130 PTEROYLGLUTAMIC ACID 



found that it had the same properties as the compound isolated by Stetten 

 and Fox.^- Subsequently it was observed that the synthesis of this amine 

 by E. coli is increased by addition of glycine and to a lesser extent by thre- 

 onine but not by serine. ^^ 



PGA has not been shown to be directly involved in the synthesis of 

 purines in the sulfonamide-inhibited E. coli system.'" Addition of PGA to 

 this system at the points where methionine, purine, or serine synthesis is 

 critical has no effect on the inhibition ratio.'" The inactivity of added 

 PGA, however, does not exclude the possibility that a more highly active 

 metabolic form or conjugate of PGA may be formed from p-aminobenzoic 

 acid which cannot be replaced by PGA itself. Support for such a view comes 

 from the observations of Woolley and Pringle,'^ who fovmd that the amino- 

 imidazolecarboxamide accumulates in the medium when E. coli is grown in 

 the presence of sufficient 4-aminopteroylglutamic acid to inhibit growth 

 slightly. In the presence of larger amounts of 4-aminopteroylglutamic acid 

 (250 7 per milliliter) the growth of E. coli is inhibited in a medium contain- 

 ing purines. Growth can be restored by the addition of thymidine but not 

 by thymine or the desoxyribosides of hypoxanthine or guanine (Franklin 

 et al}^). This evidence favors the view that p-aminobenzoic acid acts in the 

 synthesis of purines and pyrimidines by way of an intermediate synthesis 

 through PGA. This is in accord with the hypothesis of Woods'^ that the 

 primary action of sulfonamides is the inhibition of PGA formation. The 

 only place where PGA has thus far been shown to be directly involved in 

 E. coli metabolism is in the synthesis of thymine where either thymine or 

 PGA affects the inhibition index. However, the role of PGA in purine and 

 thymine synthesis in both L. casei and S. faecalis, the presence of p-amino- 

 benzoic acid in PGA, and the action of -i-aminopteroylglutamic acid in 

 stimulating production of the amine in E. coli certainly invite the view that 

 PGA may be an intermediary in the synthesis of purine by p-aminobenzoic 

 acid. 



Aminoimidazolecarboxamide can replace purine in the nutrition of cer- 

 tain organisms. Shive^ reported that the amine promotes growth of /-. 

 arabinosus in a maimer similar to purines and disappears from the medium. 

 It also replaces hypoxanthine or adenine in stimulating growth of Ophi- 

 astoma multiannulatum if a large inoculum is employed or- if suboptimal 

 concentrations of these purines are added to the medium.'^ 



1" J. M. Ravel, R. E. Eakin, and W. Shive, /. Biol. Chem. 172, 67 (1948). 



16 D. W. Woolley and R. B. Pringle, J. Am. Chem. Soc. 72, 634 (1950). 



16 A. L. Franklin, E. L. R. Stokstad, C. E. Hoffman, M. Belt, and T. H. Jukes. 

 J. Am. Chem. Soc. 71, 3549 (1949). 



1' D. D. Woods, Bull. soc. chim. bioL 30, 730 (1948), cited l)y D. W. Woolley and H. 

 B. Pringle, /. Am. Chem. Soc. 72, 634 (1950). 



'« N. Fries, Physiol. Plantarum 2, 78 (1949), cited by W. Shive, Vikwiins and Hor- 

 mones 9, 75 (1951). 



