500 NIACIN 



yield acetyl coenzyme A. DPN is known to be reduced in the processes 

 which lead to the formation of acetyl coenzyme A, although the exact bio- 

 chemical mechanism is not known. The acetyl groups transferred by co- 

 enzyrne A can be converted to acetic acid or commonly oxidized to carbon 

 dioxide and water via the tricarboxylic acid cycle.^-^ ■ ^-* In this complicated 

 series of reactions DPN is required for the interconversion of malic and 

 oxalacetic acids as described previously, and TPN is required for the con- 

 version of isocitric to oxalosuccinic acid. 



4. In Pentose Biosynthesis 



Because of the importance of ribose (and desoxyribose) in nucleotide 

 metabolism, the biological origin of this substance has considerable interest. 

 The exact mechanism of its synthesis has been obscure until recently. A 

 series of reactions requiring TPN as a coenzyme has now been demonstrated 

 which can account for the biosynthesis of pentose. The initial reaction: 



Glucose-6-phosphate + TPN ^ TPNH2 + 6-phosphogluconic acid 



was known from the early work of Warburg and associates.^ ■ * It was also 

 known from studies by Lipmann^" and by Warburg and Christian^-"- ^^^ 

 that yeast extracts were capable of further oxidizing 6-phosphogluconic 

 acid in TPN-linked reactions. Dickens^^s, 129 identified a pentose in similar 

 reaction mixtures and postulated the formation of ribose-5-phosphate. 



23 H. A. Krebs, Advances in Enzymol. 3, 247 (1943). 



2* H. A. Lardy and C. A. Elvehjem, Ann. Rev. Biochem. 14, 1 (1945). 



25 F. Lipmann, Nature 138, 588 (1936). 



26 O. Warburg and W. Christian, Biochem. Z. 287, 440 1936). 



27 O. Warburg and W. Christian, Biochem. Z. 292, 287 (1937). 



28 F. Dickens, Biochem. J. 32, 1626, 1636 (1938). 



29 F. Dickens and G. E. Clock, Nature 166, 33 (1950). 



