Inhibition of DNA-ethidium bromide intercalation due to free radical attack upon DNA. Pt. 2

The fluorescent intercalation complex of ethidium bromide with DNA was used as a probe to demonstrate damage in the base-pair region of DNA, due to the action of superoxide radicals. The O2sup(.-) radical itself, generated by γ-radiolysis of oxygenated aqueous Na-formate solutions, is rather ineffective with respect to impairment of DNA. Copper(II) ions, known to interact with DNA by coordinate binding at purines, enthance the damaging effect of O2sup(.-). Addition of H2O2 to the DNA/Cu(II) system gives rise to further enhancement, so that DNA impairment by O2sup(.-) becomes comparable to that initiated by sup(.)OH radicals. These results suggest that the modified, CU(II)-catalysed, Haber-Weiss process transforms O2sup(.-) into sup(.)OH radicals directly at the target molecule, DNA-Cu2++O2sup(.-)->DNA-Cu++O2 DNA-Cu++H2O2->DNAsup(...)OH+Cu2++OH- in a ''site-specific'' mechanism as proposed for other systems (Samuni et al. 1981; Aronovitch et al. 1983). Slow DNA decomposition also occurs without γ-irradiation by autocatalysis of DNA/Cu(II)/H2O2 systems. In this context we observed that Cu(II) in the DNA-Cu2+ complex (unlike free Cu2+) is capable of oxidizing Fe(II) to Fe(III), thus the redox potential of the Cu2+/Cu+ couple appears to be higher than that of the Fe3+/Fe2+ couple when the ions are complexed with DNA. Metal-catalysed DNA damage by O2sup(.-) also occurs with Fe(III), but not with AgI or Cd(II) ions. It was also observed that Cu(II) ions (but neither Ag(I) nor Cd(II)) efficiently inhibit the fluorescence of the intercalation complex of ethidium bromide with DNA. (orig.)