The role of free radicals in gamma-ray-induced DNA lesions

Studies of free radicals have gained impetus from accumulating evidence implicating them as an intermediate step in the production of DNA damage. Moreover, it has been well documented that free radicals are involved in radiation-induced carcinogenesis. Such documentation implies that radical-scavenging agents may be effective anticarcinogens and antimutagens. We have examined at a molecular level the effects of radical scavengers on the induction of DNA damage by 60Co gamma-rays. Using irradiated human lymphoblasts and dilute solutions of purified hamster cell DNA, two classes of DNA lesions were examined by velocity sedimentation: strand breaks and DNA base damage recognized and converted to single strand breaks by enzymatic activities present in extracts of Micrococcus luteus. In irradiated lymphoblasts, strand breakage exhibited the expected sensitivity to oxygen and OH. scavengers, but no difference in the yields under N2 or N2O irradiation conditions was observed. In contrast to strand break induction, enzyme sensitive sites (ENSSs) had no oxygen enhancement ratio, but decreased when OH. scavengers were present during cellular irradiation under O2, N2, or N2O. In dilute solutions of purified DNA, radiation-induced strand breaks displayed little quantitative change under either O2 or N2, but increased appreciably under N2O. Hypoxic irradiation of purified DNA resulted in a 2-fold increase in ENSSs compared to the yield detected under O2. These sites were shown to decrease in number when DNA was irradiated in the presence of several OH. or aqueous electron scavengers. Our results suggest a role for both OH radicals and aqueous electrons in the formation of base damage recognized by M. luteus gamma endonuclease, and provide a strong foundation for further studies on the antimutagenic effects of radical scavengers