616 THE BIOCHEMISTRY OF B VITAMINS 



exert an inhibition either directly or indirectly on nicotinic acid synthesis 

 or functioning in animals. This type of action is probably more wide- 

 spread than has been generally recognized. 



The synthesis of nicotinamide from ornithine or ammonium lactate by 

 Escherichia coli or by mixed cultures of rat caecum contents is inhibited 

 by either 2-,4-,5-, or 7-methyltryptophan. 97 The involvement of ornithine 

 and 8-amino-n-valeric acid in the biosynthesis of guvacin (1,2,5,6- 

 tetrahydronicotinic acid) and nicotinic acid had been suggested before 

 the interrelationship of tryptophan and nicotinic acid was discovered. 98 

 The effect of these substances on nicotinic acid synthesis and its relation- 

 ship to the tryptophan process is still obscure. 



Analogues of 3-Hydroxyanthranilic Acid. A study of the requirements 

 of a number of mutant strains of Neurospora revealed that one strain 

 could utilize either tryptophan, kynurenine, 3-hydroxyanthranilic acid, or 

 nicotinic acid for growth. 99 Another strain could utilize only hydroxy an - 

 thranilic acid or nicotinic acid, 100, 101 and still another required nicotinic 

 acid and could not utilize hydroxyanthranilic acid, which accumulated in 

 the medium. 100 - 101 These results indicate that kynurenine and hydroxyan- 

 thranilic acid represent successive steps in the conversion of tryptophan 

 to nicotinic acid or nicotinamide. 



It has been reported that 3-methoxyanthranilic acid is significantly 

 inhibitory to the conversion of tryptophan to nicotinic acid by Neuro- 

 spora. 102 Since methylation of 3-hydroxyanthranilic acid may occur in 

 nature, and since methyl-2-methylamino-3-methoxybenzoate has been 

 isolated from the seeds of two species of Nigella, these or similar naturally 

 occurring substances as dietary constituents may affect the transformation 

 of tryptophan to nicotinic acid. 



Inhibitions Involving Coenzyme I. The functioning of coenzyme I in 

 glucose dehydrogenase and lactic acid dehydrogenase is prevented compet- 

 itively by 3-pyridinesulfonic acid. 103 The ratios of inhibitor to coenzyme 

 for half-maximum inhibition are approximately 730 and 780, respectively. 

 Nicotinic acid and nicotinamide also effectively inhibit these enzymes, 

 but nicotinamide methiodide is inactive. 3-Pyridinesulfonamide or its 

 methiodide is somewhat less active than the corresponding sulfonic acid. 

 Although a number of other compounds were inhibitory for these enzymes, 

 the inhibitory effects of many of the substances, e.g., salicylic acid and 

 adenosine, were not prevented by coenzyme I. 



Salicylic acid prevents the function of coenzyme I in glucose fermenta- 

 tion by a zymase preparation (yeast). 104 The inhibition is prevented in 

 a competitive manner by coenzyme I over a range of concentrations. The 

 ratio of concentration of salicylic acid to coenzyme I necessary for half- 

 maximum inhibition of the fermentation is 662-692. At high concentra- 



