938 RADIATION BIOLOGY 



hydroxyl radicals. Although free radicals result from the ionization of 

 water, they may also be formed in the ambient liquid when ionization 

 occurs directly in a biological particle. Depending upon the initial 

 spatial distribution of the ions and upon the presence and configuration 

 of dissolved substances, recombination or further reaction of the radicals 

 takes place. Alpha rays may produce peroxides (presumably from 

 hydroxyl radicals) even in pure water, whereas with y and X rays, 

 peroxides do not appear unless dissolved oxygen is present (Bonet- 

 Maury and Lefort, 1948; Allen, 1948). Obviously, purity conditions 

 such as these do not obtain in biological systems, where many acceptors 

 are available for reaction with the products of activated water. More- 

 over, the products of irradiated water may not be of equal consequence 

 inside and outside of a cell. It is well to recall, in addition, that the cell 

 represents a nonhomogeneous system with varying gradients of concen- 

 tration and solubility. 



The activated-water concept of radiation action has received ample 

 confirmation in a variety of in vitro systems in which the dilution effect 

 described by Dale (1947) (independence of ionic yield and solute con- 

 centration except at very low and high concentrations) and the protection 

 effect (competition between solutes for the activated solvent molecules) 

 have been demonstrated. As the solute concentration increases or with 

 increase in radiation dosage, there is a transition from the activated- 

 water type of reaction to the direct-hit type characteristic of nonaqueous 

 systems. Demonstration of these phenomena in vivo is not an easy 

 task, however. The inability to detect an immediate oxidation of 

 sulfhydryl groups in tissues obtained from heavily irradiated animals is 

 consistent with theoretical considerations which reveal that only an 

 exceedingly small fraction of the available sulfhydryl reservoir could be 

 oxidized even in the absence of the naturally occurring protective sub- 

 stances (Patt, Straube, et al., 1950). Perhaps the most convincing evi- 

 dence, admittedly indirect, in support of the theory of in vivo effects of 

 activated water comes from studies with anaerobiosis and with protective 

 substances. It will be remembered that direct effects of radiation on 

 solutes also play a role, but one that cannot be well understood in the 

 present state of knowledge regarding exact physicochemical effects and 

 the nature of the biological targets. 



Water. It was recognized long ago that desiccation of biological 

 systems favors radioresistance. Cells exposed to ionizing radiation are 

 frequently seen to swell and this may be a factor in, or manifestation of, 

 cell death (Failla, 1940; Buchsbaum and Zirkle, 1949). A hypothesis 

 has been advanced to account for this hydration phenomenon, i.e., ion- 

 ization and subsequent intra- and extracellular partition of ions, followed 

 by removal of the latter with consequent increase in cellular osmotic 

 pressure (Failla, 1940). However, there is little evidence in support of 



