Analysis of biological effects of high LET radiation

Biological effects of high linear energy transfer (LET) were studied at the level of DNA strand breaks and their repair. The results were compared with those of Co-60 gamma radiation with low LET. Thermal neutron generated from the Research Reactor at the Kyoto University was used. Sedimentation profiles in alkaline sucrose gradient of DNA were investigated in E. coli strains, including AB2497(thy-, pol+, rec+), P3478(thy-, polA, rec+), and AB2487(thy-, pol+, recA13). For gamma-irradiated cells, although DNA strand breaks were observed in both AB2497 and P3478, they were repaired by 30 minute post-irradiation incubation. Sedimentation profiles in all of the three strains irradiated with high LET showed no repair, as well as AB2487 irradiated with gamma rays. High LET irradiation seemed to induce double strand breaks, deletion, and base damage more frequently than low LET irradiation. In the Deinococcus radiodurans cells, DNA strand breaks induced by either gamma radiation or thermal neutrons were repaired. However, the repair was completely inhibited by adding DNA synthetic inhibitors, such as nalidixic acid, novobiocin, and chloramphenicol. Double strand breaks repair was delayed in D. radiodurans wild type strain irradiated with thermal neutrons, as compared with than with gamma radiation. There were great differences in survival curves between wild type cells and gamma radiation-sensitive cells. When exposed to thermal neutrons, survival curves did not differ. For D. radiodurans exposed to high LET radiation, repair process of single strand breaks was different from that of double strand breaks. (N.K.)