242 FACTORS INFLUENCING CELL RADIOSENSITIVITY 



Number of Cellular Units Involved 



Any lesion results from the alteration of a certain type of cellular 

 element. Three possibilities must be considered : 



(a) The element is represented by several units (n), and the con- 

 sidered function acts with an intensity proportional to the number of 

 these units. Thus the lesion shows up progressively according to the 

 number of altered units. An example is cell respiration, in relation to 

 the number of respiratory enzymes, which decreases with increasing 

 dose, as an increasing number of enzymes is altered. 



(6) The element is again represented by several units (n), but the 

 considered function remains intact as long as n remains greater than a 

 certain threshold value, z. The lesion is then an all-or-non-phenomenon. 

 An example is the infective power of a cell infected with several virus 

 particles. 



(c) A particular case of (6), n = 2 = 1, arises when the element is 

 represented by a single unit, as, for example, a gene in a haploid cell. 



In the case of (6), which is very frequent, the number n of units 

 present in the cell at the time of irradiation contributes to resistance if 

 the radiation inactivates these units by discontinuous individual actions: 

 the greater this number, the lower the sensitivity. One might be able 

 to relate the rate of production of lesions by a certain dose within a 

 homogeneous population of cells to the number n. When certain con- 

 ditions are fulfilled— which cannot be discussed here— this relation is 

 given by the classical Poisson formulae. Two experimental examples 

 will illustrate and define quantitatively the influence of this factor on 

 sensitivity. 



VIRUS-INFECTED CELLS 



Radiations affect many metabolic functions through their action on 

 the enzymes responsible for these functions; thus certain physiological 

 changes in the cell influence sensitivity by modifying conditions of 

 enzymatic activity. It would be interesting to consider here the in- 

 fluence of the number n of molecules of a certain enzyme on the disap- 

 pearance of the function governed by that enzyme. Hevesy has pointed 

 out that the low concentration of enzymes renders the function that 

 they control particularly sensitive to radiation, and that this sensitivity 

 decreases as the concentration of enzyme increases. This consideration 

 applies to aU other constituents controlling well-defined cellular func- 

 tions. One would like to express this quantitatively. Unfortunately, 

 we cannot determine the number of intracellular molecules of an enzyme 



