Oxidative DNA damage associated with combination of guanine and superoxide radicals and repair mechanisms via radical trapping

In living tissues under inflammatory conditions, superoxide radicals (O₂^-.) are generated and are known to cause oxidative DNA damage. However, the mechanisms of action are poorly understood. It is shown here that the combination of O₂^-. with guanine neutral radicals, G(-H)^- in single- or double-stranded oligodeoxyribonucleotides (rate constant of 4.7 ±1.0 × 10 ⁸ M^-1 s^-1 in both cases), culminates in the formation of oxidatively modified guanine bases (major product, imidazolone; minor product, 8-oxo-7,8-dihydroguanine). The G(-H)^. and O ₂^-. radicals were generated by intense 308 nm excimer laser pulses resulting in the one-electron oxidation and deprotonation of guanine in the 5′-d(CC[2AP]-TCGCTACC) strands and the trapping of the ejected electrons by molecular oxygen (Shafirovich, V., Dourandin, A., Huang, W., Luneva, N. P., and Geacintov, N. E. (2000) Phys. Chem. Chem. Phys. 2, 4399-4408). The addition of Cu,Zn-superoxide dismutase, known to react rapidly with Superoxide, dramatically enhances the lifetimes of guanine radicals from 4 to 7 ms to 0.2-0.6 s in the presence of 5 μM Superoxide dismutase. Oxygen-18 isotope labeling experiments reveal two pathways of 8-oxo-7,8-dihydroguanine formation including either addition of O₂^-. to the C-8 position of G(-H)^. (in the presence of oxygen), or the hydration of G(-H)^. (in the absence of oxygen). The formation of the guanine lesions via combination of guanine and superoxide radicals is greatly reduced in the presence of typical antioxidants such as trolox and catechol that rapidly regenerate guanine by the reductive "repair" of G(-H)^. radicals. The mechanistic aspects of the radical reactions that either regenerate undamaged guanine in DNA or lead to oxidatively modified guanine bases are discussed.