288 



RADIATION BIOLOGY 



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Dihydrodiphosphopyridine nucleotide (DPNH2) is also easily oxidized 

 on irradiation with X rays, as shown by measurement of the absorption 

 spectrum of the reduced compound (Fig. 5-1). The extent of oxidation is 



proportional to the radiation dose 

 (Barron and Johnson, 1951). Ascor- 

 bic acid, another sluggish oxidation- 

 reduction system, is also oxidized by 

 X irradiation (Anderson and Harri- 

 son, 1943-1944). 



Of the oxidizing agents produced 

 on irradiation of water by X rays, 

 two of them — the O2H radical and 

 H2O2 — occur only in the presence of 

 oxygen. Therefore, irradiation in 

 the presence and in the absence of 

 oxygen may be used to distinguish 

 the effect of OH radicals from that 

 of O2H and of O2H2. Moreover, this 

 oxygen test can also be used to 

 differentiate the direct effect from 

 the indirect effect whenever O2H 

 and O2H2 act upon the substance to 

 be irradiated in aciueous solution. 



240 



440 



280 320 360 400 

 WAVE LENGTH, m/i 

 Fig. 5-1. Oxidation of dihydrodiphos- 

 phopyridine nucleotide (DPNH) by 

 X rays. X-ray dose 10,000 r. Curve 

 1, DPNH, 5 X 10-5 M in 0.005 M 

 phosphate buffer, pH 7.0. Curve 2, 

 DPNH irradiated with X rays. 



The effect of O2H2 may be distin- 

 guished from the effect of the radical O2H by means of catalase in 

 experiments in the presence of oxygen. In the absence of oxygen oxi- 

 dation of thiol compounds by ionizing radiations is considerably reduced, 

 33 per cent of that obtained in the presence of dissolved oxygen. When 

 the thiol is irradiated in the presence of dissolved oxygen, catalase pro- 

 tects it from oxidation by 22 per cent. From these experiments (Barron 

 and Flood, 1950) it can be calculated that on oxidation of thiols by X rays, 

 OH radicals contributed 33 per cent; O2H radicals, 45 per cent; and H2O2, 

 22 per cent. The oxidation of reduced DPNH2 is also protected by 

 catalase and is less in the absence of oxygen. 



Formic acid is also oxidized by ionizing radiations (Fricke and Hart, 

 1934) to H2 and CO2. Below pH 3 equal volumes of H2 and CO2 are 

 obtained, corresponding to the equation 



HCOOH = CO2 + H2 



The effect of the hydroxyl radicals produced on irradiation may be 

 increased 50-fold by the addition of O2H2 to aqueous solutions of formic 

 acid. The oxidation is nearly independent of the dose rate, and it seems 

 to be a chain reaction in which the radicals OH and HCOO alternate in 

 the H202-sensitized reaction (Hart, 1951) : 



