BASIC RADIATION BIOCHEMISTRY 265 



only if the substance injected is highly protective and nontoxic. In addi- 

 tion it must be able to penetrate unchanged to the site of action, wherever 

 that may be. 



SPECIFICITY OF THE REACTION OF RADICALS WITH ONE SOLUTE IN 



RELATION TO ITS STRUCTURE 



We have seen that the competition between two solutes can be used to 

 obtain information indirectly as to how the specific structure of a solute 

 molecule affects its over-all abiUty to react with radicals. But specificity 

 can be shown to exist for the reactivity of certain atomic groups within 

 one molecule in respect to the structure of the remainder of this molecule. 

 Few examples have so far been studied. One is the reaction of the sulf- 

 hydryl group in the so-called "SH enzymes" which are oxidized by radia- 

 tion to the disulfide form; this is discussed in detail in the chapter by 

 Barron. Other examples are the deamination of amino acids, peptides, 

 and related nitrogenous compounds (Dale and Davies, 1949; Dale, 

 Davies, and Gilbert, 1949a) and the liberation of sulfuretted hydrogen 

 from cysteine and reduced glutathione (Dale and Davies, 1951). 



DEAMINATION BY X RAYS OF AMINO ACIDS 

 AND RELATED NITROGENOUS COMPOUNDS 



The characteristic features of these experiments are that there is only 

 one solute present in any one experiment and that the formation of 

 ammonia is directly measured. This direct method makes it possible to 

 measure the reaction when only a few parts in a thousand of the initial 

 concentration of the solute has changed, in contrast to the two-solute 

 system with an enzyme as the indicator; there the reaction is measured 

 in terms of a change of the initial concentration, a change which has to 

 become fairly large in order to yield accurate results. 



Table 4-3. Ammonia Yield from Amino Acids and Other Nitrogen-containing 

 Compounds in 0.13 M Solution after Exposure to an X-ray Dose of 166,000 r 

 {Reproduced by permission of the editors of the Biochemical Journal.) 

 Nitrogen compound NH3, Mg/ml Nitrogen compound NH3, /ug/ml 



Glycine 8.1 Glyoxaline 3.0 



Glycine-HCl (1 equiv) 5.0 Glycylglycine 12.5 



Glycine-NaOH (1 equiv) 8.0 Glycylglycine-HCl 3.8 



Arginine 7.5 Leucylglycine 6.6 



Alanine 7.2 Diglycylglycine 7.0 



jS- Alanine 4.3 Proline 



Histidine 11.8 Urea 0.2 



Histidine-HCl 10.6 Guanidine carbonate 



Lysine-HCl 3.7 Thiourea 1.4 



Glycine anhydride 1.3 



Typical experimental results are shown in Table 4-3, where it will be 

 seen that the a-amino acids glycine, alanine, and arginine are deaminated 



