854 ADVENTURES IN RADIOISOTOPE RESEARCH 



hexosediphosphate and labelled inorganic phosphate in yeast, one-half 

 of the phosphorus atoms present in the adenylphosphate were those 

 originally located in the hexosediphosphate molecules, while the other 

 half only were originally present as orthophosphate. Later, when investi- 

 gating the formation of phospholipids in the liver, it was observed that 

 it is the glycerophosphate phosphorus which is incorporated into phos- 

 pholipids (Chaikoff, 1942) and as the formation of labelled glycero- 

 phosphate is a comparatively slow process, formation of phospholipids 

 after administration of ^^P proceeds first for a while from inactive glycero- 

 phosphate. Thus, for a while, phospholipid formation is not indicated 

 by the radioactive tracer and correspondingly, when calculating its 

 rate from the specific activity of the cellular orthosphosphate P and 

 the lipid P we underrate the rate of the formation of the phospholipids. 

 When calculating the rate of formation of DNA as described above, 

 the possibility must therefore be envisaged that the ratio 



specific actiivity of DNA P 

 mean specific activity of orthos phosphate P 



supplies the lower limit of the formation of DNA only. 



Incorporation of ^^C Into DNA 



Results similar to those obtained when we were following up the effect 

 of irradiation on the incorporation of ^^p into DNA were obtained when 

 the effect of exposure on the incorporation of i^C into DNA of the organs 

 of the mouse was investigated. When i'*C became available we injected 

 growing mice with labelled sodium acetate (CHa.i^COONa) and compared 

 the rate of incorporation of ^^C into the DNA of the organs of animals 

 exposed to 700 r of X-rays with that of non-exposed mice (Hevesy, 

 1949). As can be seen in Fig. 1, exposure depresses the incorporation 

 of i*C into DNi\. of the intestinal mucosa, liver, muscles, and kidney of 

 growing mice to about one half of that of controls. Incorporation of 

 i^C into proteins, however, is not depressed by irradiation: it is even 

 somewhat enhanced. 



In early investigations on radiation effects Mitchell (1940, 1942), 

 applying ultraviolet micrography used in conjunction with photographic 

 photometry, observed a striking increase in ultraviolet absorption 

 of the cytoplasm of proliferating and incompletely differentiated irra- 

 diated tumour cells due to the accumulation of ribonucleotides. These 

 early investigations and many subsequent ones revealed that irradiation 

 with ionizing radiations influences ribosenucleic acid formation as well. 



The influence of radiation on the rate of incorporation of precursors 

 into ribosenucleic acid (RNA) was often found to be much less marked 

 than in DNA, Holmes (1949) being the first to make this observation. 



