Biosynthesis of Pyrimidines 153 



donor and in our experiment was incorporated in purines to 

 give high specific activities, not sufficient radioactivity was 

 found in the thymine to be detected. This is not in agreement 

 with the results of Elwyn and Sprinson (1950) who used 

 [)S-i*C]-L-serine or [a-i*C]-glycine for methylation or those of 

 Totter, Volkin, and Carter (1950) who used formate. There 

 is agreement with LePage and Heidelberger (1951) who used 

 [a-^*C]-glycine. 



Hammarsten and his group (Arvidson et aL, 1949) have 

 demonstrated that orotic acid labelled with ^^N enters the 

 pyrimidines when it is fed to rats. Mary Edmonds (1950) 

 has obtained a similar result when yeast is grown in a medium 

 containing orotic acid labelled with ^^C in carbon 4. Glycine 

 carbon is not incorporated in pyrimidines. Lactate labelled 

 in 2 and 3 positions with ^^C is incorporated probably in 

 positions other than in the ureide group. 



Recently Dr. Lawrence L. Weed in my laboratory has 

 prepared orotic acid with ^^C in position 2, and has been 

 studying pyrimidine formation in vitro (Weed, Edmonds, 

 and Wilson, 1950). He has found that slices of tissues, when 

 incubated with the radioactive orotic acid will incorporate 

 the radioactive carbon in the pyrimidines. Cy tidy lie and 

 uridylic acids of ribonucleic acid (RNA) have been studied 

 mainly, but thymidylic and cytidylic acids of deoxyribo- 

 nucleic acid (DNA) have been examined to some extent. We 

 have carried out experiments as follows: Slices of tissue 

 suspended in Krebs saline with added phosphate at pH 7 • 4 

 were digested with 1-10 mg. of radioactive orotic acid at 

 38° for 4 hours. After digestion the sUces were washed 

 several times, homogenized and precipitated with trichloracetic 

 acid. The precipitate was freed from lipids by heating with 

 alcohol-ether mixture. Nucleic acid was extracted from the 

 precipitate with hot 10 per cent NaCl solution for 24 hours 

 with continuous stirring. The nucleic acid was precipitated 

 from the filtered solution with 2.5 volumes of alcohol. 

 Sometimes DNA and RNA were separated by means of the 

 Schmidt-Thannhauser procedure (1945). We feel that the 



