GENERAL DISCUSSION 331 



the trypsin acts. A study of the products of the digestion of the irra- 

 diated ribonuclease by paper chromotography, shows that they are 

 the same as are obtained with the digestion by trypsin of the chemically 

 restored ribonuclease. This is the work of Rey, Hutchinson and 

 HoroM'itz.f 



GRAY : The point is that the ESR measurements are carried out several 

 minutes after the irradiation. They can be compared quite correctly 

 with the results of the irradiation of spores about which Powers spoke. 

 We know, however, that in normal cells with a large water content 

 many reactions are carried out in a fraction of a second. In order to 

 compare the results obtained in such material with the ESR data we 

 must wait until we can carry out the measurements of the paramag- 

 netic resonance within a very short period of time after the irradiation. 



ALEXANDER: Blumenfcld states that our ESR data do not exclude 

 the possibility that the electrons in proteins wander to the sulphur 

 atom. He states that the dissimilarity between protein and cysteine 

 patterns may well arise from secondary factors. I, unlike Blumenfald, 

 am not competent to deal with these fine points of ESR, but I would 

 like to point out that our data do invalidate the argument used by 

 Gordy and Shields to establish that the electrons wander to the sulphur 

 in proteins. Their argument rested entirely on the fact that the ESR of 

 irradiated protein and of irradiated cysteine were similar. We have now 

 shown this similarity to be an artefact. Our chemical data indicate 

 that there is no preferential attack on cysteine and it is now up to the 

 people who claim that there is a localization on the sulphur to provide 

 the evidence. 



Pollard states that in his laboratory it has been found that the sul- 

 phur in proteins is attacked preferentially by radiation and he attri- 

 butes the inactivation of enzymes to this preferential attack on the 

 sulphur. We have published a detailed investigation of the effect of 

 radiation on serum albumin in which we have shown that the cysteine 

 residues are not exceptionally sensitive. Naturally, they are attacked 

 together with all the other amino acid residues and the fact that a 

 protein like ribonuclease which contains a lot of cysteine should have 

 some of this altered, does not provide evidence for migration of energy 

 to the cysteine. Our most extensive data is with serum albumin, but 

 we have made similar observations which show that the cysteine is 

 not exceptionally sensitive in y-globulin, trypsin, lysozyme and 

 keratin. 



•j- Nature, Lond. (in press). 



