39. ANTIMETABOLITES AND NUCLEIC ACID METABOLISM 509 



while analysis for thymine clearly indicated that these compounds specifi- 

 cally replaced this normal pyrimidine. 425 ' 426 The incorporation of chloro- 

 uracil into the RNA of E. coli 15T^ has also been documented by the iso- 

 lation of 5-chlorouridylic acid; however, only 2 % of the uracil residues were 

 replaced by the chloro derivative. 433 Incorporation of iododeoxyuridine has 

 been demonstrated with Ehrlich ascites cells in vivo, i2i and with L-5178-Y 

 murine lymphoblasts 423 and H.Ep. # 1 cells growing in culture 428 ; in the 

 latter case up to 38% of the thymine of the DNA was replaced by the 

 analog. These results are in contrast to the nonutilization of the 5-iodo 

 derivatives of uracil, uridine, and orotic acid in a number of mammalian 

 systems. 435 Enzymic synthesis of the deoxyribonucleosides of chloro-, 

 bromo-, and iodouracil by thymidine phosphorylase and deoxyribose-1- 

 phosphate can be demonstrated, but the yields were low and the products 

 were not isolated. 436 Although the intermediate steps have not been demon- 

 strated in cell-free systems, bromodeoxyuridine triphosphate serves as a 

 precursor in the enzymic synthesis of a DNA-like polymer by the poly- 

 nucleotide phosphorylase from E. coli B. Under growing cell conditions the 

 incorporation of bromouracil into DNA is paralled by the uptake of phos- 

 phate-P 32 into DNA. 437 However, with resting cell suspensions of E. coli 

 in which little if any DNA synthesis is occurring, there appears to be an 

 exchange of the pyrimidines, thymine and bromouracil, such that the 

 bromouracil content of the DNA can increase from 8 % to 48 % of the nor- 

 mal thymine component. 432 



Bromodeoxyuridine and iododeoxyuridine are not only incorporated into 

 DNA, but also block the incorporation of normal thymine derivatives. At 

 present it is difficult to say whether this is because of their replacing thy- 

 mine residues in the DNA or because they actually inhibit the incorpora- 

 tion of the normal precursors. Evidence obtained with cell suspensions of 

 Gardner's lymphoma and of spleen has indicated that incorporation of 

 formaldehyde-C 14 into acid-soluble thymine nucleotides was not depressed 

 by the analogs, but that subsequent incorporation of the radioactivity into 

 the thymine (but not the purines) of the DNA of these tissues, in vivo or 

 in vitro, was markedly depressed. 419 ' 420 Substantially the same results with 

 respect to DNA thymine have been obtained using Ehrlich cells in vitro 

 and exposing them to iododeoxyuridine. 421 Accordingly, the evidence sug- 

 gests that these compounds block phosphorylation or polymerization of 

 thymidine derivatives and either replace them in what may be a functional 



433 I). B. Dunn, Trans. Faraday Sac 53, 259 (1957). 

 1,1 \Y. H. Prusoff, Federation Proc. 18, 305 (1959). 



435 W. H. Prusoff, W. L. Holmes, and A. D. Welch, Cancer Research 13, 221 (1953). 

 J36 M. Friedkin and D. Roberts, J. Biol. ('hem. 207, 257 (1954). 



437 T. D. Price, P. B. Hudson, H. A. Hinds, R. A. Darmstadt, and S. Zamenhof, 

 Xature 178, 684 (1956). 



