Prise K M, Gillies N E, Michael B D
Gray Laboratory Cancer Research Trust, Mount Vernon Hospital, Northwood, Middlesex, UK.
Int J Radiat Biol. 1998 Jul;74(1):53-9. doi: 10.1080/095530098141726.
To measure hypoxic chemical fixation processes of radiation damage in both isolated plasmid DNA and in GSH-depleted E. coli cells.
Plasmid pBR322 DNA was irradiated with a single 5 ns pulse of 400 keV electrons under hypoxic conditions. At pre-set times, immediately before or after the electron pulse, the chamber containing the DNA was exposed to a high-pressure shot of hydrogen sulphide (H2S) gas.
DNA irradiated before contact with the H2S pulse was more sensitive to the production of both single strand breaks (ssb) and double strand breaks (dsb) than DNA irradiated after the addition of H2S. The post-irradiation protection of DNA by H2S was time-dependent, having first-order rate constants of 21 s(-1) for ssb and 10 s(-1) for dsb.
This is the first direct kinetic evidence for the involvement of a hypoxic fixation reaction in the production of DNA damage by ionizing radiation. It indicates that long-lived radical damage is induced in DNA which, even at times of 20-50ms after irradiation, can be chemically repaired, or rescued, by the addition of a thiol agent. This reaction may partially explain the predicted decrease in oxygen enhancement ratio (OER) with linear energy transfer (LET) on the basis of the increased clustering of radicals produced on the DNA by tracks of ionizing radiation. As radical multiplicity increases with LET there is a greater chance that some of the radicals will become fixed in the absence of oxygen leading to an increased probability of damage under hypoxia and a reduction in the OER.
测量在分离的质粒DNA和谷胱甘肽（GSH）缺失的大肠杆菌细胞中辐射损伤的缺氧化学固定过程。
在缺氧条件下，用单个5 ns脉冲的400 keV电子辐照质粒pBR322 DNA。在预设时间，即在电子脉冲之前或之后立即将装有DNA的腔室暴露于硫化氢（H₂S）气体的高压喷射下。
与添加H₂S后辐照的DNA相比，在与H₂S脉冲接触之前辐照的DNA对单链断裂（ssb）和双链断裂（dsb）的产生更敏感。H₂S对DNA的辐照后保护作用是时间依赖性的，单链断裂的一级速率常数为21 s⁻¹，双链断裂的一级速率常数为10 s⁻¹。
这是缺氧固定反应参与电离辐射引起的DNA损伤产生的首个直接动力学证据。它表明在DNA中诱导了长寿命自由基损伤，即使在辐照后20 - 50毫秒时，通过添加硫醇试剂也可以进行化学修复或挽救。基于电离辐射径迹在DNA上产生的自由基聚集增加，该反应可能部分解释了预测的氧增强比（OER）随线性能量转移（LET）的降低。随着自由基多重性随LET增加，一些自由基在无氧情况下固定的可能性更大，导致缺氧条件下损伤概率增加以及OER降低。
