Intracellular ascorbic acid enhances the DNA single-strand breakage and toxicity induced by peroxynitrite in U937 cells

A well-established protocol to increase the intracellular content of ascorbic acid was used to investigate the effects of the vitamin on DNA single-strand breakage and toxicity mediated by authentic peroxynitrite (ONOO⁻) in U937 cells. This protocol involved exposure for 60min to 100μM dehydroascorbic acid, which was taken up by the cells and converted into ascorbic acid via a GSH-independent mechanism. At the time of exposure to ONOO⁻, which was performed in fresh saline immediately after loading with dehydroascorbic acid, the vitamin present in the cells was all in its reduced form. It was found that, in cells that are otherwise ascorbate-deficient, an increase in their ascorbic acid content does not prevent, but rather enhances, the DNA-damaging and lethal responses mediated by exogenous ONOO⁻. These results therefore suggest that acute supplementation of ascorbic acid can be detrimental for individuals with pathologies associated with a decrease in ascorbic acid and in which ONOO⁻ is known to promote deleterious effects.