RENEWAL OF DESOXYRIBO NUCLEIC ACIDS 677 



value of this magnitude is about almost half of its end value, we have 

 therefore to multiply the figures mentioned above (3.6 and 2.2 respec- 

 tively) by about 2 to obtain an approximate value of the percentage 

 renewal of the ribo- resp. desoxyribo nucleic acid in the course of 2 hours. 

 It is improbable that a so highly active free phosphate is utilized 

 in the building up of the nucleic acid molecules as found in a 2 hours 

 experiment in the liver. Liver and kidneys have a privileged position 

 concerning the rate of intrusion of phosphate. The amount of nucleic 

 acid present in the liver and the kidneys makes out, furthermore, only 

 a small percentage of the total nucleic acid content of the organism. 

 It is much more probable that free P of similar specific activity as found 

 in the intestine is applied in the building up of the labelled nucleic 

 acid molecules. In fact, the amount of nucleic acid present in the mucosa 

 of the digestive tract makes out a large percentage of the body nucleic 

 acid. While the body nucleic acid contains also slightly radioactive 

 fractions, viz. those originating from the liver, the kidneys, and the 

 brain and fractions of restricted radioactivity originating from the 

 muscles (Hevesy and Ottesen, 1943), it contains also fractions of 

 higher activity than found in the intestinal mucosa, viz. those originating 

 from the bone marrow, the thymus and lymph nodes (Andreasen and 

 Ottesen, 1944, 1945). The lymphocytes secreted into the organism can 

 also be expected to contain pronouncedly active nucleic acid. This makes 

 it understandable that the rate of renewal of the average body nucleic 

 acid corresponds to about the rate of renewal of the intestinal nucleic 

 acid and is thus quite pronounced for both types of nucleic acid in 

 contradistinction to the rate of renewal found in the liver, which is 

 very low in the case of desoxyribo nucleic acid and appreciably higher 

 in the case of ribose nucleic acid. 



DISCUSSION 



In view of the high desoxyribo nucleic acid content of the lympho- 

 cytes and because they are partly produced in the spleen, the compa- 

 ratively high rate of turnover of desoxyribo nucleic acid in the spleen is 

 in agreement with our expectance. 



As to the high figures found for the rate of renewal of desoxyribo 

 nucleic acid in the intestinal mucosa, those are presumably due to the 

 rapid new-formation of cells mechanically destroyed in the course of 

 the digesture success. 



If we accept the view put forward by Casperssox, the high rate of 

 renewal of ribo nucleic acid is in no way surprising. The high figures 

 found for the rate of renewal of ribo nucleic acid in the intestine, the 

 .spleen and the liver is just what we would expect in view of the impor- 



