198 PTEROYLGLUTAMIC ACID 



with Ak4 leukemia. The toxic effect of 4-amino PGA for rats/^^ the chick 

 embryo/-® Drosophila,™ and human beings^^^ has also been demonstrated 

 to be effectively blocked by CF. It is apparent, then, that 4-amino PGA 

 may more correctly be termed a CF antagonist than an antagonist of PGA. 



a. Enzymatic Conversion of PGA to Citrovorum Factor 



From evidence so far published it appears that, before PGA can carry 

 out its catalytic functions, the organisms must first convert PGA to CF, 

 and that CF either represents the catalytically active form of PGA or is 

 more closely related to this form than PGA itself. Nichol and Welch^"^ 

 have presented evidence for the existence in liver of an enzyme (s) that is 

 involved in the conversion of PGA to CF; of particular interest was their 

 finding that a greater amount of CF was formed when ascorbic acid was 

 present. Apparently ascorbic acid brings about favorable reducing condi- 

 tions essential for the conversion. These workers also found that 4-amino 

 PGA was markedly effective in blocking the conversion of PGA to CF by 

 liver slices ;^^^ thus it is apparent that 4-amino PGA has two sites of action 

 in the cell: (1) to block the formation of CF and (2) to compete with CF 

 for essential reactions in the cell. 



The knowledge that CF contains a formyl group is of considerable in- 

 terest from a biochemical standpoint in that it provides a structural basis 

 for the concept that CF is concerned with the transfer of the 1-carbon unit 

 in metabolic reactions. If CF functions by shuttling a formyl group from 

 one substrate to another, it might be predicted that tetrahydro PGA 

 might also have activity comparable to CF. Indeed such has been found 

 to be the case,^^^ for it has been reported that tetrahydro PGA has about 

 2.5 % of the activity of leucovorin for Le. citrovorum and about one-third 

 the activity of leucovorin in counteracting 4-amino PGA toxicity in the 

 mouse. However, Elwyn et aZ.^^" found that the jS-hydrogen atoms of serine 

 accompanied the j8-carbon atom during the conversion to the methyl 

 groups of choline and thymine. Such a finding precludes the oxidation of 

 the /3-carbon to the formate oxidation level unless the /3-hydrogen atoms 

 are held in escrow by the citrovorum factor and returned to the jS-carbon 

 atom when the formyl group of CF is reduced back to the methyl group 

 to be incorporated into choline or thymine. 



'25 C. A. Nichol and A. D. Welch, Proc. Soc. Exptl. Biol. Med. 74, 403 (1950). 



126 W. W. Cravens and E. E. Snell, Proc. Soc. Exptl. Biol. Med. 75, 43 (1950). 



1" E. D. Goldsmith and M. H. Ilarnly, Cancer Research 11, 251 (1951). 



128 E. B. Schoenbach, E. M. Greenspan, and J. Colsky, J. Am. Med. Assoc. 144, 1558 



(1950). 

 '29 H. P. Hroquist, M. J. Fahrenl)ach, J. A. Brockniati, Jr., E. E. T{. Stokstaii, and 



T. H. Jukos, J. Am. Chem. Soc. 73, 3535 (1951). 

 '3« D. Elwyn, A. Weissbach, and D. B. Sprinson, ./. Am. Chem. Soc. 73, 5509 (1951). 



