NUCLEIC ACID IX THE ORGANS OF THE RAT 669 



course of 1 hour, a very large part of the acid-soluble P compounds 

 and a few per cent of the phosphatides present in the liver are renewed. 

 Compared with these figures, the rate of renewal of nucleic acid in the 

 liver is negligible. 



The percentage ratio of the specific activity of the nucleic acid P and 

 the free P indicates the percentage of new nucleic acid present, i.e. 

 nucleic acid formed in the course of the experiment. We cannot state 

 with certainty whether this new nucleic acid is formed in the organ in 

 which it is found or transported from another organ in which it was 

 built up. It would be conceivable that the nucleic acid molecules built 

 up in the intestinal mucosa, for example, where we find far the greatest 

 rate of renewal of nucleic acid, reach the circulation and are deposited 

 in the muscles. Information on this point can be obtained on the same 

 line or on similar lines on which the origin of the phosphatides present 

 in the yolk was investigated (Hevesy and Hahn, 1938). The nucleo- 

 proteins are probably built up in the nuclei of the cells and not carried 

 from organ to organ. The low new (labelled) nucleic acid content of 

 the liver can be interpreted as an argument against the last mentioned 

 interpretation. The liver takes up easily constituents present in the cir- 

 culation and, if any organ takes up from the circulation nucleoproteins 

 and thus nucleic acids, we would expect the liver to do so. The active 

 nucleic acid content of the liver nucleic acid is, however, very low 

 and this fact supports the view that the active nucleic acid molecules 

 present in the liver are synthesized in this organ. The rate of renewal 

 of the nucleic acid molecules in the liver may be identical with the 

 rate of new formation of liver cells^^^ 



The figures for the rate of formation of nucleic acid in the organs of 

 the rat found in this investigation are very much lower than those 

 for the renewal of nucleic acid or of "nucleoproteins" by different 

 experiments both in the organs of the rabbit and in the organs of the 

 mouse. In the liver of the rabbit (Hahn and Hevesy, 1940), for example, 

 6 per cent of the nucleic acid present were found to be renewed in the 

 course of 11.5 hours. In the liver of the mouse (Tuttle, Erf and 

 Lawrence, 1941), in the course of 6 hours, about 40 per cent of the 

 "nucleoproteins" present w^ere found to be labelled. In these experiments, 

 the nucleic acid P and the "nucleoprotein P", respectively, contained 

 presumably some strongly active acid-soluble or phosphatide phospho- 

 rus, the presence of which was presumably responsible for the high values 

 obtained for the rate of renewal of the nucleic acid and the "nucleo- 

 proteins". 



^1^ The rate at which hver cells are renewed is not known. While this rate may- 

 be smaller than the rate of formation of nucleic acid in the liver cells, it can hardly 

 he larger. 



