GLYCOGEN-SYNTHASE - EXISTENCE OF A SINGLE DEMONSTRABLE FORM IN BOVINE ADIPOSE-TISSUE

Glycogen synthase from bovine adipose tissue has been kinetically characterized. Glucose 6-phosphate increased enzyme activity 50-fold with an activation constant (A0.5) of 2.6 mm. Mg2+ reversibly decreased this A0.5 to 0.75 mm without changing the amount of stimulation by glucose 6-phosphate. Mg2+ did not alter the apparent Km for UDP-glucose (0.13 mm). The pH optimum was broad and centered at pH 7.6. The glucose 6-phosphate activation of the enzyme was reversible and competitively inhibited by ATP (Ki = 0.6 mm) and Pi(Ki = 2.0 mm). The use of exogenous sources of glycogen synthase and glycogen synthase phosphatase suggests that (i) adipose tissue glycogen synthase phosphatase activity in fed mature steers is low or undetectable, and (ii) endogenous bovine adipose tissue glycogen synthase can be activated to other glucose 6-phosphate-dependent forms by addition of adipose tissue extracts from fasted steers or fed rats. © 1979.