332 GEORGE W. CROSBIE 



ADP or UDP assay. The uridylate kinase [reaction (3)] can be distinguished 

 from adenylate kinase on the basis of differential heat lability at pH 7. 

 The possibility of the catalytic involvement of an adenine nucleotide in 

 reaction (3) could not however be dismissed. 67 Strominger et a/. 65 have re- 

 ported similar transphosphorylation reactions in calf liver preparations but 

 no evidence for uridylate kinase activity [reaction (3)] was obtained. Berg 

 and Joklik 68 ' 69 and Lieberman et a/. 67 have described a nucleosides-diphos- 

 phate kinase of widespread occurrence which effects the following trans- 

 phosphorylation : 



ATP + UDP ^ ADP + UTP 



UTP synthesis from UDP by the action of pyruvate phosphokinase has also 

 been reported. 70 In contrast to these enzymes of an anabolic character, a 

 Mg ++ -requiring nucleoside-5'-diphosphatase has been described by Gibson 

 etal. n : 



UDP h!o > UMP + P(V " 

 ADP is not a substrate of the enzyme. It is apparent that there are several 

 mechanisms available for the synthesis and rearrangements of uridine-5'- 

 di- and triphosphate. To these must be added polynucleotide phosphorylase 

 first described in Azotobacter vinelandii by Grunberg-Manago and co- 

 workers. 72, 73 The enzyme is capable (/) of exchange of inorganic phosphate- 

 P 32 with UDP and (2) of synthesis of a 3',5'-linked polynucleotide from 

 UDP or other nucleoside-5'-diphosphates. 



The biosynthesis of uridine nucleotide coenzymes has been reviewed re- 

 cently 74 " 77 and consequently will not be discussed here. 



III. Biosynthesis of Cytidine and Deoxycytidine Nucleotides 



1. Biosynthesis of Cytidine-5'-mono-, di-, and triphosphates 



Herbert et al. K have extended their investigations of uracil nucleotide 

 phosphorylations by rat liver fractions to a study of cytidine-5'-phosphate 



68 P. Berg and W. K. Joklik, Nature 172, 1008 (1953). 



69 P. Berg and W. K. Joklik, J. Biol. Chem. 210, 657 (1954). 



78 A. Romberg, in "Phosphorus Metabolism" (W. D. McElroy and B. Glass, eds.), 

 Vol. I, p. 392. Johns Hopkins Press, Baltimore, 1951. 



71 D. M. Gibson, P. Ayangar, and D. R. Sanadi, Biochim. et Biophys. Acta 16, 536 

 (1955). 



72 M. Grunberg-Manago and S. Ochoa, J. Am. Chem. Soc. 77, 3165 (1955). 



73 M. Grunberg-Manago, P. J. Ortiz, and S. Ochoa, Science 122, 907 (1955). 



74 H. M. Ralckar and H. Rlenow, Ann. Rev. Biochem. 23, 527 (1954). 



75 C. E. Carter, Ann. Rev. Biochem. 25, 123 (1956). 



76 J. Baddiley and J. G. Buchanan, Quart. Revs. 12, 152 (1958). 



77 M. F. Utter, Ann. Rev. Biochem. 27, 245 (1958). 



78 E. Herbert and V. R. Potter, /. Biol. Chem. 222, 453 (1956). 



