Kinetic Properties of Glycogen Synthase and Phosphorylase and Structural Aspects of Glycogen in the db / db Mouse Liver

Abstract

Kinetic studies were carried out on liver glycogen synthase and phosphorylase isolated from genetically diabetic db/db mice. Glycogen synthase a and b enzymes from diabetic mice had V_max values 30% and 20% lower, respectively, than the enzymes from normal mice. Glycogen synthase b from diabetic mice also had a 30% lower l_0.5 for P_i and ATP at physiologic concentrations of UDP-glucose (0.25 mM) compared with the normal enzyme. Kinetic studies of phosphorylase a showed that, at low glycogen concentrations (0.25 mg/ml), the V_max of the diabetic enzyme was twofold greater than that of the normal enzyme. This was probably related to the diabetic phosphorylase a having a lower apparent K_m for glycogen. This enzyme also had a slightly higher l_0.5 for ATP compared with the enzyme from normal mice.

Structural studies of liver glycogen isolated from these diabetic mice showed differences from normal mouse glycogen. Both the α- and β-amylase limits were lower in the diabetic glycogen, and the average chain lengths, exterior chain lengths, and interior chain lengths calculated from these limits were all shorter in the glycogen from diabetic mice. Although both normal and diabetic glycogen absorbed light maximally at 430 nm when complexed with iodine, the absolute absorbance value was significantly lower for the diabetic glycogen. These data suggest an altered branching pattern of liver glycogen from the diabetic mice and it is suggested that this altered structure may ultimately influence the activities of glycogen-metabolizing enzymes. These results provide further characterization of the db/db mouse and show heretofore undescribed changes in phosphorylase a kinetics and glycogen Structure that occur in diabetes.