Study on biological effectiveness of organic bound tritium by using DNA deficient cells.

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Tritium is one of radioactive isotopic element, which constitute of the cell DNA.There is two majour factors for the risk of tritium in environment, which should enhance the biological effects, namely ; (1) DNA (organic)-bound tritium microdosimetricaly cause the large lession of DNA,(2) amount of tritium incorperated into organisms are increased through a food chain. In previous paper, we reported that a food chain results in 2.1-fold increase in liver cell nucleus and DNA,leading to several times higher biological effects in comparison with that of tritiated water. Our review study confirmed these high biological effectiveness of organic-bound tritium, except that tritiated amino acid have tremendous effectiveness in embryo.It is known that a low concentration or low dose rate of organic-bound tritium from environment will result in reduction of biological effectiveness, so called, dose rate effect. DNA repair from radiation-induced damage is majour factor of this low dose rate. In this experiment we estimated the effect of DNA repair on tritium biological effectiveness by using DNA repair deficient mutants, mouse scid cells and human Nijmegen Breakage Syndrome cells. The mean lethal dose Do of scid cells and NBS cells were 0.7 and 0.6 Gy, respectively and they were about half of that of normal cells. Hence, we concluded that an adequate ALI for organic bound tritium in environment is a quarter of 2.8 x 10^9 Bq presently recommended by ICRP.