Protective effect of adenine on DNA synthesis in irradiated Ehrlich ascites tumor cells

Ehrlich ascites tumor cells were cultured in the presence of adenine for several hours before gamma irradiation. The inhibition of DNA synthesis that is normally observed when these cells are irradiated was prevented. More detailed analysis using the velocity sedimentation technique provided evidence that this protective effect occurred at the level of initiation of DNA replication. This conclusion is supported by the fact that adenine failed to prevent the inhibition of DNA synthesis induced by hydroxyurea. The ability of adenine to suppress the gamma-ray-induced inhibition of DNA synthesis was dependent on both its concentration and the exposure time. This suppression was already pronounced by about 4 h after exposure of the cells to adenine, and the maximum effect was observed within 6-20 h. The optimum concentration of adenine was about 0.1 mM. Among other purines tested, a similar effect was caused only by guanine.