Glycogen synthetase of rat skeletal muscle and hepatomas and its comparison with the enzyme of rat liver

Abstract

1. 1.Properties of glycogen synthetase of rat skeletal muscle and two ascites hepatomas (AH-66F and AH-130) were studied in comparison with rat liver enzyme. The standard assay employed the Tris-maleate buffer of pH 7.4 and 5 mM UDP-glucose. 2. 2.When fresh extracts of muscle and hepatomas were incubated at 30 °C, glycogen synthetase underwent an activation due to the D to I conversion, but unlike the activation observed in the liver enzyme, only the activity measured without glucose 6-phosphate was increased with time. 3. 3.The D and I forms of glycogen synthetase were prepared from muscle, AH-66F and AH-130. The pH-activity curves of the D forms from these tissues differed from that of liver enzyme in that in the presence of glucose 6-phosphate, the activity measured at pH 7.4 was as high as that at pH 8.6. 4. 4.The apparent Michaelis constants for UDP-glucose of the D forms of muscle, AH-66F and AH-130 determined in the presence of glucose 6-phosphate were 0.50, 0.22 and 0.15 mM, respectively. These values are much lower than that reported for the D form of liver (8 mM). 5. 5.Glycogen synthetases of muscle and liver are thus mutually different proteins. There was a close resemblance between the muscle and hepatoma enzymes, suggesting that hepatocarcinogenesis may have resulted in the replacement of liver-specific synthetase by some other type, possibly of muscle.