Morita H, Yano Y, Niswender KD, May JM, Whitesell RR, Wu L, Printz RL, Granner DK, Magnuson MA, Powers AC
J Clin Invest. 1994 Oct;94(4):1373–82
A Xenopus oocyte expression system was used to examine how glucose transporters (GLUT 2 and GLUT 3) and glucokinase (GK) activity affectglucose utilization. Uninjected oocytes and low rates of both glucose transport and phosphorylation; expression of GLUT 2 or GLUT 3 increasedglucose phosphorylation approximately 20-fold by a low Km, endogenous hexokinase at glucose concentrations < or = 1 mM, but not at higherglucose concentrations. Coexpression of functional GK isoforms with GLUT 2 or 3 increased glucose utilization approximately an additional two- to threefold primarily at the physiologic glucose concentrations of 5-20 mM. The Km for glucose of both the hepatic and beta cell isoforms of GK, determined in situ, was approximately 5-10 mM when coexpressed with either GLUT 2 or GLUT 3. The increase in glucose utilization bycoexpression of GLUT 3 and GK was dependent upon glucose phosphorylation since two missense GK mutations linked with maturity-onset diabetes, 182: Val–>Met and 228:Thr–>Met, did not increase glucose utilization despite accumulation of both a similar amount of immunoreactive 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 normal 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 transporter and GK.