Accelerated net efflux of 3-O-methylglucose from rat adipocytes: a reevaluation.
In a study of 3-O-methylglucose transport in insulin-stimulated rat adipocytes (catalyzed primarily by the GLUT4 isoform), it was reported that at 37 degrees C the Km and Vmax were 2.8-fold higher for net efflux than for equilibrium exchange (Vinten, J. (1984) Biochim. Biophys. Acta 772, 244-250). Because of its implications for the relative sizes of steps in the transport cycle, we reinvestigated this phenomenon. Accelerated net efflux was apparent when the extracellular methylglucose was diluted 26-fold but not when it was diluted 11-fold. When analyzed according to the one-site alternating conformation model, the data indicate about a 1.7-fold higher Vmax for efflux than for exchange, only about 40% of the difference reported previously. Together with other results in the literature, the accelerated net flux indicates that the conformational change of the loaded transporter from its outward-facing to its inward-facing form is likely the slowest step in the transport cycle, in contrast to the case for GLUT1. Experiments at 25 degrees C indicate a lower degree of accelerated net flux than at 37 degrees C. This is consistent with the above conformational change being the step with the lowest activation energy, as for GLUT1.
Wheeler
T J
TJ
Department of Biochemistry, University of Louisville School of Medicine, KY 40292.
D019325:3-O-Methylglucose; D000273:Adipose Tissue / Q000378:metabolism*; D000818:Animals; D001692:Biological Transport; D002478:Cells, Cultured; D004912:Erythrocytes / Q000378:metabolism; D051272:Glucose Transporter Type 1; D051275:Glucose Transporter Type 4; D007700:Kinetics; D008757:Methylglucosides / Q000378:metabolism*; D009004:Monosaccharide Transport Proteins / Q000737:chemistry / Q000378:metabolism*; D009124:Muscle Proteins; D011487:Protein Conformation; D051381:Rats; D017207:Rats, Sprague-Dawley; D013696:Temperature
D051272:Glucose Transporter Type 1; D051275:Glucose Transporter Type 4; D008757:Methylglucosides; D009004:Monosaccharide Transport Proteins; D009124:Muscle Proteins; C495679:Slc2a1 protein, rat; C495694:Slc2a4 protein, rat; D019325:3-O-Methylglucose