57 



nitrogen and normal yeast. However, fermentation was inhibited by much low- 

 er doses of X radiation in low-nitrogen than in normal yeast cells. We have no 

 data on the effect of temperature; however, if the period of normal respiration, 

 or fermentation, represents the survival time of the enzymes directly concerned, 

 there might be a considerable difference between normal and low-nitrogen yeast. 



COHN: What is the relation between this curve and the curve you get 

 from nonirradiated cells? Is there any evidence that you have impaired enzy- 

 matic activity from the initial portion of the curve? 



SHERMAN: No. These curves (Figures 1 and Z) indicate that the ac- 

 tivity of the enzymes associated with respiration are not impaired immediately 

 after irradiation. However, there is evidence that other cellular activities are 

 delayed (9). 



COHN: You don't really know that you have done anything to the en- 

 zymes except impaired their replacement. 



ZIRKLE: Isn't there evidence that the actual enzyme content is not im- 

 paired to any appreciable extent by doses that you are talking about. If you break 

 the cells open and analyze for enzyme content, wouldn't you find practically 100 

 percent of the control value? In that case, wouldn't the difference in these 

 curves be due to something else? 



SHERMAN: Yes, in fact, experiments recently reported by Bair and 

 Stannard (10), indicate that alteration in the composition of the medium may 

 change radiation effects on fermentation from inhibition to enhancement. Their 

 data indicate that some enzymes are able to function at 100 percent or more of 

 the control value. 



POTTER: I was about to ask if you intend to get into a discussion of 

 coenzymes, and if you are, then it might be premature to comment. 



SHERMAN: As a matter of fact, this would be a good time to consider 

 the role of cofactors. 



POTTER: Well, it seems to me that before one can speculate on the 

 formation of wrong enzymes or the failure of synthesis of right enzymes, one 

 has to break the cells open and find out whether there actually is less of the en- 

 zyme that you are measuring. Of course, this must be done in the presence of 

 appropriate coenzymes and substrate. If the enzyme is present, then one would 

 have to reject the idea that wrong enzymes have been formed and that the right 

 ones have not. My own explanation is that this is more likely to be an interfer- 

 ence with continuing synthesis of the necessary coenzymes. 



SHERMAN: I think this interpretation is as reasonable as the one sug- 

 gested by Billen and the modification of it that I have suggested. 



HOLLAENDER: I believe that you will find that not only one enzyme is 

 affected by the radiation but probably a large number of enzymes and coenzymes. 

 We have tried to give irradiated cells an opportunity to repair some of the dam- 

 age. The experiments which Dr. Sherman discussed really grew out of these 

 studies. If a cell is given a chance to repair some of the damage before cell di- 

 vision is initiated, some recovery may be possible. We have studied this in 

 E. coli , B/r. This organism has its optimum growth at 37°C. If our strain of 

 coli is kept in the presence of certain nutrients at 1 8°C instead of 37°C, 100 

 times as many colony-forming organisms will be obtained from cells irradiated 



