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



methyl (h) derivative, either /i 10 FH 4 or /i 510 FH 4 (i.e., a compound with a 

 — CH 2 — group linking nitrogens 5 and 10), in which the substituent is at 

 the formaldehyde level of oxidation; (3) a formimino (fi) derivative (/i 5 FH 4 ), 

 in which the substituent at position 5 has the structure — CH=NH, (4) 

 folinic acid (CF), the relatively stable form which may be designated as 

 / 5 FH 4 ; and (5) a phosphorylated derivative, pFH 4 , which is believed to be 

 N 10 -phosphoryl FH 4 . 



Recent articles 12, 16, 18 have summarized much of our present knowledge 

 of the enzymic interconversions of the derivatives of FH 4 and their function 

 in the biosynthesis and degradation of various metabolites. Perhaps the 

 first indications of the involvement of folic acid in the metabolism of formate 

 were obtained (a) in studies of the utilization by F-deficient and repleted 

 rats of formate-C 14 for amino acid synthesis, 39 (6) in the initial demonstra- 

 tion of the biosynthesis from formate-C 14 of labile methyl groups by rat 

 liver slices, 40 and (c) in studies of the incorporation of formate-C 14 into the 

 total nucleoprotein and nucleic acid purines of mice treated with aminop- 

 terin and amethopterin. 41 However, studies of purine metabolism made 

 definitive the participation of folic acid in the activation of formate. As 

 discussed in Chapter 35, recent developments in our knowledge of the bio- 

 synthesis of purines have shown that the conversion of glycinamide ribo- 

 nucleotide to the formyl derivative is dependent upon FH 4 . The introduc- 

 tion of this formate-carbon into what is to become carbon 8 of the 

 hypoxanthine moiety of inosine-5'-phosphate (i.e., inosinic acid), is to be 

 compared to the subsequent addition, by an apparently analogous reaction 

 which also involves /FH 4 , of formate-carbon to the amino group of 4-amino- 

 5-imidazolecarboxamide ribonucleotide, which after a TPXH-dependent 

 ring closure becomes inosinic acid. Possibly the most susceptible to inhibi- 

 tion by amethopterin, of the FH 4 -dependent reactions concerned with the 

 formation of nucleotides, is the conversion of deoxyuridine-5'-phosphate 

 and formaldehyde to thymidylic acid and, therefore, the synthesis of DNA. 

 Early studies with formate-C 14 and deoxyuridine, 42 " 46 which served as the 



39 G. W.E . Plaut, J. J. Betheil, and H. A. Lardy, Abstr. 116th Meeting Am. Chem. 

 Soc, Atlantic City, New Jersey, p. 65C (1949); J. Biol. Chem. 184, 795 (1950). 



40 A. D. Welch and W. Sakami, Federation Proc. 9, 245 (1950) ; W. Sakami and A. D. 

 Welch, J. Biol. Chem. 187, 379 (1950). 



41 H. E. Skipper, J. H. Mitchell, Jr., and L. L. Bennett, Jr., Cancer Research 10, 510 

 (1950). 



42 J. R. Totter, E. Volkin, and C. E. Carter, J. Am. Chem. Soc. 73, 1521 (1951) ; J. R. 

 Totter, ibid. 76, 2196 (1954). 



43 P. Reichard, Acta Chem. Scand. 9, 1275 (1955). 



44 M. Friedkin and D. Roberts, J. Biol. Chem. 220, 653 (1956); M. Friedkin and A. 

 Romberg, in "The Chemical Basis of Heredity" (W. D. McElroy and B. Glass, 

 eds.), p. 609. Johns Hopkins Press, Baltimore, 1957. 



46 R. L. Blakley, Biochim. et Biophys. Acta 24, 224 (1957); Nature 182, 1719 (1958). 

 48 E. A. Phear and D. M. Greenberg, ./. Am. Chem. Soc. 79, 3737 (1957). 



