| As a significant outcome of oxidative damage to genomes, DNA-protein cross-links (DPCs) have been observed under a variety of conditions, but still remain the least well-characterized lesions. There are two most possible pathways for the formation of DPCs in living cells. Oxidation of nucleobases, usually G or 8-oxo-7,8-dihydroguanine (OG) results in an electron-deficient species which is then subject to nucleophilic attack by amino acid side chains from surrounding proteins giving rise to the generation of DPCs. On the other hand, the initial formation of protein radicals, possibly on lysine or tyrosine residues, can also lead to DPCs by subsequent reactions with DNA bases.;While in a DNA-protein complex devoid of OG, lysine is relatively more susceptible to the one-electron oxidation processes due to a lower redox-potential for the primary amines than the four DNA bases. We identified various Sp-like guanine-lysine adducts (5-Lys-Sp, 8-Lys-Sp, and 5,8-diLys-Sp) in a wide range of oxidation systems and demonstrated that the formation of C5 vs. C8 adducts depends on the mechanism of oxidation. It is concluded that the aminyl radical resulting from lysine oxidation leads to an 8-Lys-G adduct, and further oxidation produces 8-Lys-Sp and 5,8-diLys-Sp since lysine serves as a competitive nucleophile with water. 1O2 oxidation of G generates 5-Lys-Sp exclusively, and one-electron oxidants react with G to yield both C5 and C8 adducts in the presence of lysine.;With a lower redox-potential than the four natural DNA bases and most amino acids, OG is regarded as a major target for oxidative processes when it is present in a DNA-protein complex. Model studies for oxidatively induced DPCs have been conducted by employing single amino acids and an OG-containing DNA oligomer, and high yielding adducts were observed when OG was oxidized in the presence of lysine or tyrosine. Further structural characterizations support that under oxidative conditions, lysine adds to C5 of OG via its epsilon-amine group and the base portion rearranges to a spirocyclic structure giving rise to 5-Lys-Sp as a major adduct, while tyrosine reacts with C5 of OG through the para-carbon first followed by nucleophilic addition of phenolic oxygen to C4 leading to an unusual tricyclic adduct 4,5-Tyr-OG. Decomposition of both 5-Lys-Sp and 4,5-Tyr-OG ultimately yields spiroiminodihydantoin (Sp), a product of 4-electron oxidation of G or 2-electron oxidation of OG. |
