Ascorbate recycling in human erythrocytes: role of GSH in reducing dehydroascorbate

Abstract

Human erythrocytes regenerate ascorbate from its oxidized product, dehydroascorbate. The extent to which such ascorbate recycling occurs by a GSH-dependent mechanism was investigated. In the presence of glucose, erythrocytes took up over 90% of extracellular [14C]dehydroascorbate and rapidly converted it to [14C]ascorbate, which was trapped within the cells. Dehydroascorbate uptake and reduction was not associated with generation of a monoascorbyl free radical intermediate. Uptake and reduction of dehydroascorbate by glucose-depleted erythrocytes coordinately decreased GSH and raised GSSG concentrations in erythrocytes. This effect was reversed by D-glucose, but not by L-lactate. Conversely, depletion of cellular GSH decreased the ability of cells to recycle dehydroascorbate to ascorbate, as reflected in the extent to which cells were able to reduce extracellular ferricyanide. Monoascorbyl free radical was formed during the reduction of extracellular ferricyanide, indicating that one electron transfer steps were involved in this process. In GSH-depleted cells, addition of L-lactate as an energy source for glycolysis-dependent NADH regeneration did cause a partial recovery of the ability of cells to reduce ferricyanide. However, in resealed erythrocyte ghosts containing either 4 mM GSH or 400 mu M NADH, only the GSH-containing ghosts supported regeneration of ascorbate from added dehydroascorbate. These results suggest that in human erythrocytes ascorbate regeneration from dehydroascorbate is largely GSH dependent, and that it occurs through either enzymatic or nonenzymatic reactions not involving the monoascorbyl free radical.