134 PTEROYLGLUTAMIC ACID 



3. Resting cell suspensions of Streptohacterium ylaniarum synthesize 

 PGA (measured by L. casei assay) in a system containing only buffer, glu- 

 cose, glutamic acid, and p-aminobenzoic acid.^^ A quantitative relationship 

 exists between the amount of PGA synthesized and the concentration of 

 7>-aminobenzoic acid used. It is also significant that synthesis of PGA 

 begins at that level at which p-aminobenzoic acid becomes limiting for the 

 growth of organism. Synthesis of PGA is inhibited by sulfonamide and 

 reversed in a competitive manner by p-aminobenzoic acid. 



4. The p-aminobenzoic acid requirement (0.1 y per milliliter) of L. 

 arabinosus can be replaced by approximately thirty times as much PGA 

 (Lampen and Jones^^) or by much larger quantities of thymine (50 y per 

 milliliter) in the presence of a purine. The amount of PGA needed is thus 

 approximately 3.0 m7 per milliliter or about five times the requirement of 

 S. faecalis for this vitamin. The ability of a combination of thymine plus a 

 purine to produce maximum growth with L. arabinosus in the absence of 

 PGA parallels thr.t of S. faecalis. In L. arabinosus the function of p-amino- 

 benzoic acid seems solely that of PGA synthesis which in turn mediates 

 the synthesis of thymine plus purine. The low activity of PGA compared 

 to p-aminobenzoic acid suggests that PGA is not the active biological form 

 synthesized by the organism from y>-aminobenzoic acid. Citrovorum factor 

 is no more active than PGA in this system.*^ 



Sulfapyridine inhibits the growth of L. arabinosus and is competitively 

 reversed by p-aminobenzoic acid and non-competitively by both PGA (10 

 my per milliliter) and thymine (30 y per milliliter). ^^ The amount of PGA 

 required to induce growth in the presence of a sulfonamide approximates 

 that which is effective in its absence. Similar sulfonamide-p-aminobenzoic- 

 PGA relationship results are obtained with Streptobacteriurn planiarum and 

 with a strain of L. arabinosus trained to grow in the absence of p-amino- 

 benzoic acid. 



5. Lampen and Jones^^ observed that L. casei and S. faecalis R which 

 require preformed PGA are relatively insensitive to sulfonamides. S. 

 faecalis Raltson, however, which requires PGA for rapid early growth, is 

 sensitive to sulfonamides, and this inhibition is reversed non-competitively 

 by PGA or by thymine. p-Aminobenzoic acid at a level equal to that of 

 PGA has only a small effect on sulfonamide toxicity. The amount of PGA 

 required to reverse the effect of sulfonamide corresponds to that required 

 for growth of S. faecalis R. 



Three pairs of different strains of enterococci (S. faecalis, S. zymogenes, 



32 R. H. Nimmo-Smifh, J. Lasccllcs, and D. D. Woods, Brit. J. Exptl. Pathol. 29, 



264 (1948). 

 " J. O. Lampon and M. J. Jonos, ./. Biol. Chnn. 170, 133 (1947). 

 34 H. P. Sarett, Arch. Biochcm. 34, 378 (1951). 

 56 J. O. Lampen and M. J. Jones, /. Biol. Chem. 166, 435 (1946). 



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