236 THE EFFECT OF IONIZING RADIATIONS 



of the stimulated frog muscle. The authors attribute this effect to in- 

 hibition of glycolysis. A simpler explanation is that of oxidation of the 

 — SH groups of myosin, which are essential not only for the activity of 

 adenosinetriphosphatase, but also for the formation of actomyosin. 

 There is no necessity to bring forth H2O2, since the, radicals OH and 

 O2H have a greater oxidizing power towards thiols. 



From the definition of the term "ionic yield," / = M/N, it will be 

 seen that recognition of changes in the protein molecule, when small 

 doses of irradiation are used, will depend on the method of detection. 

 In fact, the excellent work of Dale on inhibition of enzymes by irradia- 

 tion with small doses of x-rays was possible because he used amounts 

 of protein in the vicinity of 1 X 10~^° moles. 



When the concentration of the protein is increased, and the dose of 

 radiation is increased, the probability of direct collision also increases. 

 Under those conditions we have protein denaturation and precipitation. 

 Drastic changes also occur in the amino acid molecules, as has been 

 observed by Dale and his coworkers on irradiation of glycine and other 

 amino acids. Splitting of the protein molecule on irradiation with high 

 doses of alpha rays was observed by Swedberg. Hemocyanine was 

 split into two. Since the process was temperature-independent, Swed- 

 berg beheves that splitting was produced by direct collision, or "hit," 

 of the ionizing particle and the hemocyanine molecule. Similar molecular 

 disintegrations were also observed by Sparrow and Rosenfeld (18) on 

 irradiation of thymonucleohistone and thymonucleate with large doses 

 of x-rays. Viscosity and birefringency fell on irradiation. Whether 

 such a drop in viscosity is due to direct collision alone or to reactions 

 produced by the ionization products of water remains to be found. 



It is well known that hemorrhages and diminished coagulation time 

 are symptoms of radiation sickness. It is quite possible that the lack 

 of coagulation may be due to oxidation of the — SH groups of the 

 fibrinogen molecule. 



In conclusion it may be stated that ionizing radiations may affect 

 proteins either indirectly by the products of water irradiation or directly 

 by collision. The indirect action produces mainly oxidation of polar 

 groups such as — SH and OH groups; the denaturation and precipitation 

 phenomena are perhaps due to rupture of hydrogen bonds by direct 

 collision. 



Ionizing Radiations and Cell Metabolism 



It has been known for a long time that ionizing radiations inhibit 

 catabolic as well as anabolic reactions, and that, as Hevesy discusses, 

 synthesis reactions are inhibited with relatively small doses of x-rays. 



