NICOTINIC ACID (NIACIN) 



liberate formaldehyde.^ Alkaline hydrolysis yielded adenosine-di- 

 phosphoric acid,'' indicating the presence of a pyrophosphoric acid 

 group. It was assumed that one of the phosphoric acid groups in 

 cozymase (but not in dihydro-cozymase) was neutralised by the nitrogen 

 atom of the pyridine ring. That ribose is attached directly to the nico- 

 tinamide was established by the isolation of nicotinamide riboside 

 from yeast codehydrogenase I, following hydrolysis with an enzyme 

 preparation made from almond press-cake.® 



Codehydrogenase II on hydrolysis gave i mole of adenine, i mole 

 of nicotinamide, 2 moles of D-ribose and 3 moles of phosphoric acid.^ 

 It differed from codehydrogenase I therefore in the presence of an 

 additional phosphoric acid group. Codehydrogenase II was dibasic. 



Neither coenzyme has been synthesised, but it has been claimed 

 that codehydrogenase I can be converted into cozymase by treatment 

 with phosphorus oxychloride in ether or by enzymic phosphorylation, ^^^ 

 an observation which is hardly consistent with the above structure 

 for triphosphopyridine nucleotide. 



The synthesis of cozymase by red blood cells was enhanced both 

 in vitro and in vivo by the presence of nicotinic acid or nicotinamide, 

 the former being at least three times as effective as the latter ; ^^ the 

 cells were freely permeable to both compounds. The enzymic de- 

 struction of cozymase, which follows haemolysis, was inhibited by 

 nicotinamide, but not by the free acid. Cozymase synthesis by 

 erythrocytes was not merely a reversal of the process of decomposition. 



An aqueous extract of pigeon liver was found to contain a thermo- 

 labile enz5mie system capable of synthesising triphosphopyridine 

 nucleotide from ribose, nicotinamide and adenosine triphosphate.^^ 



Function of Coenzyme I 



It appears probable that the two coenz5niies combine with a 

 variety of protein carriers, the apoenz5mies, each of which is specific 

 for a particular reaction ; they are thus enabled to effect the dehydro- 

 genation of a large number of substrates. Coenzyme I effects the 

 conversion of j8-hydroxybutyric acid into acetoacetic acid,^^ formic 

 acid into carbon dioxide and water,^^ lactic acid into pyruvic acid,^^ 

 malic acid into oxaloacetic acid,^^ alcohol into acetaldehyde,^^ glucose 

 into gluconic acid,^' glutamic acid into a-ketoglutaric acid,^® 

 a-glycerophosphoric acid into phosphoglyceric acid,^^ and phospho- 

 glyceric aldehyde into diphosphoglyceric acid.^^ It also effects the 

 dismutation of aldehyde into alcohol and acid,^^ and the conversion of 

 retinene into vitamin A.^i" It is also said to be involved in the meta- 

 bolism of testosterone by the liver. 2^* 



Codehydrogenase I (coenzyme I or cozymase) occurs in all animal 

 and plant ceUs in which carbohydrates are metabolised. Fresh yeast 



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