334 GEORGE W. CROSBIE 



et a/., 86 who have reported that the incorporation of carbamylaspartic acid 

 and orotic acid into the polynucleotide cytosine of some rat tissues was 

 significantly depressed relative to the effect on uracil and thymine by the 

 glutamine analog, 6-diazo-5-oxo-L-norleucine (DON) (see Chapter 39). DON 

 had little effect on the utilization of cytidine-3'(2'-)-phosphate-G-C 14 by 

 tumor slices for polynucleotide pyrimidine synthesis in marked contrast to 

 its effect on the incorporation of uridine-3'(2'-)-phosphate-G-C 14 into poly- 

 nucleotide cytosine. Salzman et al. s7 have also reported that the 6-amino 

 group of polynucleotide cytosine is derived from the glutamine amide group 

 rather than ammonia. Kammen and Hurlbert 88 have recently extended 

 these observations and have described a synthesis of cytidine nucleotides 

 from orotic acid-G-C 14 and uridine-5'-phosphate-6-C 14 by a soluble enzyme 

 system from the Novikoff hepatoma. For full activity, the Dowex-l-(for- 

 mate)-treated enzyme required glutamine, ATP, guanosine-5'-phosphate 

 (GMP), and an ATP-generative system. GMP could be replaced by the 

 corresponding triphosphate. Low levels of DON were inhibitory (cf. the 

 antagonistic effect of DON on other glutamine amide transfer reactions 89 ). 

 The complexity of kinase and pyrophosphatase activities present in the 

 system obscured the nature of the substrate of the animation reaction but 

 the involvement of UTP is considered likely on the basis of the evidence 

 available. 



The uracil and cytosine ring systems are also interrelated through the 

 activities of the cytosine and cytidine deaminases described in a variety 

 of mammalian and microbial cells. The systems are, however, generally 

 considered to be catabolic with respect to the cytosine ring rather than ana- 

 bolic with respect to uracil derivatives. Thus cytidine-3'-phosphate is 

 catabolized by a high speed supernatant fraction of rat liver homogenates, 

 the products being uracil, ammonia, and inorganic phosphate. 90 The avail- 

 able evidence implicated cytidine and uridine but not uridine-3'-phosphate 

 or cytosine as intermediates. 



Both CDP and CTP have been shown to participate in polynucleotide 

 synthesis. CDP is a substrate of polynucleotide phosphorylase 72 and CTP 

 has been implicated 91 as a proximal precursor during the incorporation of 

 CMP into end groups of the RNA of the supernatant fraction of Ehrlich 

 ascites carcinoma cells. 



86 M. L. Eidinoff, J. E. Knoll, B. Marano, and L. Cheong, Cancer Research 18, 105 



(1958). 

 « N. P. Salzman, H. Eagle, and E. D. Sebring, J. Biol. Chem. 230, 1001 (1958). 



88 H. O. Kammen and R. B. Hurlbert, Biochim. et Biophys. Acta 30, 195 (1958). 



89 B. Levenberg, I. Melnick, and J. M. Buchanan, J. Biol. Chem. 225, 163 (1957). 



90 M. J. Simcock, G. Sneyd, and R. D. Blatt, Arch. Biochem. Biophys. 71, 62 (1957). 



91 L. I. Hecht, P. C. Zamecnik, M. L. Stephenson, and J. F. Scott, J. Biol. Chem. 

 233, 954 (1958). 



