Inactivation of glycogen synthase [formula omitted] by the catalytic subunit of cyclic AMP-dependent protein kinase. Kinetics of inactivated forms

Abstract

Rabbit skeletal muscle glycogen synthase was phosphorylated to varying degrees with [γ- 32P]ATP and the catalytic subunit of the cyclic AMP-dependent protein kinase. Phosphorylation of glycogen synthase up to 1 mol phosphate per mol subunit had very little effect on the activity ratio (activity measured in the absence of glucose 6-phosphate divided by activity measured in the presence of glucose 6-phosphate), the A 0.5 for glucose 6-phosphate (concentration of glucose 6-phosphate which gives half maximal activation), or the K m for UDPGlc measured in the absence of glucose 6-phosphate. Phosphorylation to the extent of 1.8 mol phosphate per mol subunit resulted in partial inactivation of glycogen synthase ( activity ratio = 0.6 ) due primarily to an increase in the K m for UDPGlc measured in the absence of glucose 6-phosphate. Phosphorylation to the extent of 2.6–2.8 mol phosphate per mol subunit resulted in further inactivation ( activity ratio = 0.05–0.13 ) due primarily to a decrease in V (maximal velocity) measured in the absence of glucose 6-phosphate and partly to an additional increase in the K m for UDPGlc measured in the absence of glucose 6-phosphate. The form of glycogen synthase containing 2.6–2.8 mol phosphate per mol subunit was unique in that activation by glucose 6-phosphate showed little or no positive cooperativity, A 0.5 for glucose 6-phosphate was relatively high, and V measured in the absence of glucose 6-phosphate was reduced. Phosphorylation of glycogen synthase had very little effect on either the K m for UDPGlc or the V measured in the presence of glucose 6-phosphate. We conclude from these studies that inactivation of glycogen synthase a by the catalytic subunit of cyclic AMP-dependent protein kinase in vitro is complex, and that no single kinetic parameter is the best index of inactivation.