Both the glucose 6-phosphate independent (I) and the glucose 6-phosphate dependent (D) forms of rat muscle glycogen synthetase are inhibited by adenosine triphosphate (ATP), adenosine diphosphate (ADP), and P¡. The inhibition is, kinetically, of the competitive type with respect to uridine diphosphate glucose (UDP-glucose). Glucose 6-phosphate reverses this inhibition, without affecting, at pH 6.6, the Km for UDP-glucose. UDP, one of the reaction products, is a strong inhibitor and its effect is not reversed by glucose 6-phosphate. Photooxidation of the I form in the presence of methylene blue causes a preferential loss of the inhibition by ATP, as compared to that by UDP. The sum of this evidence suggests that UDP-

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