Guanine-specific DNA oxidation photosensitized by the tetraphenylporphyrinP(V) complex

Abstract

Porphyrins have been studied as photosensitizers in photodynamic therapy. DNA is one of the most important targets of the sensitizer. In the present study, we have examined the photosensitized DNA damage caused by dihydroxo P ( V ) tetraphenylporphyrin ( P ( V ) TPP ), a cationic water-soluble porphyrin. P ( V ) TPP photosensitized guanine-specific damage to the DNA fragment. P ( V ) TPP induced severe photodamage to single-stranded rather than to double-stranded DNA. High performance liquid chromatography measurements confirmed the formation of 8-oxo-7,8-dihydro-2'-deoxyguanosine (8-oxo-G), an oxidized product of 2'-deoxyguanosine, and showed that the content of 8-oxo-G in single-stranded DNA is larger than that in double-stranded DNA. The effects of reactive oxygen scavengers on DNA damage suggested the involvement of singlet oxygen (1O2). Photosensitized1O2formation was confirmed by near-infrared emission measurements. The results showed that1O2formation mainly contributes to the mechanism of DNA photodamage by P ( V ) TPP . Absorption spectrum measurements showed the interaction between P ( V ) TPP and DNA. This interaction is expected to enhance the1O2-mediated DNA damage since the lifetime of1O2in a cell is very short. On the other hand, P ( V ) TPP induced DNA damage at the consecutive guanines in double-stranded DNA. Because the consecutive guanines act as a hole trap, this DNA-damaging pattern suggests the partial involvement of photo-induced electron transfer. The fluorescence of P ( V ) TPP was quenched by DNA, supporting the electron transfer mechanism. However, DNA damage by electron transfer was not a main mechanism possibly due to reverse electron transfer. In conclusion, P ( V ) TPP binds to DNA and induces guanine-specific, photo-oxidation mainly via1O2generation.