36. BIOSYNTHESIS OF PYRIMIDINE NUCLEOTIDES 339 



ene-bridge derivative — in the "methylation" reaction [cf. the role of iV 10 - 

 formyltetrahydrofolic acid (N 10 — CHO — FH 4 ) in purine biosynthesis, see 

 Greenberg et al., m Buchanan et al., 129 - 13 ° and Chapter 35]. Several authors 

 have reported N— CH 2 OH— FH 4 (or JV 5 , A^ 10 -methylene-FH 4 ) as a product 

 (a) of serine hydroxymethylase action on serine, 131133 (6) of nonenzymic in- 

 teraction 132 ' 134 of HCHO and FH 4 , and (c) of 'W-hydroxymethyltetrahy- 

 drofolic acid" dehydrogenase action 135 ' 136 on N 10 -CHO — FH 4 (or the iV 5 , 

 A/^-methenyl-bridge derivative). The pathway of glycine-2-C 14 incorpora- 

 tion into the thymine methyl group can be inferred from the work of Nakada 

 and Weinhouse 137 to involve an initial transamination to glyoxylate-2-C 14 

 followed by oxidation to formate-C 14 and then by way of N 10 — C 14 HO — FH 4 

 formation and the steps outlined above. The evidence relating to glyoxylate 

 involvement in formate production is based solely on experiments employ- 

 ing the trapping technique. The recent observation by Fleming and Cros- 

 bie 138 of the randomization of activity between glycine-2-C 14 and glyoxylate 

 due to facile nonenzymic transamination thereby renders the proposed de- 

 tails of this pathway suspect. The observation 127 of formate utilization for 

 purine "1-C" positions but not for the thymine methyl group in an organism 

 (E. coli) in which glycine-2-C 14 is utilized for all "1-C" positions also strongly 

 indicates that free formate (and probably N 10 — CHO — FH 4 ) is not involved 

 in the glycine incorporation pathway. Consideration must be given to the 

 possible role of 5-aminolevulinic acid or aminoacetone 138a ' 138b as intermedi- 

 ates between glycine and a compound at the oxidation level of formalde- 

 hyde. 



Experiments designed to illuminate the nature of the "1-C" acceptor 

 molecule in thymine ring synthesis have been described by Reichard. 96 

 A comparison of the utilization of 5-methyluridine-2-C 14 , deoxyuridine-2- 

 C 14 , uridine-2-C 14 , thymine-2-C 14 , and thymidine-2-C 14 for polynucleotide 

 pyrimidine synthesis in regenerating rat liver and intestinal mucosa has 



128 G. R. Greenberg, L. Jaenicke, and M. Silverman, Biochim. et Biophys. Acta 17, 

 589 (1955). 



29 J. G. Flaks, M. J. Erwin, and J. M. Buchanan, J. Biol. Chem. 229, 603 (1957). 



30 L. Warren, J. G. Flaks, and J. M. Buchanan, /. Biol. Chem. 229, 627 (1957). 



31 L. Jaenicke, Biochim. et Biophys. Acta 17, 588 (1955). 



32 L. Jaenicke, Federation Proc. 15, 281 (1956). 



33 J. M. Huennekens, Y. Hatefi, and L. D. Kay, J. Biol. Chem. 224, 435 (1957). 



34 R. L. Blakley, Nature 182, 1719 (1958). 



35 G. R. Greenberg, L. Jaenicke, and M. Silverman, Biochim. et Biophys. Ada 17, 

 589 (1955). 



36 Y. Hatefi, M. J. Osborn, L. D. Kay, and F. M. Huennekens, J. Biol. Chem. 227, 

 637 (1957). 



37 H. I. Nakada and S. Weinhouse, Arch. Biochem. Biophys. 42, 257 (1953). 



38 L. Fleming and G. W. Crosbie, Biochim. et Biophys. Acta (in press). 



38a K. D. Gibson, W. G. Laver, and A. Neuberger, Biochem. J. 70, 71 (1958). 

 38b W. H. Elliot, Biochem. J. 74, 478 (1960). 



