DESOXYPENTOSE NUCLEIC ACID 419 



desoxy pentose nucleic acid molecule (10). The following summarizes 

 some of our findings (11) : 320 mice gave an average value of 2.50 ± 0.045 

 X 10~^ specific activity of P^^ in tumor * desoxyribose nucleic acid; 92 

 animals receiving 60-r whole-body irradiation gave an average nucleic 

 acid specific activity of 2.23 ± 0.041 X 10~^ The difference is signifi- 

 cant at the 1 per cent level, and the magnitude of the effect is 0.18 per 

 cent depression per roentgen, which is of the same order as the depres- 

 sive effect of radiation on red- and white-cell formation. Presumably 

 the synthesis of desoxypentose nucleic acid is a function of the mitotic 

 activity of the tissue, so that this also represents the effect of radiation 

 on a proliferative process. In the same group of animals the specific 

 activities of liver desoxypentose nucleic acid were examined; they are: 



Controls (188 animals) 3.83 ± 0. 19 X lO"" 



60-r total-body x-irradiation equiva- 

 lent to 1.4 X 10^ ergs (84 animals) 3 . 16 ± . 26 X 10^" 



Muscles irradiated with 5.25 X 10* 

 ergs (148 animals, radioyttrium 

 colloid) 2.83z±z0.12 X 10^" 



The difference of liver specific activities of desoxypentose nucleic acid 

 between unirradiated controls and the 60-r whole-body irradiated ani- 

 mals is in the expected direction and magnitude, but it is of doubtful 

 significance statistically; the depression of desoxypentose nucleic acid 

 turnover is 0.28 per cent per roentgen. When (see above data) muscles 

 are irradiated by an infiltration of radioactive yttrium colloid, a signifi- 

 cant depression of the desoxypentose nucleic acid specific activity is 

 observed. The effect obviously must have been indirect, as in this case 

 the liver was unirradiated. If the ionization energy had been expended 

 as average whole-body irradiation, the observed effect would have been 

 a 0.16 per cent depression of specific activity per rep. 



In the same animals (irradiation localized to muscle) the specific ac- 

 tivity of the tumors was 2.10 zfc 0.042 X 10~^ (that in the control ani- 

 mals, above, was 2.50 ± 0.045) ; the difference is significant. The effect, 

 had the same ionization been expended over the whole of the animal, 

 would have been 0.07 per cent depression per rep. The liver is appar- 

 ently relatively more sensitive than the tumor to the indirect effects of 

 radiation on the formation of desoxypentose nucleic acid, but this might 

 relate to such facts as that the liver has a much greater blood supply 

 than the tumor and that the tumor has 10 times more relative turnover 

 of this nucleic acid than the liver. When livers are specifically irradi- 

 ated with radioactive colloid (4.25 X 10^ ergs), the specific activity of 



* Mammary carcinoma transplants in A strain mice. 



