284 CHROMOSOME ABERRATION PRODUCTION 



to form the powerful radical O2H: O2 + H -» O2H. The presence of oxygen, 

 therefore, increases considerably the oxidizing power of ionizing radiations. 

 Molecular oxygen can be used, for these reasons, as a test for the mechanism of 

 action of ionizing radiations. If the effects obtained on irradiation are greater 

 in the presence of oxygen, there will be no doubt that the effect was due to prod- 

 ucts of the irradiation of water. The radical OH and jitomic hydrogen are 

 formed regardless of the nature of gas dissolved in water ; O2H and H2O2, how- 

 ever, are formed only in the presence of oxygen. Thiol compounds, such as 

 glutathione, — SH enzymes, — SH proteins, are oxidized by all three agents, 

 and irradiation will be more effective in the presence of oxygen. Ferrocyto- 

 chrome c is not oxidized by H2O2; in this case the presence of oxygen would 

 increase only oxidation due to the radical O2H. Finally, there is the possibility, 

 which must be investigated, that the H2O2 produced on irradiation may act as 

 an oxidizing agent when combined with catalase. 



In summary, the presence of oxygen introduces two more oxidizing agents. 

 It is obvious, therefore, that the absence of oxygen will decrease the toxic effects 

 of ionizing radiations, a decrease which reaches 70 per cent in the case of the 

 oxidation of thiol compounds. If ionizing radiations have the same effect in the 

 presence as well as in the absence of oxygen, oxidation processes cannot be dis- 

 missed because the powerful OH radicals are still formed. It would be inter- 

 esting to study the effect of low oxygen tensions on irradiation of tissues with 

 small doses of radiation. It is quite possible that x-irradiation of bacteria at low 

 oxygen tensions, such as those prevailing in the high plateaus of the Andes and 

 the Himalayas, would produce fewer mutations than irradiation at sea level. 



