PEROXY RADICAL OXIDATION OF THYMIDINE

The peroxy radical (ROO) is unique among reactive oxygen species impl icated in the production of DNA damage in that it possesses an extreme ly long half-life (order of seconds) and is predicted to have a relati vely greater chemical selectivity in its reactions relative to other r adical intermediates. Yet no product studies of the reactions of ROO with bases, nucleosides, or DNA have appeared, and thus no meaningful predictions can be made regarding its potential involvement in the pro duction of DNA base damage and the mutagenic process. We report here o n the reaction products formed by peroxy radical with thymidine, a maj or target of oxidative base damage. ROO reacts with thymine to yield predominantly 5-Me oxidation products. The highly mutagenic 5-(hydrope roxymethyl)-2'-deoxyuridine, 5-formyl-2'-deoxyuridine, and 5-(hydroxym ethyl)-2'-deoxyuridine are produced by peroxy radical oxidation. In co ntrast, 5-Me oxidation products are minor products of thymidine oxidat ion by OH, which yields predominantly saturated derivatives via addit ion to the 5,6 double bond. A plausible mechanistic scheme for the for mation of the base oxidation products of thymidine by peroxy radicals is presented. Attack at the deoxyribose moiety resulting in oxidative depyrimidination is also found to occur, as indicated by free base-rel ease. Phosphodiester backbone cleavage resulting in single and double strand breaks is also catalyzed by peroxy radical, as demonstrated usi ng a plasmid nicking assay.