494 NIACIN 



being removed in the process. In the pig heart lactic dehydrogenase system, 

 desamino DPNH2 is actually more active than DPNH2 for pyruvate reduc- 

 tion but is less active for lactate oxidation. In other dehydrogenase systems 

 desamino DPN has activity sometimes equal to, but generally less than, 

 the parent material.** Whether desamino DPN has any normal function, 

 or even exists in tissues, is not known. Desamino TPN cannot be formed 

 by the same procedures which yield desamino DPN apparently due to the 

 influence of the third phosphate attached to the ribose adenine group. 



3. Nicotinamide Mononucleotide 



It has been shown that nicotinamide mononucleotide (nicotinamide- 

 ribose-5-phosphate) accumulates in red blood cells incubated with nico- 

 tinamide and glucose.*^ Furthermore, certain tissues have been shown to 

 contain a nucleotide pyrophosphatase which splits DPN to yield this com- 

 pound. ^^' ^^ Although this substance cannot function enzymatically as 

 DPN or TPN, it may have considerable significance as an intermediate in 

 the biosynthesis of DPN and TPN (p. 506). 



H. BIOCHEMICAL REACTIONS 



DPN and TPN function in oxidation-reduction systems (although no 

 oxygen is actually transferred). The redox potentials of the systems cata- 

 lyzed by these coenzymes are generally quite similar to the redox potentials 

 of the coenzymes themselves. Thus the reactions are generally reversible, 

 depending on the concentration of the reactants and the products as well 

 as changes in redox potential induced by other reactions in the cell. 



Eakin^^ has classified the important reactions catalyzed by DPN and 

 TPN into five categories. These are listed below, with some of the more 

 important specific systems cited as examples. In addition, a miscellaneous 

 category may be added to include reactions less clearly characterized or 

 which are not included in this classification. Schlenk^^ j^^s carefully de- 

 scribed the most important of the enzyme reaction systems listed below. 



1 . Aldehyde ;=i Primary alcohol 



Acetaldehyde— ethanol (DPN) 



2. Ketone ^^ Secondary alcohol 



Pyruvic acid— lactic acid (DPN) 

 Pyruvic acid — malic acid (TPN) 



88 M. E. Pullman, S. P. Colowick, and N. O. Kaplan. J. Biol. Chem. 194, 593 (1952). 



89 I. G. Leder and P. Handler, ./. Biol. Chem. 189. 889 (1951). 



9» A. Romberg and O. Lindberg, J. Biol. Chein. 176, 665 (1948). 



81 R. E. Eakin in The Biochemistry of the B Vitamins, American Chemical Society 



Monograph No. 110, p. 141. Reinhold Publishing Corp., New York, 1950. 

 '2 F. Schlenk in The Enzymes, Vol. II, Part 1, pp. 278-315. Academic Press, New 



York, 1951. 



