EFFECT OF X RAYS ON BIOLOGICAL SYSTEMS 289 



HCOOH + OH = H2O + HCOO 

 HCOO + H2O2 = H2O + CO2 + OH 



Reduction by ionizing radiations is mueh more difficult to obtain than 

 oxidation, probably because atomic hydrogen reacts readily with oxygen 

 to give the O2H radical. Oxidized thiols, ferricytochrome c, and diphos- 

 phopyridine nucleotide are not reduced when irradiated in aqueous solu- 

 tions with X-ray doses up to 100,000 r. Yet it is possible to reduce 

 oxidation-reduction systems, as can be demonstrated on irradiation of 

 dyestuffs such as methylene blue (Loiseleur, 1943; Gallico and Camerino, 

 1948). This "reduction," however, seems to go beyond the true reduc- 

 tion process, because after irradiation the "reduced" dye cannot be com- 

 pletely reoxidized by oxygen. 



EFFECT OF IONIZING RADIATIONS ON NUCLEIC ACIDS 

 AND NUCLEOPROTEINS 



The inhibition of cell division and of mitosis and the production of 

 chromosome aberrations and of mutation by X rays have increased 

 interest in the study of the changes produced by ionizing radiations on 

 nucleic acids and nucleoproteins in the belief that these substances are the 

 most important components of chromosomes and do take part in cell 

 division. Irradiation of thymonucleic acid (TNA) produces depoly- 

 merization, drop in viscosity, and diminution of birefringency (Sparrow 

 and Rosenfeld, 1946; Taylor et al, 1947, 1948; Errera, 1947). The 

 anomalous viscosity (greatly dependent on the velocity gradient or the 

 applied pressure) of 0.2 per cent solutions of TNA is completely oblit- 

 erated by X-ray doses above 22,400 r. The drop in viscosity continues 

 after cessation of irradiation for a period of about 8 hr, when it levels off 

 and approaches the spontaneous drop of the control. TNA solutions at 

 the concentration used by Taylor et al. show no changes in the absorption 

 spectrum. More dilute solutions (0.003 per cent) show on X irradiation a 

 decrease in the absorption spectrum at 2600 A, the decrease being propor- 

 tional to the radiation dose. The same decrease in the absorption spec- 

 trum is observed on irradiation of dilute solutions of ribonucleic acid, 

 adenylic acid, and adenosine triphosphate. This depression can be 

 attributed to oxidation of the double bonds of the chromophore groups 

 responsible for the absorption of light in the ultraviolet, since the absence 

 of oxygen and the addition of catalase have a protective action against 

 these radiation effects (Barron and Bonzell, 1950b). Lowering of vis- 

 cosity, however, seems unaffected by the absence of oxygen (Smith and 

 Butler, 1951). 



The participation of free radicals in these changes has been demon- 

 strated by the action of OH radicals produced either by exposing H2O2 to 



