230 



THE EFFECT OF IONIZING RADIATIONS 



in vitro, will oxidize thiol compounds; the experiments in which H2O2 

 or other peroxides produced mutations or chromosome breaks show only 

 that oxidation reactions are responsible for these phenomena. There 

 is no plausible reason, however, to attribute such oxidations to H2O2 

 when there are two other more powerful oxidation products formed on 

 irradiation of water, namely, the radicals OH and O2H. 



Radiations and Coenzymes 



Ionizing radiations attack not only easily oxidizable systems, but also 

 a number of substances of biological importance. The adenine residue 

 of adenosinetriphosphoric acid can be oxidized by x-ray irradiation, 



X 10'' r 



Fig. 10. The x-ray efficiency on the destruction of adenosinetriphosphoric acid as 



measured by the absorption spectrum at 2600 A. Abscissa: X-ray dose in roentgen 



units; ordinate: per cent difference in the absorption spectrum. 



although the ionic yield is rather low, about 0.09 (Fig. 10). That the 

 measured change (height of the absorption spectrum at 2600 A) (Fig. 

 11) is due to oxidation is demonstrated by the reduction produced by 

 catalase (46 per cent) and by the exclusion of oxygen (70 per cent). 

 The pyrimidine-pyridine residues of diphosphopyridine nucleotide, how- 

 ever, are more resistant to the action of ionizing radiations (Fig. 12) 

 (ionic yield 0.01). If the intensity of irradiation is increased, there will 

 be more profound alterations, as has been shown by Scholes et al. (14) 



