252 



FACTORS INFLUENCING CELL RADIOSENSITIVITY 



role played by oxygen in the reaction has been verified (Loiseleur et al.), 

 but has not been found in the case of alpha rays. With x-rays, for 

 example, about 50 times less H2O2 is obtained in the absence of oxygen. 

 It is mainly in the presence of dissolved oxygen that irradiation pro- 

 vides aqueous media with the properties of an active oxidation system. 

 This fact can be demonstrated directly by recording the platinum po- 

 tential of an aqueous redox system during irradiation. This was done 

 by Loiseleur and Latarjet (19) by x-raying an aqueous solution of quin- 

 hydrone either saturated with or completely deprived of oxygen. The 



100 



E 90 



Fig. 7. Conversion of hydroquinone to quinone by irradiation in the presence of 

 oxygen (curve A), but not in the absence of oxygen (curve B). [Loiseleur and 



Latarjet (19).] 



platinum potential of the solution permits observation of the trend of 

 the system, either towards quinone (oxidation) or towards hydroquinone 

 (reduction). One observes (Fig. 7) a progressive oxidation of the satu- 

 rated solution, whereas, in the alternate case, no oxidation takes place. 

 This proves that irradiation of water alone does not liberate oxidative 

 radicals in sufficient quantity to displace the equilibrium in the direction 

 of oxidation, whereas dissolved oxygen insures the phenomenon. 



Although a mechanism involving free oxygen may not explain all 

 radio-oxidations, this constituent is responsible for a high proportion 

 of them. This is proved by a number of experiments carried out on 

 various biological systems. We may cite, as examples, inactivation of 

 auxine by x-rays (27), and the experiments of Thoday and Read (29) 

 on chromosome alterations produced by x-rays and alpha rays in bean 

 roots suspended in water saturated with nitrogen or oxygen : with x-rays 

 the lesions are more severe in samples irradiated in the presence of 

 oxygen, whereas with alpha rays this gas has almost no effect. Giles 

 and Riley (8) observed in Tradescantia that chromosome sensitivity is 



