260 



RADIATION BIOLOGY 



tion acted directly on the solute molecules without an intermediary agent 

 (radicals), the proportion of solute molecules changed chemically would 

 remain constant, i.e., independent of the initial concentration. 



In cases in which the experimental method is sensitive enough to record 

 changes at extremely low concentrations of solute, it has been noticed 

 (Fig. 4-3), for enzymes (Dale, Davies, and Meredith, 1949) and viruses, 



to 

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20 



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CONCENTRATION, ^g/ml 



Fig. 4-3. Relation between reciprocal of the ionic yield and concentration for car- 

 boxypeptidase (C.P.). {Dale, Davies, and Meredith, 1949. Reproduced by permission 

 of the editors of the British Journal of Cancer.) 



that below a certain low concentration the radiation becomes less and less 

 efficient the more the solution is diluted. At these high dilutions the 

 separation of the solute molecules is so great that radicals have a better 

 chance of colliding with each other, and thus of recombining, than of col- 

 liding and reacting with solute molecules. An alternative explanation, 

 however, would be the presence of minute traces of impurities in the solu- 

 tion, which probably no experimental procedure can entirely remove. 

 These impurities would act as an additional solute which would be in 

 reactive competition with the enzyme for radicals, thus reducing the 

 effect on the enzyme molecules. This competition will be the more effec- 

 tive the lower the concentration of the original solute relative to the con- 

 stant amount of impurity. The latter explanation leads on to and is 

 actually part of what is known as the "protection effect" in solutions of 

 two or more solutes. 



PROTECTION EFFECTS IN SOLUTIONS OF TWO OR MORE SOLUTES 



Two or more solutes in one solution, all assumed to be capable of react- 

 ing with radicals, share the total of radicals available; in other words, they 

 compete for radicals. There are, therefore, fewer radicals left for reaction 

 with any one solute than there would be if this solute were present alone. 

 As a result the radiation effect on each of them, singly, is reduced, i.e., the 

 solutes appear to "protect" each other (Dale, 1942). This protection 

 will vary not only in proportion to their relative amounts but also accord- 

 ing to their respective capabilities to act as acceptors. The exponential 



