26 



W. M. Dale 



radiation doses of the order of 100 r can cause appreciable 

 percentage destruction in a dilute solution (Dale, 1940). 



One will have to consider in the discussion whether this 

 dilution effect can be operative in the inhomogeneous interior 

 of a cell. Fig. 1, which refers to the enzyme carboxypeptidase, 

 shows that the efficiency of the radiation decreases only in 

 extremely dilute solutions. This is usually interpreted as 

 being due to the fact that the distance between solute 

 molecules is so great that part of the radicals recombine 

 before they have a chance of reacting with solute. 



25 



20 



O 15 



10 

 5 



X 



10 



.6 



10" 



»-4 



\Q 



_i 



10^ 



10"" 10 10 



ENZYME CONC.ing./ml. 

 Fig. 1. Yield: concentration relationship for carboxypeptidase. 



From a certain concentration onwards, however, the yield 

 remains constant over a wide range of concentrations (Dale, 

 Gray and Meredith, 1949). Other enzymes, e.g. trypsin 

 (McDonald, 1954&, 1955) and chymotrypsin (McDonald and 

 Moore, 1955a) show some increase in the ionic yield when this 

 concentration is increased. The cause of this is not quite 

 certain though it may be connected with a lower probability 

 of elimination of radicals by the enzyme molecule. 



An interesting occurrence of two different ionic yields has 

 been found by Aronson, Mee and Smith (1955), working with 

 a-chymotrypsin, which has an esterase and protease function. 



