268 RADIATION BIOLOGY 



ing section. No H2S is given off by the oxidized forms of either com- 

 pound, and it is interesting to note that no deamination takes place in 

 the reduced form. 



We have seen in the foregoing that the indirect action of X radiation 

 can be markedly specific, and, although the ionization tracks, and there- 

 fore the radicals, are distributed at random, their action has not the same 

 random distribution owing to the selective specificity of interaction 

 between radicals and solute molecules. Research on specificity effects 

 when further developed and extended may teach us which type of reaction 

 in cells, tissues, and organisms is most likely to occur, which type is less 

 likely, and which type will not occur at all. It will further show what 

 kind of reaction products may be produced. These reaction products 

 may be of great biological significance, especially as they are generated 

 within the cell and are therefore not limited by permeability barriers. 

 The concentration of these substances may be high locally when con- 

 sidered on a submicroscopic scale of dimensions. 



DEVIATION FROM THE BASIC TYPE OF THE 

 YIELD-CONCENTRATION RELATION 



The inactivation of an enzyme and the oxidation of ferrous ions to 

 ferric ions were suitable examples from which the indirect action of 

 ionizing radiations could be deduced. The characteristic feature, how- 

 ever, i.e., that the yield is independent of the concentration over a wide 

 range of concentrations, does not hold invariably. A number of reac- 

 tions have become known in which the yield depends very much on the 

 concentration of the solute. Examples of such reactions are the inactiva- 

 tion of trypsin (Kaufmann, McDonald, et al, 1950), the deamination of 

 glycine (Dale et al, 1949a; Stein and Weiss, 1949), and L-serine (Dale and 

 Davies, 1950; Stein and Weiss, 1949), and the liberation of sulfuretted 

 hydrogen from cysteine hydrochloride and glutathione (Dale and Davies, 



1951). 



The yield-concentration relation for glycine and L-serine is shown in 

 Fig. 4-7. The deamination curve of glycine rises continuously to an 

 ionic yield of 2.9 without showing a tendency of reaching a constant level, 

 even at the highest concentration possible, i.e., its hmit of solubility. 

 This is in contrast with the constant ionic yield of 0.18 of carboxy- 

 peptidase down to very much lower concentrations (see Fig. 4-3). In 

 the latter case we assumed that all radicals reacted ^vith enzyme mole- 

 cules, rather than with themselves, at all concentrations above 0.1 per 



cent protein. 



In the case of L-serine (Avith its high solubility and high ionic yield) it 

 has been shown (Dale and Davies, 1950) that there is a tendency for the 

 vield-concentration curve to level off at high concentrations (see Fig. 



