A Comparison of the Chemical Repair Rates of Free Radical Precursors of DNA Damage and Cell Killing in Chinese Hamster V79 Cells

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Abstract
One of the important temporal stages of radiation action in cellular systems is the chemical phase, where oxygen fixation reactions compete with chemical repair reactions involving reducing agents such as GSH. Using the gas explosion technique it is possible to follow the kinetics of these fast (> 1 ms) reactions in intact cells. We have compared the chemical repair kinetics of the oxygen-dependent free radical precursors leading to DNA single-strand and double-strand breaks, measured using filter elution techniques, with those leading to cell killing in V79 cells. The chemical repair rates for DNA dsb (670 s−1 at pH 7·2 and 380 s−1 at pH 9·6) and cell killing (530 s−1) were similar. This is in agreement with the important role of DNA dsb in radiation induced cell lethality. The rate for DNA ssb precursors was significantly slower (210 s−1). The difference in rate between DNA ssb and dsb precursors may be explained on the basis of a dsb free radical precursor consisting of a paired radical, one radical on each strand. The instantaneous probability of one or other of these radicals being chemically repaired and not proceeding to form a dsb will be twice that of a ssb radical precursor. This agrees well with the concept of locally multiply damaged sites (LMDS) produced from clusters of ionizations in DNA (Ward 1985).

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