58 Discussion 



to a sol. If the intact cells are irradiated and the nuclei isolated sub- 

 sequently, then the gel obtained is partially damaged. Irradiation of 

 the isolated nuclei with small doses does not have this effect on the 

 nucleoprotein gel. This may be interpreted as the release, on irradiation 

 of the cell, of an enzyme which is able to break down this nucleoprotein 

 gel, turning it into a sol. 



Butler: The question which Dr. Pirie has been discussing is whether 

 in addition to effects on preformed enzymes there are effects on the 

 synthesis of enzymes, and one step in the synthetic process is, or may 

 be, the incorporation of amino acid. We have done a few experiments 

 in which we looked at the effects of irradiation on the incorporation of 

 amino acid in the proteins, actually of the rat liver. I know that other 

 experiments of that type have been done with rather inconsistent 

 results. It might be of interest to you in connection with the stimula- 

 tions you mentioned that we do find a stimulation of incorporation by 

 rather small doses of X-rays. It is not known what effect this has on 

 synthesis of proteins, but the radiation certainly has some effect on the 

 incorporation reaction. 



Pirie: I think that the question of whether enzyme synthesis is upset 

 or not is one of the most interesting ones, and I wonder whether the 

 fact that in bacteria and yeast it is not affected is really relevant for the 

 very much more radiosensitive mammalian cell. It is difficult to get 

 data about enzyme lifespans in mammalian cells, but there certainly 

 are situations where enzymes are synthesized very rapidly. Miller found 

 that if he starved a rat, i.e. gave it a low protein diet, then there were 

 quite rapid changes in enzymes of the liver, and on replacing protein in 

 food the enzymes returned to normal activity in a few days. That is 

 a situation where enzyme synthesis is going on and could be studied. 



Alper: Since the question of relative sensitivity of bacteria and 

 mammalian cells comes up quite often, I would like to point out here 

 that the sensitivity is not as different as is generally supposed. The 

 sort of doses which give, for instance, chromosome breaks in cells, are not 

 very far off the sort of doses with which you get long forms and much 

 increased lag. The doses which give killing effects in bacteria are not 

 really so different from those which kill mammalian cells. It is often 

 said that you cannot compare them, but I think this is not correct. 



