202 CELLULAR METABOLISM 



the rate of nuclear synthesis due to lack of oxygen or low temperature 

 often reduces considerably the sensitivity of the tissue to the effects of 

 irradiation. 



Barley seeds show a reduced sensitivity when irradiated in vacuum 

 (103). Inhibition of the growth of Vicia faha root tips is materially 

 reduced if the exposure is made under anaerobic conditions (55). Similar 

 results obtained by other experimenters will be mentioned later. Torula 

 cremonas is more sensitive to x-rays in the presence of oxygen (56). Five 

 times as many roentgens are necessary to produce an equivalent killing 

 effect when irradiation of bacteria is done in the absence of oxygen than 

 when oxygen is present (104). Cells forming the margins of carcinoma- 

 tous cell masses are more sensitive to gamma radiation than the cen- 

 tral cells. A reasonable explanation for this fact is that the marginal 

 cells, being near the blood vessels, have a more abundant oxygen supply 

 and are, therefore, more radiosensitive. Treatment of mice with thy- 

 roxin greatly increases the lethality of a given dose (105). Cells under 

 anaerobiosis are relatively insensitive to radiation (57, 139, 140). Varia- 

 tions in blood supply, and hence in oxygen supply, control radiosensi- 

 tivity of tumors. Embryos become more radiosensitive as soon as a 

 blood supply is established (56a, 57, 136). There is a striking reduction 

 in the number of recessive lethal mutations induced in Drosophila males 

 when they are exposed to x-rays while in an atmosphere of low oxygen 

 concentration (76a). The protecting effect of cyanide mentioned on 

 p. 200 may also be interpreted as due to a reduced oxygen con- 

 sumption. To protect against radiation damage, the radiation has 

 to take place shortly after the administration of cyanide, thus when 

 oxygen consumption is markedly reduced. The protecting effect is 

 only minute when the cyanide is injected 15 min after irradiation (41). 

 The protecting effect of cysteine may also be partly or wholly ascribed 

 to oxygen starvation produced by the easily oxidized cysteine. 



Only when the solution contained oxygen did Butler and Conway (87) 

 observe an after-effect of irradiation with 7000 r on a thymus nucleic 

 acid solution. They found the effect not to be due to the formation 

 of hydrogen peroxide, but possibly to the HO2 radical. 



Studies on inactivation of some enzymes in vitro (106) show no in- 

 fluence of the oxygen concentration of the medium on their radiosensi- 

 tivity. In the case of ribonuclease (107), which is inactivated under 

 the action of hydroxyl radicals in the medium, dissolved oxygen was 

 found not to play an essential part. Reduced oxygen supply may slow 

 down the metabolic rate, reducing the rate of formation of noxious 

 products or giving more time to wipe out lesions. Reduced oxygen 

 supply can, however, interfere with radiation sensitivity in different 



