Mechanism of DNA damage photosensitized by trisbipyrazyl ruthenium complex. Unusual role of Cu/Zn superoxide dismutase

Trisbipyrazyl ruthenium(II) (Ru[bpz](3)(2+)) was examined as DNA photosensi tizer. Damage resulting from the photolysis of synthetic oligonucleotides h as been monitored by polyacrylamide gel electrophoresis. Photoadduct format ion is found on both single- and double-stranded oligonucleotides, On oligo nucleotide duplex, oxidative damage occurs selectively at the 5'G of the 5' GG3' site and to a lesser extent at the 5'G of a GA sequence. These finding s suggest the involvement of electron transfer and show that this mechanism is the main DNA damaging process involved in Ru(bpz)(3)(2+) photosensitiza tion, In addition, photoadducts and oxidative damage are both highly affect ed by an increase of salt concentration in the reaction medium, stressing t he importance of direct interactions between nucleic acid bases and the exc ited ruthenium complex for efficient electron transfer. On single-stranded oligonucleotides, all the guanines are oxidized to the same extent. In this case, oxidative damage, which is not affected by an increase of salt in th e solution, has been attributed, in part, to singlet oxygen. More important ly, Cu/Zn superoxide dismutase (SOD) strongly enhances the yield of all dam age, correlated to an increase of both electron transfer and singlet oxygen production. This original activity of SOD is the first example of bioactiv ation of a polyazaaromatic ruthenium complex.