DNA-REPAIR BY THIOLS IN AIR SHOWS 2 RADICALS MAKE A DOUBLE-STRAND BREAK

Using agarose gel electrophoresis, we have measured the yields of DNA single- and double-strand breaks (SSBs and DSBs) for plasmid DNA gamma -irradiated in aerobic aqueous solution. The presence during irradiati on of either of the thiols cysteamine or N-(2-thioethyl)-1,3-diaminopr opane (WR-1O65) resulted in a concentration-dependent decrease in the yield of SSBs and a much greater decrease in the yield of DSBs. This l arge differential protective effect was not produced by thioethers or an alcohol of structural similarity to the two thiols, suggesting that repair of DSB radical precursors by thiols is more efficient than for SSB precursors. These observations suggest the existence of a diradic al intermediate in the formation of DSBs. The results argue against a major contribution by a single radical mechanism involving interstrand radical transfer via hydrogen abstraction by a peroxyl intermediate, since the half-life of this radical transfer reaction appears to be si gnificantly greater than the lifetime of the intermediate. (C) 1995 by Radiation Research Society