IV. BIOCHEMICAL SYSTEMS 503 



O H H NHo O 



II I I I II 



HO— C— C— C— C C— OH + DPN ^ 



H H H 



H 

 O H H N O 



II I I II II 

 DPNHo + HO— C— C— C— C— C— OH 



H H U 



O H H O O 



II I I II II 

 HO— C— C— C— C— C— OH + NHs 



H H 



vides a means whereby the organism can derive energy from protein by 

 converting the protein to glycogenic fragments^® This reaction probably 

 has considerable biological significance, since glutamic acid is currently 

 believed to constitute a central pool to receive or donate amino groups. 



Indirectly, DPN and TPN are important in protein metabolism when 

 the organism breaks down amino acids to provide energy. The deaminated 

 derivatives of the glycogenic amino acids are further oxidized via conven- 

 tional carbohydrate oxidative pathways as already described. 



Meister^- has recently shown that crystalline lactic dehydrogenase and 

 DPNH2 are capable of reducing a number of a, 7-diketo, and a-keto acids 

 to products as yet unidentified (but not lactic acid). TPNH2 can substitute 

 for DPNHo, but the reaction rate is much slower. This reaction may have 

 biological significance in the utilization of the keto derivatives of amino 

 acids as well as other metabolites. 



7. In Photosynthesis 



The biochemical mechanism of photosynthesis has been a subject of 

 great interest but little information for many years. It is now known that 

 animals as well as plants and bacteria can incorporate atmospheric carbon 

 dioxide into organic compounds. Wood'^^ has listed seventeen compounds 

 in which carbon dioxide can be fixed, although only a few of these provide 

 for primary fixation of carbon dioxide. 



Two recent research findings have emphasized the fact that the pyridine 

 nucleotides may have an important role in photosynthesis. Horecker and 

 Symrniotis'^^ have proved the reversibility of the following reaction: 



Ribulose-S-phosphale + CO-. + TPNH + H+ ^ 6-Phosphogluconate + TPN 



139 H. G. Wood, Physiol. Revs. 26, 198 (1946). 



