Human erythrocytes transport dehydroascorbic acid and sugars using the same transporter complex

GLUT1, the primary glucose transport protein in human erythrocytes [red blood cells (RBCs)], also transports oxidized vitamin C [dehydroascorbic acid (DHA)]. A recent study suggests that RBC GLUT1 transports DHA as its primary substrate and that only a subpopulation of GLUT1 transports sugars. This conclusion is based on measurements of cellular glucose and DHA equilibrium spaces, rather than steady-state transport rates. We have characterized RBC transport of DHA and 3-O-methylglucose (3-OMG), a transported, nonmetabolizable sugar. Steady-state 3-OMG and DHA uptake in the absence of intracellular substrate are characterized by similar V-max (0.16 +/- 0.01 and 0.13 +/- 0.02 mmol.l(-1).min(-1), respectively) and apparent K-m (1.4 +/- 0.2 and 1.6 +/- 0.7 mM, respectively). 3-OMG and DHA compete for uptake, with K-i(app) of 0.7 +/- 0.4 and 1.1 +/- 0.1 mM, respectively. Uptake measurements using RBC inside-out-membrane vesicles demonstrate that 3-OMG and DHA compete at the cytoplasmic surface of the membrane, with K-i(app) of 0.7 +/- 0.1 and 0.6 +/- 0.1 mM, respectively. Intracellular 3-OMG stimulates unidirectional uptake of 3-OMG and DHA. These findings indicate that DHA and 3-OMG bind at mutually exclusive sites at exo- and endofacial surfaces of GLUT1 and are transported via the same GLUT1 complex.