Direct and hydrogen peroxide-induced chromium(V) oxidation of deoxyribose in single-stranded and double-stranded calf thymus DNA

Oxidative DNA damage by a model Cr(V) complex, [CrO(ehba)₂]^-, with and without added H₂O₂, was investigated for the formation of base and sugar products derived from C1', C4', and C5' hydrogen atom abstraction mechanisms. EPR studies with 5,5-dimethylpyrroline N-oxide (DMPO) have shown that Cr(V)- ehba alone can oxidize the spin trap via a direct chromium pathway, whereas reactions of Cr(V)-ehba in the presence of H₂O₂ generated the hydroxyl radical. Direct (or metal-centered) Cr(V)-ehba oxidation of single-stranded (ss) and double-stranded (ds) calf thymus DNA demonstrated the formation of thiobarbituric acid-reactive species (TBARS) and glycolic acid in an O₂- dependent manner, consistent with abstraction of the C4' H atom. A minor C1' H atom abstraction mechanism was also observed for direct Cr(V) oxidation of DNA, but no C5' H atom abstraction product was observed. Direct Cr(V) oxidation of ss- and ds-DNA also caused the release of all four nucleic acid bases with a preference for the pyrimidines cytosine and thymine in ds-DNA, but no base release preference was observed in ss-DNA. This base release was O₂-independent and could not be accounted for by the H atom abstraction mechanisms in this study. Reaction of Cr(V)-ehba with H₂O₂ and DNA yielded products consistent with all three DNA oxidation pathways measured, namely, C1', C4', and C5' H atom abstractions. Cr(V)-ehba and H₂O₂ also mediated a nonpreferential release of DNA bases with the exception of the oxidatively sensitive purine, guanine. Direct and H₂O₂-induced Cr(V) DNA oxidation had opposing substrate preferences, with direct Cr(V) oxidation favoring ss-DNA while H₂O₂-induced Cr(V) oxidative damage favored ds-DNA. These results may help explain the carcinogenic mechanism of chromium(VI) and serve to highlight the differences and similarities in DNA oxidation between high- valent chromium and oxygen-based radicals.