Biosynthesis of Pyrimidines 



155 



1951). We calculate that the washings alone in our procedure 

 will leave less than 1 • 10* per cent of the original orotic acid. 

 We have demonstrated that the paper will separate orotic 

 from uridylic acids (Table I) and the column will separate 

 orotic and cytidylic acids. 



Table I 



Separation of Uridylic and Cytidylic Acids from Radioactive Orotic 



Acid 



*The cytidylic acid was removed from paper and run through a resin column before the 

 radioactivity was determined. 



The isolations are moderately quantitative. Table II 

 shows recoveries of pyrimidines and pyrimidine nucleotides 

 as determined by ultraviolet absorptions. They were from 

 90 per cent to 100 per cent. 



The combination of paper chromatography and ion ex- 

 change is important for the separation of certain constituents 

 and for obtaining good planchejts for radioactivity determina- 

 tion. In our experiments with radioactive orotic acid no 

 radioactivity was found in the purines and all the activity 

 of the uridylic and cytidylic acids was found in the pyrimidine 

 bases. It is, therefore, reasonable to conclude that the orotic 

 acid enters the pyrimidines as a whole and does not break up 

 to give appreciable amounts of radioactive CO2 in our 

 experiments. 



