Extracellular Reduction of the Ascorbate Free Radical by Human Erythrocytes

We investigated the possibility that human erythrocytes can reduce extracellular ascorbate free radical (AFR). When the AFR was generated from ascorbate by ascorbate oxidase, intact cells slowed the loss of extracellular ascorbate, an effect that could not be explained by changes in enzyme activity or by release of ascorbate from the cells. If cells preserve extracellular ascorbate by regenerating it from the AFR, then they should decrease the steady-state concentration of the AFR. This was confirmed directly by electron paramagnetic resonance spectroscopy, in which the steady-state extracellular AFR signal varied inversely with the cell concentration and was a saturable function of the absolute AFR concentration. Treatment of cells N-ethylmaleimide (2 mM) impaired their ability both to preserve extracellular ascorbate, and to decrease the extracellular AFR concentration. These results suggest that erythrocytes spare extracellular ascorbate by enhancing recycling of the AFR, which could help to maintain extracellular concentrations of the vitamin.