NUCLEIC ACID AND IRRADIATION 183 



ments have also shown the stabihty of the peptide Hnkage in poly- 

 peptides. The practical value of all these experiments — and a great 

 deal still remains to be done on these lines — is that they furnish an in- 

 dication of the likelihood or otherwise of reactions which could occur 

 within the cell. 



Nucleic Acid and Irradiation 



I now turn to investigations of radiation effects on the important cell 

 constituent, nucleic acid. Euler and Hevesy (10) studied the formation 

 of desoxyribonucleic acid in unirradiated and irradiated Jensen Sarcoma 

 of rats with the aid of P^^ and found a retardation of its formation 

 within ]/2 hr after a dose of 1000 r. This effect declined with time, two- 

 thirds of the inhibition disappearing within 2 hr after irradiation. A 

 possible mechanism of the inhibition is suggested by the work of Mitchell 

 (11), who followed the nucleic acid metabolism in proliferating and in- 

 completely differentiated cells. According to Mitchell, the drop in des- 

 oxyribonucleic acid content is due to an inhibition of the synthesis of 

 this compound in the nucleus and the accumulation of ribonucleotides 

 in the cytoplasm, based on the inability of the irradiated cell to reduce 

 ribonucleotide to desoxyribonucleotides in the nucleus. This very proba- 

 bly means that the radiation has interfered with an enzymatic reaction. 

 Holmes (12), however, found ribonucleotides unaltered or diminished in 

 the cytoplasm of Jensen rat sarcoma after irradiation. Hevesy (13) has 

 also proved the x-ray effect on the formation of desoxyribonucleic acid 

 by following the uptake of C^* into this substance. In other experi- 

 ments the uptake of P^^ in the phosphatides of the liver or of the intesti- 

 nal mucosa of irradiated mice was shown to be unaffected by irradi- 

 ation. These metabolic changes appear to stress the importance of nu- 

 cleic acid metabolism and are examples of experiments in vivo. 



There is, however, an investigation on nucleic acid done in vitro which 

 I should like to mention because it will lead us on to another aspect of 

 radiation effects which in my opinion should attract more attention than 

 it has received so far. Hollaender, Greenstein, and Taylor (14) investi- 

 gated the change in the viscosity of sodium thymus nucleate occurring 

 with x-radiation. They also found a change in the ability of the solu- 

 tion to precipitate out in 95 per cent alcohol in the presence of salt. 

 This ability was completely lost in the irradiated material, but they did 

 not find any change in the chemical properties of the nucleic acid mole- 

 cule or in its digestibility by the enzyme desoxyribonuclease. These 

 carefully conducted experiments led the authors to the conclusion that 

 the effect was apparently due to breaking up of the particles into shorter 



