Effect of pigment on photomediated production of thymine dimers in cultured melanoma cells.

It was the aim of these studies to determine whether the presence of intracellular melanin quantitatively alters the rate of production of thymine dimers in DNA of irradiated cells in culture. Pigmented and nonpigmented Cloudman mouse melanoma cells were selected assuming that the two cell lines differ primarily in their content of melanin pigment. Cells were cultivated in tritiated thymine in order to label their DNA and were then exposed to ultraviolet (UV) irradiation (260 nm, 500-2000 ergs/mm2). Neither cell line survived these doses of irradiation. DNA was extracted immediately following irradiation and was subjected to acid hydrolysis. The presence of thymine dimers was determined by two-dimensional paper chromatography. The percent of labeled thymine recovered as thymine dimer was calculated and was found to be a linear function of UV dose for both cell lines. The rate of formation of dimers in the nonpigmented cells was nearly twice that in the pigmented cells. These data demonstrate the photoprotective property of intracellular melanin in shielding isolated cells from one type of photomediated injury to DNA.