100 HOLLAENDER, StAPLETON AND BURNETT 



Chemical Protection 



We noted early in our work that bacteria irradiated in 

 broth are more resistant than those irradiated in buffer. 

 Furthermore, it was found that most of the amino-acids 

 added to suspensions of aerobically-grown bacteria decrease 

 to some extent the X-ray sensitivity of the bacteria. Very 

 good protection was afforded by cysteine and glutamic acid. 

 This suggested that an examination of the protective ability 

 of a wide variety of compounds would be profitable. 



Results. This portion of the study indicated that many 

 compounds are able to give some degree of protection. Com- 

 pounds found to be protective fall into several classes on the 

 basis of their chemical structures. 



A crude classification follows: — 



1. Compounds containing sulphydryl groups, viz., cysteine, 

 mercaptosuccinate, 2,3-dimercaptopropanol (BAL), and 

 2- (2-mercaptoethoxy )-ethanol. 



2. Alcohols and glycols, viz., methyl alcohol, ethyl alcohol, 

 isopropryl alcohol, propanediol, glycerol, triethylene 

 glycol, and propylene glycol. 



3. Sodium hydrosulphite. 



4. Metabolic intermediates and products, viz., formate, 

 succinate, pyruvate, fumarate, lactate, and malate. 



Characterization — Results. Characterization of the 

 protective agents is still in the preliminary stages. However, 

 typical data are given in graphic form in Figs. 3 and 4, for the 

 sulphydryl compounds, cysteine and BAL. Fig. 3 shows the 

 survival fraction of the bacterial cells as a function of X-ray 

 dose in the presence and absence of BAL. Fig. 4 is a plot of 

 relative survival as a function of molar concentration of the 

 two compounds, at a constant dose of X-rays, with suspen- 

 sions in equilibrium with atmospheric air. The differences 

 in the slopes of the curves for the two compounds indicate 

 a twofold efficiency of BAL over cysteine. These curves 

 are of interest since the optimum or limiting concentration 



