112 



H. Laser 



(2) The oxygen effect, which is very marked with fresh 

 yeast (Fig. 3), was similarly abolished with yeast {Candida 

 utilis) which had been starved of nitrogenous reserves ("low- 

 nitrogen" yeast) by depriving it, in presence of oxygen and a 

 carbon source (glucose), of added nitrogenous substrate for 

 1-2 days prior to irradiation (Fig. 4). Such a yeast has a very 

 low (resting) metabolism. When it is brought into a nutrient 



lOO 



Of 2 3 4 5 6 7 



time(h) 



Fig. 3. Percentage increase in Og uptake ( = growth) of yeast (Candida 

 utilis) after irradiation in air and in nitrogen with 24 kr X-rays. 



(nitrogenous) medium the oxygen uptake per cell increases 

 before growth sets in. It is reasonable to assume that the 

 nitrogen-depleted cell has to replenish its relevant enzymic 

 make-up before starting to divide. As the depleted cell shows 

 no oxygen effect, it follows that the sensitive structure was 

 either not present or was in a state in which the presence of 

 oxygen did not affect its radiosensitivity. 



Lastly, I would like to discuss experiments dealing with 

 protection from irradiation by means other than reducing 

 agents, such as cysteine. Dale (1940, 1942) has already shown 



