Biosynthesis of Pyrimidines 



157 



recovered may be of some value. In this experiment, one 

 may note that the specific radioactivity calculated per milli- 

 gram of base was much higher for uridylic than for RNA 

 cytidylic acid, the former being 3-4 times higher than the 

 latter. There was isolated about twice as much cytidylic 

 acid as uridylic acid. This was true for the separation by 

 resin alone as well as for the separation by paper and resin. 

 We have come to believe, however, that the combination 

 of paper and resin is the most satisfactory procedure. 



Some results of our studies on tumour tissue from rats are 

 given in Table IV. The specific activity of uridylic acid was 



Table IV 



Rat Walker Carcinoma 256 Incubated with 3 mg. of [2-^^C] Orotic Acid 



about 9 times that of the cytidylic acid, although, again, 

 there was twice as much cytidylic as uridylic acid. A small 

 amount of thymidylic acid was obtained which had a very 

 small specific activity. In part II of the experiment, alternate 

 slices of tissue were taken, a small amount of methopterin 

 added, and a parallel digestion and determination carried out. 

 The results indicate, we believe, that methopterin had no 

 effect and the data therefore show the accuracy of our 

 duplicate determinations. 



