36. BIOSYNTHESIS OF PYRIMIDINE NUCLEOTIDES 327 



The interconversion of carbamylaspartic acid and orotic acid in cell-free 

 extracts of Z. oroticum has been reported by Lieberman and Romberg.' 28 

 The extracts contain dihydroorotase and dihydroorotic acid dehydrogenase 

 (a flavin nucleotide-linked enzyme 29 ) catalyzing, respectively, the inter- 

 conversion of carbamylaspartic acid and dihydroorotic acid and of dihydro- 

 orotic acid and orotic acid. The adaptive nature of these Z ' ymobacterium 

 enzymes has been demonstrated 30 and the possibility that they may not be 

 concerned with normal pyrimidine biosynthetic pathways may be con- 

 sidered. Yates and Pardee 30 have, however, demonstrated the presence of 

 corresponding enzymes in extracts of E. coli B grown on minimal medium, 

 the activities of the isolated enzymes being sufficient to account for their 

 participation in polynucleotide pyrimidine synthesis in the parent cells. 



The pathway of conversion of orotic acid to uridine-5'-phosphate has 

 been clearly defined by the recent work of Lieberman et al. u These authors 

 have described a purified yeast enzyme, orotidine-5'-phosphate pyrophos- 

 phorylase, which effects a condensation of orotic acid with 5-phosphori- 

 bosylpyrophosphate 32 to yield orotidine-5'-phosphate which in turn is con- 

 verted to uridine-o'-phosphate irreversibly by orotidine-S'-phosphate 

 decarboxylase. The occurrence of these enzymes in mammalian liver 33 

 satisfactorily explains the precursor-product relationship between orotic 

 acid and acid-soluble uridine-5'-nucleotides previously noted. 16 ' 33 



The structure of orotidine-5'-phosphate as the 5'-phosphate ester of 3-d- 

 ribofuranosyluracil-4-carboxylic acid is indicated 31 by its apparent ident- 

 ity with the product obtained enzymically by phosphate transfer to orot- 

 idine. 34 The structure of orotidine as 3-D-ribofuranosyluracil-4-carboxylic 

 acid has been substantiated recently on the basis of a spectrophotometry 

 study of N-alkylorotic acid derivatives. 34a 



The yeast orotidine-5'-phosphate pyrophosphorylase is specific to orotic 

 acid, no reaction with DL-carbamylaspartic acid, L-dihydroorotic acid, ura- 

 cil, or cytosine having been observed. No evidence has been adduced for 

 the participation of an open-chain glycoside in the reactions leading to 

 orotidine-5'-phosphate formation. 



It may be noted, parenthetically, that Handschumacher 35 has presented 

 evidence indicating that the growth inhibitory action of as-triazine-3 , 5- 



28 I. Lieberman and A. Kornberg, Biochim. et Biophys. Acta 12, 223 (1953). 



29 H. C. Friedman and B. Vennesland, J. Biol. Chem. 233, 1398 (1958). 



30 R. A. Yates and A. B. Pardee, J. Biol. Chem. 221, 743 (1956). 



31 I. Lieberman, A. Kornberg, and E. S. Simms, J. Biol. Chem. 215, 403 (1955). 



32 I. Lieberman, A. Kornberg, and E. S. Simms, J. Biol. Chem. 215, 389 (1955). 



33 R. B. Hurlbert and P. Reichard, Acta Chem. Scand. 9, 251 (1955). 



3 < A. M. Michelson, W. Drell, and H. K. Mitchell, Proc. Natl. Acad. Sci. U. S. 37, 



396 (1951). 

 34a J. J. Fox, N. Yung, and I. Wempen, Biochim. et Biophys. Acta 23, 295 (1957). 

 35 R. E. Handschumacher, Federation Proc. 17, 236 (1958). 



