Role of monosaccharide transporter in vitamin C uptake by placental membrane vesicles

Dehydroascorbic acid (DHA), the reversibly oxidized form of vitamin C, was taken up much more rapidly than L-glucose into membrane vesicles prepared from the maternal face of the human placental syncytiotrophoblast. DHA uptake was sensitive to inhibition by cytochalasin B and was independent of a sodium concentration gradient. At equilibrium, the concentration of DHA in the vesicles did not exceed that of the medium. DHA and the D-glucose analogue, 3-O-methyl-D-glucose (3-O-MG) appeared to compete with one another for the transporter. The 3-O-MG and DHA inhibitory constants were indistinguishable. Vesicles loaded with a high concentration of 3-O-MG and suspended in low 3-O-MG displayed a marked, transitory enhancement of DHA, but not L-glucose uptake. These findings suggest that DHA is taken into the first cellular boundary of the placenta between maternal and fetal circulations by the sodium-independent monosaccharide transporter. In contrast to DHA, L-ascorbic acid, the reversibly reduced form of vitamin C, was taken into these vesicles much more slowly. This uptake was not affected by cytochalasin B nor by a sodium concentration gradient; it appeared to occur by simple diffusion.