BASIC RADIATION BIOCHEMISTRY 



269 



4-7). The explanation of the difference between the radiation effect on 

 deamination and the inactivation of an enzyme with regard to the respec- 

 tive shape of the curves, as well as to the ionic yield, may involve a com- 

 bination of various factors. At present the situation is not clear enough 

 for a definite answer to be given, though a discussion of these factors will 

 still be useful. 



The deamination reaction is notable for an ionic yield well above 

 unity {M/N is 2.9 for a 20 per cent solution of glycine and approximately 

 3 for dry glycine, the corresponding figures for DL-serine being 4.2 and 

 approximately 8), whereas the ionic yield for carboxypeptidase is only 

 0.18 and for trypsin in a 1 per cent solution is still lower, 0.01. In the 



10 10'' lO" 10^ 



CONCENTRATION, juq/tn\ 



o L-SERINE, • GLYCINE 



Fig. 4-7. Yield-concentration curve for the deamination of aqueous solutions of 

 L-serine and glycine; X-ray dose = 166 X 10^ r. {Dale and Davies, 1950. Repro- 

 duced by permission of the editors of Nature.) 



latter case we do not know whether the ionic yield would reach constancy 

 on raising the concentration beyond a 1 per cent solution, which was the 

 highest concentration examined by McDonald. Furthermore, we do not 

 know how much of the yield in concentrated solutions has to be ascribed 

 to the direct action of radiation and how much to the indirect action, since 

 it is by no means certain (Krenz, 1949) that the ionic yield for direct action 

 is the same for a substance in solution as it is for the dry state — water 

 of hydration, for instance, may cause a difference. Yet to assume a 

 direct-action yield for the dissolved state of the solute equal to that in 

 its dry state is at present the only means available of estimating the 

 magnitude of the yield to be allocated to the indirect action. 



Stating, as we have done, that the maximum ionic yields of the inac- 

 tivation of enzymes, i.e., of big biologically active protein molecules, are 

 much lower than those obtained in the deamination reactions, implies 

 that collisions of active radicals with solute molecules do not always lead 



