5-Carboxamido-5-formamido-2-iminohydantoinin Addition to 8-oxo-78-Dihydroguanine Is the Major Productof the Iron-Fenton or X-ray Radiation-Induced Oxidation ofGuanine under Aerobic Reducing Conditions in Nucleoside and DNA Contexts

摘要

Exogenously and endogenously produced reactive oxygen species attack the base and sugar moieties of DNA showing a preference for reaction at 2′-deoxyguanosine (>dG) sites. In the present work, >dG was oxidized by HO^• via the Fe(II)-Fenton reaction or by X-ray radiolysis of water. The oxidized lesions observed include the 2′-deoxynucleosides of 8-oxo-7,8-dihydroguanine (>dOG), spiroiminodihydantoin (>dSp), 5-guanidinohydantoin (>dGh), oxazolone (>dZ), 5-carboxamido-5-formamido-2-iminohydantoin (>d2Ih), 5′,8-cyclo-2′-deoxyguanosine (>cyclo-dG), and the free base guanine (>Gua). Reactions conducted with ascorbate or N-acetylcysteine as a reductant under aerobic conditions identified >d2Ih as the major lesion formed. Studies were conducted to identify the role of O2 and the reductant in product formation. From these studies, mechanisms are proposed to support >d2Ih as a major oxidation product detected under aerobic conditions in the presence of the reductant. These nucleoside observations were then validated in oxidations of oligodeoxynucleotide and λ-DNA contexts that demonstrated high yields of >d2Ih in tandem with >dOG, >dSp, and >dGh. These results identify >dG oxidation to >d2Ih to occur in high yields leading toa hypothesis that >d2Ih could be found from in cells stressedwith HO^•. Further, the distorted ring structureof >d2Ih likely causes this lesion to be highly mutagenic.