852 



ADVEN-TUEES IN" RADIOISOTOPE RESEARCH 



which labelled phosphate is incorporated into the desoxyribonucleic acid 

 present in various tissues (Hahn and Hevesy, 1940). The nucleus of 

 the cell being its most radiosensitive part and desoxyribonucleic acid, 

 DNA, being one of the main constituents of the nucleus, it seemed to 

 be pertinent to investigate if the rate of incorporation of phosphate into 

 DNA is influenced by exposure to radiation. As phosphorus is one of 

 the constituents of DNA, no DNA molecule can be formed without 

 incorporation of phosphorus and thus, if we label the plasma phosphate 

 by administering radioactive phosphorus with the food or by injection, 

 the phosphorus utilised in DNA formation becomes labelled and ^ap 

 is bound to enter the newly formed DNA molecule. 



In early studies incorporation of ^sp into the Jensen-sarcoma of the 

 rat was studied (Euler and Hevesy, 1942, 1944). Half of the 156 rats 

 investigated were exposed to a dose of 335 to 1500 r. They showed 

 a depression of ^^p incorporation into the DNA secured from the sar- 

 coma compared with the incorporation into the DNA of the sarcomata 

 of non-irradiated controls. A very similar result was obtained in studies 

 carried out by Holmes (1947, 1949). 



When the activity of 1 mgm of DNA phosphorus of the Jensen-sarcoma 

 of the rats exposed to total body radiation of 335 r or more is compared 

 with the labelled ^^p content of the DNA of the Jensen-sarcoma of unir- 

 radiated rats, the former was found to be about 40 per cent only of the 

 latter; thus the rate of DNA formation was depressed by exposure to 

 less than half its normal value. Table 1 shows values of this depression 

 obtained at different times after exposure of the rat. 



Similar results were obtained by us (Ahlstrom, Euler and Hevesy, 

 1945) in the investigation of the formation of labelled DNx\ in. the normal 

 organs of 250 rats. In liver of 3.5 to 4.5-clay old rats in which the percent- 

 age increase in the labelled, thus additionally formed, DNA was found 

 to be 20 times larger than in the one-year-old animal; the percentage 

 depression of ^^P incorporation due to irradiation differed only slightly 

 from that observed in the outgrown animal. Very numerous invest!- 



