96 E. L. POWERS 



presented to the spore after irradiation. In Fig. 2 we show that apjjroxi- 

 mately 50 per cent of the total effect of irradiation can be removed by 

 post-radiation treatment of spores with nitric oxide. This effect is inde- 

 pendent of the temperature during irradiation, and the effect of expos- 

 ing the spores to X-rays over the temperature range and treating them 

 with nitric oxide after irradiation is to shift the entire curve down 

 without changing its general form. The same figure demonstrates that 

 coincident values are obtained by post-irradiation heating of the 

 irradiated sjDores for 15 minutes at 80°C. 



Heat and nitric oxide exposure after irradiation accomplish the same 

 tiling; namely, they induce changes in the irradiated spore that pre- 

 vent the development of part of the damage expected from the given 

 radiation dose. The coincidence of these results, the well-known action 

 of nitric oxide in reducing free radical concentrations in physical 

 systems, and the well-known effect of heat in reducing free radical con- 

 centrations, lead us to conclude that the portion of radiation-induced 

 damage that is not observed following these treatments is caused by 

 radiation-induced free radicals that have appreciably long lives. It 

 should be noted that this radical component is independent of the 

 temperature (below 30°C) at which the spores are irradiated — the over- 

 all I'elationship between temperature and radiation sensitivity is the 

 same in the restored and in the unrestored spoi'es. 



When nitric oxide is present during irradiation, the degi'ee of pro- 

 tection observed is less than that seen when nitric oxide is given after 

 (Powers et al., 1960b). In Fig. 3 we see that the level (inactivation con- 

 stant) to which nitric oxide protects the cells when present during 

 irradiation is approximately 25 per cent higher than that seen when 

 nitric oxide is given after irradiation. This result is understandable if 

 we postulate two actions of nitric oxide : one, a protective action that is 

 due to the scavenging of radicals by nitric oxide that prevents their 

 becoming toxic to the cell ; and the other an enhancing effect of nitric 

 oxide on tlie action of X-rays. The enhancing effect is much smaller 

 than the protective effect, and the net result of the two actions is a 

 lower protective action of nitric oxide when present during irradiation 

 compared to its action afterwards. 



Hydrogen suliMde 



Because of the interest in protective chemicals containing sulphydryl 

 groups, and because of the possibility that these may act by scavenging 

 radicals, we have tested a number of sulphydryls for their action 

 against the long-lived radical component. Using the gas hydrogen 

 suljihide (Powers and Kaleta, 1960), we have been able to show that 



