<SESSION>CELL KILLING AND MUTATION TO 6-THIOGUANINE RESISTANCE AFTER EXPOSURE TO TRITIATED AMINO ACIDS AND TRITIATED THYMIDINE IN CULTURED MAMMALIAN CELLS

Cell killing and mutation to 6-thioguanine resistance were studied in growing mouse leukemia cells (L5178Y) in culture after exposure to tritiated amino acids and tritiated thymidine, and compared to those following HTO and γ ray exposure. These effects varied widely among the tritiated compounds tested, being greatest for tritiated thymidine followed by tritiated arginine and tritiated lysine, tritiated leucine, in that order, for a given concentration of tritium expressed in kBq/ml of tritium in the medium. A difference of 3 order of magnitude was found between tritiated thymidine and HTO. The differences between each tritiated amion acid and those between tritiated amino acids and HTO disappeared almost totally when the effects were compared on the basis of the absorbed dose to the cell. The effects of tritiated thymidine, however, remained more than two-fold greater compared to other tritiated compounds including HTO. The results indicate the importance of determining the absorbed dose for assesment of the radiotoxicity of tritiated compounds. For an exceptional case (tritiated thymidine), contribution of a mechanism(s) other than β irradiation should also be taken into account. Similar studies are being carried out in contact inhibited near-diploid mouse embryonic skin cells (m5s), kindly provided by Dr. M. S. Sasaki. For cell killing, results essentially similar to those found in the L5178Y cells were obtained for tritiated compounds tested : tritiated leucine, tritiated arginine and tritiated lysine.