514 R. E. HANDSCHUMACHER AND A. D. WELCH 



azauridine was an effective inhibitor 471 ; when tested against the growth of 

 sarcoma-180, or of L-5178-Y lymphoblasts in culture, azauracil was com- 

 pletely inactive, but the growth of the cells was extremely sensitive to in- 

 hibition by ribonucleoside. 475, 48] 



The accumulation of orotidine 478 " 480 in tumors inhibited by azauracil or 

 azauridine and the amassing of orotic acid 472 ' 482 ' 483 and orotidylic acid 472 

 in cultures of E. coli inhibited by azauracil suggested a blockade in the 

 synthesis de novo of pyrimidines, rather than interference with uracil uti- 

 lization. Independently, orotidine and orotic acid were noted in the urine 

 of animals receiving azauracil. 477 Subsequent studies showed that incorpora- 

 tion of orotic acid into nucleic acid pyrimidines was markedly depressed 

 in mice treated with azauridine, while the incorporation of uridine was 

 even greater than in control animals. 480 Such a blockade by azauridine 

 (but not by azauracil) also has been demonstrated in the conversion of 

 orotic acid to uridine nucleotides by soluble extracts from tumors. This 

 system accumulates orotidylic acid in the presence of azauridine, a result 

 which implies inhibition of orotidylic acid decarboxylase. 471, 480 Preparations 

 of this enzyme from either yeast 479 or tumor tissues 480 were inhibited in a 

 competitive manner by either the naturally formed or the synthetic 

 o'-phosphate of azauridine (2 X 10~ 6 M), the only metabolite of azauridine 

 found in mammalian tissues; however, neither azauracil nor azauridine, 

 nor the 5'-di- and triphosphates obtained from bacteria, significantly in- 

 hibited these enzymes. 484 These results offer confirmation of microbial in- 

 hibition studies in which only those organisms which have the ability to 

 utilize exogenous uracil (and hence possess the capacity to form the ribo- 

 nucleotide of azauracil), but ordinarily depend upon the synthesis of pyrimi- 

 dines de novo, displayed sensitivity to the antimetabolite. 462 Further support 

 for the concept that the same enzymes are involved was afforded by the 

 finding that strains of S. faecalis selected for resistance to azauracil were 

 unable to form nucleotides of this analog or to utilize exogenous uracil. 471 

 Although orotidylic acid decarboxylase appears to be the primary site of 

 the action of metabolically formed azauridylic acid, other sites of inhibition 

 have been observed. In adenocarcinoma-755 the incorporation of uracil over 

 a 16-hour period following treatment with azauracil was depressed, but it 

 is difficult to separate this finding from the inhibition of tumor growth dur- 

 ing this period. 468 A similar depression in the uptake of uracil has been ob- 



479 R. E. Handschumacher and C. A. Pasternak, Biochim. et Biophys. Acta 30, 451 

 (1958). 



480 C. A. Pasternak and R. E. Handschumacher, /. Biol. Chem. 234, 2992 (1959). 



481 R. Schindler and A. D. Welch, Science 125, 548 (1957). 



482 J. Skoda and F. Sorm, Biochim. et Biophys. Ada 28, 659 (1958). 



483 J. Skoda and F. Sorm, Collection Czechoslov. ('hem. Communs. 24, 1331 (1959). 



484 R. E. Handschumacher, /. Biol. Chem., in press. 



