Seasonal variations of glycogen synthase and phosphorylase activities in the liver of the frog Rana esculenta

Abstract

Glycogen metabolism has been studied in the liver of the frog Rana esculenta throughout the annual cycle. Hepatic glycogen reserves are high during winter and decrease during spring and summer, the changes being reflected by variations in the activities of the rate-limiting enzymes glycogen synthase and phosphorylase. Glycogen deposition is associated with increased levels of synthase D activity and no changes in the relative proportions of the active-inactive form of the enzyme, whereas during glycogen mobilization significant changes in both total phosphorylase and in the percent phosphorylase a are observed. Regression analysis indicates a positive correlation between glycogen content and synthase activity, and an inverse relationship between glycogen content and either total phosphorylase or phosphorylase a. The results suggest that glycogen deposition and/or mobilization could be mainly controlled by mechanisms related to either changes in the amount of enzyme protein or to allosteric effects of the metabolite glucose-6-phosphate on glycogen synthase and phorphorylase.