8-Hydroxydeoxyguanosine formation at the 5' site of 5'-GG-3' sequences in double-stranded DNA by UV radiation with riboflavin.

DNA damage caused by UV radiation in the presence of riboflavin or hematoporphyrin was characterized by the DNA sequencing technique using 32P-labeled DNA fragments and the analysis of 8-hydroxydeoxyguanosine (8-OH-dG) formation in calf thymus DNA. Exposure of double-stranded DNA to 365 or 302 nm radiation in the presence of riboflavin induced the sequence-specific DNA cleavage which is different from that caused by 302 or 254 nm irradiation in the absence of a sensitizer. The specific cleavage sites were the guanine residues located 5' to guanine. On the other hand, when denatured single-stranded DNA was irradiated at 365 nm with riboflavin or hematoporphyrin, cleavages occurred at most guanine residues. With D2O, the sequence-specific damage of double-stranded DNA by riboflavin was not enhanced, whereas the damage to single-stranded DNA by hematoporphyrin was greatly enhanced. Photodynamic action of riboflavin caused the formation of 8-OH-dG in double-stranded DNA. The enhancing effect of D2O on 8-OH-dG formation was not observed with riboflavin. By contrast, hematoporphyrin plus 365-nm light induced the 8-OH-dG formation only in denatured single-stranded DNA and the 8-OH-dG yield was increased about 2-fold in D2O. ESR spin destruction experiments suggested that photoexcited riboflavin reacts with dGMP to produce riboflavin anion radical and guanine cation radical, but not with other mononucleotides. The estimated ratio of 8-OH-dG yield to total guanine loss indicates that the photoexcited riboflavin induces 8-OH-dG formation specifically at the guanine residue located 5' to guanine through electron transfer. The mechanism was discussed in relation to UV carcinogenesis.