200 



THE BIOCHEMISTRY OF B VITAMINS 



OH 



-I 



H— C 



O 



II 

 H C H 



\/ \ I 



-N C— N 



\ 



O 



H C H 



\/ \ I 

 N C— N 



CH 



C— N 



V 



CH + H 2 



formic 4(5)-arafno-5(4)- 



acid imidazolcarboxamide 



H— C C— N 



N 



hypoxanthine 



The possible involvement of folic acid in the metabolism of formic 

 acid derivatives was indicated a few months later when the structure of 

 the factor which could be substituted for folic acid in culturing Strepto- 

 coccus faecalis R. (SLR factor) was shown to be formylpteroic acid 

 (I). 292 The glutamic acid peptide of this substance, formylfolic acid (II), 

 was synthesized and shown to be more active than folic acid itself in 

 reversing inhibitors structurally related to folic acid. 293 



OH 

 I H 



C N 0=C . . O 



/ \ / \ I /TA II 



N C C— CH 2 — N— f J— C— OH 



H 2 N— C C CH 



OH 



I 

 C N 



formylpteroic acid (I) 



H 

 0=C 



^C— CH 2 — N— f 7— O— NH— CH— (CH 2 ) 2 — COOH 



1 II I 



H 2 N— C C CH 



VV 





^OOH 



formyl folic acid (II) 



Since p-aminobenzoic acid derivatives occur in both a formylated and 

 unformylated state, and since both states are biologically active, it is 

 tempting to postulate that the coenzymes of this vitamin catalyze the 

 reactions of single carbon units by serving as their carriers in the same 

 way that the nicotinic acid coenzymes are considered to be hydrogen 

 carriers. As yet, however, no direct evidence has been offered to prove 

 or disprove the hypothesis that the coenzymes mediating the single carbon 



