Quantitation of glycogen synthase and phosphorylase protein in mouse liver: Correlation between enzymatic protein and enzyme activity

Abstract

Phosphorylase and glycogen synthase protein were measured in normal and genetically diabetic (C57BL/KsJ db db ) mice liver extracts using rocket immunoelectrophoresis, and these data correlated with measurements of total phosphorylase and total glycogen synthase activities, respectively. Phosphorylase protein in 5-week-old normal mice was about 5 μg/mg protein and reached 8 μg/mg protein by 9 weeks. In comparison, the diabetic mice had elevated levels of phosphorylase protein (11–13 μg/mg protein) which correlated with an increased total phosphorylase activity compared to normals. The correlation coefficient for the phosphorylase activity vs protein plot was highly significant ( r = 0.73, P < 0.001). The molar concentration of phosphorylase subunit in normal mouse liver was calculated to be 11 μ m and up to 23 μ m in the diabetic mice. The liver concentration of glycogen synthase was relatively constant in normal mice at 400 ng/mg protein (corresponding to approximately 1.4 μ m) but varied from 230 to 441 ng/mg protein (0.9 to 1.8 μ m) in diabetic mice. There was little correlation between glycogen synthase activity and enzymatic protein ( r = 0.15). These results indicate (1) that phosphorylase is present at concentrations approximately 10 times that of glycogen synthase, and (2) that glycogen synthase activity is relatively more dependent upon factors other than the amount of enzymatic protein.