INDUCTION OF SLOWLY DEVELOPING ALKALI-LABILE SITES IN HUMAN P3 CELL-DNA BY UVA AND BLUE-LIGHT AND GREEN-LIGHT PHOTONS - ACTION SPECTRUM

Action spectra (365-520 nm) for the formation of DNA single-strand bre aks (SSB) and slowly developing alkali-labile sites (SDALS) in human t eratocarcinoma P3 cells in culture were determined. Induction of SDALS results from the absorption of blue- and green-light photons. The spe ctrum has a broad peak that is maximal between 400 nm to 500 nm and de clines sharply above and below these wavelength regions. Negligible yi elds of SDALS were produced by photons at wavelengths of 365 nm or sho rter and at 520 nm or longer, whereas for SSB, the action increases wi th shorter wavelength throughout the whole spectral range studied. The configuration of the SDALS action spectrum suggests that the primary chromophore, and therefore possibly the photosensitizer, is a mixture of porphyrin and flavin residues.