PHOTOSENSITIZATION OF SUPERCOILED DNA-DAMAGE BY 5,6-DIHYDROXYINDOLE-2-CARBOXYLIC ACID, A PRECURSOR OF EUMELANIN

The photosensitizing or photoprotecting action of 5,6-dihyandroxyindol e-2-carboxylic acid (DICA), an intermediate in the biosynthesis of eum elanins, was investigated. Under irradiation at 313 nm, aqueous buffer ed solutions of DICA (22.5 mu M) photosensitized the cleavage of phage Phi X174 DNA. The number of single strand breaks (SSB) depended on th e dose of irradiation and was more important in the absence than in th e presence of oxygen. In the presence of oxygen, the quantum yield of SSB was around 6 x 10(-7) (Phi(SSB)) The influence of specific scaveng ers, such as mannitol, sodium azide or superoxide dismutase, indicated that hydroxyl radicals, superoxide anions and perhaps singlet oxygen were involved in these processes. The increase in SSB in D2O was also indicative of the participation of singlet oxygen. Comparative experim ents performed with indole-2-carboxylic acid (IC), a dehydroxylated an alog of DICA, showed that this compound, although lacking a phenol gro up, also photosensitized DNA cleavage via a mechanism involving hydrox yl radicals. Various sources of these radicals were envisioned. Furthe rmore, under our conditions, DICA was not found to photoinduce the for mation of DNA dimers: No increase in SSB was observed in DNA irradiate d in the presence of DICA, after treatment by phage T4 endonuclease V (an enzyme that selectively cuts DNA at dimer sites), whereas, in cont rast, a significant increase in SSB was detected after treatment of DN A irradiated alone. So it appears that DICA may both photosensitize DN A cleavage and reduce UV-induced DNA dimer formation.