucose in order to increase or decrease the phosphorylase a concentration, respectively, glucose stimulation of synthase phosphatase activity was observed. In preparations from glucagon-treated rats, addition of glucose stimulated synthase and phosphorylase phosphatase simultaneously and not sequentially. Synthase phosphatase stimulation was glucose concentration dependent even when phosphorylase a had been rapidly reduced to a low level. The estimated A_0.5 for glucose stimulation of synthase phosphatase activity was 27 mm. An A_0.5 for glucose stimulation of phosphorylase phosphatase activity could not be estimated since activity was still increasing with concentrations of glucose as high as 200 mm. In preparations from glucose-treated rats which contain virtually no phosphorylase a, glucose stimulation was still apparent but the A_0.5 was increased modestly (36 mm). Stimulation of synthase phosphatase activity was specific for glucose. Several other monosaccharides and the polyhydric alcohol sorbitol were ineffective.