The effects of coal tar and ultraviolet radiation upon the incorporation of thymidine into DNA by Ehrlich ascites carcinoma cells in vitro.

The effects of coal tar with and without radiation from three types of sources upon the incorporation of thymidine into DNA by Ehrlich ascites carcinoma cells were investigated. It was found that addition of varying concentrations of coal tar inhibited the thymidine incorporation by progressively greater amounts. Irradiation by mercury vapor lamp or fluorescent tubes, both emitting predominantly UVA (320-400 nm) and visible radiations, inhibited the DNA synthesis to a small extent. However irradiation with either of these sources in the presence of tar gave a synergistic effect on the DNA synthesis. Irradiation with a predominantly UVB (260--320nm) source inhibited the DNA synthesis to a large extent. Irradiation with this source in the presence of coal tar did not give a synergistic effect but gave a summation effect. These findings have relevance in helping to choose the approporate radiation source for tar phototherapy of psoriasis. This in vitro system could also be used for other applications such as determining potencies of various tars or tar fractions.