Rapid Assay for Glycogen-Cycle Enzymes in Small Samples of Muscle

A rapid assay is described for the glycogen-cycle enzymes glycogen synthetase and α-glucan phosphorylase (UDP glucose:glycogen α-4-glucosyltransferase, EC 2.4.1.11 and α-1,4-glucan: orthophosphate glucosyltransferase, EC 2.4.1.1) in muscle tissue. UDP glucose-14C and G-1-P-14C are used as substrates for reaction with muscle homogenates, and the rate of incorporation of labeled glucose into primer glycogen is measured. Muscle samples weighing 25 to 75 mg are suitable for assay. We found activities in rat gastrocnemius muscle of 0.37 ± 0.02 µmol of glucose per minute per gram of tissue for synthetase I, 2.84 ± 0.29 µmol per minute per gram of tissue for synthetase D, and 34.5 ± 2.8 µmol per minute per gram of tissue for phosphorylase b. Corresponding values for human muscle tissue for phosphorylase a and b were 12.6 and 10.6 µmol per minute per gram of tissue, and synthetase activity of 0.011 and 0.399 µmol per minute per gram of tissue.