EFFECT OF X RAYS ON BIOLOGICAL SYSTEMS 



307 



susceptible, and amino acid oxidation comes next. The rate of absorp- 

 tion of glucose by the small intestines is decreased soon after irradiation, 

 inhibition being proportional to the irradiation dose in the range 500 to 

 50 r. At the same time there is inhibition of respiration, whereas glycol- 

 ysis remains unaffected. On irradiation with 500 r, inhibition persists 

 throughout the duration of the experiments (four days) ; on irradiation 

 with 50 r, respiration comes back to normal 5 hours after irradiation. 



Table 5-9. Effect of X Rays on Respiration of Tissues (Q02 Values) Soon 



AFTER Irradiation 



PROTECTION AGAINST IONIZING RADIATIONS. REACTIVATION 



Protection of a given system in solution against the effects of ionizing 

 radiations by addition of other systems has been known for a long time 

 (Fricke and Washburn, 1932). Dale (1943) was, however, the first who 

 attempted a study of this phenomenon by the addition of the "protecting 

 agent" to carboxypeptidase. He found that a number of organic sub- 

 stances (proteins, sugars, amino acids) as well as inorganic substances 

 may prevent inhibition of the enzyme by X irradiation. No attempt was 

 made to interpret this effect, except by presenting it as evidence of the 

 indirect action of radiations. It has been shown that the indirect action 

 is mainly due to oxidation by the OH and OoH radicals and by H2O2. 

 The oxidation-reduction potentials of these oxidizing agents is so high 

 (Gurin, 1940) that in theory any reducing agent could be used to protect 

 a solute from the indirect action of moderate radiation doses. There 

 are, however, some substances that are not readily attacked by H2O2 or 

 by O2H (ferrocytochrome c, for example, is not oxidized by O2H radicals 

 or by H2O2). The presence of electronegative groups close to the group 

 that is going to be oxidized, as well as other factors, may produce steric 

 hindrances that alter the rate of reaction with the oxidizing agents just 

 mentioned (Barron, Miller, and Kalnitsky, 1947). The effective pro- 

 tective agent must be a substance readily oxidized by the three oxidizing 

 agents but one that is not easily oxidized by atmospheric oxygen ; other- 

 wise the protecting power would be quickly lost. Thiol compounds that 

 are sluggish oxidation-reduction systems fulfill these criteria; their high 



