CHEMICAL SPECIES INDUCED BY X-RAYS IN CELLS 



97 



when given after irradiation it is equivalent to nitric oxide given after 

 irradiation, and to post-irradiation heat. The level to which the cells 

 are protected by the gas is a]>proximately the same as that to which 

 radiation sensitivity is reduced by heat and NO. Tliis result is consistent 

 with the interpretation that this part of the effect is due to long-lived 

 free radicals : in this instance H2S is donating a hydrogen atom to the 

 free radicals, repairing these before they can become damaging. 



When hydrogen sulphide is present at the time of irradiation, a degree 

 of protection is seen exceeding any that we have observed in any other 



O - 



o 



•4- 



100 



jrig_ 3. — Changes in I'adiation sensitivity with concentration of O2 and NO present at 

 the time of irradiation. (From Powers et al., 1960b). 



circumstance. This indicates the presence of another kind of radical 

 that can accept hydrogen atoms from hydrogen sulphide. The lifetime 

 of this species must be very short compared with the lifetime of the 

 radicals that can be removed by post -radiation treatment. This is dis- 

 cussed below. 



The role of oxygen 



The relationship of oxygen to these effects is revealed by the follow- 

 ing series of experiments. When we irradiate spores in the presence of 

 oxygen, we see a gradual increase in radiation sensitivity with an in- 

 crease in oxygen concentration to a constant level that is reached at 

 about 10 per cent oxygen (Powers et al, 1960a, b and Fig. 3). The ratio 



