178 ASPECTS OF BIOCHEMICAL EFFECTS 



molecules transformed. There are even reactions in which ionization 

 plus excitation may not be adequate to account for the magnitude of 

 the effect and one may be led to invoke chain reactions (5). 



Further progress has been made by recognizing the dependence of 

 biological and biochemical effects on the specific ionization along the 

 tracks of the ionizing particles (electrons, protons, alpha particles) and 

 on the separation of these tracks. This dependence has been investi- 

 gated for solutions and radiation effects in tissues by comparing the 

 efficiency of alpha and of x-radiation. In the not very numerous experi- 

 ments with solutions it was nearly always found that the ionic yield for 

 alpha radiation was considerably lower than that for x-radiation; in fact, 

 so much lower that the delta rays may account for the whole effect (3) . 

 Similar results were obtained in some biological experiments; in others, 

 however, the efficiency of alpha radiation proved to be greater than that 

 of x-radiation (4). In the latter case biologists usually postulate the 

 necessity for producing within a particular molecule multiple local high 

 spots of energy which the densely ionizing alpha rays can provide. One 

 has also to think of a possible H2O2 effect, since II2O2 is generated in 

 much higher concentration in the vicinity of the alpha-ray track than 

 in the case of x-rays. 



If we now deal with the direct action first, that is the released energy 

 within any molecule, we can say that the biochemical effect is unpre- 

 dictable in the sense that we do not know where and in what way the 

 molecule will be affected. It is, however, worth mentioning that in the 

 deamination of glycine (5) the ionic yield is approximately the same for 

 the dry material as the yield for an only 20 per cent solution (Fig. 1). 

 One cannot at present obtain absolute values for each of the two modes 

 of action (direct and indirect) in solutions experimentally. The only 

 way of arriving at an estimate is via the hypothesis that the dry material 

 when dissolved does not change its properties and therefore its response 

 to a direct hit. But is this assumption justified? One could be tempted 

 to argue (6) whether a hydration shell could raise the direct-action ionic 

 yield of a dissolved substance so that the combined indirect- and direct- 

 action yields for the dissolved substance apparently equal the ionic yield 

 of the dry substance. What really matters, however, from the biochemi- 

 cal point of view is the combined effect, since dry substances do not 

 occur in living tissues. On the contrary, the average water content is 

 of the order of 75-80 per cent, and for young and embryonic tissues 

 much higher still. If one allows for the inhomogeneity of the cell con- 

 tent, some components, for example the nucleus, granules, and mito- 

 chondriae, probably contain less water. How far water of hydration 

 and non-solvent water play a special part remains a problem to be solved 



