EFFECT OF X RAYS ON BIOLOGICAL SYSTEMS 



287 



Theoretically there could be four molecules of thiol oxidized per ion 

 pair produced on irradiation. The great sensitivity of thiols to the oxi- 

 dizing action of ionizing radiations is shown by the large ionic yield (more 

 than 3) that is found on oxidation of glutathione and propane-l,3-dithiol 

 by X rays, 7 rays, or particles. In fact, thiols seem to be the most 

 sensitive systems of all those which have been studied (Table 5-1). The 



Table 5-1. Efficiency of X-ray Irradiation on Oxidation-reduction Processes 



efficiency of oxidation of the sulfhydryl groups by ionizing radiations 

 depends on the pH of the solution ; it is low at pH 5 to 6, the amount of 

 oxidation increasing with the pH from 6 on, an indication that the degree 

 of oxidation is related to the degree of dissociation of the sulfhydryl group 

 (Barron and Flood, 1950). The great importance of this oxidation is the 

 result of the role of sulfhydryl groups in cellular division and in cell 

 metabolism (Barron, 1951). 



Cytochrome c, another sluggish oxidation-reduction system, can also be 

 oxidized by ionizing radiations when in the reduced state, the oxidation 

 being directly proportional to the radiation dose. Since this iron- 

 porphyrin compound contains a protein moiety, there is on irradiation 

 not only oxidation of the metal nucleus but also of the protein and por- 

 phyrin components. The changes produced by irradiation of ferrocyto- 

 chrome with 10,000 r are completely reversible. Oxidation of ferrocyto- 

 chrome c seems to be produced only by the hydroxyl radicals because the 

 yield does not decrease in the absence of oxygen and in the presence of 

 catalase. The porphyrin moiety is more sensitive to the action of ioniz- 

 ing radiations when in acid solution, so that in 0.005 M HCl there is 

 almost complete disappearance of the big Soret band at 4100 A on irra- 

 diation with 100,000 r (Barron and Bonzell, 1950a). 



