Mammalian facilitative hexose transporters mediate the transport of dehydroascorbic acid

Abstract

Although vitamin C is critical to human physiology, it is not clear how it is taken up into cells. The kinetics of cell and tissue accumulation of ascorbic acid in vitro indicate that the process is mediated by specific transporters at the cell membrane. Some experimental observations have linked the transport of ascorbic acid with hexose transport systems in mammalian cells, although no clear information is available regarding the specific role(s) of these transporters, if any, in this process. Here we use the Xenopus laevis oocyte expression system to show that the mammalian facilitative hexose transporters are efficient transporters of the oxidized form of vitamin C (dehydroascorbic acid). Two transport pathways, one with low affinity and one with high affinity for dehydroascorbic acid, were found in oocytes expressing the mammalian transporters, and these oocytes accumulated vitamin C against a concentration gradient when supplied with dehydroascorbic acid. We obtained similar results in experiments using normal human neutrophils. These observations indicate that mammalian facilitative hexose transporters are a physiologically significant pathway for the uptake and accumulation of vitamin C by cells, and suggest a mechanism for the accumulation of ascorbic acid against a concentration gradient.

Efflux of hepatic ascorbate: a potential contributor to the maintenance of plasma vitamin C

“…Uptake and release of the two forms of vitamin C were found to be differentially affected by various reagents (Table 3), suggesting two separate transport mechanisms. In agreement with previous studies [29], DHA uptake was inhibited by reagents known to block mammalian glucose transport, including cytochalasin B and phloretin (Table 3). Importantly, cytochalasin D did not affect transport, precluding a non-specific interaction of the cytochalasins with the cell cytoskeleton to affect DHA uptake or AH release.…”

“…Further, perfusion studies in situ revealed that livers obtained from both rats and guinea-pigs released AH into the circulating perfusate. In agreement with previous studies [29], DHA uptake was mediated by glucose transporters in HepG2 cells. However, AH release from HepG2 cells seemed to be mediated by a separate temperature-sensitive mechanism that was susceptible to inhibition by phloretin and DIDS but was not affected by the glucose transport inhibitor cytochalasin B.…”

“…However, despite this rapid and complete intracellular reduction of DHA in a wide variety of cells, including whole blood cells, AH release from these cells has not been reported. Furthermore, frog oocytes transfected with the mammalian glucose (GLUT 1) transporter isoform take up DHA but specifically fail to release intracellularly reduced AH [29]. Thus cells need to be specifically equipped with a mechanism to mediate AH efflux, a process that seems to be biochemically and biophysically distinct from the transport of DHA (oxidized vitamin C).…”

Identification of putative downstream genes of Oct‐3, a pluripotent cell‐specific transcription factor

“…However, the substrate specificity of each isoform has n ot been firmly demonstrated. In fact, certain glucose transporters can transport a vitamin C derivative, an agent structurally unrelated to glucose (Vera et al 1993). Thus, it is possible that Glut-3 can transport an unknown material which is required for specialized cells such as the inner cell mass and pachytene spermatocytes.…”

Dehydroascorbic acid uptake in a human keratinocyte cell line (HaCaT) is glutathione-independent

“…Several studies have demonstrated that the uptake and intracellular reduction of DHA from the surrounding interstitial space contributes to the maintenance of intracellular concentrations of AA. Transport studies have revealed that there are at least two different systems involved in the cellular uptake of vitamin C in mammalian cells ; a Na + -dependent co-transporter for AA [6], and a facilitative glucose transporter for DHA [7]. Following its uptake, DHA can be reduced to ascorbate in an enzymic or non-enzymic manner through a direct chemical reaction with reduced glutathione [8].…”