Micromolar concentrations of hydrogen peroxide induce oxidative DNA lesions more efficiently than millimolar concentrations in mammalian cells

Reactive oxygen species produce oxidized bases, deoxyribose lesions and DNA strand breaks in mammalian cells. Previously, we demonstrated that aldehydic DNA lesions (ADLs) were induced in mammalian cells by 10 mM hydrogen peroxide (H2O2). Interestingly, a bimodal H2O2 dose-response relationship in cell toxicity has been reported for Escherichia coli deficient in DNA repair as well as Chinese hamster ovary (CHO) cells. Furthermore, it has been demonstrated that H2O2 causes single-strand breaks in purified DNA in the presence of iron and induces mitochondrial DNA damage in CHO cells with a biphasic dose-response curve. Here we show that H2O2 produces ADLs at concentrations as low as 0.06 mM in HeLa cells and that lower concentrations of H2O2 were much more efficient at inducing ADLs than higher concentrations. This dose-response curve is strikingly similar to that for cell killing effects in E.coli deficient in DNA repair exposed to H2O2. Interestingly, serial treatment of submillimolar levels of H2O2 induced a massive accumulation of ADLs. The toxicity arising from H2O2 determined by intracellular NAD(P)H in cells correlated well with the formation of ADLs. The addition of dipyridyl, an iron (II)-specific chelator, significantly protected against DNA damage and cell toxicity from submillimolar, but not millimolar, amounts of H2O2. These results suggest that ADLs induced by submillimolar levels of H2O2 may be due to a Fenton-type reaction between H2O2 and intracellular iron ions in mammalian cells.