Frog oocyte glycogen synthase: enzyme regulation under in vitro and in vivo conditions.
Frog oocyte glycogen synthase properties differ significantly under in vitro or in vivo conditions. The K(mapp) for UDP-glucose in vivo was 1.4mM (in the presence or absence of glucose-6-P). The in vitro value was 6mM and was reduced by glucose-6-P to 0.8mM. Under both conditions (in vitro and in vivo) V(max) was 0.2 m Units per oocyte in the absence of glucose-6-P. V(max) in vivo was stimulated 2-fold by glucose-6-P, whereas, in vitro, a 10-fold increase was obtained. Glucose-6-P required for 50% activation in vivo was 15 microM and, depending on substrate concentrations, 50-100 microM in vitro. The prevailing enzyme obtained in vitro was the glucose-6-P-dependent form, which may be converted to the independent species by dephosphorylation. This transformation could not be observed in vivo. We suggest that enzyme activation by glucose-6-P in vivo is due to allosteric effects rather than to dephosphorylation of the enzyme. Regulatory mechanisms other than allosteric activation and covalent phosphorylation are discussed.
Báez
Mauricio
M
Departamento de Biología, Facultad de Ciencias, Universidad de Chile, P.O. Box 653, Santiago, Chile.
Preller
Ana
A
Ureta
Tito
T
D000818:Animals; D001001:Anura / Q000378:metabolism*; D004789:Enzyme Activation; D004795:Enzyme Stability; D005260:Female; D019298:Glucose-6-Phosphate / Q000378:metabolism; D006006:Glycogen Synthase / Q000737:chemistry / Q000378:metabolism*; D066298:In Vitro Techniques; D007700:Kinetics; D009865:Oocytes / Q000201:enzymology*; D010766:Phosphorylation; D013379:Substrate Specificity; D014532:Uridine Diphosphate Glucose / Q000378:metabolism
D019298:Glucose-6-Phosphate; D006006:Glycogen Synthase; D014532:Uridine Diphosphate Glucose