108 H. Laser 



It has previously been shown (Laser, 1954) with the bacter- 

 ium Sarcina lutea, that the oxygen effect on the rate of repro- 

 duction could be largely abolished by doses up to 26 kr if cell 

 respiration was inhibited by respiratory poisons during the 

 irradiation. Thus, after removal of the poison, the cells 

 behaved as if they had been irradiated in nitrogen, when 

 judged by the degree of growth inhibition. The effective 

 inhibitors were carbon monoxide, potassium cyanide, hydro- 

 xylamine and sodium azide. Urethane did not diminish the 

 oxygen effect (Fig. 1). The mode of action of the effective 

 poisons in their role as respiratory inhibitors is known (Keilin, 

 1933; Keilin and Hartree, 1939; Keilin and Slater, 1953). They 

 all block hydrogen transfer through the respiratory enzymic 

 system by combining with cytochrome % and stabilizing the 

 remaining respiratory enzymic chain in the reduced form. 

 Taking this mode of action as a guide in advancing a possible 

 explanation for the oxygen effect in irradiation, it is suggested, 

 at least for this bacterium, that the enhancement of irradia- 

 tion damage (1) involves the enzymic respiratory mechanism, 

 (2) requires at least part of the enzymic respiratory chain to 

 be in the oxidized form during irradiation. This supports the 

 view that the impedance has been caused by a reducing agent. 

 These results and their interpretation have recently been 

 corroborated by Tahmisian and Devine (1955) who have 

 shown that grasshopper eggs which show a certain regression, 

 *' negative growth", when irradiated in air are less affected if 

 the nitrogen of the air is replaced by carbon monoxide, i.e., in 

 20 per cent Og/CO. The protection by carbon monoxide is 

 light-sensitive, being effective only in the dark. That X-ray 

 induced inactivation of biological materia] is brought about 

 by reduction has also been shown for two different strains of 

 bacteriophage, by Ebert and Alper (1954) and by Bachofer 

 and Pottinger (1954). It should, however, be added that the 

 view that reducing agents are generally responsible for irradia- 

 tion damage has been contradicted. Thus, Forssberg's (1947) 

 claim, that catalase in aqueous solution is inactivated by 

 means of reducing hydrogen atoms produced by X-irradiation, 



