Human erythrocyte sugar transport is incompatible with available carrier models

GLUT1-mediated, passive D-glucose transport in human erythrocytes is asymmetric. V-max and K-m(app) for D-glucose uptake at 4 degrees C are 10-fold lower than V-max and K-m(app) for D-glucose export. Transport asymmetry is not observed for GLUT1-mediated 3-O-methylglucose transport in rat, rabbit, and avian erythrocytes and rat adipocytes where V-max for sugar uptake and exit are identical. This suggests that transport asymmetry is either an intrinsic catalytic property of human GLUT1 or that factors present in human erythrocytes affect GLUT1-mediated sugar transport. In the present study we assess human erythrocyte sugar transport asymmetry by direct measurement of sugar transport rates and by analysis of the effects of intra- and extracellular sugars on cytochalasin B binding to the sugar export site. We also perform internal consistency tests to determine whether the measured, steady-state 3-O-methylglucose transport properties of human erythrocytes agree with those expected of two hypothetical models for protein-mediated sugar transport. The simple-carrier hypothesis describes a transporter that alternately exposes sugar import and sugar export pathways. The fixed-site carrier hypothesis describes a sugar transporter that simultaneously exposes sugar import and sugar export pathways. Steady-state 3-O-methylglucose transport in human erythrocytes at 4 degrees C is asymmetric. V-max and K-m(app) for sugar uptake are 10-fold lower than V-max and K-m(app) for sugar export. Phloretin-inhibitable cytochalasin B binding to intact red cells is unaffected by extracellular D-glucose but is competitively inhibited by intracellular D-glucose. This inhibition is reduced by 13% +/- 4% when saturating extracellular D-glucose levels are also present. Assuming transport is mediated by a simple-carrier and that cytochalasin B and intracellular D-glucose binding sites are mutually exclusive, the cytochalasin B binding data are explained only if transport is almost symmetric (V-max exit = 1.4 V-max entry). The cytochalasin B binding data are consistent with both symmetric and asymmetric fixed-site carriers. Analysis of 3-O-methylglucose, 2-deoxy-D-glucose, and D-glucose uptake in the presence of intracellular 3-O-methylglucose demonstrates significant divergence in experimental and theoretical transport behaviors: We conclude either that human erythrocyte sugar transport is mediated by a carrier mechanism that is fundamentally different from those considered previously or that human erythrocyte-specific factors prevent accurate determination of GLUT1-mediated sugar translocation across the cell membrane. We suggest that GLUT1-mediated sugar transport in all cells is an intrinsically symmetric process but that intracellular sugar complexation in human red cells prevents accurate determination of transport rates.