264 RADIATION BIOLOGY 



a new radical may be formed from a protector molecule, which can react 

 with the indicator, thereby initiating a chain reaction. It suffices here to 

 quote a table, comparing the protective powers of various compounds 

 relative to carboxypeptidase (C.P.) and alloxazinadeninedinucleotide (D) 

 (Table 4-2). It can be seen that the protective powers can differ by a 

 factor as large as 10,000. On the other hand, certain inorganic salts, e.g., 

 NaCl, are so inert that no protective effect can be detected. 

 From Table 4-2 it can be seen that urea 



NH2 



/ 

 C=0 



\ 

 NHo 



has hardly any protective effect, whereas thiourea 



NH2 



/ 



C=S 



\ 

 NH2 



is the most powerful protector so far found. Thus the simple substitution 

 of the oxygen atom in urea by sulfur raises the protective power 10,000- 

 fold. Other sulfur-containing substances, e.g., thiosulfate and elemental 

 sulfur itself, are also highly protective. Similarly the change from oxalic 



COOH 

 acid I , to formic acid, HCOOH, increases the protective power 



COOH 

 considerably. The carboxyHc group and the C — C bond in oxalic acid 

 are apparently more resistant to attack by radicals than is the Hnkage of 

 an H atom to the carboxylic group in formic acid. 



It is perhaps not surprising that the specificity which is so marked in 

 small molecules, where a special group of atoms forms a large part of the 

 total molecule, is not displayed in big molecules, e.g., in proteins, because 

 their uniform average structure outweighs the effect of a few special 

 groups. 



The value of these experiments is more than of purely theoretical inter- 

 est since they have given an insight into the marked specificity of the 

 radiation effect in relation to the chemical structure of the compound. 

 This may have practical applications. Encouraging results have been 

 obtained in experiments in which the mean lethal X-ray dose for mice or 

 rats was raised on injecting glutathione (Chapman et al., 1949) and 

 cysteine (Patt et al, 1949) immediately before irradiation. Similar 

 results were obtained with thiourea and sodium ethane diphosphonate 

 (Mole, Philpot, and Hodges, 1950). Such experiments can be successful 



