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.