Résumé
Glucose provides a key supply of energy and carbon and its transport is achieved via multimembrane-spanning glucose transporters (GLUTs). The human erythrocyte is the cell type expressing the highest level of the GLUT1 glucose transporter, harboring greater than 200,000 molecules per cell. We now demonstrate that GLUT1 transcripts increase by 3-logs during erythropoiesis and high GLUT1 surface expression is observed following passage through the basophilic erythroblast stage. Paradoxically though, glucose transport significantly decreases. As GLUT1 also transports L-dehydroascorbic acid (DHA), the oxidized form of ascorbic acid (AA), transport of this molecule was assessed and indeed, it increases dramatically during erythropoiesis. The switch from glucose to DHA transport is coupled to the physical association of GLUT1 with stomatin, an integral erythrocyte membrane protein. We find that stomatin inversely regulates the relative transports of glucose and DHA by GLUT1. Moreover, in a patient with overhydrated hereditary stomatocytosis, a rare genetic disorder of red cell membrane permeability wherein stomatin is absent, glucose uptake is significantly higher but there is a concomitant 50% reduction in DHA transport. Intriguingly, erythrocyte-specific DHA transport is not conserved amongst all mammalian species; we did not detect GLUT1 on mature murine erythrocytes where DHA uptake is minimal. Notably though, humans differ from the vast majority of the greater than 5,000 mammalian species in that they are unable to synthesize ascorbic acid from glucose. This trait is shared only with other higher primates, guinea pigs and and fruit bats. We have determined that erythrocyte GLUT1 expression and associated DHA transport are specific features of these diverse ascorbic acid-deficient mammals. Within the primate order, defective ascorbic acid synthesis is due to an inactivation of the L-gulonolactone oxidase (GLO) enzyme at the Haplorrhini-Strespsirrhini split. In accord with the data presented above, we find that erythrocyte GLUT1 expression and associated DHA transport are characteristic of erythrocytes from primates within the Haplorrhini suborder but not Strepsirrhini lemurs. Thus, the erythrocyte-specific co-expression of GLUT1 and stomatin constitutes a compensatory mechanism in mammals that are unable to synthesize the essential ascorbic acid metabolite.