CHAPTER D 



The Effect of X Rays on Systems 

 of Biological Importance 



E. S. Guzman Barron 

 Chemical Division, Department of Medicine, University of Chicago 



Introduction. The two theories of biological action of radiations. Products of irradia- 

 tion of water. Effect of ionizing radiations on oxidation-reduction systems of biological 

 im'portance. Effect of ionizing radiations on nucleic acids and nucleoproteins. Mis- 

 cellaneous effects. Effect of ionizing radiations on proteins. Effect of ionizing radia- 

 tions on amino acids. Effect of ionizing radiations on enztjmes. Effect of ionizing 

 radiations on metabolism of single cells. Effect of ionizing radiations on tissue metabo- 

 lism. Protection against ionizing radiations. Reactivation. References. 



INTRODUCTION 



The use of ionizing radiations in medicine as a therapeutic and diagnos- 

 tic agent, and in biology as a producer of mutations, greatly stimulated 

 the study of the mechanism of action of these radiations on hving cells. 

 Their introduction as a warfare agent has increased the interest in these 

 studies. The subject is indeed of considerable biological importance, for 

 we have here an agent which may produce mutations, stop cellular divi- 

 sions, or act as a cancerogenic agent when applied in small and repeated 

 doses, act as a cancerolytic agent when applied in moderate amounts, and 

 produce death in large amounts. 



In spite of a large amount of work, little progress had been made in this 

 field up to 1940, probably because of the dominant concepts about the 

 nature of the effects of ionizing radiations. With no systematic study of 

 the alterations produced by these radiations, attention was focused on the 

 search for the "sensitive spot," the "point sensible" hit by the ionizing 

 radiation. The dominant thought up to 1937 was, in fact, well expressed 

 by Scott (1937), who concluded that metabohc reactions play no part in 

 the primary action of irradiation on cells and that any detected metabolic 

 disturbances are a manifestation of degenerative processes which succeed 

 the primary damage to cell growth and the nucleus. The target theory 

 dominated the field, and some biologists maintained that only it could 

 explain the production of mutations and the shape of the "survival 

 curves" caused by ionizing radiations (Lea, 1946; Crowther, 1938). The 

 classical experiments of Dale in 1940 broke the ground for the introduc- 



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