39. ANTIMETABOLITES AND NUCLEIC ACID METABOLISM 461 



basis of these findings, have now been made much more definitive (see Chap- 

 ter 36). Recently, a transfer of H 3 from substrate amounts of FH 4 3 to thy- 

 midine-5'-phosphate has been shown to occur during its formation from 

 deoxyuridine-5'-phosphate and formaldehyde. 47 It was proposed that "hy- 

 droxymethyl-FH 4 " (presumably as /i 5 - 10 FH 4 ) and deoxyuridylic acid are 

 linked, each through their respective position 5, by the "active methylene" 

 group derived from formaldehyde. Cleavage of the complex to form thy- 

 midylic acid is thought to involve the transfer of one atom of H 3 (from 

 position 6 of the FH 4 3 -derivative) to yield the 5-methyl group, with the 

 separation of 7,8-FH 2 3 . Such a mechanism would require the continued 

 action of dihydrofolic acid reductase for regeneration of FH 4 and this situa- 

 tion would result in the process being exceedingly sensitive to inhibition 

 by the 4-amino analogs of folic acid. 



The importance to nucleic acid biosynthesis in mammalian cells of inter- 

 ference with the formation of FH 4 was convincingly demonstrated when 

 the inhibition of reproduction of sarcoma- 180 cells in culture by amethop- 

 terin was shown to be prevented completely (if glycine and other amino 

 acids were present) by the simultaneous addition of a utilizable purine and 

 thymidine. 48 



Both severe depletion of folic acid by dietary means and the administra- 

 tion of 4-amino analogs of the vitamin cause remarkably variable effects in 

 different organs and tissues, and considerable variations also are seen among 

 various animal species. 14 The rates of formation and degradation of FH 4 and 

 its derivatives probably are quite variable in various tissues and under 

 varying conditions. Also, marked differences exist with respect to the opera- 

 tion in various tissues, under varying circumstances, of the many metabolic 

 reactions which are dependent on coenzymes derived from folic acid. 14 In 

 almost all cases, however, the most profound effects of administration of 

 aminopterin and amethopterin are observed in those tissues in which 

 proliferation is essential and continuous (e.g., the bone marrow and the 

 intestinal mucosa) . In the liver, in which there exists not only an extremely 

 active system for the enzymic reduction of folic acid, but also many of the 

 metabolic reactions which are dependent upon folic acid-derived coenzymes, 

 the uptake of formate-C 14 into purine-containing nucleotides is usually 

 sharply depressed by the administration of the antagonist. In addition, 

 liver homogenates inactivate irreversibly a folic acid-derived, normal con- 

 stituent of that tissue, the enzymic formation of which is blocked by 

 amethopterin. 30 The enzymic inactivation is diphosphopyridine nucleo- 

 tide (DPN) -dependent 30 and is inhibited by xanthopterin, 14 ' 30 ' 49 - 51 as well 

 as by arsenite and tetraethylthiuram disulfide. 49 



47 M. Friedkin, Federation Proc. 18, 230 (1959). 



48 M. T. Hakala, Science 126, 255 (1957). 



49 M. Silverman and J. C. Keresztesy, Federation Proc. 12, 268 (1953). 



