The effects of ionizing radiation on deoxyribonucleic acid. Part 5. The rôle of thiols in chemical repair

Abstract

Exposure of frozen, deoxygenated, aqueous solutions of DNA to 60Co γ-rays at 77 K results in the formation of guanine-centred radical-cations(G˙+) and thymine-centred radical-anions (T˙–). Both these primary centres are thought to be capable of inducing DNA strand-breaks, both single (SSB) and double (DSB). When low concentrations of a range of water-soluble thiols were added, there was no change in the initial yield of G˙+ and T˙– as judged from the e.s.r. spectra. However, on annealing, the normal pattern of radical reactions was abruptly modified at ca. 200 ± 5 K, with the DNA-centred radicals being dramatically reduced in concentration with the concomitant growth of e.s.r. signals characteristic of RS˙–SR– radical-anions. For example, for solutions containing one thiol molecule per 25 base-pairs, there was a loss of ca. 50% in the concentration of DNA radicals at this temperature.Using plasmid DNA, the change in the numbers of SSBs and DSBs was monitored when various thiols were present. There was a marked fall in the yields of both these events, in accord with the e.s.r. results.It is concluded that these thiols react by hydrogen-atom donation to various DNA radicals thereby forming RS˙ radicals which rapidly from RS˙SR– radical-anions. It seems that, under our conditions, neither of these sulphur radicals is able to react with DNA.In the presence of oxygen, the results are less definitive, the degree of repair being a function of the relative concentrations of oxygen and thiol. E.s.r. evidence for the formation of DNA-centered peroxy radicals and their reaction with thiols is presented, and also there is evidence for the addition of oxygen to RS˙ radicals to give RSO2˙ radicals. The latter are probably able to react with DNA.