Enzymes in Irradiated Tissues 53 



extracts of cabbage and find that both the naturally occurring 

 and synthetic substances are as active as auxin itself in some 

 of the growth assay procedures, though not in others. On the 

 other hand Gordon and Nieva (1949a and b) in two full 

 papers have produced evidence that pineapple leaves, if 

 extracted with ether, yield an aldehyde in the neutral fraction 

 and this can be partly purified by reaction with dimedon or 

 bisulphite. The aldehyde, when regenerated from its bisul- 

 phite compound, will react with soil enzymes or leaf enzymes 

 to form auxin, but the conversion is not more than 10 per cent 

 after 24 hours. 



This subject, therefore, is in a state of flux. That radiation 

 causes an immediate change in auxin formation seems 

 certain but there still seem to be doubts as to what is the 

 exact enzyme inhibited. Since this is such a fascinating 

 piece of work one looks forward with particular interest to 

 full publication. 



What conclusions, if any, may be drawn from this partial 

 survey? Two things stand out in my mind: first, that we 

 cannot yet say in a single case that radiation directly or 

 indirectly damages an enzyme; second, that this present 

 position by no means rules out the possibility that enzymic 

 damage is important. The study of the effect of radiation on 

 enzymes is still in its infancy in spite of all the work that has 

 been done. 



Gray has pointed out that the amount of energy from 

 radiation that is capable of preventing cell division is very 

 small indeed and that damage must either be to a key enzyme, 

 or be of a self-multiplying nature — or a break in continuity of 

 an important structure. 



Inactivation of a key enzyme might be the cause of the 

 failure of DNA synthesis; break in continuity of a struc- 

 ture could be break up of DNA itself, or, for example, change 

 in structure of mitochondria. The self-duplicating form of 

 damage can be pictured as failure of enzyme synthesis. 



Unfortunately we do not know what is the lifespan of 

 cellular enzymes or at what stage new synthesis takes place. 



