36. BIOSYNTHESIS OF PYRIMIDINE NUCLEOTIDES 331 



kis 49 has reported that sonic extracts of a uracil-requiring L. bulgaricus 

 contain uridine-5'-phosphate pyrophosphorylase. E. coli B extracts have 

 been shown 58 to contain no uridine-5'-phosphate pyrophosphorylase. On 

 the other hand, extracts of E. coli W and of its cytosineless, uracilless 

 mutant E. coli Wc~ show a small but significant conversion of uracil to 

 the 5'-nucleotide in the presence of 5-phosphoribosylpyrophosphate. 



2. Biosynthesis of Uridine-5'-di- and triphosphates 



The isolation by ion-exchange procedures of purine and pyrimidine nu- 

 cleoside-5'-mono-, di-, and triphosphates from several sources has been re- 

 ported. 59 " 62 Potter et al. 63 have shown that inorganic phosphate-P 32 rapidly 

 equilibrates with the di- and triphosphates of rat liver in vivo. Herbert et 

 al. &i have subsequently investigated nucleotide phosphorylation in various 

 cellular subfractions obtained by differential centrifugation of rat liver 

 homogenates. They have reported that the cytoplasmic fraction can phos- 

 phorylate uridine-5'-phosphate (UMP) to uridine-5'-diphosphate (UDP) 

 and uridine-o'-triphosphate (UTP). Isolated mitochondria effect a phos- 

 phorylation of UDP to UTP but have no action on UMP, while the super- 

 natant fraction from the mitochondria is able to phosphorylate UMP to 

 UDP and some UTP. The phosphorylation capacity of the cytoplasmic 

 fraction toward uridine nucleotides is not unexpectedly dependent upon 

 oxidative phosphorylation reactions or when these are prevented upon ad- 

 dition of ATP. Other groups 6567 have reported enzymic interactions of ATP 

 and UMP analogous to adenylate kinase (myokinase). Lieberman et a 7 .. 67 

 have described a partially purified yeast enzyme which effects the follow- 

 ing interconversions : 



ATP + UMP ^ ADP + UDP (1) 



UTP + AMP ^ UDP + ADP (2) 



UTP + UMP ;=± 2 UDP (3) 



The reactions may be followed using the coupled pyruvate phospho- 

 kinase-lactic dehydrogenase reactions as a spectrophotometric method for 



69 H. Schmitz, R. B. Hurlbert, and V. R. Potter, /. Biol. Chem. 209, 41 (1954). 



60 L. I. Hecht, V. R. Potter, and E. Herbert, Biochim. et Biophys. Acta 15, 134 (1954). 



61 H. Schmitz, Biochim. et Biophys. Acta 14, 160 (1954). 



62 R. Bergkvist and A. Deutsch, Acta Chem. Scand. 7, 1307 (1953) ; 8, 1880, 1889 (1954). 



63 V. R. Potter, E. Herbert, Y. Takagi, P. Siekevitz, and A. F. Brumm, Federation 

 Proc. 13, 276 (1954). 



64 E. Herbert, V. R. Potter, and Y. Takagi, J. Biol. Chem. 213, 923 (1955). 



65 J. L. Strominger, L. A. Heppel, and E. S. Maxwell, Arch. Biochem. Biophys. 52, 

 488 (1954). 



66 A. Munch-Petersen, Acta Chem. Scand. 8, 1102 (1954). 



67 I. Lieberman, A. Romberg, and E. S. Simms, </. Biol. Chem. 215, 429 (1955). 



