ENZYMES AS INDICATORS 181 



It was possible in this way to examine the effect of radiation on the 

 components separately and when combined together. Both moieties 

 were radiosensitive, and a summation of deactivation occurred when 

 each component was irradiated singly and then joined after irradiation 

 to form the complete enzyme. The protein appeared to be more radio- 

 sensitive than the prosthetic group, weight for weight. If, however, 

 the difference in the size of the respective molecules is taken into account 

 and the results are calculated in terms of collision frequencies with radi- 

 cals, the prosthetic group shows the greater radiosensitivity of the two. 

 We have seen in these two examples that the inactivation of enzymes by 

 radiation can be caused by different chemical reactions. Generally the 

 type of reaction, as well as the size of the macromolecule, may be a 

 determining factor for variations in the radiosensitivity. Carboxy-pep- 

 tidase has an ionic yield of 0.18, which is 6 times greater than that of 

 ribonuclease, which has the lowest figure of the literature. 



Enzymes as Indicators 



Experiments mentioned so far aimed at establishing quantitative re- 

 lationships between radiation and its effect on the enzymes themselves. 

 Another type of experiment uses the enzymes as indicators to investi- 

 gate radiation effects on other non-enzymatic substances via the protec- 

 tion effect, and this kind of experiment has served to reveal the remark- 

 able specificity of the indirect action of radiation on substances of 

 special chemical composition. The basic idea is that, on adding the 

 substance to be tested for its ability to react with radicals to the so- 

 lution of the indicator substance (enzyme), the indicator will be pro- 

 tected and thus require a bigger dose of radiation in order to be inac- 

 tivated to the same extent as in the absence of protector. From experi- 

 ments of this kind it has been found that the protective powder can vary 

 over a ten-thousand-fold range for different substances (Table 1). Of 

 these, compounds containing sulfur in organic linkage are at the top of 

 the scale. This fact, taken together with the important role of sulfur, 

 groups in proteins, the occurrence of sulfur in higher proportion in the 

 skin, and the further fact that the experiments made by the Chicago 

 Laboratory (9), in which the injection of cysteine increases the mean 

 lethal dose for mice by more than 50 per cent, seems to be a useful 

 opening for additional research which may become of practical impor- 

 tance. The further analysis of the protection effect has shown that 

 present concepts of radical action and the formulae developed in the 

 past for a straightforward sharing mechanism of radiation energy are 

 not adequate, without modification, to cover all phenomena observed. 



