32 W. M. Dale 



enzyme protein. When phosphoglyceraldehyde dehydro- 

 genase was inactivated by 100 r, complete reactivation by 

 glutathione was possible, but after 200 r only 62 per cent 

 reactivation occurred. Thus the differentiation between 

 action on SH groups and denaturation of protein lies within 

 rather narrow limits and and does not seem fully justified. 



Closely linked with the question of denaturation of protein 

 is the observation of after-effects. Continued inactivation 

 after cessation of irradiation has been found with pepsin 

 (Anderson, 1954), and with trypsin (McDonald, 1954a). Some 

 modification of the enzyme molecule has been produced which 

 makes the molecule more sensitive to thermal denaturation, 

 similar to the action of radiation on the albumin examined by 

 Fricke (1952). 



Effect of oL-radiation. With regard to the effect of radiation 

 of different ionization density, only one extensive study on the 

 effect of a-radiation as compared with X-radiation, on carb- 

 oxypeptidase, has been carried out (Dale, Gray and Meredith, 

 1949). The result was that the efficiency of a-rays was shown 

 to be only one twentieth that of X-rays, and this low efficiency 

 could be accounted for by the S-radiation which accompanies 

 the a-radiation, and is similar in ion density to X-radiation. 



We have so far dealt with enzymes and co-solutes in solu- 

 tion. There is, however, evidence that there are many 

 enzymes firmly bound to cell structure. Not very much is 

 known about these from in vitro experiments. Mazia and 

 Blumenthal (1950) made an attempt to investigate a mono- 

 molecular film of pepsin-albumin on the surface of water, 

 and exposed it to radiation. This difficult experiment is open 

 to some criticism since the substrate, which was not in great 

 excess of the enzyme, was also exposed to radiation. However, 

 they reported an inactivation of a thousand enzyme mole- 

 cules per ionization under the circumstances of the experi- 

 ment. More model experiments with phase-bound enzymes 

 would be valuable. 



In conclusion of this survey of in vitro experiments with 

 enzymes it may be useful to stress the main features of the 



