Influence of Oxygen on the Repair of Direct Radiation Damage to DNA by Thiols in Model Systems

Abstract

Here the reactions of thiols with DNA primary radical intermediates formed after γ-irradiation of frozen (77 K) anoxic and oxic solutions of DNA/thiol mixtures are investigated. Through analysis of the experimental composite spectra at each annealing temperature, the relative concentrations of individual radicals present are estimated and reaction sequences inferred. In all samples the primary DNA radical anions and cations (DNA*⁺ and DNA*⁻) are suggested to be the predominant radicals at low temperatures. In anoxic samples, TH* (5,6-dihydrothym-5-yl radical), RSSR*⁻ and, in glutathione samples, *GSH [γ-glu-NHC(CH₂SH)CO-gly] radicals are observed as the temperature is increased. The presence of oxygen efficiently suppresses the formation of RSSR*⁻ and *GSH; instead, in oxic samples, O*⁻₂, DNAOO*, RSOO* and RSO* are observed at higher temperatures. The photolytic conversion of RSOO* to RSO*₂ is used to verify the presence of RSOO* in γ-irradiated DNA/thiol systems and confirm that the computer analysis employed yields reasonable estimates of the relative DNAOO* and RSOO* concentrations. From the relative concentrations of radicals present, it is clear that the radicals observed at higher temperatures originate from the radical reactions of the primary DNA*⁺ and DNA*⁻ radicals. Based on the reaction sequences inferred and previous work with thiols alone, it is concluded that TH*, DNAOO* and RSOO* (in part) originate largely with DNA*⁻, whereas RSSR*⁻, *GSH and RSOO* (in part) originate largely with DNA*⁺. The possible roles of DNAOO*, RSOO*, RSO*, RSO₂* and *OOGSH in the chemical oxygen enhancement effect at biologically realistic temperatures are discussed.