Influence of Oxygen on Radiation Effects 107 



agents affect biological systems which are in any event in a 

 predominantly oxidized steady-state equilibrium ; or that they 

 oxidize reduced substances, e.g. SH groups (Barron, 1952) 

 which are normally reversibly oxidized and reduced. Although 

 such groups may be oxidized by radiation-produced radicals, 

 they will only be eliminated from participation in further 

 metabolic reactions if the oxidation thus produced is irrevers- 

 ible. That that is not generally the case follows, e.g., from the 

 fact that anaerobic fermentation of yeast is not affected by 

 fairly large X-ray doses which, however, strongly inhibit the 

 ability to reproduce. Similarly the work of Pirie, van Heyn- 

 ingen and Boag (1953) and van Heyningen, Pirie and Boag 

 (1954) on cataract induction by X-rays has shown that the 

 glutathione content, total protein-SH and the activity of 

 glutathione reductase in the lens were unaffected during the 

 first 20 hours after irradiation, while the activity of a number 

 of SH-enzymes begins to fall together with the onset of 

 clinical cataract only weeks after irradiation. The authors 

 believe that these changes may not constitute primary effects. 

 The biochemical approach which I propose to adopt with 

 regard to the oxygen effect visualizes that : 



(1) only in the presence of oxygen is the enzymic equili- 

 brium within the cells such that they are most severely affected 

 by irradiation products of water, the primary step being 

 reduction by hydrogen atoms followed by secondary oxidation 

 through either molecular oxygen or oxidizing radicals, which, 

 however, leads to abnormal, irreversibly oxidized products; 



(2) the oxygen effect, as expressed, e.g., by inhibition of 

 bacterial growth, occurs only if the cells maintain a certain 

 minimal metabolic activity and possess the entire enzymic 

 make-up (or at least its precursors) necessary for subsequent 

 growth and protein synthesis, during the actual irradiation; 



(3) many substances and cell constituents which protect 

 the cell from irradiation do not do so effectively in the presence 

 of oxygen. 



Experimental data in support of these three propositions 

 will be given. 



