BASIC RADIATION BIOCHEMISTEY 275 



ency to attribute too much weight to reactions with hydrogen peroxide. 

 Many radiation effects may become less but do not disappear in absence 

 of oxygen. Others are not affected at all either by the presence or 

 absence of oxygen, or by hydrogen peroxide in concentrations built up by 

 the radiation dose used. This has been shown for the inactivation of 

 carboxypeptidase (Dale, Gray, and Meredith, 1949), for the deamination 

 of amino acids (Dale, Davies, and Gilbert, 1949b), and for the deactiva- 

 tion of ribonuclease (Holmes, 1950). There are, however, other reactions 

 in which hydrogen peroxide does play a part. This has been shown, for 

 instance, for bacteriophage, S 13 (Alper, 1948), which is not only inactiv- 

 ated by X rays but also by hydrogen peroxide of such concentration as is 

 generated by the inactivating X-ray dose in aerated solution. In living 

 matter accumulation of hydrogen peroxide would not be expected because 

 it would be broken down at the instant of its formation by the powerful 

 enzyme catalase which is present in all tissues. 



Hydrogen peroxide therefore complicates the unraveling of the mecha- 

 nism of the response to radiation of those substances which are sensitive 

 to hydrogen peroxide and requires special attention from the experi- 

 mental point of view in order to separate the reaction of radicals with 

 solute from the reaction of hydrogen peroxide with solute. Any tendency 

 to overemphasize the hydrogen peroxide effect carries the danger of 

 obscuring the progress made in understanding the mode of action of ion- 

 izing radiations, i.e., that these radiations can have an effect of their own. 



A few examples of the effect of X radiation on biological systems under 

 anerobic conditions may be given. Crabtree and Cramer (1933) found 

 that tumor cells of rats became less sensitive to radiation w^hen irradiated 

 in absence of oxygen. Thoday and Read (1947) report that the growth 

 rate of root tips of Viciafaha was less retarded when irradiated in absence 

 of oxygen than in its presence and therefore consider an effect of hydrogen 

 peroxide probable. Schrek (1946) showed that lymphocytes are not 

 radiosensitive under anaerobic conditions. 



These results indicate an effect of oxygen (and reaction products like 

 H2O2 and HO2) in some biological radiation effects of X rays. This ques- 

 tion cannot, however, be considered as finally settled, especially in view 

 of conflicting results obtained in experiments with addition of cyanide 

 which also causes anaerobiosis. Cyanide increased the radiosensitivity 

 of tumor tissues (Crabtree and Cramer, 1933) and the radiosensitivity 

 of growing tips of bean roots (Mottram, 1935). On the other hand it 

 reduced mutation rate in X-irradiated barley (d'Amato and Gustafsson, 

 1948) and raised the mean lethal dose of rats exposed to X rays (Herve 

 and Bacq, 1949). 



Other Suhstajices. The steadily increasing knowledge of cleavage 

 products formed by radiation helps us to come nearer to an understand- 

 ing of the biological implications. As already briefly mentioned earlier 



