Oxidative damage to DNA: formation, measurement and biochemical features.

Emphasis is placed in the first part of this survey on mechanistic aspects of the formation of 8-oxo-7,8-dihydroguanine (8-oxoGua) as the result of exposure to z.rad;OH radical, one-electron oxidants and singlet oxygen (1O(2)) oxidation. It was found that 8-oxoGua, which is generated by either hydration of the guanine radical cation or .OH addition at C8 of the imidazole ring, is a preferential target for further reactions with 1O(2) and one-electron oxidants, including the highly oxidizing oxyl-type guanine radical. Interestingly, tandem base lesions that involve 8-oxoGua and a vicinal formylamine residue were found to be generated within DNA as the result of a single .OH radical hit. The likely mechanism of formation of the latter lesions involves the transient generation of 5-(6)-peroxy-6-(5)-hydroxy-5,6-dihydropyrimidyl radicals that may add to the C8 of a vicinal guanine base before undergoing rearrangement. Another major topic which is addressed deals with recent developments in the measurement of oxidative base damage to cellular DNA. This was mostly achieved using the accurate and highly specific HPLC method coupled with the tandem mass spectrometry detection technique. Interestingly, optimized conditions of DNA extraction and subsequent work-up allow the accurate measurement of 11 modified nucleosides and bases within cellular DNA upon exposure to oxidizing agents including UVA and ionizing radiations. Finally, recently available data on the substrate specificity of DNA repair enzymes belonging to the base excision and nucleotide excision pathways are briefly reviewed. For this purpose modified oligonucleotides in which cyclopurine, and cyclopyrimidine nucleosides were site-specifically inserted were synthesized.