944 RADIATION BIOLOGY 



with either the radiation or its toxic intermediates or with the biological 

 system. 



It is not known whether reducing substances and oxygen deprivation 

 protect in the same manner. There is, of course, no compelling reason 

 to believe that the mode of action of either factor is identical in all living 

 systems or indeed in a specific system under different conditions. X-ray 

 inactivation of ribonuclease is prevented by glutathione but not by oxygen 

 lack (Holmes, 1950). It is perhaps significant that resistance of the rat 

 to the acute lethal action of X rays is increased to the same degree by 

 anoxic anoxia or cysteine (Dowdy et al., 1950; Smith, Patt, Tyree, and 

 Straube, 1950) and that the effect of cysteine is enhanced in mice breath- 

 ing 10 per cent oxygen during the irradiation, although the latter by itself 

 does not offer any protection (Mayer and Patt, 1953). Certain additive 

 effects have been observed in bacteria. The sensitivity of aerobically 

 grown E. coli irradiated in phosphate buffer is reduced beyond the value 

 obtained with anoxia alone when cysteine is present during the exposure 

 (Hollaender et al., 1952). This is not the case, however, when the organ- 

 isms have been grown anaerobically. Thymic cells resemble the anaer- 

 obic bacteria in the lack of additivity (Patt, Blackford, and Straube, 

 1952). These considerations suggest perhaps that some of the protec- 

 tive substances may act by diminishing the availability of oxygen in the 

 biological system. This does not imply, however, that oxygen depriva- 

 tion per se is necessarily the decisive event in the protection afforded by 

 such agents. Information relating to the relative effectiveness of oxygen 

 and protective substances for the different qualities of radiation, to their 

 additivity or lack of additivity for a specific radiation quality, and their 

 temperature dependence should aid materially in the elucidation of the 

 mechanism of action. A survey of some of the more important chemical 

 and metabolic factors that have been employed in studies of animal radio- 

 sensitivity is presented in Table 14-3. Although it is not yet possible 

 to classify these factors in terms either of their biochemical action or of 

 radiation action, their significance is obvious since they provide a real 

 basis for the study of the immediate chemical effects of ionization and 

 excitation in living systems. The more recent contributions in this field 

 have been reviewed by Patt (1953). 



SUMMARY 



The physical and biological factors affecting sensitivity to ionizing radi- 

 ations have been discussed with emphasis directed toward the response 

 of mammalian tissue. It has been shown that the biological effect may 

 be influenced by the quality, quantity, manner, and completeness of 

 irradiation. Although it is generally true that radiosensitivity is related 

 to growth rate, it is clear that the relation is not a simple one. In view 



