Photoinduced Hydroxylation of Deoxyguanosine in DNA by Pterins: Sequence Specificity and Mechanism

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Abstract
Pterin-sensitized DNA photodamage has been characterized by a DNA sequencing technique. Exposure of double-stranded DNA to 365-nm light in the presence of pterin, 6-carboxypterin, biopterin, neopterin, and folic acid produced sequence-specific DNA lesions, whereas photoinduced DNA lesions were not observed in the presence of xanthopterin or isoxanthopterin. The DNA photodamage induced by these pterin derivatives occurred specifically at the guanine located 5‘ to guanine. High-pressure liquid chromatography (HPLC) analysis revealed that the pterin-sensitized DNA photodamage was predominantly due to the formation of 7,8-dihydro-8-oxo-2‘-deoxyguanosine (8-oxo-dG). The DNA photodamage with pterin was not enhanced in D2O, suggesting that 1O2 is not the main active species. Electron spin resonance (ESR) spin destruction experiments demonstrated that the photoexcited pterins reacted specifically with dGMP to produce pterin anion radicals. In addition, the reactivities of the photoexcited pterin derivatives with dGMP were found to correlate well with their efficiencies of DNA photodamage induction. These results indicate that the photoexcited pterins specifically oxidize deoxyguanosine in DNA to produce 8-oxo-dG through an electron transfer reaction. With denatured single-stranded DNA, the extent of pterin-sensitized photodamage was decreased and the damage occurred at most guanine residues without specificity for those located 5‘ to guanine. The mechanism of pterin-induced sequence-specific guanine photodamage could be explained on the basis of a recent theoretical study [Sugiyama, H., & Saito, I. (1996) J. Am. Chem. Soc. 118, 7063−7068] concerning the ionization potentials of stacked dinucleotide base pairs. Sepiapterin, a model compound for the dihydropterins, induced similar sequence-specific photolesions in double-stranded DNA. However, DNA photodamage by sepiapterin was more extensive in the presence of Cu(II), and the sites of the photolesions were different from those induced in the absence of Cu(II). These data may provide a basis for the elucidation of the molecular mechanism of solar UV carcinogenesis.