PHOSPHORYLATION AND INACTIVATION OF RAT-HEART GLYCOGEN-SYNTHASE BY CAMP-DEPENDENT AND CAMP-INDEPENDENT PROTEIN-KINASES

The regulation of cardiac muscle glycogen metabolism is not well under stood. Previous studies have indicated that heart glycogen synthase is heavily phosphorylated in vivo on multiple sites. Using purified enzy mes, we have investigated the effect of phosphorylation of different s ites on the activity of rat heart glycogen synthase. A convenient proc edure was developed for the purification of rat heart glycogen synthas e. The enzyme was phosphorylated by selected kinases, and glycogen syn thase activity, extent of phosphorylation, and phosphopeptide maps wer e analyzed. Rat heart glycogen synthase, purified to apparent homogene ity (M(r) 87,000 on SDS-PAGE), had a specific activity of 18 U/mg prot ein and had an activity ratio of 0.74 (activity in the absence divided by the activity in the presence of glucose 6-P). cAMP-dependent prote in kinase, glycogen synthase kinase 3, Ca2+/calmodulin-dependent prote in kinase II, protein kinase C, and phosphorylase kinase phosphorylate d the enzyme with a concomitant decrease in the activity ratio to valu es ranging from 0.1 to 0.4. Casein kinase II phosphorylated but did no t inactivate glycogen synthase. Six tryptic phosphopeptides, obtained from heart glycogen synthase phosphorylated by the various kinases, we re separated by reverse-phase chromatography. The phosphopeptide(s) ob tained with each kinase eluted at the same position(s) as correspondin g phosphopeptides obtained from rat skeletal muscle glycogen synthase. The study shows that the pattern of phosphorylation and effects on ac tivity are very similar for cardiac and skeletal muscle glycogen synth ase. It is suggested that the well known differences in heart and glyc ogen metabolism may be due to the interplay of kinases and phosphatase s which could lead to different phosphorylation and activity states of glycogen synthase.