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 Km app for UDP–glucose in vivo was 1.4 mM (in the presence or absence of glucose-6-P). The in vitro value was 6 mM and was reduced by glucose-6-P to 0.8 mM. Under both conditions (in vitro and in vivo) Vmax was 0.2 mUnits per oocyte in the absence of glucose-6-P. Vmax 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 μM and, depending on substrate concentrations, 50–100 μM 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.