754 RADIATION BIOLOGY 



ammonia yield) when hydrogen was present during irradiation, as com- 

 pared with irradiation in a vacuum or in oxygen. The negative results 

 of the Tradescantia experiment may be taken to indicate that OH radicals 

 are in fact not effective in producing chromosome breaks, and that the 

 X-ray effect in the absence of oxygen is primarily a direct one. However, 

 there is no unequivocal evidence that hydrogen is actually present in the 

 cell in sufficient amounts or in the appropriate locations to ensure that the 

 back reaction will occur, although the evidence indicating a rapid penetra- 

 tion and effectiveness of oxygen would suggest that a similar situation 

 exists for hydrogen. Furthermore, it is also possible that H atoms rather 

 than OH radicals are responsible for the biological effect. That this may 

 be true for certain chemical reactions is again suggested by the experi- 

 ments of Scholes and Weiss (1950), in which evidence is presented that the 

 liberation of phosphate from X-irradiated nucleic acid may result from 

 reactions involving H atoms rather than OH radicals. The conclusion is 

 thus not yet warranted that all or most of the X-ray effect on chromo- 

 somes in the absence of oxygen is a direct one. In other biological studies, 

 such as those involving the killing of bacteria by X rays (Hollaender, 

 1952), there is evidence that chemical protection in the absence of oxygen 

 can occur. 



Further general evidence for the probable importance of the indirect 

 effect comes from observations on the relatively greater radioresistance of 

 dry as compared with soaked seeds (cf. Gustaffson, 1947), even though 

 these experiments have been performed in air and not in the absence of 

 oxygen. Regardless of the relative magnitude of the direct and indirect 

 effects in the absence of oxygen, it appears to be quite clear that the 

 indirect effect is of major importance when oxygen is present. 



Even if the identity of the intermediate substances responsible for 

 radiation-induced chromosome breakage were unequivocally established, 

 the problem of determining the chemical structures involved and the 

 chemical reactions leading to breakage would remain to be elucidated. 

 It is not known, for example, whether the protein or the nucleic acid or 

 both components of chromosomes are ruptured in the initial breakage 

 reactions. Certain studies on the effects of irradiation on nucleic acid 

 and proteins in vitro and in vivo are pertinent to this problem, however. 

 The experiments of Sparrow and Rosenfeld (1946) demonstrated that 

 X rays can induce depolymerization of thymonucleohistone and of 

 sodium thymonucleate. The studies of Scholes and Weiss (1950) already 

 referred to indicate that this effect on nucleic acid may result largely from 

 indirect radical action. This conclusion is supported by the observation 

 (G. C. Butler, 1949) that the presence of glucose or methanol in the solu- 

 tion during irradiation partially protects the nucleic acid. J. A. V. 

 Butler and Smith (1950) conclude that the degradation of DNA can be 

 produced by the action of OH radicals. Taylor et al. (1948) demon- 

 strated that there is a continuing depolymerization of nucleic acid after 



