ALEXANDER HOLLAENDER AND C. O. DOUDNEY 



Figure 2 shows a comparison of the protective ability of the two compounds 

 on E. coli irradiated in a balanced salt solution at 2°C. More than 99-9 

 per cent of the irradiated bacteria are killed at 60 kr, but in cysteamine at 

 the same dose, only about half of the bacteria are killed. 



With increasing energy in excess of 60 kr the protective ability of cysteamine 

 decreases markedly and reaches another level of protection at about 90 kr. 

 At submaximally protective concentrations of cysteamine the decline in 

 protective ability comes at correspondingly lower energy levels. However, 



Figure 2. The effect of cysteamine and 



^-mercaptoethanol on survival of X-irradiated 



E. coli 5/r 



TO-^ 



10' 



10' 



Cysfsaminc fo OS M ) 



\ p-Mercapfoethanol 

 \ (o-m) 





20 f-0 60 80 100 120 



Energy kr 



even at comparatively low concentrations of cysteamine (0-002M), a very 

 high initial level of protection is maintained. In contrast to the peculiar 

 survival curve obtained with cysteamine, mercaptoethanol protects linearly 

 with increasing energy and fails to show a change in its protective ability 

 at higher energy levels. Of all the compounds tested, cysteamine is the 

 only one which produces this peculiar response to radiation. Up to 60 kr, 

 the dose reduction factor (DRF) for cysteamine is about 12, falling to about 

 6 at 90 kr. Mercaptoethanol shows a DRF of about 8. Up to 60 kr, the 

 addition of mercaptoethanol or any other protective compound to cysteamine 

 either at maximally or submaximally protective concentrations does not add 

 to the level of protection obtained. This applies also to oxygen removal. 

 In excess of 60kr, mercaptoethanol, as well as other protective compounds, 

 is additive to cysteamine protection. Also, at these energy levels oxygen 

 removal adds to the protet:tive effect of cysteamine. 



Cysteamine in the presence of phosphate ions loses much of its protective 

 ability, the DRF being changed from 12 to about 6 in its presence. We 

 believe that phosphate interferes in some way with the mechanism by which 



113 I 



