Time-dependent pseudo-activation of hepatic glycogen synthase b by glucose 6-phosphate without involvement of protein phosphatases.
During a 30 min incubation at 25 degrees C in the presence of 5-10 mM glucose 6-phosphate, pure glycogen-bound glycogen synthase b from dog liver was progressively converted into a form that was fully catalytically active in the presence of 10 mM Na2SO4 plus 0.5 mM glucose 6-phosphate. The latter enzyme was unlike synthase a (which does not require glucose 6-phosphate for activity), and unlike synthase b (which is strongly inhibited by sulphate). The conversion was insensitive to various inhibitors of Ser/Thr-protein phosphatases and alkaline phosphatases, and was therefore termed 'pseudo-activation'. Kinetically, pseudo-activation increased the V(max) 4-fold without affecting the K(m) for the substrate UDP-glucose. Pseudo-activation appeared to be an irreversible process, but several lines of evidence argue against a limited proteolysis. Pseudo-activation of glycogen synthase occurred also readily in a rat liver cytosol, but it was not observed with purified synthase from skeletal muscle. These observations have important implications for the assay of liver gycogen-synthase phosphatase; the possible physiological implications remain to be explored.
Wera
S
S
Afdeling Biochemie, Faculteit Geneeskunde, Katholieke Universiteit Leuven, Belgium.
Bollen
M
M
Moens
L
L
Stalmans
W
W
D000818:Animals; D004285:Dogs; D004789:Enzyme Activation / Q000187:drug effects; D019298:Glucose-6-Phosphate; D005958:Glucosephosphates / Q000494:pharmacology*; D006006:Glycogen Synthase / Q000187:drug effects / Q000378:metabolism*; D007527:Isoenzymes / Q000187:drug effects / Q000378:metabolism*; D007700:Kinetics; D008099:Liver / Q000187:drug effects / Q000201:enzymology*; D010749:Phosphoprotein Phosphatases / Q000378:metabolism*; D010766:Phosphorylation; D011817:Rabbits; D051381:Rats; D013552:Swine
D005958:Glucosephosphates; D007527:Isoenzymes; D019298:Glucose-6-Phosphate; D006006:Glycogen Synthase; D010749:Phosphoprotein Phosphatases