X-Rays and Cell Metabolism 115 



tissue and a cessation of mitosis, which usually occurs rapidly, 

 and by which the epithelium is supplied with young cells. 

 These changes must account for part, at least, of the metabolic 

 changes which have been described as directly due to enzyme 

 destruction. 



Immediate effects, on excised tissue, for instance, cannot 

 usually be demonstrated unless a very large dose is used and 

 even in this case there is selective action and only some 

 enzymes appear to be inactivated (Holmes, 1939). 



A biological illustration of the lack of effect on the ordinary 

 processes of the cell is given by the difficulty of producing 

 any visible action on a non-dividing tissue, while the biological 

 physicists would agree that the number of ionizations pro- 

 duced by ordinary clinical doses could not be expected to 

 produce any obvious effect on the complex mixture of mole- 

 cules in the cell cytoplasm. 



Biological studies had shown, on the other hand, that quite 

 small doses do affect the nucleus and the chromosomes and 

 prevent or make abnormal the division of cells. Thus it was 

 plain that the various components of the nucleus had to be 

 the objects of metabolic study. 



The work of Mitchell (1942), who demonstrated a dis- 

 turbance in nucleic metabolism after X-rays, and also the 

 early work of the Lawrence school in Berkeley, which had 

 demonstrated the uptake of radioactive phosphorus into the 

 nucleic acids of dividing nuclei, immediately suggested that 

 32p might be used to follow events after irradiation. At that 

 time, just at the end of the war, we did not know that Hevesy 

 (1945) had just completed work along the same lines. 



The use of tracer phosphorus is- essential for this work, in 

 order that the new formation of nucleic acid may be deter- 

 mined. Taking the average of the cell population, as one 

 does when estimating nucleic acid in the tissues by other 

 methods, the actual amount of deoxyribonucleic per unit wet 

 weight of tissue does not change after irradiation, since it is 

 produced in proportion to the amount of new cell material 

 formed. 



