Mechanism of the induction of DNA strand breaks by low energy radiation

电离辐射的大部分能量转移给许多低能二次电子，然后产生大部分放射生物学效应。真空和远紫外光子提供了一种独特的工具来控制能量吸收过程、激发和电离。因此，单色低能电子和光子对于阐明放射生物学效应的机制至关重要，但由于缺乏方便的来源，它们很少用于放射生物学。我们的项目是一项国际合作，旨在利用两种独特的辐射源来阐明诱导 DNA 链断裂的机制：单色低能电子（英国格雷实验室）和单色真空紫外光子（日本光子工厂）。质粒DNA链断裂诱导的主要结果如下： (1) 低至15 Ev的电子诱导ssb和dsb与注量呈线性关系，表明诱导ssb和dsb的阈值能量低于15 Ev； DSB值的实验测定是该项目的主要成果之一。 （2）dsb/ssb之比约为0.01，与D1Co γ射线相似。 (3) 低至 4.3 Ev（测试的最低能量）的低能量光子诱发单边带和双带带，量子产率非常低。该结果表明，从这个意义上讲，阈值能量低于 4.3 Ev，但通常定义的阈值能量约为 10 Ev。 (4)我们采用格雷实验室开发的方法，用真空紫外光子照射缓冲溶液中的质粒DNA；单色 7-Ev 光子诱导 ssb 和 DSb。 (5)这些阈值能量对于DNA链断裂模型的模拟计算至关重要。

Most energy of ionizing radiation transfers to many low energy secondary electrons, and then they produce most radiobiological effects. Vacuum-and far-UV photons provide an unique tool to control the energy absorbing processes, excitation and ionization. Monochromatic low energy electrons and photons, thus, are essential to elucidate mechanism of radiobiological effects, but, however, they are scarcely used in radiation biology because of lack of convenient sources. Our project is an international cooperation to elucidate mechanism of the induction of DNA strand breaks using two unique radiation sources : monochromatic low energy electrons (Gray Laboratory, UK) and monochromatic vacuum-UV photons (Photon Factory, Japan). Major results on the strand break induction in plasmid DNA are as follows ; (1) electrons as low as 15 Ev induced ssb and dsb linearly with the fluence, indicating that the threshold energy for the induction of ssb and dsb is below 15 Ev ; experimental determination of the value for dsb is one of the major result of this project. (2) The ratio of dsb / ssb was about 0.01, which was similar to that for ィイD160ィエD1Co γ-rays. (3) Low energy photons down to 4.3 Ev (the lowest energy tested) induced ssb and dsb with very low quantum yield. This result indicated that the threshold energy was lower than 4.3 Ev in this sense, but that defined usual was about 10 Ev. (4) We irradiated plasmid DNA in buffer solution by vacuum-UV photons using the method that was developed in the Gray Laboratory ; monochromatic 7-Ev photons induced ssb and dsb. (5) These threshold energies are crucially important to simulation calculation of models for DNA strand break.