286 RADIATION BIOLOGY 



oxygen is negligible. In the presence of oxygen, hydrogen peroxide for- 

 mation for a given dose of radiation depends on the concentration of dis- 

 solved oxygen. In water saturated with oxygen hydrogen peroxide for- 

 mation depends on the intensity and the rate dose of irradiation. In 

 aqueous solutions, when the rate dose is kept constant, hydrogen peroxide 

 formation depends on the pH, temperature, and presence of dissolved sub- 

 stances, even of those which do not combine with hydrogen peroxide, 

 such as electrolytes. Hydrogen peroxide formation is maximum at acid 

 pH values, at low temperatures, and in the absence of added substances 

 (Bonet-Maury and Lefort, 19^0). Of the electrolytes, potassium nitrate 

 has the greatest inhibitory power on hydrogen peroxide formation (Barron 

 and Redfield, 1950). When aqueous solutions are irradiated with a par- 

 ticles, hydrogen peroxide formation is independent of the presence of dis- 

 solved oxygen (Duane and Scheuer, 1913). When biological fluids with 

 pH values from 5 to 7 are irradiated in the presence of proteins and elec- 

 trolytes, the amount of hydrogen peroxide formed is only half that formed 

 on irradiation of distilled water. In the study of the mechanism of ioniz- 

 ing radiations, the possibility of the effect of hydrogen peroxide must be 

 taken into consideration whenever a substance reacts with hydrogen per- 

 oxide. At the same time, account must be taken of the effect of catalase, 

 which either destroys hydrogen peroxide or induces oxidations when com- 

 bined with hydrogen peroxide. 



EFFECT OF IONIZING RADIATIONS ON OXIDATION-REDUCTION SYSTEMS 



OF BIOLOGICAL IMPORTANCE 



It has been shown that when water saturated mth oxygen is irradiated, 

 there is formation of the free radicals OH and O2H, of hydrogen peroxide, 

 and of hydrogen atoms. Of these four products of the irradiation of 

 w^ater, the first three are oxidizing agents, whereas the last is a reducing 

 agent. Moreover, hydrogen atoms combine readily with oxygen to give 

 O2H radical. It might be predicted that ionizing radiations will oxidize 

 oxidation-reduction systems of biological importance but will have little 

 effect on the reverse process, reduction. There are a number of examples 

 which support this contention. 



Thiol compounds are sluggish oxidation-reduction systems, which are 

 not oxidized by atmospheric oxygen but are easily oxidized by catalytic 

 amounts of some heavy metals or by some enzyme systems. They are 

 also oxidized by ionizing radiations when in aqueous solutions. Under 

 these conditions the following reactions may take place: 



2RSH + 20H = RS- SR + 2H2O 

 2RSH + 2O2H = RS SR + 2H2O2 

 2RSH + H2O2 - RS^ SR + 2H2O 



