Glycogen‐Synthase Phosphatase Activity in Rat Liver

Three subfractions of glycogen synthase b (termed b 1 , b 2 , b 3 ) have been isolated from the glycogen fraction of dog liver on the basis of a different affinity for DEAE‐cellulose. Their kinetic properties and chromatographic behaviour are compatible with the presence of an increasing number of phosphorylated sites from synthase b 1 towards b 3 . Synthase phosphatase activity in rat liver stems from two 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 b 2 and b 3 , the S‐component had a low phosphatase activity that was increased 7‐fold and 11‐fold, respectively, upon addition of the G‐component. Synthase b 1 , however, 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 analysed by sucrose‐gradient centrifugation a single peak of phosphatase activity ( s 20, w = 10.2 S; provisional M r = 254000) was detected with synthase b 2 as substrate. In addition to this peak, presumably an S‐G complex, synthase b 1 also identified free S‐component of lower and heterogeneous molecular weight. Our results illustrate in general the influence of the type of synthase b on the detection of synthase phosphatase activity, and specifically may provide an explanation for some discrepant reports on the subcellular distribution of the enzyme.