, BASIC RADIATION BIOCHEMISTRY 259 



plotted against the X-ray dose (Dale, 1940; Dale, Meredith, and Tweedie, 

 1943). In this case, as with many other biologically active compounds, 

 e.g., viruses or drugs, the chemical change is unknown. What is actually 



corded is the loss of activity, infectivity, etc.« 



Analysis of the results of experiments with solutions of several solutes 

 suggests that the exponential relation between X-ray dose and activity of 

 the enzyme, which when considered on its own could be due to a direct 

 radiation effect, should be explained by assuming the interference of 

 already inactivated enzyme molecules which are still capable of reacting 

 with radicals (Dale, Meredith, and Tweedie, 1943). This reaction, how- 

 ever, is not detected by activity measurements and becomes apparent 

 only by a reduction in the number of radicals available for reaction with 

 the still active enzyme molecules, a phenomenon which will be discussed 

 in greater detail in the next section on solutions with several solutes 

 (protection effect). In fact, however, the exponential relation found for 

 any one concentration of an organic solute does not alter the basic propor- 

 tionality between X-ray dose and effect, as can be demonstrated by 

 examining the radiation effect on solutions differing in their initial 

 concentrations. Only experiments of this kind can prove that the action 

 of radiation on dilute aciueous solutions is indirect. If different initial 

 concentrations are used in our two examples of an inorganic and an 

 organic solute, it is found as the characteristic feature that the number of 

 solute molecules changed for a given dose of X radiation, i.e., the ionic 

 yield, ^ is independent of initial concentration over a wide range. It 

 follows that the 'percentage change for a given X-ray dose is higher, the 

 more dilute the initial concentration; and, vice versa, that the X-ray dose 

 which produces a given percentage change increases with increasing initial 

 concentration. Consequently, when a dilute solution of the enzyme is 

 irradiated by an X-ray dose which is sufficient to inactivate the greater 

 part of the enzyme, a concentrated solution would seem practically 

 unchanged when irradiated by the same X-ray dose. This phenomenon 

 is generally known as the "dilution effect" (Dale, 1943). If the radia- 



' The use of the term "ionic yield" is subject to a certain amount of controversy. 

 Ionic yield is defined as M/N, where M is the number of molecules changed and N is 

 the number of ion pairs formed. Those who advocate the use of this term point to 

 the easily measurable and reproducible number of ionizations in gases, which is the 

 firm basis of physical measurement of X-ray dosage in roentgens. Those, however, 

 who recommend that the word "ion" should disappear from the definition wish to 

 stress that ionization is not the only mode of action of ionizing radiations and fear 

 that the mention of ions diverts attention from excitation of molecules, which can 

 also lead to chemical change. The American usage of G as the energy represented by 

 100 ev is noncommittal in this respect, but the absorption of energy is less easily 

 measured accurately. The uncertainty of how many primary events lead to chemical 

 action in solution is not removed by either notation, and the use of M/N or of G only 

 involves a factor subject to the same uncertainty. This author uses the definition 

 M/N. 



