INFLUENCE OF OXYGEN ON THE REPAIR OF DIRECT-RADIATION DAMAGE TO DNA BY THIOLS IN MODEL SYSTEMS

Here the reactions of thiols with DNA primary radical intermediates fo rmed after gamma-irradiation of frozen (77 K) anoxic and oxic solution s of DNA/thiol mixtures are investigated. Through analysis of the expe rimental composite spectra at each annealing temperature, the relative concentrations of individual radicals present are estimated and react ion 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 [gamma-glu -NHC(CH2SH) CO-gly] radicals are observed as the temperature is increa sed. The presence of oxygen efficiently suppresses the formation of RS SR(.-) and (.)GSH; instead, in oxic samples, O-2(.-), DNAOO(.), RSOO(. ) and RSO(.) are observed at higher temperatures. The photolytic conve rsion of RSOO(.) to RSO(2)(.) is used to verify the presence of RSOO(. ) in gamma-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 radic als present, it is clear that the radicals observed at higher temperat ures originate from the radical reactions of the primary DNA(.+) and D NA(.-) 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 a nd RSOO(.) (in part) originate largely with DNA(.+). The possible role s of DNAOO(.), RSOO(.), RSO(.), RSO(2)(.) and (.)OOGSH in the chemical oxygen enhancement effect at biologically realistic temperatures are discussed.