Glucagon-stimulated phosphorylation of rat liver glycogen synthase in isolated hepatocytes.
Addition of glucagon (20 nM) to the isolated hepatocytes from 24-h starved male rats results in an inactivation of glycogen synthase. The A0.5 for glucose-6-P is increased 2-fold over the control but the S0.5 for UDP-glucose is not significantly affected. The glucagon-stimulated inactivation of glycogen synthase is also accompanied by a 60-120% increase in the phosphorylation of the synthase. Glycogen synthase labeled with 32P by incubation of the hepatocytes with [32P] PO4(3-) was recovered by immunoprecipitation and the resulting immunoprecipitate was subjected to tryptic digestion. Analysis of the 32P-labeled peptides reveals that the sites corresponding to those phosphorylated by cAMP-dependent protein kinase and glycogen synthase (casein) kinase-1 (Itarte, E., and Huang, K.-P. (1979) J. Biol. Chem. 254, 4052-4057) are rapidly phosphorylated in response to glucagon. These results demonstrate that glucagon not only triggers the activation of cAMP-dependent protein kinase through an increase in the intracellular level of cAMP but also, by an unknown mechanism, activates a Ca2+- and cAMP-independent protein kinase.
D000818:Animals; D002855:Chromatography, Thin Layer; D005934:Glucagon / Q000494:pharmacology*; D006006:Glycogen Synthase / Q000378:metabolism*; D007525:Isoelectric Focusing; D008099:Liver / Q000187:drug effects / Q000201:enzymology*; D008297:Male; D010766:Phosphorylation; D011494:Protein Kinases / Q000378:metabolism; D051381:Rats; D011919:Rats, Inbred Strains; D014357:Trypsin / Q000378:metabolism
D005934:Glucagon; D006006:Glycogen Synthase; D011494:Protein Kinases; D014357:Trypsin