IONIZING RADIATIONS AND THIOL GROUPS 219 



result in the formation of — S — S — bridges between molecules with pro- 

 duction of asymmetry and "breakage." Thoday and Read (8), it must 

 be recalled, reported that x-ray irradiation of root tips of Vicia faha 

 under anaerobic conditions had a much reduced effect on the growth 

 rate; there was also considerable reduction in the number of cells show- 

 ing chromosome bridges or fragments at anaphase. Lack of oyxgen 

 prevented the formation of the O2H radicals and of H2O2; hence, an 

 inhibition of oxidation processes would be the consequence. I might 

 go as far as to postulate the existence of an — SH enzyme for the syn- 

 thesis of ribonucleic acid, an enzyme containing freely reacting — SH 

 groups, easily attacked by alkylating and oxidizing agents. This would 

 explain the inhibition of cell growth produced by alkylating agents 

 (mustard, nitrogen mustard) and by oxidizing agents (ionizing radia- 

 tions). It must be emphasized that, whenever there is a diminished 

 effect in the absence of oxygen, this effect must be attributed to oxida- 

 tion and not to direct collision or "hit." 



The rapid oxidation of thiols by ionizing radiations was shown in 

 experiments in which glutathione, BAL, and other dithiols were irradi- 

 ated with x-rays, gamma rays, and beta rays. In all cases, the ionic 

 yield was over 3, which means a 75 per cent efficiency of the oxidation 

 reaction (Table 3). It was possible also to measure the part played 



TABLE 3 



Ionic Yields of Thiol Compounds Oxidized by X-Rays at pH 7.0 



(From Barron et al, J. Gen. Physiol, 33: 229, 1950) 



Thiol Ionic Yield 

 Glutathione 3.35 



Propane 1,3-dithiol 3.5 



2,3-Dithiopropanol (BAL) 3.72 



by the three oxidizing agents produced on irradiation of oxygen-con- 

 taining water. In the presence of catalase there was 86 per cent oxida- 

 tion; in the absence of oxygen there was 33 per cent oxidation as com- 

 pared to the oxidation of control solutions containing oxygen and no 

 catalase. From these two series of experiments it is possible to calculate 

 the efficiency of the three oxidizing agents in oxidation of thiols: H2O2 

 contributes 24 per cent, O2H 43 per cent, and OH 23 per cent, of the 

 total oxidation in an oxygenated aqueous milieu. 



There is no doubt now of the importance of the thiol groups in ionizing 

 radiations. It must be emphasized, however, that not much attention 

 has been paid to the oxidation of the soluble thiol compounds, those 



