Repair and mutagenic potential of oxaluric acid, a major product of singlet oxygen-mediated oxidation of 8-oxo-7,8-dihydroguanine.

Oxidative reactions within DNA commonly result in base modifications. Among the four DNA bases, guanine is the most susceptible to various oxidants, and its related oxidized form, 8-oxo-7,8-dihydroguanine, has been extensively studied in terms of repair and mutagenicity. However, 8-oxo-7,8-dihydroguanine is readily subjected to further oxidation, and this has become a point of interest. We recently found that singlet oxygen oxidation of 8-oxo-7,8-dihydroguanine led to the predominant formation of oxaluric acid as the final product. We report herein on the biological features of oxaluric acid dealing with in vitro DNA synthesis and its removal from DNA by repair enzymes. Nucleotide insertion opposite oxaluric acid, catalyzed by Kf exo(-) and Taq indicates, that oxaluric acid induces G to T and G to C transversions. On the other hand, oxaluric acid represents a block when synthesis is performed with pol beta. Interestingly, DNA repair experiments carried out with formamidopyrimidine DNA N-glycosylase (Fpg) and endonuclease III (endo III) show that oxaluric acid is a substrate for both enzymes. Values of k(cat)/K(m) for the Fpg-mediated removal of oxidative guanine lesions revealed that 8-oxoGua is only a slightly better substrate than oxaluric acid. Interestingly, the results obtained with endo III suggest that oxaluric acid is a much better substrate than is 5-hydroxycytosine (5-OHC), an oxidized pyrimidine base.