NICOTINIC ACID (NIACIN) 



nicotinic acid synthesis, none being formed when vitamin Bg-deficient 

 rats were given tryptophan.^ It is known that pyridoxine-like 

 compounds are involved in amino acid metabolism (page 330). 



Further evidence has been obtained by C. Heidelberger and his 

 colleagues ® from a study of the metabolism of tryptophan containing 

 isotopic carbon, C^^, at various positions in the molecule. When 

 DL-tryptophan-jS-C^* (I) was fed to rabbits, dogs and rats, kynurenine 

 (II) and kynurenic acid (III) containing C^^ were isolated together 

 with nicotinic acid from which C^* was absent. On the other hand, 

 when DL-tryptophan-3-Ci* (IV) was fed to the animals, kynurenine, 

 3-hydroxyanthranilic acid (V) and nicotinic acid, all containing C^^, 

 were isolated from the urine ; in the last two compounds, the C^^ was 

 in the carboxyl groups. These changes can be represented as follows : 



OH 



H 



NH, 



Y' 



(I) 



,CO • CHa • CH . COOH 

 NH2 



(11) 



COOH 



(III) 



COOH 



NH, 



COOH 



U 



H 



Ij ij— CHa • CH . COOH 



H 



.. ^CO • CH2 • CH • COOH 

 -.( Y NH, 



(IV) 



COOH 



NHa 



N. 



COOH 



\/\ 

 OH 

 (V) 



\\hen DL-tryptophan containing C^^ in the carboxyl group was 

 administered to rats,i^ the N^-methylnicotinamide excreted in the 

 urine did not contain C^^. 



That 3-hydroxyanthranilic acid is indeed an intermediate in the 

 conversion of tryptophan into nicotinic acid is confirmed by the fact 



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