OXYGEN ACCEPTORS AS RESISTANCE FACTORS 253 



markedly decreased in the absence of oxygen, the ratio of sensitivities 

 between O2 and N2 being about 5 to 1.* 



From the results obtained in these experiments, it is almost certain 

 that this influence of O2, even in complex systems, is expressed at the 

 level of the primary effect rather than during the dark reactions. 

 Weiss's (30) interpretation of this phenomenon during the primary 

 effect in aqueous systems attributes to the O2 molecule the role of a 

 buffer in the course of the reactions initiated in water. This inter- 

 pretation has the advantage of explaining the disappearance of this 

 factor in the case of alpha rays. It is probable, nevertheless, that O2 

 acts also by other processes. Direct oxidation of a biological molecule 

 bj^ O2, after activation of one of these two molecules, also plays an 

 important part, according to the theory of Loiseleur and Latarjet. 

 This second mechanism leads to a better understanding of the decrease 

 of sensitivity caused by oxygen acceptors, a phenomenon which we will 

 now discuss. 



OXYGEN ACCEPTORS AS RESISTANCE FACTORS 



Whatever the nature of the oxidating agents produced in irradiated 

 medium, whether activated oxygen or free radicals are involved, these 

 agents may reach the oxidizable molecules at the mercy of unknown 

 chemical affinities. 



A true competition is established in any complex medium, and the 

 oxidation of a biological molecule has to contend with the presence of 

 other oxidizable molecules which more or less protect it. These pro- 

 tection phenomena have been thoroughly studied, and we will consider 

 them here only in so far as they concern oxygen acceptors (without 

 underestimating all other types of protection which are pointed out by 

 Dale, Hevesy, and Barron). 



When desensitization is obtained, by means of prior injection of pro- 

 teins, serum, or hormones (9), in animals irradiated in toto, it is im- 

 possible, in the present state of our knowledge, to decide which sensi- 

 tivity factor is involved. In certain cases, however, the properties of 

 the injected substance may suggest a mechanism. Thus radiologists 

 long ago observed that diabetics are less sensitive than normal subjects. 

 This fact has been experimentally verified in animals (1, 20); in this 

 particular case it was assumed that glucose acted as an oxidation buffer 

 at the level of primary processes. 



In fact, working with simple systems, it has been observed that glucose 

 acts as a buffer in that it behaves as a hydrogen donor (17). In water 



* Cf. Giles's paper. 



