674 ADVENTURES IN RADIOISOTOPE RESEARCH 



In contradistinction to the desoxyribo nucleic acid, a large part of 

 ribo nucleic acid is located in the cytoplasm and, according to the view 

 developed by Caspersson (1940), is involved in protein synthesis. As 

 such a synthesis takes place at a marked rate in the liver, the rate of 

 renewal of the ribo nucleic acid can be expected to be larger than the 

 rate renewal of desoxyribo nucleic acid in this organ. The present note 

 contains the results of experiments in which the rate of renewal of both 

 types of nucleic acid was determined, viz. in the liver, the spleen, and 

 the intestinal mucosa of the rat and also in the total rat body. The 

 methods applied in the isolation of the nucleic acids will shortly be 

 published by one of the authors. 



RESULTS 



If we assume the free phosphate to be built into the nucleic acid 

 molecules without the formation of an intermediary phosphorus com- 

 pound of comparatively long life, the ratio of the specific activity of 

 the nucleic acid P and the free P is a measure of the rate of formation of 

 new nucleic acid molecules and thus of the extent of renewal of such 

 molecules. If, previous to the formation of labelled nucleic acid mole- 

 cules, labelled precursors would be built up at a rate slower than the 

 rate of formation of new nucleic acid molecules, the ratio of the specific 

 activity of the nucleic acid P and that of the free P would no longer be 

 a proper measure of the rate of renewal of the nucleic acid. In the latter 

 case, namely, during part of or possibly throughout the whole experiment, 

 new nucleic acid molecules would be built up without participation of 

 =^2P. The participation of such labelled intermediary compounds of a 

 very long life in the formation of desoxyribo nucleic acid in the liver is, 

 however, highly improbable in view of the comparatively short hfetime 

 of most of the acid-soluble phosphorus compounds and the very long 

 lifetime of desoxyribo nucleic acid molecules present in the liver. 



In our experiments the specific activity of the nucleic acid P is com- 

 pared with the specific activity of the free P at the end of the experiment. 

 As the specific activity of the free P changes throughout the experiment, 

 the specific activity of the nucleic acid P at the end of the experiment 

 should, however, be compared with the mean specific activity of the 

 free P during the experiment. As the liver takes up ^ap at a very rapid 

 rate, and its free ^^p content culminates within the first 2 hours, the end 

 value and the average value of the specific activity of the free liver P 

 do not differ essentially, the average value being about 5 per cent lower 

 than the end value (Ahlstrom, Euler and Hevesy, 1944). In the case 

 of the spleen the corresponding figure is about 25, and even larger 

 differences are found in the case of the intestinal mucosa. The figures 



