Glycogen‐Synthase Phosphatase Activity in Rat Liver

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Abstract
Three subfractions of glycogen synthase b (termed b1, b2, b3) were isolated from the glycogen fraction of dog liver on the basis of a different affinity for DEAE-cellulose. Their kinetic properties and chromatographic behavior are compatible with the presence of an increasing number of phosphorylated sites from synthase b1 towards b3. Synthase phosphatase activity in rat liver stems from 2 heat-labile and trypsin-labile proteins. These components are conveniently prepared from the cytosolic fraction of glycogen-depleted liver: the G-component of the phosphatase co-sediments with added particulate glycogen, whereas the S-component remains in the supernatant. The G-component alone did not convert any available synthase b to the a form. The synthase phosphatase activity of the S-component was variable according to the actual type of substrate. When acting on synthase b2 and b3, the S-component had a low phosphatase activity that was increased 7-fold and 11-fold, respectively, upon addition of the G-component. Synthase b1 was efficiently activated by the S-component, and only 35% faster in the presence of both components. When the cytosolic fraction of glycogen-depleted livers was analyzed by sucrose-gradient centrifugation a single peak of phosphatase activity (s20,w = 10.2 S; provisional MW = 254,000) was detected with synthase b2 as substrate. In addition to this peak, presumably an S-G complex, synthase b1 also identified free S-component of lower and heterogeneous MW. Results illustrate the influence of the type of synthase b on detection of synthase phosphatase activity, and may provide an explanation for some discrepant reports on subcellular distribution of the enzyme.

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