Discussion 91 



logical changes, even if we are not prepared to accept a lot of cell death 

 within the first two hours, are still very significant indeed to the inter- 

 pretation of the results. 



Van Bekkum: What time do you suppose it takes for a lymphoid cell 

 to go through mitosis? 



Howard: I think the cycle must be fairly rapid, of the order of a few 

 hours. 



Van Bekkum: Well, in our spleen tissues, at least, the number of 

 mitotic figures is very low compared to the total number of cells, of 

 the order of 2 or 3 per thousand. I don't think that the explanation you 

 suggest can cover this if you consider what a small number of cells is 

 moved by it per hour. 



Howard: The whole cycle probably takes a few hours, but there may 

 be a very short time spent in division. I would imagine that cell turn- 

 over is quite rapid. 



de Hevesy: There is a large fraction of lymphocytes having a very 

 long life-time, as found by Ottesen, the existence of which may explain 

 Dr. Van Bekkum's finding. 



Laser: Dr. Van Bekkum, I take it that you agree that irradiation of 

 isolated mitochondria has no effect on oxidative phosphorylation. In 

 this connection, I would like to mention one result which has rather 

 surprised us. Dr. Slater and I have measured the actual oxidative 

 phosphorylation during radiation. The system was so adapted that 

 within six minutes, during which we applied 30,000 r, we could measure 

 the disappearance of a-ketoglutarate and the phosphorylation. There 

 was no significant effect at all, which means that these processes went 

 on undisturbed during the actual application of 30,000 r to the isolated 

 material. 



Van Bekkum: We have done the same sort of thing with similarly 

 negative results. Did you add cytochrome c to the system during the 

 measurement of phosphorylation ? 



Laser: Yes. 



Van Bekkum: So did we. 



Popjak: Dr. Van Bekkum, have you an explanation for the rather 

 low P/O ratio in this particular type of mitochondria? It seems to me 

 that from spleen and thymus you are getting a P /O ratio of barely over 1 . 

 One normally gets higher P/O ratios with liver mitochondria. 



Van Bekkum: I only know that Dr. Slater has also found, in the case 

 of heart mitochondria, a rather low P/O ratio of about 1 in the presence 

 of succinate. I don't know of any explanation for that. 



Loutit: With this level of dose the period of two hours seems to me 

 a very critical one. There is also evidence, in addition to histological 

 and biochemical evidence, for cell death at this time. The ordinary 

 sodium pump mechanism has been shown to break doAvn at this time 

 and sodium leaks into the cell and potassium leaks out. Furthermore, 

 we have already discussed the loosening of the enzyme systems, that 

 enzymes are getting into places where they should not be, which might 

 be the preliminary stage for this final blow-up and deathblow to the 

 cell. 



