Discussion 139 



in fact form the so-called "acid-soluble RNA" in these irradiated 

 organisms. 



Stapleton: No, we have not looked at this, but we certainly intend to 

 do so, especially on the basis of some of the work that has been done 

 with U.V., which indicates that irradiated cells pile up DNA precursors. 

 The point Dr. Gale refers to is the only indication in all these experiments 

 that there is any pile-up of anything. 



Gray: In support of what Popjak has just been saying, Burns did an 

 experiment with yeast in which he obtained quite appreciable protection 

 with 2?-chloromercuribenzoate which did not, however, eliminate the 

 oxygen effect. The oxygen effect was still shov/n in the protected cells. 

 I think in the case of cysteine we have got to be extremely careful in 

 any given case to be sure that it is not operating simply by removal of 

 oxygen from the solution. 



Whether or not cysteine protects mammals by inducing a state of 

 tissue anoxia I do not know, but an examination of cases reported in 

 the literature in which the addition of cysteine to solutions in which 

 bacteria were suspended or roots immersed reveals that the experimental 

 conditions were in each case such as to produce partial or complete 

 anoxia at the time of irradiation. 



Popjak: The first oxidation product of cysteine would be cystine. 

 There are some enzymes which can reduce disulphides very effectively 

 and very rapidly, the typical example, of course, being the glutathione 

 reductase which has been described in wheat germ and pea seedlings, 

 but which Dr. Hele and I find to be present also in animal tissues and 

 which has a high specificity for TPN. So even though it may get oxi- 

 dized, in the cells it will get reduced again to cysteine. 



Ch-ay: I am not speaking of possible chemical mechanisms of cysteine 

 protection, that is quite beyond me; I am just saying that before the 

 occasion for a chemical theory arises we must first be sure that we are 

 not dealing with simple anoxia in any given case. This has very often 

 not been established. 



Hollaender: May I say that I don't want to leave the impression that 

 we have been able to reverse mutations. All we believe that we have 

 been able to do is to stop the effect before the mutation has been carried 

 through, before the damage has been completed. But what we have 

 done with these chemicals, either before or especially after irradiation, 

 is to interfere with the completion of the mutation process. 



