Glycogen Synthase Activity in Adipose Tissue: Methods for Freeze-Clamping and Assay

One of the best markers of in vivo insulin-action at insulin sensitive tissues is an increase in glycogen synthase (GS) activity. GS is the rate-limiting enzyme of GS. The activity of glycogen synthase is increased by dephosphorylation, which converts the dependent form (dependent on glucose 6-phosphate [G6P]) to the independent form. GS activity can be increased by an increase in the activities of protein (serine/threonine) phosphatases, which dephosphorylate the enzyme, and/or by a decrease in the activities of protein (serine/threo-nine) kinases, which phosphorylate the enzyme.