COEXPRESSION OF GLUCOSE TRANSPORTERS AND GLUCOKINASE IN XENOPUS OOCYTES INDICATES THAT BOTH GLUCOSE-TRANSPORT AND PHOSPHORYLATION DETERMINEGLUCOSE-UTILIZATION

A Xenopus oocyte expression system was used to examine how glucose tra nsporters (GLUT 2 and GLUT 3) and glucokinase (GK) activity affect glu cose utilization. Uninjected oocytes had low rates of both glucose tra nsport and phosphorylation; expression of GLUT 2 or GLUT 3 increased g lucose phosphorylation similar to 20-fold by a low K-m, endogenous hex okinase at glucose concentrations less than or equal to 1 mM, but not at higher glucose concentrations. Coexpression of functional GK isofor ms with GLUT 2 or 3 increased glucose utilization approximately an add itional two- to threefold primarily at the physiologic glucose concent rations of 5-20 mM. The K-m for glucose of both the hepatic and beta c ell isoforms of GK, determined in situ, was similar to 5-10 mM when co expressed with either GLUT 2 or GLUT 3. The increase in glucose utiliz ation by coexpression of GLUT 3 and GK was dependent upon glucose phos phorylation since two missense GK mutations linked with maturity-onset diabetes, 182:Val->Met and 228:Thr->Met, did not increase glucose uti lization despite accumulation of both a similar amount of immunoreacti ve GK protein and glucose inside the cell. Coexpression of a mutant GK and a normal GK isoform did not interfere with the function of the no rmal GK enzyme. Since the coexpression of GK and a glucose transporter in oocytes resembles conditions in the hepatocyte and pancreatic beta cell, these results indicate that increases in glucose utilization at glucose concentrations > 1 mM depend upon both a functional glucose t ransporter and GK.