Ultraviolet Radiation-induced DNA Damage and Its Photorepair in the Skin of the Platyfish Xiphophorus1

Fluence response relationships for the induction of DNA damage in the skin of I \ -irradiali-«!Xiphophorus fish were obtained by quantitative gel electrophoresis of unlabeled DNA following extraction and treatment with an enzyme preparation that makes single strand breaks next to cyclobutane pyrimidine dimers. A buffer containing 7 M urea minimized the degradation of DNA during extraction and gave reproducible results. The shapes of fluence response curves for the production of dimers by sun lamp irradiation (A > 290 nm) or 302 nm in the dermis of grown fish were similar. Photoreversal of dimers was readily observed by black light ex posure or from the longer wavelengths (>304 nm) from sun lamps. As expected, the number of pyrimidine dimers/incident fluence in young fish skin was considerably higher on the irradiated side of immobilized fish than it was in swimming (randomly moving) fish, and the shape of the fluence response curves was linear for all wavelengths used A >290, 302, and 313 nm. On the other hand, young fish irradiated from above with A >290 nm showed a less than linear relationship between pyrimidine dimers in their skin and radiation fluence because most exposure occurred on the dorsal rim of fish skin; thus, some cells in that skin were exposed to high fluences while others were not, leading to a heterogenous population of cells. Values of dimers produced were also much less than in immobilized fish. The pigment melanin decreased the number of dimers in the epidermis of grown fish exposed to A >290, 302, or 313 nm, or in the dermis of fish following 302 nm, thus conferring protection against this kind of damage. No dimers were detected in the epidermis of fish exposed to 365 nm. The dimers produced at 302 and 313 nm at tumoricidal exposures correspond to 1 dimer in 10* base pairs.