490 THE BIOCHEMISTRY OF B VITAMINS 



p-Aminobenzoylpolyglutamyl Peptide from Yeast. A conjugate of 

 p-aminobenzoic acid with 10 or 11 L-glutamic acid residues and one un- 

 known amino acid residue presumably acidic in nature has been isolated 

 from yeast. 75 - 76 This conjugate accounts for 20 to 30 per cent of the total 

 amount of p-aminobenzoic acid occurring in yeast. Approximately 400 

 mg of the purified peptide was obtained from 50 kg of dried yeast. As 

 the amino group of p-aminobenzoyl peptide is diazotizable, it appears that 

 the aromatic acid is conjugated by means of the carboxyl group to a 

 peptide chain of 10 or 11 L-glutamic acids with presumably a terminal 

 unidentified amino acid. By analogy with the folic acid group, it seems 

 probable that the conjugate may have y-glutamyl units in the pep- 

 tide chain. The ability of this conjugate to prevent the hydrolysis of a 

 conjugate of folic acid (p. 571) further emphasizes this similarity. 



As indicated in Table 3, the conjugate of p-aminobenzoic acid is rela- 

 tively inert in promoting growth of organisms requiring p-aminobenzoic 

 acid or in preventing the toxicity of sulfonamides. 75 



Folic Acid and Related Compounds. The first conclusive indication of 

 an interrelationship of p-aminobenzoic acid and folic acid was the report 

 of Miller, 77 who indicated that for both normal and resistant strains of 

 Escherichia coli, sulfonamides at concentrations which do not affect 

 growth decrease markedly the production of microbiologically active 

 forms of folic acid. The biosynthesis of biotin is not affected under these 

 conditions. Earlier work had indicated that p-aminobenzoic acid stimu- 

 lated growth and increased approximately threefold the formation of 

 microbiologically active forms of folic acid in mixed culture of bacteria 

 from the fowl intestine. 78 p-Aminobenzoic acid was also reported to en- 

 hance the synthesis by Mycobacterium tuberculosis of factors with ac- 

 tivities of vitamin Bi and B n , which are chick factors replaceable by 

 folic acid. 79 



OH 



h 



N C C— CH 2 — NH— (/ N >— CO— NH— CH— CH 2 — CH 2 — COOH 



H 2 N— C C CH COO 



\ / \ s 



N N 



folic acid 



The elucidation of the structure of folic acid (p. 565) made the relation- 

 ship apparent, since folic acid contains p-aminobenzoic acid in combina- 

 tion with a pteridine group and glutamic acid. Initially the possibility 

 that p-aminobenzoic acid functioned solely in the biosynthesis of folic 

 acid presented itself. This possibility was also indicated by the resistance 

 to sulfonamide-inhibition of organisms requiring folic acid for growth. 80 



