Direct evidence for hydroxyl radical-induced damage to nucleic acids by chromium(VI)-derived species: implications for chromium carcinogenesis

Reduction of Cr(VI) by NADH and NADPH has been shown to yieldCr(V) species, which have been detected by electron paramagneticresonance (EPR) spectroscopy. The fine structure on the EPRsignal of the Cr(V) species is consistent with the presenceof two NAD(P)H ligands in a square-pyramidal arrangement witha single oxygen (oxo) group at the apex. Neither this speciesnor the initial Cr(VI) complex damage DNA components as evidencedby the lack of effect of these compounds on the optical andEPR signals of the Cr(VI) and Cr(V) species respectively. Additionof hydrogen peroxide to the Cr(V) species is shown to resultin the formation of a further transient EPR signal, the parametersof which are consistent with an assignment to a Cr(V)-peroxidecomplex. Inclusion of the spin trap 5,5-dimethyl-l-pyrroline-N-oxidein this system demonstrates that hydroxyl radicals are alsogenerated, possibly via the decomposition of the peroxide complex.Inclusion of DNA components in this system together with thespin trap 2-methyl-2-nitrosopropane results in the detectionof base- and sugar-derived radicals; the characteristic EPRsignals of these species have allowed both the identificationof these species and their mechanism of formation to be determined.The signals from the former species are consistent with radicaladdition to the base, whereas the sugar-derived species arebelieved to be formed via hydrogen atom abstraction. In eachcase, this behaviour is consistent with hydroxyl radicals beingthe damaging species in systems where Cr(V) is generated inthe presence of hydrogen peroxide. These results therefore suggestthat it may be the hydroxyl radical that is the ultimate carcinogenicspecies in cells and systems exposed to Cr(VI).