Kinetics of GLUT1 and GLUT4 glucose transporters expressed in Xenopus oocytes.
The predominant mechanism by which insulin activates glucose transport in muscle and adipose tissue is by affecting the redistribution of the facilitated hexose carriers, GLUT1 and GLUT4, from an intracellular site to the plasma membrane. A quantitative analysis of this process has been hampered by the lack of reliable determinations for kinetic constants catalyzed by each of these isoforms. In order to obtain such information, each transporter was expressed in Xenopus oocytes by the injection of mRNA encoding rat GLUT1 or GLUT4. Equilibrium exchange 3-O-methylglucose uptake was measured and the data fitted to a two-compartment model, yielding Km = 26.2 mM and Vmax = 3.5 nmol/min/cell for GLUT1 and Km = 4.3 mM and Vmax = 0.7 nmol/min/cell for GLUT4. Measurement of the abundance of cell surface transporters was accomplished by two independent protocols: photolabeling with the impermeant hexose analog 2-N-4-(1-azi-2,2,2-trifluoroethyl)benzoyl-1,3-bis(D-mannos-4 -yloxy)-2-propylamine and subcellular fractionation of oocytes. Data obtained by either technique revealed that the ratio of plasma membrane GLUT1 to GLUT4 was about 4; this paralleled the relative maximal velocities for hexose transport, indicating that the turn-over numbers for the two isoforms were the same. Moreover, measurement of the concentration of exofacially disposed transporters with 2-N-4-(1-azi-2,2,2-trifluoroethyl)benzoyl-1,3-bis(D-mannos-4 -yloxy)-2-propylamine allowed calculation of the turnover number to be about 20,000 min-1. These data indicate that, at low substrate concentrations, the catalytic efficiency of GLUT4 is significantly greater than GLUT1. Extrapolation to mammalian systems suggests that GLUT4 is responsible for virtually all of the hexose uptake in insulin-responsive targets, particularly in the presence of hormone.
Nishimura
H
H
Experimental Diabetes, Metabolism and Nutrition Section, National Institute of Diabetes and Digestive and Kidney Disease, National Institute of Health, Bethesda, Maryland 20892.
Pallardo
F V
FV
Seidner
G A
GA
Vannucci
S
S
Simpson
I A
IA
Birnbaum
M J
MJ
D019325:3-O-Methylglucose; D000345:Affinity Labels; D000818:Animals; D001386:Azides; D001692:Biological Transport; D015153:Blotting, Western; D002462:Cell Membrane / Q000378:metabolism; D003001:Cloning, Molecular; D003847:Deoxyglucose / Q000378:metabolism; D004187:Disaccharides; D005260:Female; D051272:Glucose Transporter Type 1; D051275:Glucose Transporter Type 4; D006027:Glycosides; D006801:Humans; D007700:Kinetics; D008757:Methylglucosides / Q000378:metabolism; D009004:Monosaccharide Transport Proteins / Q000235:genetics / Q000378:metabolism*; D009124:Muscle Proteins; D009865:Oocytes / Q000378:metabolism*; D011233:Precipitin Tests; D011437:Propylamines; D012333:RNA, Messenger / Q000235:genetics; D051381:Rats; D014982:Xenopus laevis
D000345:Affinity Labels; D001386:Azides; D004187:Disaccharides; D051272:Glucose Transporter Type 1; D051275:Glucose Transporter Type 4; D006027:Glycosides; D008757:Methylglucosides; D009004:Monosaccharide Transport Proteins; D009124:Muscle Proteins; D011437:Propylamines; D012333:RNA, Messenger; C495677:SLC2A1 protein, human; C495692:SLC2A4 protein, human; C495679:Slc2a1 protein, rat; C495694:Slc2a4 protein, rat; C065412:2-N-(4-(1-azitrifluoroethyl)benzoyl)-1,3-bis-(mannos-4-yloxy)-2-propylamine; D019325:3-O-Methylglucose; D003847:Deoxyglucose