RADIOACTIVE TRACERS 



863 



sence of a great variety of protecting substances, is prevented from reach- 

 ing the enzymes present in the tissue. By producing cell damage, for 

 example through lesion of the DNA nucleotides, the normal enzymatic 

 pattern gets changed and chains of pathochemical effects may set in 

 as in the cases mentioned above. 



Radiosensitivity 



A correlation between radiosensitivity and vulnerability of desoxy- 

 ribopolynucleotides, between radiosensitivity and macro-molecular lesion 

 of DNA, to use an expression introduced by Mitchell (1956), is so far 

 a hypothetical one, not so the correlation between radiosensivivity and 

 rate of formation of DNA. It is easy to find deviations from such a 

 correlation but as is demonstrated by the data in Table 2, in a great 

 number of cases a striking correlation is found between radiosensitivity 

 of the organ and the turnover rate of its DNA. Table 2 shows the per- 

 centage of DNA formed in the organs of the adult rat in the course of 

 24 hours. The sequence of radiosensitivity is that stated by War- 

 ren and Boners (1950). This is in spite of the fact that radiation sen- 

 sitivity depends, in addition to genetically determined factors, on cellular 

 composition, on the metabolic state of the cell, on the rate at which 

 precursors and humoral agencies reach the cell and waste products are 

 removed, and on other factors. When Bergonie and Tribondeau 

 pointed out 50 years ago that radiosensitivity increases with decreasing 

 age and decreasing differentiation of the cell and with increasing mitotic 

 figure they made a most pertinent observation. 



Table 2. 



Turnover Figures 



