940 RADIATION BIOLOGY 



cells. The possible relation of this aspect of radiosensitivity to the 

 reactions of activated water was suggested by Thoday and Read (1947, 

 1949) and Read (1950). They confirmed the observation of Mottram 

 (1935a) that oxygen deprivation decreases the growth reduction of broad 

 bean roots following X irradiation and observed, in addition, that 

 oxygen lack did not afford significant protection following a irradiation. 

 This is a biological parallel to the radiochemical reactions involving 

 oxygen in aqueous solutions, since oxygen lack would be expected to 

 exert a minor influence on the formation of the hydroperoxyl radical 

 and hydrogen peroxide by a rays. 



Deprivation of oxygen during exposure to y and X radiations dimin- 

 ishes their action in a variety of biological systems. This is true for 

 yeast (Anderson and Turkowitz, 1941), E. coli (Hollaender et al., 1951), 

 barley seeds (Hayden and Smith, 1949), Vicia faba (Mottram, 1935a; 

 Thoday and Read, 1947), Drosophila (Baker and Sgourakis, 1950), 

 Tradescantia (Giles and Riley, 1950), tumor cells (Crabtree and Cramer, 

 1933), and mice and rats (Lacassagne, 1942; Dowdy et al., 1950). Radio- 

 resistance is said to be the same whether anaerobiosis is induced by 

 nitrogen, helium, argon, or hydrogen. Reduced oxygen tension has been 

 shown to decrease a number of radiation effects, including lethality 

 (Dowdy et al., 1950), growth reduction (Thoday and Read, 1947), 

 chromosome aberrations (Giles and Riley, 1950), and sex-linked lethal 

 mutations (Baker and Sgourakis, 1950). However, under conditions in 

 which the radiation effect is considered to arise from the direct ionization 

 of a biological particle, the oxygen level is without influence (Hewitt and 

 Read, 1950). 



It is well known that radiation injury to specific sites parallels the 

 blood flow during exposure (Schwarz, 1909; Carty, 1930; Mottram, 1924; 

 Jolly, 1924; Evans et al., 1942). Thus, damage to a limb is greatly 

 diminished when the limb circulation is blocked, and tumor sensitivity 

 varies with its vascularity. Mechanical retardation of breathing also 

 increases resistance to irradiation (Evans et al., 1942). Since a factor 

 common to all these examples is a reduced oxygen tension, it is assumed 

 that anoxia accounts for the modification of sensitivity. While this 

 seems reasonable from the preponderant evidence in support of anaerobio- 

 sis cited previously, it has not been proved. 



The question naturally arises as to whether anoxia modifies radiation 

 injury by interfering with the radiochemical reactions that involve free 

 oxygen or by inducing a more specific biological effect, e.g., on enzymes, 

 metabolism, or cell division. The precise mechanism must be regarded 

 as unsettled ; there is reason to think, however, that both modes of action 

 may be involved in the oxygen effect. Although Dowdy et al. (1950) have 

 shown that the 30-day LD 5 o for rats X-irradiated in 5 per cent oxygen is 

 about twice that for rats exposed in air, it is not known whether breathing 



