56 



at 26*^C instead of 37°C, had a longer periad of normal respiration (Figure Z). 

 The total amount of oxygen consumed by irradiated cells at 26°C was larger than 

 that at 37°C. The processes resulting in deterioration of enzymes associated 

 with respiration appear to have a higher temperature coefficient than respiration. 



A point I would like to emphasize is that microorganisms that have re- 

 ceived large doses of X- rays may keep a large part of their metabolic machinery 

 intact and functional even though, for example, they may not be able to divide. 

 Yeast cells can utilize glucose and phosphate for periods of at least 24 hours 

 after irradiation. They are not dead cells. It is only when they are asked to do 

 something they can no longer do that they seem to be dead. 



PATT: I should like to ask whether oxygen consumption levels off at 

 the same place at both temperatures or at a somewhat higher level with the low- 

 er temperature. 



SHERMAN: It looks as if it might level off at a higher value. 



PATT: Perhaps then it is not entirely a matter of slowing up the de- 

 terioration but also of promoting some recovery at the lower temperature. 



SHERMAN: That may be the case, but the experiment wasn't continued 

 long enough to establish whether the respiration of irradiated cells incubated at 

 26*-'C would eventually fall off to the same level as the irradiated cells incubated 

 at 37°C. 



BARRON: In our early work, we observed also that the ability of cells 

 to divide and form colonies might be inhibited even though respiration was per- 

 fectly normal. At that time we pointed out that we have to differentiate between 

 the death of the cell and the loss of ability to divide. 



ZIRKLE: Don't be too hard on us biologists. We are just lazy like 

 everybody else, and sometimes use terms like "death" and "lethal action" loose- 

 ly. In precise discourse, itis.of course, necessary to state just what we mean 

 by "death" in that particular context. In one context, e.g. , a person may use 

 the term to mean inhibition of cell division, in another to mean cessation of mo- 

 tility. 



SHERMAN: In our experiments with yeast, there was an initial period 

 during which the rate of fermentation of glucose was normal. There was little 

 inhibition for a period of about 2 hours, but after 4 hours, the inhibition of fer- 

 mentation reached a maximum and this maximum did not change for over 24 

 hours (7). 



It would be interesting to compare the life span of irradiated and non- 

 irradiated cells from the standpoint of maintenance of their capacity to ferment 

 glucose. 



COHN: I am not clear about these curves. If enzyme replacement has 

 been inhibited, does not this delay period simply constitute a measure of the sur- 

 vival time of the enzymes that are there to start with, and, couldn't this survival 

 be longer at lower temperatures? 



SHERMAN: I would go along with this interpretation of Billen's experi- 

 ments. A couple of years ago, we published a paper on the effect of X radiation 

 on the fermentation of glucose by "low nitrogen" and normal yeast (8). The peri- 

 od of "normal" fermentation after irradiation appeared to be similar in both low- 



