DAMAGE IN AEROBIC AND ANAEROBIC SYSTEMS 35 



haem pigments, including cytochrome oxidase (Keilin, 1055), the 

 possibility of a hiocliemical role cannot be ignored, especially since 

 post -irradiation respiration sometimes has an important influence on 

 radiosensitivity ()). 41). In all the experiments with vegetative 

 bactei'ia, and with normal and malignant mammalian cells referred to 

 in this section, this ])ossibility was taken into account by control 

 experiments in \\ Inch the nitric oxide was added after irradiation. 



One further investigation with Serratia marcescens, carried out by 

 Dewey (1960b), may be mentioned as possibly indicating the extent 

 to which the radiolysis of water contributes to loss of reproductive 

 integrity in this organism. 



A number of j^ears ago Burnett et al. (1951) studied the influence of 

 high concentrations of alcohol on the radiosensitivity of E. coli B/r, and 

 these mvestigations were later extended by Marcovich (1958). In re- 

 peating the experiments under controlled oxygen tensions Dewey 

 (1960b) finds: 



1. A progressive increase in the 37 per cent inactivation dose D (i.e. 

 a progressive decrease in sensitivity) with increasing glycerine 

 concentrations up to 2M, which conforms to the relation 



D G 



— = l + (in-l) 



Do G + K 



where Do is the inactivation dose in the absence of glycerine. 



2. That when the bacteria are aerobic at the time of irradiation the 

 influence of a given concentration of glycerine is completely inde- 

 pendent of (a) oxygen concentration in the range 14 to 1400 

 /zmoles/1, and (b) temperature over the range 20 to 37°C (Fig. 5). 



Under all these conditions jx = 5-25 and K = 0*9 mole/1. 



3. Increasing concentrations of glycerine also progressively lower the 

 sensitivity of bacteria irradiated under strictly anoxic conditions. 

 A two-fold depression of sensitivity below the normally anoxic 

 level has been observed, and extrapolation to infinite glycerine 

 concentration indicates a maximum depression of 2-66. The con- 

 stant K was estimated to be 0*8 mole/1, and does not differ 

 significantly from that for aerobic irradiations. Both constants 

 are independent of temperature over the range 20 to 37°C. 



4. The influence of glycerine is essentially the same for cells which 

 are irradiated in the presence of nitric oxide as for cells which are 

 irradiated in the presence of oxygen. 



5. Equal molarities of ethyl alcohol, glycol and glycerine, which 

 have respectively 1 , 2 and 3 OH groups per molecule, are of com- 

 parable, though not exactly equal, efficiency. 



