Influence of Oxygen on Radiation Effects 111 



unaffected by sodium azide which, with Esch. coli, is an 

 inhibitor of anaerobic glycolysis. 



It is conceivable that haematin compounds in the cell 

 behave in the same way as haemoglobin and cytochrome c, 

 which are first reduced by X-radiation and then partly 

 reoxidized in a secondary reaction by molecular oxygen and 

 by radicals to an unnatural, green pigment, to which extent 

 they lose their oxygen-carrying and catalytic properties (Laser, 

 1955). The fact that, with the exception of Esch. coli, the 

 aerobic and anaerobic metabolism of non-growing suspensions 

 of a fairly large and representative number of bacteria and of 

 yeast was not affected — even to the extent of uninhibited 

 adaptive enzyme formation (Baron, Spiegelman and Quastler, 

 1952-53) — by X-ray doses which inhibit growth by more than 

 90 per cent may be taken to indicate that the complement of 

 haematin compounds in the cell involved in these reactions is 

 not a limiting factor but may be so in relation to induction of 

 growth. 



That the prerequisite for the oxygen effect is a certain state 

 and/or metabolic activity of the cell during irradiation is 

 demonstrated by the following two types of experiment : 



(1) Spores of Bacterium suhtilis were irradiated under 

 varying conditions. They were then transferred to a growth- 

 promoting medium in manometer flasks, and the rate of 

 growth, i.e. the formation of vegetative forms and subsequent 

 reproduction, was determined by measuring the increase in 

 oxygen uptake with time, which under normal conditions 

 follows a logarithmic course and is a true measure of the 

 increase in the number of cells. Fresh spores, i.e., those har- 

 vested soon after sporulation, irradiated either dry (not dried) 

 or suspended in phosphate buffer^, showed about the same 

 radiosensitivity as vegetative forms and an oxygen effect of 

 the same order. However, if the spores, after thorough 

 washing, had been freeze-dried prior to being irradiated, they 

 were somewhat less radiosensitive and showed no oxygen 

 effect (with doses up to 36 kr) when resuspended in buffer 

 and irradiated in the liquid phase. 



