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

氚是构成细胞DNA的放射性同位素元素之一，环境中氚的危险性有两个主要因素，即：(1)DNA(有机)结合的氚微量剂量导致DNA的大量损失；(2)通过食物链进入生物体内的氚数量增加。在以前的论文中，我们报道了食物链导致肝细胞核和DNA增加2.1倍，导致与氚水相比，生物效应高出数倍。我们的综述研究证实了有机结合氚的这些高生物学效应，除了氚氨基酸在胚胎中具有巨大的效应外。众所周知，环境中有机结合氚的低浓度或低剂量率会导致生物有效性的降低，即剂量率效应。辐射损伤的DNA修复是造成这种低剂量率的主要因素。在本实验中，我们利用DNA修复缺陷突变体、小鼠SCID细胞和人Nijmegen断裂综合征细胞来评估DNA修复对氚生物学效应的影响。SCID细胞和NBS细胞的平均致死剂量剂量分别为0.7和0.6GY，约为正常细胞的一半。因此，我们得出结论：ICRP目前推荐的环境中有机结合氚的适宜ALI为2.8×10^9Bq的四分之一。

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.