Association between 8-hydroxy-2'-deoxyguanosine formation and DNA strand breaks mediated by copper and iron

Oxidative DNA damage is involved in diverse biological phenomena and consists of several kinds of lesions, mainly, strand breaks, base modifications, and DNA-protein crosslinking. However, little is known about the existence of a chemical relationship among them or the ratio by which these different types of lesions are produced. In the present study we investigated whether a relationship exists between DNA strand breakage and base modification. We selected cupric [Cu(II)] and ferric [Fe(III)] ions for this study because these transition metals are active catalysts of DNA damage in vivo. Supercoiled plasmid DNA pZ189 was treated with Cu(II) or Fe(III) in the presence of different reducing agents. We measured in each sample both the number of DNA single-strand breaks (SSB) by quantitative electrophoresis and the amount of a modified DNA base, 8-hydroxy-2'-deoxyguanosine (8-OHdG) by HPLC with simultaneous electrochemical (EC) and spectrophotometric detection. The number of DNA SSBs produced was linearly related to the number of 8-OHdG present. The regression of the number of SSBs as a function of the number of 8-OHdG is expressed by the equation [SSBs] = b x [8-OHdG], where b = 1.7, 2.0, 2.7, 1.7, and 9.4, for Cu(II) in the presence of H2O2, L-cysteine and L-ascorbate, and for Fe(III) in the presence of H2O2 and L-ascorbate, respectively. The linear correlation we observed between the production of SSB and 8-OHdG mediated by Fe(III) and by Cu(II) suggests that these products may arise via a common chemical mechanism and could allow an easier and more precise estimation of DNA breakage.