338 



ANIMAL BIOCHEMISTRY 



Some of these reactions probably consist of more than one step. The 

 nicotinamide is then converted into DPN+ and TPN + . See pages 140 

 and 170 for the structures of these coenzymes, which are the only 

 known forms in which the nicotinamide functions in cells. 



CONH^ 



CONH2 



-N 



,<^ 



CH.,OH 



CH.,OH 



ribose-l-phosphate 



HO HO 



nicotinamide riboside 



TPN' 



ATP 



DPN+ ^-^^ 



CHoOP03= 



HO HO 



nicotinamide 

 mononucleotide 



More is known about the metabolism of nicotinamide than about 

 that of the other vitamins. This substance is methylated before its 

 excretion by man in urine. 



/-^CONH2 



+ 



^N=^ 



Cri3SCH2 



1 



CH2 



1 



CHNHs^ 



1 

 COO- 



iiietliioiiiiio 



/-^GONHa 



GH3 



methyl 

 nicotinamide 



HSCH2 



1 

 GH2 



GHNH3+ 



GOO- 



homocysteine 



Other transformations are known to occur in plants, birds, and dogs. 

 These details may be obtained from reviews covering the biochemistry 

 of nicotinamide and nicotinic acid. 



Function 



As an essential part of both DPN+ and TPN + , nicotinamide partic- 

 ipates in a great variety of important oxidation-reduction processes. 

 No other role is known. The two coenzymes are quite indispensable 

 in all known metabolic systems where they serve as hydrogen-electron 

 carriers in many enzymatic reactions. See particularly pages 172 

 and 222. It will be recalled that the major terminal oxidation of 



