504 NIACIN 



Since 6-phosphogluconate can be converted to glucose-6-phosphate in 

 a TPN-linked reaction (p. 500), it is evident that this could provide a 

 mechanism for the incorporation of carbon dioxide directly into glucose. 



An even more pertinent development was the demonstration by Ochoa 

 et al.^~^ of the following TPN-linked reaction catalyzed by "mahc" enzyme: 



CO,, + pyruvate + TPNHo =^ L-Malate + TPN 



When this reaction was coupled with a glucose-6-phosphate-TPN system 

 to provide a steady supply of TPNH2, a highly efficient means of fixing 

 carbon dioxide resulted. ^^^ A similar reaction occurs in the formation of 

 isocitric acid.''^ 



Mn++ 



CO2 + a-ketoglutarate + TPNH2 ^ — D-lsocitrate + TPN 



Furthermore, evidence has been obtained that green grana from spinach 

 chloroplasts can effect a reduction of DPN and TPN, presumably by utiliz- 

 ing the energy of light to transfer hydrogens from water to the coenzymes, 

 thus supplying the reduced coenzymes needed for the above reactions. ^*^"^^' 

 Carbon dioxide fixation forming malate or isocitrate was shown to proceed 

 in these green grana suspensions when illuminated, but not in the dark. 

 Ochoa and Vishniac^'*" also demonstrated that the DPNH2 generated in 

 the green grana could be utilized to catalyze other DPN linked reactions 

 such as pyruvate ^ lactate, oxalacetate :^ malate, and a-ketoglutarate :^ 

 L-glutamate. It should also be noted that DPNH2 and TPNHo produced 

 by the action of light could be used to generate high phosphate bond energy, 

 i.e., convert light energy to chemical energy (p. 496). Tolmach""* and 

 Arnon^^^ have confirmed and extended much of the work reported above 

 and have provided additional evidence for the view that photosynthesis in 

 plants may operate basically through a reversal of a respiratory cycle 

 which is powered by radiant energy rather than chemical energy. ^^^ The 

 light-induced reduction of the pyridine nucleotides may be a key step in 

 these processes. 



J. BIOLOGICAL SYNTHESIS OF DPN AND TPN 



It is now possible to account rather completely for the biological synthe- 

 sis of these coenzymes. Reactions other than, or in addition to, the ones 



"0 S. Ochoa and W. Vishniac, Science 115, 297 (1952). 



1" W. Vishniac, Federation Proc. 10, 265 (1951). 



i« W. Vishniac and S. Ochoa, Nature 167, 768 (1951). 



i« W. Vishniac and S. Ochoa, /. Biol. Chem. 195, 75 (1952). 



"4 L. J. Tolmach, Nature 167, 946 (1951). 



"5 D. I. Arnon, Nature 167, 1008 (1951). 



i« M. Calvin, J. A. Bassham, and A. A. Benson, Federation Proc. 9, 524 (1950). 



