580 RADIATION BIOLOGY 



2. Strong reducers, especially sodium hydrosulfite (Na2S204), highly 

 effective at 0.04 M without prior incubation. 



3. Alcohols, including, for instance, ethyl alcohol, glycerine, and 

 glycols, which require higher concentrations and are much more effective 

 after the organisms are incubated for 3^^ hour in a medium containing 

 them. 



4. Salts of carboxylic acids, effective at very low concentrations, e.g., 

 0.0001 M, but requiring preincubation. 



Whereas the actions of two substances of the same group have been found 

 so far to be additive, those of groups 1, 2, and 3, tried in combination, 

 act synergistically, so that it can be concluded that their effects are 

 different in kind or on different sites. As pointed out by these investi- 

 gators, group 1 evidently protects by sparing the SH groups naturally 

 present in cell proteins containing them, which Barron and co-workers 

 (Barron, Dickman, and Singer, 1947; Barron and Dickman, 1949; 

 Barron et al., 1949; Barron and Flood, 1950) had found to be especially 

 sensitive to the oxidizing effect of radiation, while group 2 obviously 

 protects directly through its reducing activity. The related groups 3 and 

 4, requiring incubation, evidently are subjected to metabolic changes 

 before they can exercise effective protection, and it has been found by 

 Stapleton and Billen that respiratory inhibitors such as cyanide prevent 

 these changes from occurring and thus block the development of the 

 protective property. Since the substances of group 3 are in cell metab- 

 olism changed in part to those of group 4, their action may be through this 

 pathway. It is suggested by Hollaender, Baker, and Anderson that 

 possibly the carboxylic acids enter into some reaction in the nucleo- 

 protein cycle. 



In onion root tips, Riley (1952) has tested the influence of group 1, 

 represented by BAL, and group 2, represented by sodium hydrosulfite 

 (Na2S204), on the production of chromosome changes by y rays and has 

 found both to be distinctly protective in this respect. It was further 

 noted, in the case of the hydrosulfite at least, that protection was also 

 afforded against the inhibition of mitosis by the radiation. 



It is already evident that not all four of these groups of compounds will 

 afford protection against radiation mutagenesis in all organisms. This 

 is well illustrated in the recent work of Kimball and Gaither (1951, 1952) 

 on P. aurelia which had been treated with X rays. For one thing, it was 

 shown that the killing effect of the X rays is entirely or almost entirely 

 due to material produced by the radiation in the extracellular medium 

 since the organisms are killed as effectively if they are not irradiated 

 themselves but placed in a medium that had received the given amount of 

 radiation. Moreover, this same amount of killing effect can be brought 

 about by treating the medium with an amoimt of hydrogen peroxide not 



