28 P-AMINOBENZOIC ACID 



bound PABA. The conjugated form was thought to be a peptide involving 

 the arylamino group, since diazotization was found to occur only after 

 autolysis. 



A second form of bound PABA was discovered by Ratner and her col- 

 laborators.'"' " This conjugate is believed to consist of one PABA moiety 

 associated with ten or eleven glutamic acid residues and one other acidic 

 amino acid. In contrast to the complex reported by Loomis^ and by Blan- 

 chard/ the PABA in the peptide described by Ratner is linked to the rest 

 of the molecule through the carboxyl group. The amino group is free, as 

 shown by its positive response to the Bratton-Marshall test. 



In recent years PABA has been shown to occur as a component of sev- 

 eral biologically important entities. These include the group of factors that 

 comprise the fohc acid family, namely, folic acid (the liver L. casei factor), '^ 

 pteroyl-7-glutamyl-7-glutamylglutamic acid (the fermentation L. casei fac- 

 tor), '^''^ pteroylhexagiutamylglutamic acid (Be conjugate),'^ rhizopterin 

 (the S. faecalis R factor),''^ and folinic acid (the L. citrovorum factor).'^ 



COOH 



I 

 CH2 



O CH2 



11 I 

 — CNHCHCOOH 



Folic acid 



COOH 



1 

 CH2 



0/ COOH \ CH2 



OH X=X 'I I /I 



I CHoNH— 4 >— CVNHCHCH.CHoCO/.NHCHCOOH 



^\/^Y — 



H2N 



Fermentation L. casei factor 



1" S. Ratner, M. Blanchard, A. D. Coburn, and D. E. Green, /. Biol. Chem. 155, 689 



(1944). 

 " S. Ratner, M. Blanchard, and D. E. Green, /. Biol. Chem. 164, 691 (1946). 



