PART IV 

 NUCLEIC ACIDS 



THE BIOSYNTHESIS OF PYRIMIDINES IN VITRO* 



D. WRIGHT WILSON 



The metabolism of the pyrimidines seems to be quite 

 different from that of the purines. For many years it has 

 been known that while purines are excreted as uric acid or 

 allantoin, the pyrimidines form neither of these but are 

 excreted as urea and CO2. With the introduction of the use 

 of the rare isotopes as tracers it became possible to study the 

 precursors of the pyrimidines. It was early shown that 

 ammonium salts labelled with ^^N could become incorporated 

 in pyrimidines of nucleic acid (Barnes and Schoenheimer, 

 1943). If, however, a labelled pyrimidine, uracil or thymine 

 or cytosine, is fed to rats, it does not appear in the nucleic 

 acids (Plentl and Schoenheimer, 1944; Bendich et al., 1949). 

 Purines are not the source of pyrimidines nor are pyrimidines 

 the source of purines. Although pyrimidines are catabolized 

 to urea, urea is not one of the building stones of pyrimidines. 



Heinrich (Heinrich and Wilson, 1950) has shown that CO2 

 is incorporated into the pyrimidine molecule in rats when 

 bicarbonate is given. Most of it enters position 2 (the ureide 

 carbon) although some may enter position 4 ( Lager kvist, 

 1950). Carbon of radioactive formate was not found in 

 thymine. Although formate is known to be a good methyl 



♦Aided by a grant from the American Cancer Society administered by the 

 Committee on Growi:h of the National Research Council. The i*C used in 

 this investigation was obtained on allocation by the United States Atomic 

 Energy Commission. 



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