Hormonal regulation of glycogen synthase phosphorylation in rabbit skeletal muscle.

Phosphorylation of rabbit skeletal muscle glycogen synthase in vivo in response to hormones was investigated. Methods were developed to purify the synthase without altering its kinetic parameters, thereby indicating constancy of the synthase phosphorylation state. The enzyme from control rabbits had an activity ratio (-glucose-6-P/+glucose-6-P) of 0.24, K, for glucose-6P of 0.17 m ~ , and K , for UDP-glucose of 1.56 m. Analysis of alkali-labile phosphate gave a value of 2.4 mol/mol of subunit of which 1.0 mol was in the trypsinsensitive (T.S.) domain and 1.4 mol in the trypsin-insensitive (T.I.) domain. The synthase activity ratio was decreased and K, for glucose-6-P and K , for UDP-glucose increased by epinephrine-treatment or induction of alloxan diabetes in the rabbits. Although both of these conditions elevated total phosphate content to about 3.9 mol/mol of subunit, the distribution of this phosphate between the two domains was very different. With epinephrine treatment, T.S. phosphate doubled (2.1 mol/mol) with little change in T.I. phosphate (1.8 mol/mol) whereas in diabetes, T.S. phosphate was unchanged (1.1 mol/mol) but T.I. phosphate doubled (2.8 mol/mol). Treatment of the diabetics with insulin returned the blood glucose, tissue glycogen, and synthase kinetic parameters to control values. There was also a specific dephosphorylation of the T.I. domain of synthase. Administration of insulin to control rabbits resulted in an elevation of the synthase activity ratio and decreases in K, for glucose-6-P and K, for UDP-glucose within 10 min. This was associated with a significant decrease in T.I. phosphate (0.8 mol/mol) in rabbits treated for 4 days. Plots of synthase kinetic properties against total phosphate and T.I. phosphate were fairly linear. However, the data from the epinephrine group indicated that T.S. phosphate also had some influence on K , for UDP-glucose and activity ratio and a strong influence on K, for glucose-6-P.