34 L. H. GRAY 



first 2 to 3 krad of the total dose. Thereafter the cell reacted as an 

 anoxic system. 



One of the most striking differences between damage induced in cells 

 of high water content and low water content (dry spores and dry seeds) 

 M'hen each is irradiated in the absence of oxj'gen, is that nitric oxide 

 enhances the radiation damage in the former, and depresses it ia the 

 latter. In dry spores of B megaterimn Powers and his co-workers 

 observed less depression when the nitric oxide was present during 

 irradiation than when it was added after irradiation and from this con- 

 cluded that the nitric oxide slightly enhanced the toxicity of some inter- 

 mediates while greatly depressing that of the others. The enhancement 

 was first observed by How^ard-Flanders in Shigella flexneri (1957). As 

 regards the magnitude of its influence on sensitivity at any given con- 

 centration, Howard-Flanders and Jockey (1960) find the molecule of 

 nitric oxide to be essentially equivalent to a molecule of oxygen. After 

 the discovery of this phenomenon by Howard-Flanders, a similar effect 

 of nitric oxide on the sensitivity of Vicia faba roots was observed by 

 Kihlman (1958) : on the sensitivity of tumour cells by Gray et al. (1958) 

 and in human liver cells by Dewey (1960a); (see p. 38). In Serratia 

 marcescens and Proteus vulgaris, Dewey (unpublished) has observed 

 that nitric oxide substantially enhances the sensitivity of anoxic cells, 

 but falls a little short of oxygen in its effectiveness. In each case cells 

 which are exposed to nitric oxide after the end of an anoxic irradiation, 

 show only the normal anoxic sensitivity. Nitric oxide, having one un- 

 paired electron in an outer orbital, readily coml)ines with free radicals 

 to form stable compounds. In this respect it differs from oxygen, which 

 has certain of the characteristics of a bi-radical, so that the product of 

 a reaction between oxygen and a free radical is itself a free radical. 



H H 



C-+0.i C-0-0. 



i 1 



H H 



By virtue of its paramagnetism, nitric oxide, like oxygen, readily 

 catalyses transitions in either direction between singlet and triplet 

 states (Porter and Windsor, 1958). Nitric oxide is also a very effective 

 substitute for oxygen in reactions leading to the production of chromo- 

 some structural damage by visible light absorbed in Vicia faba meris- 

 tem cells which had ju'eviously been exposed to acridine orange (Kihl- 

 man, 1959). 



Since niti-ic oxide is known to combine rather I'eadily with certain 



