220 



THE EFFECT OF IONIZING RADIATIONS 



acting as regulators of cellular metabolism and cell division. The thiol 

 reagents increase cell respiration and inhibit cell division when used 

 in small concentrations, whereas they inhibit cell respiration when the 

 concentration is increased. In the same manner, ionizing radiations 

 used in small doses might increase respiration and inhibit cell division. 

 It is quite possible that the effects of small amounts of ionizing radia- 

 tions, those which produce nuclear changes, inhibition of mitosis, and 

 production of mutations, may be due to the oxidation of the soluble 

 thiol groups, and not to direct effect on the nucleoproteins. 



The Oxidation of Substances of Biological Importance 



Ascorbic acid is another oxidation-reduction system that exists in the 

 tissues mostly in the reduced state. Anderson and Harrison (9) found 

 it to be oxidized on x-ray irradiation; the ionic yield, however, was 

 much lower than that of glutathione oxidation, 0.7. Reduced cyto- 

 chrome c is also oxidized on irradiation with x-rays (Fig. 1). Because 



Fig. 1. Oxidation of ferrocytochrome c by x-ray irradiation. Solvent 0.005 M 



pho.sphate, pH 7.0. Cytochrome 1.6 X 10"^ M. Abscissa: X-ray dose in roentgen 



units; ordinate: per cent oxidation. 



of the characteristic absorption spectrum of the different components 

 of the molecule (protein, porphyrin, iron) the effect of irradiation can 

 be easily distinguished. By spectrophotometric measurements it was 

 possible to observe that oxidation of the Fe++ cytochrome c occurred 

 before there was any change in the porphyrin or in the protein molecule. 

 Oxidation of ferrocytochrome by ionizing radiations is reversible (Fig. 

 2). The ionic yield of the oxidation reaction is 0.55. The protein 

 moiety of ferricytochrome c was altered on irradiation with large doses 

 of x-rays, the alteration being more marked in alkaline solutions 



