244 BIOLOGICAL EFFECTS OF IONIZING RADIATIONS 



known. By one view they scavenge free radicals H, OH, H0 2 , etc., pro- 

 duced in the radiolysis. By another view they attach themselves to enzymes 

 or nucleic acids at just the spots most sensitive to radical attack ( — SH 

 groups) and thereby reduce the effects of irradiation on the big molecules. 

 For example, even impure acetylcholinesterase, in a solution with much 

 other protein, is only half as sensitive to irradiation damage in the presence 

 of 10" 3 M-cysteine as in its absence. In living cells the enzymes are well 

 protected, and seem to be resistant to much larger doses of radiation than 

 the same molecule in vitro. 



The "Oxygen Effect" 



The radiation sensitivity of most molecules is greater the higher the oxy- 

 gen content of the solution. Thus, the rate of oxidation of Fe +2 to Fe +3 by 

 X rays is twice as high in the presence, than in the absence, of oxygen. For 

 small molecules like phenol and the amino acids the rate is often even more 

 enhanced by 2 . This increased radiation sensitivity in the presence of oxy- 

 gen is observed right on up the hierarchy of structures — viruses, bacteria, 

 cells, tissues, to whole animals. 



A striking practical demonstration of this effect has been shown with 

 carcinoma tissue. Due to necrosis, many parts of a tumor can become 

 anoxic. By increasing the pressure of the respired air this anoxia can be re- 

 duced, with a consequent increase in the radiosensitivity of the carcinoma 

 cells. 



Now, oxygen itself is known to accelerate many metabolic reactions, and 

 the effect of oxygen in increasing radiation damage is thought by some to 

 result from this fact. However, in other quarters the effect is thought to 

 occur through the radical, H0 2 . This radical is produced from the reaction 



H + 2 — H0 2 



after the radiolysis reaction has produced the hydrogen atoms as follows: 



H 2 -^ H 2 0* — H + OH 



The radical HO, is a strong oxidizing agent, since it readily accepts an elec- 

 tron from any source to become the peroxide ion, H0 2 ~. Hence, one can 

 consider that the H atom simply puts 2 into a form in which it can react 

 faster. Since 2 is used up in the reaction, it must be supplied continuously 

 if advantage is to be continuously taken of enhanced rate of destruction. 

 Conversely, of course, oxygen scavenger molecules increase the protection of 

 macromolecules against ionizing radiations. 



The mode of action of oxygen is one of the most intriguing practical prob- 

 lems of radiology. Once it is understood, it can be controlled and utilized 



