Effects of mixing metal ions on oxidative DNA damage mediated by a Fenton-type reduction

The formation of 8-hydroxy-deoxyguanosine (8-OHdG) and strand breaks in DNA by Fenton-type reactions by mixtures of two of five metal ions, iron (II), cadmium (II), nickel (II), chromium (III) or copper (II), has been investigated and compared to their formation by each single metal ion. Salmon sperm DNA and pBluescript K+ plasmid were each incubated with hydrogen peroxide and metal ions. The formation of 8-OHdG declined in the Fe (II) or Cu (II) Fenton reaction upon addition of Cd (II) or Ni (II) ion. In contrast, the Fe (II) reaction upon addition of Cr (III) ion showed an additive influence on the formation of 8-OHdG. Furthermore, the Cu (II) plus Cr (III) reaction showed a synergistic effect. These influences relate to the interaction of metal ions with DNA, the potentials of the metal ions to generate activated oxygen and electron transfer between metal ions. The formation of DNA strand breaks was investigated in plasmid DNA by agarose gel electrophoresis and subsequent densitometry. The formation of DNA strand breaks in the Fe (II) or Cu (II) Fenton reaction decreased upon the addition of Ni (II) ion, as with the formation of 8-OHdG mediated by these metal ions. On the other hand, the formation of DNA strand breaks in the Fe (II) reaction decreased upon addition of Cr (III) ion, and the Cu (II) plus Cr (III) reaction did not show the synergistic influence on DNA strand breaks. These results suggest that interactions between two metal ions can influence the generation of 8-OHdG and the formation of DNA strand breaks and demonstrate that these lesions can arise by different mechanisms.