Transcriptionally active r-chromatin from Tetrahymena has been irradiated in dilute phosphate buffer, pH 7.2, in the presence of the sulfhydryl compound 2-mercaptoethanol [MSH]. MSH was more protective against radiation-induced inactivation of transcription under N2 than under O2. The OH scavenger, t-butanol, on the other hand, gives significantly less protection under N2 than under O2, apparently due to inactivation by secondary t-butanol radicals under anoxia as shown previously (C. Herskind and O. Westergaard, Radiat. Res., 114, 28-41 (1988). However, MSH was found to restore most of the protective effect of t-butanol under N2. Inactivation was studied as a function of MSH concentration [0.03-10 mM] at different, fixed concentrations of t-butanol [3-300 mM]. The observed protection may be explained essentially in terms of (1) OH scavenging, (2) "repair" of DNA radicals by H-atom transfer from MSH under N2 in competition with fixation of damage under O2, and (3) protection against inactivation by secondary t-butanol radicals by H-atom transfer to these radicals. The sensitizing effect of oxygen in the presence of MSH is reduced by t-butanol and may even be reversed to produce an apparently protective effect. This finding is discussed in terms of residual inactivation by secondary radicals. The significance of OH scavengers as potential modifiers of oxygen enhancement ratio values is discussed.

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