Variable protection by OH scavengers against radiation-induced inactivation of isolated transcriptionally active chromatin: the influence of secondary radicals.

Isolated r-chromatin, the chromatin form of the extrachromosomal gene coding for the rRNA precursor in Tetrahymena, has been used to study radiation-induced inactivation in vitro in the presence of the OH radical scavengers, t-butanol, formate ions, and methanol. Induction of biologically important DNA lesions was detected by the effect on transcription by endogenous RNA polymerases associated with the isolated r-chromatin. The OH scavengers were found to give strong protection in the presence of oxygen as anticipated from previous results obtained with this system. By contrast, only a modest protection was observed under 100% N2 or 100% N2O, and the level of protection was different for each scavenger. The data suggest that secondary radicals may inactivate r-chromatin under anoxia. In the presence of oxygen, the secondary radicals react with O2 to form organic peroxy radicals (or O2-) which seem to be less reactive. Since the protective effect of the OH scavengers varies with the gassing conditions, the dose modifying effects of O2 and N2O relative to N2 depend on the identity and concentration of OH scavenger. The implications for radiation-chemical studies on DNA and living cells are discussed.