BIOCHEMICAL MECHANISM OF OXYGEN EFFECT 277 



water. It has already been suggested by Thoday and Read (28, 29) that 

 this product may be hydrogen peroxide. Additional evidence is now 

 available which supports this conclusion. The results of four entirely 

 unrelated types of experiments furnish evidence favoring or compatible 

 with the H2O2 hypothesis. Bonet-Maury and Lefort (5) have in- 

 vestigated the production of H2O2 in water irradiated with x-rays and 

 with alpha particles under various conditions, including the effect of 

 oxygen concentration and of temperature. In addition, data are avail- 

 able on the effect of pH on peroxide yield; see Loiseleur (21). 



At least four striking parallels exist between H2O2 production and 

 chromosome-aberration production under varying conditions of irradia- 

 tion, (a) It is found that with x-rays H2O2 is not produced in oxygen- 

 free water [or is produced in very small amounts, as shown by Allen (1)], 

 but that when oxygen is present the amount of H2O2 produced depends 

 markedly on the concentration of oxygen. The type of curve obtained 

 for the increased yield of H2O2 with increasing oxygen concentration 

 at a constant x-ray dose is generally similar to that obtained in the 

 present studies for the relation between aberration frequency and 

 the percentage of oxygen present during x-radiation. (6) In oxygen- 

 saturated w^ater, H2O2 production by x-rays decreases regularly with 

 decreasing temperature, a definite discontinuity marking the pas- 

 sage from water to ice. Below — 116°C no H2O2 can be detected. 

 Faberge (10) has shown that, when Tradescantia pollen grains are x- 

 rayed at various temperatures, the general character of the sensitivity 

 curve (as measured by the number of chromosome breaks observed in 

 pollen tube divisions) resembles that for H2O2 production. There is a 

 dip in the region of 0° C and a gradual decline thereafter. However, al- 

 though H2O2 production stops at — 116° C, chromosome breaks are still 

 produced at — 192°C, their frequency being almost one-fifth that at 

 +25° C. (c) The pH of the solution exerts an effect on the yield of 

 H2O2 with x-rays, an alkaline pH favoring a lowxr H2O2 concentration. 

 The experiments of Marshak (22) have shown that the frequency of 

 chromosome aberrations observed at anaphase in root tips of Vicia faba 

 and Allium cepa is markedly reduced when x-radiation is carried out in 

 the presence of penetrating bases, such as ammonium hydroxide, {d) 

 Undoubtedly the most cogent evidence obtained to date in favor of the 

 H2O2 hypothesis is that derived from a comparison of x-ray and alpha- 

 particle effects in the presence and absence of oxygen. With alpha par- 

 ticles, H2O2 production occurs even in oxygen-free water and the ad- 

 dition of oxygen does not increase the yield. Thoday and Read (29) 

 have shown that, for aberrations induced in the root tips of Vicia faba, 

 the removal of oxj^gen results in little if any decrease in aberration fre- 



