218 THE EFFECT OF IONIZING RADIATIONS 



glyceraldehyde dehydrogenase, adenosinetriphosphatase, and succino- 

 dehydrogenase, were reversibly inhibited by ionizing radiations (Table 

 1). Furthermore, the ionic yield was higher than that obtained by 

 Dale on inactivation of non-thiol enzymes (Table 2). Protection and 



TABLE 2 

 Ionic Yields of Enzymes Inhibited by X-Ray Irradiation 



reactivation of the enzyme were achieved by the addition of glutathione. 

 I urged at that time the use of thiols for prevention and treatment of 

 x-ray sickness. Feeble, unsuccessful attempts were made then (1944) 

 at the Metallurgical Laboratory. It was not until 1949 that Patt and 

 his coworkers (2) and Chapman and his coworkers (3) simultaneously 

 succeeded in protecting animals against lethal doses of x-rays by the 

 previous injection of thiol compounds, such as cysteine, BAL, and 

 glutathione. 



The importance of this reaction (oxidation of — SH groups by ionizing 

 radiations) is the consequence of the tremendously important role of 

 thiol groups. As soluble thiol compounds (glutathione, cysteine) they 

 are one of the regulatory mechanisms of cellular metabolism (4); they 

 take part — as yet in an unknown manner — in the processes of mitosis 

 and of cell division and cell growth. As fixed thiol groups, attached 

 to the side chains of proteins and of nucleoproteins, they contribute to 

 the activity of enzymes, to the union of muscle proteins, to blood coagu- 

 lation, and to the flexibility of fibrous proteins, such as those in the 

 skin (kerateins) and in the lens. 



I believe that the cataract produced by ionizing radiations is due 

 to oxidation of the thiol groups of the lens proteins; oxidation of the 

 — SH groups would produce intermolecular — S — S bridge formation 

 with consequent polymerization, thickening of the protein molecules, 

 and production of opacity [von Euler (5) and Bellows (6) have both 

 shown that the — SH content of the lens proteins is diminished in 

 cataract]. A similar explanation may be given for the production of 

 chromosome breaks on irradiation. According to Davidson and Lawrie 

 (7), chromosomes contain very few thiol groups. Their oxidation would 



