HYDROGEN-PEROXIDE PROTECTS YEAST-CELLS FROM INACTIVATION BY IONIZING-RADIATION - A RADIOBIOLOGICAL PARADOX

Purpose: To elucidate mechanisms of the interaction of hydrogen peroxi de with chloride-derived cytotoxins under steady-state irradiation con ditions and to determine the effects on cell viability. Materials and Methods: Yeast cells were suspended in phosphate-buffered saline and e xposed to Co-60 gamma-irradiation under different conditions. The colo ny-forming ability was determined. Results: Irradiation of PBS produce s H2O2 and HOCl simultaneously. Under slightly acidic conditions and l ow oxygen tension the yield of HOCl exceeds that of H2O2 while at phys iological pH and normoxic conditions H2O2 exceeds HOCl. Both substance s react with each other rapidly in a pH-dependent way, even during an irradiation that lasts several seconds. As HOCl is about 1000-fold mor e toxic than H2O2 to the strain of Saccharomyces cerevisiae used in th ese experiments, it is evident that in an irradiation that produces mo re HOCl than H2O2 the radiation-induced damage will be large. If, in c ontrast, the cells are irradiated under conditions in which H2O2 produ ction predominates, the damage will be small. One would therefore pred ict that addition of hydrogen peroxide to a cell suspension prior to i rradiation should result in protection for suspended cells if H2O2 int erferes with the generation of HOCl and thereby inactivates this more powerful toxin. Our data show that addition of H2O2 in sublethal conce ntration decreases radiation-induced cell death to the level that is f ound in chloride-free solution, i.e. depending on pH, reduces it by a factor of greater than or equal to 3.