120 Barbara E. Holmes 



out of gear. This, perhaps, would provide the large or diffuse 

 target which is demanded by biophysicists, Dr. L. H. Gray 

 and Dr. S. Pele among them. However, the particular 

 vulnerability of this system to irradiation may be due to its 

 position rather than to any particular sensitivity. Enzymes 

 placed on a structure such as a chromosome or chromatin 

 thread and perhaps out of reach of the protecting substances 

 in the cytoplasm might show great vulnerability. 



There are many obvious fallacies in the comparison of 

 inhibitions caused by chemical agents and by ionizing 

 radiations, but, with due regard to these, it is probably worth 

 attempting to imitate these metabolic effects of X-rays by 

 the use of inhibitors of known enzyme systems, together with 

 the tracer materials. 



REFERENCES 



BuLLOUGH, W. S., and Green, H. V. (1949). Nature, Lond., 164, 795. 



Crabtree, H. G., and Gray, L. H. (1939). Brit. J. Radiol., 12, 39. 



Hevesy, G. (1945). Rev. mod. Phys., 17, 102. 



Holmes, B. E. (1939). Proc. Roy. Soc. B., 127, 233. 



Mitchell, J. S. (1942). Brit. J. exp. Path., 23, 296. 



PopjAK, G. (1950). Nature, Lond., 166, 184. 



Sacks, J. (1948). Cold Spr. Harh. Sym. quant. Biol., 13, 180. 



DISCUSSION 



Boscott: a paper by Butler and Smith (Butler, G. C, and Smith 

 D. B., 1951, J. Amer. chem. Soc., 73, 258) states that X- and gamma-rays 

 degrade sodium thymonucleate. I wondered if that would be of any 

 interest to you. 



Holmes: I think it probably is, only as far as we know, it does usually 

 take a rather enormous dose. I was thinking of it when Dr. HoUaender 

 was saying that perhaps you have got two things to consider in irradia- 

 tion damage. It does look very like it from the biochemical data. 

 You may have to think in one case of the enzyme inhibition, which you 

 may be able to stop with something like hydrosulphite, and you may at the 

 same time have to think of some direct effect like the effect of a hydroxyl 

 radical on your sodium thymonucleate. 



Boscott: In the biochemistry of mitosis perhaps purely analytical 

 biochemistry may not give results that one would desire, because one 

 may be up against such things as the degree of polymerization and 

 depolymerization of the nucleic acid. 



