Transport of vitamin C in the lens

Mediated transport of dehydro-L-ascorbic acid (DAS) occurs in the mammalian ocular lens. At physiological pH, there is negligible cellular uptake of reduced L-ascorbate (AS). In the calf, inhibition by analogues and by cytochalasin B, and saturability with increasing concentration provide evidence of a transport-system for DAS, apparently by facilitated diffusion, since entry was independent of external Na+. The lenticular transporter for DAS evidently is interrelated with that for hexoses, as determined by kinetic studies and inhibition by analogues. Although the two ligands bind initially to separate sites, they probably permeate cellular membranes of this tissue via a common channel. Investigation of AS in the calf's lens over the range, pH 5.0-7.4, showed that uptake increased with increasing acidity in contrast to DAS and D-glucose, whose uptake decreased. This behavior of AS was attributed to back-titration of its enolic hydroxyl group to give the un-ionized moiety, which penetrated as readily as DAS under comparable able conditions, and apparently undergoes mediated transport.