Influence of Chemicals on Radiosensitivity 121 



oxygen removal is not always so efficient as chemical protec- 

 tion. As a matter of fact, the dose-reduction factor (DRF) 

 for Escherichia coli will go as high as 12 under the best condi- 

 tions for cysteamine protection (Hollaender and Doudney, 

 1955) in contrast to 3 for oxygen removal by replacement 

 (Hollaender, Stapleton, and Martin, 1951). The difference 

 between these dose-reduction factors probably is based on the 

 premise that replacement of oxygen by an inert gas (Doudney 

 and Hollaender, 1956) — for instance, nitrogen — might not 

 completely remove the oxygen, whereas sodium hydrosulphite 

 (Burnett et al., 1951) or cysteamine, since both are highly 

 water-soluble, can enter the cell and remove the oxygen, 

 especially when it is in loose association with certain com- 

 poimds or is adsorbed on the surface of particular cell struc- 

 tures. Several investigators have shown that cysteamine can 

 associate with nucleic acids and other compounds (Kluyskens, 

 1953). 



Two compounds that appear to be of greater importance, 

 because of their striking protective ability, are p-mercapto- 

 ethylamine (MEA) and /S-p-aminoethyh'^othiouronium • Br • 

 HBr (AET). The former is apparently the most successful 

 compound for Esch. coli, and the latter for mice. 



Cysteamine (MEA), first reported by Bacq and co-workers 

 (1951) to be protective for mice and since found to be an 

 effective chemical protective agent by workers in many other 

 laboratories, is an easily oxidized compound, has to be stored 

 under nitrogen, and has other characteristics that make it 

 somewhat difficult to handle. In freshly prepared water 

 suspension, there is increasing protection for Esch. coli B/r 

 (Fig.l) with increasing concentration until a plateau is reached 

 at 0-02 M. Phosphate buffer interferes somewhat with the 

 protection, and the protective ability is dependent on the 

 nutritional factors supplied after irradiation. The effect of 

 cysteamine on Esch. coli maybe summarized as follows (Fig. 2): 



1. It protects in a concentration of 0-02 m and has a dose- 

 reduction factor of 12 if broth-grown cells are incubated in a 

 complete medium. 



