36. BIOSYNTHESIS OF PYRIMIDINE NUCLEOTIDES 333 



(CMP) phosphorylation. As with the uracil analog CMP is phosphorylated 

 by a cytoplasmic fraction during oxidative phosphorylation to cytidine- 

 5'-di- and triphosphates (CDP and CTP, respectively). The adenylate ki- 

 nase of mitochondria has apparently no action on CMP. Evidence obtained 

 through the use of inorganic phosphate-P 32 is consistent with the role of 

 ATP as an intermediate phosphate acceptor during the incorporation of 

 inorganic phosphate into CDP and CTP. 



Strominger et al. 6b have described a liver enzyme preparation which ef- 

 fects the following transphosphorylation : 



ATP + CMP ^ ADP + CDP 



Maley 79 has similarly reported the preparation of a purified cytidine 

 monophosphokinase from A. vinelandii. The preparation also contains an 

 ATP-linked nucleoside diphosphokinase converting CDP to CTP. Stromin- 

 ger 80 has also reported the phosphorylation of CDP by pyruvate phospho- 

 kinase, the system apparently not involving adenine nucleotides as inter- 

 mediate phosphate donors. 



Brawerman and Chargaff 81 - 83 have described a nucleoside phosphotrans- 

 ferase in crude extracts of a variety of cell types. CMP is formed exclusively 

 from cytidine and an organic phosphate by plant and bacterial enzymes 

 whereas CMP and some cytidine-3'-phosphate are formed by the corre- 

 sponding mammalian system. The variation in transferase activity with the 

 growth phase of the parent cell points to a role of the phosphotransferase in 

 nucleotide biosynthesis. As no de novo pathway of cytidine (or uridine) syn- 

 thesis other than via the corresponding 5'-nucleotides has been described, 

 the actual role of the nucleoside phosphotransferase enzyme may be merely 

 that of a salvage mechanism. 



A connection between cytosine and uracil derivatives is achieved through 

 amination and deamination reactions. Lieberman 84 • 85 has reported the con- 

 version of UTP to CTP by an E. coli extract in the presence of NH 3 and 

 ATP. Aspartic acid, asparagine, glutamic acid, and glutamine failed to re- 

 place NH 3 . Uracil, uridine, and UMP were similarly not utilized. UDP 

 was utilized for CTP formation but the evidence available indicated its 

 prior conversion to UTP. That the amination mechanism in mammalian 

 cells may differ from that in E. coli is indicated by the work of Eidinoff 



79 F. Maley, Federation Proc. 17, 267 (1958). 



80 J. L. Strominger, Biochim. et Biophys. Acta 16, 616 (1955). 



81 G. Brawerman and E. Chargaff, Biochim. et Biophys. Acta 15, 549 (1954). 



82 G. Brawerman and E. Chargaff, Biochim. et Biophys. Acta 16, 524 (1955). 



83 M. Tunis and E. Chargaff, Biochim. et Biophys. Acta 21, 205 (1956). 



84 I. Lieberman, J. Am. Chem. Soc. 77, 2661 (1955). 



85 I. Lieberman, J. Biol. Chem. 222, 765 (1956). 



