BIOCHEMICAL PHENOMENA AND BIOLOGICAL SYSTEMS 185 



enzymes occurs. We have further mentioned the high specificity of the 

 indirect action in its relationship to the chemical structure of the mole- 

 cules concerned, and we have also seen that the present concept of radi- 

 cal mechanisms may need modification in the light of more recent de- 

 velopments. Finally we have considered the more physicochemical 

 aspects of radiation effects. This may enable us now to discuss tenta- 

 tively the consequences of irradiation on the biochemical phenomena 

 affecting tissues. 



Biochemical Phenomena and Biological Systems 



To start with, one has to distinguish between substances split off and 

 residues remaining therefrom, and the mere depletion, even though it 

 be local and temporary only, of necessary substances occurring in minute 

 amounts. All these factors, singly or together, ma^ play a part. At 

 the time when the dilution and protection effects were established it 

 was natural to speculate on possible consequences in biological systems 

 (cells). The question arose whether these phenomena could account for 

 radiation effects on living cells, and it was tentatively assumed that the 

 inhomogeneity of the cell interior could provide the necessary regions of 

 dilution through which solutes (for example enzymes) had to pass on 

 their way to their places of reaction. Then when the solutes were deci- 

 mated in these zones by incident radiation the usual delicately poised 

 balance of reaction was upset. This upset would, according to its shorter 

 or longer duration, either only disturb or completely and irreversibly de- 

 stroy cell activity. Against that it can be said that the presence of other 

 radiosensitive solutes might be sufficiently protective to minimize the di- 

 lution effect. Later experiments have shown that the protective effect is 

 by no means always proportional to the amount of protector but rela- 

 tively diminishes with increasing concentrations of protector. Further- 

 more, examples of reactions have been found in which an ionic yield 

 for indirect action of 3 and more than 3 is obtained on increasing the 

 concentration of solute. These findings could be of particular relevance 

 for the biochemical effect in cells. 



In addition to the loss of essential solute through irradiation the for- 

 mation of new products from the solvent or solute has to be considered. 

 With regard to the solvent (water) the formation of H2O2 as a result 

 of further interactions of H atoms and OH radicals, partly in reaction 

 with dissolved oxygen, has been assumed to be the actual agent of the 

 radiation effect in cells, a reaction recently more stressed by the French 

 school. There is a real danger of an undue emphasis on the effect of 

 H2O2, and the pitfall has to be avoided of thinking that the formation 



