EFFECT OF X RAYS ON BIOLOGICAL SYSTEMS 303 



effect from the same treatment of diploid cells. If ploidy is related to 

 nucleic acid content, nucleic acids must play an important role in the 

 protection against the damaging effects of ionizing radiations, and this 

 protection is not confined to cell multiplication but extends to cell 

 metabolism. As a matter of fact, irradiation with 30,000 r inhibits by 

 30 per cent the oxygen uptake of haploid yeast suspended in water in the 

 presence of glucose, whereas it has no effect on diploid cells. The extent 

 of inhibition remains constant for 24 hours. Irradiation of haploid cells 

 with 10,000 r has no effect on the oxidation of glucose, an indication that 

 the cells lose the power to multiply under these conditions, preserving 

 intact their metabolism. When yeast cells are irradiated in the presence 

 of buffers, the effect is irregular; there is some protection, which obviously 

 is due to reaction with the products of water irradiation (Barron and 

 Seki, 1951). 



Of the irradiation products of water responsible for most of the effects 

 of ionizing radiations, only hydrogen peroxide is stable. However, on 

 irradiation of aqueous solutions containing organic compounds there 

 might be formed organic peroxides capable of producing effects similar to 

 those of ionizing radiations. There are, in fact, a number of oxidizing 

 agents and free-radical-forming agents that simulate the effects produced 

 by such radiations (Brues and Barron, 1951). A number of investigators 

 have reported that on irradiation of aqueous solutions, whether with 

 X rays or with ultraviolet light, there is formation of some peroxide that 

 affects growth, mutation rate, and metabolism. Taylor et al. (1933) 

 found inhibition of growth of protozoa in media previously irradiated 

 with X rays; Stone et al. (1947) found increase in the mutation rate of 

 *S. aureus to penicillin resistance; Wyss et al. (1948), inhibition of the 

 growth of bacteria; and Evans (1947), inhibition of the fertilizing capacity 

 o| sea urchin sperm. The experiments of Evans were repeated and the 

 respiration of sea urchin sperm suspended in X-irradiated sea water was 

 measured. Addition of sea water irradiated with 100,000 r to sperm 

 inhibited respiration 38 per cent and of sea water irradiated with 200,000 r, 

 70 per cent. Inhibition is not affected by previous addition of catalase. 

 Addition of hydrogen peroxide at a concentration that would be pro- 

 duced on irradiation .of distilled water with 100,000 r (no measurable 

 amount of hydrogen peroxide is formed on X irradiation of sea water 

 with 100,000 r) has no effect (Barron, Flood, and Gasvoda, 1949). The 

 mechanism of this inhibition is still unknown. It is not due to organic 

 peroxide formation because inhibition is also produced by irradiated 

 artificial sea water and by irradiated distilled water to which the salts 

 contained in sea water are added afterwards (Barron and Gasvoda, 

 1949c). 



It has been shown that thiols are very sensitive to the action of ionizing 

 radiations, which oxidize them with great efficiency. Li\'ing cells contain 



