EFFECT OF X RAYS ON BIOLOGICAL SYSTEMS 297 



rays (Dale, Davies, and Gilbert, 1949b). The yield of ammonia depends 

 also on the pH. Starting at the lowest pH values, the yield rises to a 

 maximum at pH 3, falls to a pronounced minimum at pH 6 to 6.5, rises 

 very steeply to a second maximum around pH 9, and falls again at higher 

 pH values (Stein and Weiss, 1949c). Dale and Stein have discussed at 

 length the variation of ionic yield with concentration of solute. It is 

 well known that OH radicals chemically produced can oxidize amino acids 

 deaminatively (Dakin, 1905; Neuberg, 1909). Dale, Davies, and Gilbert 

 (1949a) have suggested the following mechanism for the oxidation: 



OH + +H3NCH2COO- = OHCH2COO- + NH+ 

 NH+ + H2O = NH+ + OH 



The re-formed hydroxyl radical can then oxidize another amino acid mol- 

 ecule. Stein and Weiss (1949c) have suggested that on irradiation of 

 glycine "not only the hydroxyl radicals, but also the hydrogen atoms 

 attack the acceptor, both leading to the formation of ammonia." If this 

 reaction is added to the above two, an ionic yield of 3 would be expected. 

 It is obvious, however, that when large amounts of amino acids are irra- 

 diated with large doses of radiation (as in these experiments) , both mech- 

 anisms take place — the indirect action of the radiation products of water 

 and the direct collision between the amino acid molecule and the ionizing 

 tracks. It must be emphasized that ionizing radiations have different 

 qualitative effects at different doses; with small doses the effects are 

 mainly indirect, while with large doses the direct effect has to be taken 

 into consideration. 



EFFECT OF IONIZING RADIATIONS ON ENZYMES 



The effect of ionizing radiations on enzyme systems has been the sub- 

 ject of numerous investigations. In the early experiments, however, the 

 amount of radiation necessary to produce inhibition was so high that 

 Scott (1937) cjuite reasonably concluded that these radiations affect 

 enzymes only in large doses. Dale, in 1940, by diminishing the concen- 

 tration of carboxypeptidase obtained inhibition on irradiation with rel- 

 atively small doses of X rays. He was the first to postulate that enzyme 

 inhibition is due to the radiation products of water. Ionizing radiations 

 may act on either of the two components of which enzymes are made up, 

 i.e., the protein or the prosthetic group. When acting on the protein 

 moiety they may oxidize reactive groups in the side chain of the molecule, 

 such as sulfhydryl groups, hydroxyl groups, amino groups, or double 

 bonds; or they may act by breaking hydrogen bonds with production of 

 denaturation or precipitation. When acting on the prosthetic group they 

 may produce chemical changes which alter the biological activity of the 

 enzyme. 



