Glycogen synthetase activity

D. If the phosphodiesterase that degrades cAMP were inhibited, cAMP levels would rise. Protein kinase A would become more active in the liver and muscle, pyruvate kinase would become less active, and glycogen synthetase activity would be decreased. 

Another hormone, namely, insulin, acts in the opposite direction, and produces a rapid rise in glycogen synthetase activity. It has been suggested that insulin acts by changing the glycogen synthetase kinase into a form having a low affinity for cyclic AMP, so that the conversion of the independent into the dependent form (that is, inactivation of the glycogen synthetase) is retarded. 

Some of the biochemical effects which have been attributed to increased cyclic AMP levels are increased phosphorylase activity, decreased glycogen synthetase activity. Increased gluconeogenesls, and Increased llpolysls. l... 

This study has revealed that 2-azetidinones are effective as antihyperglycemic agents and might act either through increased utilization of glucose or through increased insulin activity or induction of glycogen synthetase enzyme. 

Second, GSH functions, presumably nonenzymically, in the reduction of protein thiols which have become oxidized to mixed disulfides (803). In this latter function GSH in some cases converts inactive enzymes to active ones, or vice versa, and may thus serve as a means of metabolic control. Examples of this important possibility are glycogen synthetase D (EC 2.4.1.11) and fructose-1,6-diphosphatase (EC 3.1.3.11). The D form of glycogen synthetase is dependent for activity upon the presence of glucose 6-phosphate. The enzyme is inactivated by GSSG and reactivated by GSH (204). Mixed disulfide formation between thiols of the enzyme and GSSG leads to a decrease in affinity of the enzyme for its activator (205). 

The method can lead to an underestimate of the activity of glycogen synthetase in crude tissue extracts, as the presence of phosphorylase and a-amylase may cause the degradation of the radioactive glycogen [91]. 

Graham, L L, Whistler, R L, Uridine 5 -(5-Thio-a-D-glucopyranosyl pyrophosphate) chemical synthesis and activation of rat liver glycogen synthetase. Biochemistry, 15, 1189-1194, 1976. 

In muscle and adipose tissue, insulin promotes transport of glucose and other monosaccharides across cell membranes it al.so facilitates tran.sport of amino icids, potassium ion.s. nucleosides, and ionic phosphate. Insulin also activates certain enzymes—kinases and glycogen. synthetase in muscle und adipose tissue. In adipose tissue, insulin decreases the release of fatty acids induced by epinephrine or glucagon. cAMP promotes fatty acid release from adipose ti.ssue therefore. it is pos.sible that insulin decreases fatty acid release by reducing tissue levels of cAMP. Insulin also facilitates the incorporation of intracellular amino acids into protein. 

Many enzymes may be activated or inhibited by phosphorylation. -Pyruvate dehydrogenase and glycogen synthetase are inhibited... 

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