Coexpression of glucose transporters and glucokinase in Xenopus oocytes indicates that both glucose transport and phosphorylation determine glucose utilization.
A Xenopus oocyte expression system was used to examine how glucose transporters (GLUT 2 and GLUT 3) and glucokinase (GK) activity affect glucose utilization. Uninjected oocytes and low rates of both glucose transport and phosphorylation; expression of GLUT 2 or GLUT 3 increased glucose phosphorylation approximately 20-fold by a low Km, endogenous hexokinase at glucose concentrations < or = 1 mM, but not at higher glucose 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 by coexpression 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.
Morita
H
H
Department of Medicine, Vanderbilt University School of Medicine, Nashville, Tennessee 37232.
Yano
Y
Y
Niswender
K D
KD
May
J M
JM
Whitesell
R R
RR
Wu
L
L
Printz
R L
RL
Granner
D K
DK
Magnuson
M A
MA
Powers
A C
AC
D000818:Animals; D001692:Biological Transport; D005260:Female; D015870:Gene Expression; D005941:Glucokinase / Q000235:genetics* / Q000378:metabolism; D005947:Glucose / Q000378:metabolism*; D051273:Glucose Transporter Type 2; D051274:Glucose Transporter Type 3; D006801:Humans; D007700:Kinetics; D009004:Monosaccharide Transport Proteins / Q000235:genetics* / Q000378:metabolism; D009419:Nerve Tissue Proteins; D009865:Oocytes / Q000378:metabolism*; D010766:Phosphorylation; D017354:Point Mutation / Q000502:physiology; D012333:RNA, Messenger; D014982:Xenopus laevis
D051273:Glucose Transporter Type 2; D051274:Glucose Transporter Type 3; D009004:Monosaccharide Transport Proteins; D009419:Nerve Tissue Proteins; D012333:RNA, Messenger; C495687:SLC2A3 protein, human; D005941:Glucokinase; D005947:Glucose