Do.OH scavenger secondary radicals protect by competing with oxygen for cellular target sites?

Recently, using Chinese hamster V79 cells, we found no relationship between the level of protection and the overall rate for .OH removal [Ewing and Walton, Radiat. Res. 126, 187-197 (1991)]. We offered several possible interpretations for this observation, including that the scavengers may actually have multiple ways to protect, ways that would occur in addition to, or instead of, simple .OH removal. With bacterial spores, we had noted that protection occurs only with those .OH scavengers that are able to react and form secondary, reducing radicals (alpha-hydroxy radicals, RCOH), and we suggested that protection might occur if these radicals reduced cellular radical sites in competition with (damaging) reactions of O2. We have now tested that hypothesis with four .OH scavengers (DMSO, ethanol, glycerol, and methanol), and Chinese hamster V79 cells, irradiated while equilibrated with 0.9% O2 and 100% O2; our recent experiments with these scavengers in air provide data for a third O2 concentration. If these scavengers protect in vitro mammalian cells by forming secondary reducing radicals which compete with O2 for damaged cellular sites, we expect that when we reduce the O2 concentration, we will concomitantly reduce the scavenger concentrations needed for protection. If the proposed competition occurs, we expect the scavenger concentrations for 50% maximum effect to occur in the ratio of the three O2 concentrations used approximately 1:20:100. We found no evidence for such a competition as the mechanism of protection for these four .OH scavengers.