39. ANTIMETABOLITES AND NUCLEIC ACID METABOLISM 503 



deoxyribonucleoside is less toxic. The most marked inhibitory effects on 

 tumors in vivo usually (but not always) were obtained with fluorodeoxy- 

 uridine. 383 Bacterial inhibition studies 382 ' 384 indicated a similar order of 

 activity. It has been shown that fluorodeoxyuridine causes a "thymineless" 

 death of E. coli; however, there is a limitation on the duration of action 

 because of cleavage by deoxyribonucleosidases, with the release of fluoro- 

 uracil. In general, the ribonucleoside, though an .inhibitor of growth, is not 

 an active bacteriocidal agent, presumably because both it and fluorouracil 

 not only evoke a thymineless status in the cell, but also limit other aspects 

 of pyrimidine metabolism, thereby preventing an expression of the type 

 of lethal effect which results from a deprivation involving only the pre- 

 cursors of DXA thymine. 382 



With this background information on the biological and biochemical 

 effects of fluorouracil and its derivatives in vivo and in vitro, certain sites 

 have now been established with cell suspensions or soluble enzyme prep- 

 arations. Confirmation of the blockade of formate incorporation into DXA 

 thymine has been obtained with glycolyzing Ehrlich ascites tumor cells 

 in which an inhibition of 50% was produced by 5 X 10 -5 M fluorouracil 

 and 7 X 10 -9 M fluorodeoxyuridine. At these and somewhat higher con- 

 centrations little effect of the fluoropyrimidines on the conversion of uracil 

 or orotic acid to acid-soluble ribonucleotides or the utilization of formate 

 for the biosynthesis of purines was observed. However, inhibition of the 

 incorporation of orotic acid and uracil into the thymine of DXA paralleled 

 that seen with formate incorporation. 374 ' 375 These general results have been 

 confirmed and extended by using cell cultures of a human epithelioma, 

 "H.Ep. # l," 385 and slices of this and other human tumors grown in X-ir- 

 radiated rats. 386 That this does not represent inhibition of the conversion 

 of uridine-5'-phosphate to deoxyuridine-5'-phosphate is apparent from 

 studies with ascites tumor cell suspensions, in which profound blockade 

 of the incorporation of randomly labeled deoxyuridine into DXA thymine 

 was effected by fluorodeoxyuridine. 374 A more precise localization of the 

 site of action of fluorodeoxyuridine has been accomplished with partially 

 purified cell-free extracts of E. coli B infected with T6r + phage. 382 Properly 

 supplemented, these extracts converted deoxyuridylic acid into thymidylic 

 acid, and deoxycytidylic acid into 5-hydroxymethyl deoxycytidylic acid, 

 by tetrahydrofolic acid-dependent condensations with formaldehyde. Both 

 naturally formed and synthetic 387 fluorodeoxyuridylic acid at 2 X 10" 6 .1/, 



3 * A J. M. Scheiner and R. Duschinsky, Federation Proc. 17, 305 (1958). 



385 M. A. Rich, J. L. Bolaffi, J. E. Knoll, L. Cheong, and M. L. Eidinoff, Cancer Re- 

 search 18, 730 (1958). 



386 M. L. Eidinoff, J. E. Knoll, and D. Klein, Arch. Biochem. Biophys. 71, 274 (1957). 

 is7 \Y. ( 1. Farkas, L. C. Iacono, and R. Duschinsky, Abstr., 4th Intern. Congr. Biochem., 



Vienna p. 6 (1958). 



