Effects of glutathione and chelating agents on copper-mediated DNA oxidation: pro-oxidant and antioxidant properties of glutathione.

The exposure of DNA to H2O2 in the presence of Cu(II) and a reducing agent is known to result in the induction of a variety of oxidative lesions, including DNA strand breaks and base modifications. Since the reducing agent glutathione occurs in cell nuclei at relatively high concentrations, and copper exists in nuclei associated with chromatin, the present study was undertaken to evaluate the ability of GSH to promote copper-mediated free radical damage to DNA. When compared with ascorbate, GSH was found to be inefficient in the promotion of damage to DNA. Parallel ESR spin trapping measurements indicated that GSH inhibits free radical formation by copper ions in the presence of H2O2, ascorbate, and DNA. The protective effect of GSH is attributed to its stabilization of copper in the +1 oxidation state, thereby compromising its ability to participate in free radical generating reactions. Consequently, it is suggested that the GSH in cell nuclei serves to prevent, rather than promote, copper-dependent damage to DNA. In contrast, in the presence of 1,10-phenanthroline, GSH stimulated free radical formation and DNA damage. This is attributed to the failure of GSH to remove copper(I) from 1,10-phenanthroline. Therefore, under these conditions, GSH serves primarily to redox cycle the reactive 1,10-phenanthroline-copper complex.