549 



Pteridines and Flavines 



higher animals, but are not so active as folic acid for the 

 bacteria ordinarily used in bioassays. 



The functions of folic acid as a B-vitamin have been in- 

 vestigated extensively and are now largely understood. 

 Some of these have been encountered earlier in our discus- 

 sions, but the role of folic acid derivatives in one-carbon 

 metabolism has not been considered as such. 



In its coenzyme form folic acid is attached to a protein 

 apoenzyme, probably at the glutamic acid moiety, and the 

 pteridine ring is reduced. One of these pteroproteins has 

 been crystallized.^ The "active formate" form of the co- 

 enzyme has been shown to be N^'--formyltetrahydrofolic 

 acid,^- ^' ^ and the "active formaldehyde" form probably is 

 N'',N^'^-methylenetetrahydrofolic acid.^' ^- ''• * 



HoN 



/^N^-^N^ 



"Active Formaldehyde" N^ N^"- 

 Methylenetetrahydrofolic Acid 



OH 



\Xj 



CHO 

 CH.— N ^^^— C— I 



HoN 



O COOH 



I 

 NH— CH 



1 

 CH2 



I 

 CH2 



COOH 



"Active Formate" 

 N^^-Formyltetrahydrofoiic Acid 



^ Jesse C. Rabinowitz and W. E. Pricer, Jr., Federation Proc. 17 

 293 (1958). 



- H. M. Rauen and Lothar Jaenicke, Z. physiol. Chem. 293 46 

 (1953). 



3 Lothar Jaenicke, Biochim. et Biophys. Acta 17 588 (1955). 



4H. M. Rauen, Biochem. Z. 328 562 (1957). 



5R. L. Blakley, Biochem. J. 58 448 (1954). 



«Roy L. KisHuk, J. Biol. Chem. 227 805 (1957). 



^ M. J. Osborn and F. M. Huennekens, Biochim.. et Biophys. Acta 

 26 646 (1957). 



^ F. M. Huennekens and M. J. Osborn, Advances in Enzym.ology 21 

 370 (1959). 



