NICOTINIC ACID (NIACIN) 



lactic acid, oxaloacetic acid into malic acid, acetaldehyde into alcohol, 

 a-ketoglutaric acid (and ammonia) into glutamic acid, and diphospho- 

 glyceric acid into phosphoglyceraldehyde. It is possible that the 

 oxidation of dihydro-codehydrogenase I may be brought about in 

 tissues by such coenzyme-linked reactions,^^ e.g. j8-hydroxybutyric 

 acid may be oxidised to acetoacetic acid and the dihydro-coenzyme 

 thus formed may reduce an aldehyde to an alcohol. 



The inter-convertibility of codehydrogenase I and II and dihydro- 

 codehydrogenase I and II is due to the nicotinamide portion of 

 the molecule. Karrer et al."^^ studied a number of nicotinamide 

 derivatives as models for this reaction and found that only those 

 derivatives with a pentavalent nitrogen atom in the ring, e.g. nicotin- 

 amide methiodide, gave dihydro-com pounds with an absorption 

 spectrum comparable with that of the dihydro-coenzymes ; these 

 have two maxima, at 260 and 340 m^u,, whereas the coenzymes them- 

 selves have only one peak at 260 m/>t. On reduction, the nitrogen 

 atom became tervalent. By a comparison of the dihydro-coenzymes 

 with model dihydropyridine derivatives, it was concluded that the 

 former were i : 2-dihydro-derivatives, so that the reduction of the 

 coenzymes can be represented as : 



;CONH 



W. A. Waters ^o suggested that the prosthetic group of the de- 

 hydrogenases provides an initial free radical : 



^NcONH, 



\t 



^N. 



in a reaction chain. 



278 



