368 S. GARD AND O. MAAL0E 



temperatures; in fact, their data can be fitted to the common first order 

 equation by introducing the absolute temperature, T, as variable; the 

 equation then becomes: NjNo = e exp. (— Z)(0-00027 T + 0-09)), where NjN^ 

 is a fraction of phage surviving at the time when the dose D has been applied. 

 The authors suggest that the temperature dependency may be due to residual 

 indirect effects, and they proceed to show that estimates of target volume, 

 based on measurements at room temperature, may be off by as much as a 

 factor of 2. As discussed on page 365, these findings show that it is difficult 

 to decide precisely where the indirect effect of X-rays ceases to play a role, 

 and, consequently, that it remains somewhat uncertain what fraction of the 

 total effect can be ascribed to ionizations in the target material. 



Great efforts have been made to analyze the indirect effect chemically. A 

 few aspects of these studies will be mentioned here: it is known from the work 

 of Hollaender et al. (1951) that the presence of molecular oxygen during 

 irradiation greatly enhances the bacterial effect of X-rays. Parallel observa- 

 tions have been made on tumor cells (Gray et al., 1953). In contrast, several 

 phage strains (the coliphages Tl, T3, and S13) are more rapidly inactivated 

 in the absence than in the presence of oxygen (Alper and Ebert, 1954; Ebert 

 and Alper, 1954; Bachofer and Pottinger, 1954b). The conclusion drawn from 

 this, and from irradiation experiments carried out in Og, Na, or Hg atmos- 

 pheres at various pH values (Alper and Ebert, 1954), is that phage is in- 

 activated by reducing rather than oxidizing radicals. 



Alper (1956) recently suggested that the interplay between direct and 

 indirect action and protection might be interpreted in terms of target damage 

 followed by more or less efficient restoration, rather than in terms of prevented 

 or not prevented damage. 



Various reaction sequences, starting from the primary reducing radicals 

 H and OH, formed by ionizations m water, have been proposed to account for 

 the inactivation of phage (Ebert and Alper, 1954), for the fragmentation of 

 DNA in solution (Daniels et al., 1953) and for the inactivation and denatura- 

 tion of enzymes (Barron et al., 1949). For some enzymes, two different reac- 

 tions are postulated: a reversible inactivation of SH groups and, at higher 

 doses, irreversible protein denaturation. No differentiation with respect to 

 mechanism of inactivation has been made in the case of viruses, but it is 

 known that, apart from the short-lived radicals just referred to, relatively 

 stable, toxic components are produced that may interact with viruses intro- 

 duced into preirradiated medium. In general, phage inactivated by direct 

 effects, by short-lived radicals, and by stable, toxic agents, respectively, 

 differ from each other in adsorption characteristics and in other respects as 

 well (Watson, 1952). 



Some interesting hints about the nature of the long-lived, toxic products 

 come from studies of the radiomimetic effect of certain peroxides (Latarjet, 



