Biosynthesis of Pyrimidines 



159 



tumours. The ratio of activities of uridylic to cytidylic acids 

 is 3, the same that we have obtained for normal hver and 

 much less than for the tumours. On account of the larger 

 amount of thymidylic acid obtained from this growing tissue, 

 the thymidylic acid was collected in two fractions. Although 

 only small counts were obtained the specific activities were 

 higher than usual. Furst, Roll and Brown (1950) have 

 found a similar result with purines. 



Data from another normal tissue, a cat spleen, are shown in 

 Table VII. Here the ratio of specific activities of uridylic 

 and RNA cytidylic acids was 6. The specific activities of the 

 DNA cytidylic acid and thymidylic acid were very low. 



Table VII 



Normal Cat Spleen Incubated with [2-i^C] Orotic Acid 



One may not draw any startling conclusions from these 

 data. However, the following comments may be made: 

 We have developed a method for determining readily the 

 incorporation of radioactivity into pyrimidines of the nucleic 

 acids after incubation of tissue slices with radioactive orotic 

 acid. The orotic acid goes into the pyrimidines without being 

 degraded to COg. The specific a^ctivity of resulting uracil is 

 from 3 to 11 times greater than that of the RNA cytidylic 

 acid or cytosine. This ratio is associated with a concentration 

 of cytidylic twice as high as that of uridylic acid. 



The differences in incorporation of orotic acid in uridylic 

 and cytidylic acid emphasized the uncertainty of studies 

 where incorporation in total DNA is compared with incorpora- 

 tion in total RNA. These considerations lead to the question 



