Further Metabolism of Nicotinic Acid

In the greater part of the preceding discussion nicotinic acid has, for brevity, been treated as the end product of tryptophan metabolism. In fact the end products consist of several metabolites of nicotinic acid, which can 

Diagram 22. Further metabolism of nicotinic acid in animals. 

Nicotinic acid can be amidated fairly readily in mammals, but hydrolysis of the amide occurs to a lesser extent. In microorganisms (e.g., 655) and insects (e.g., 843) deamidation appears to occur more readily. Nicotinic acid is also probably more easily broken down by microorganisms, this degradation occurring via 6-hydroxynicotinic acid (420). 

Nicotinic acid is also excreted as the glucuronide by the rat (249a). 

The conversion of N-methylnicotinamide to its 6-pyridone has been studied enzymically (429). The isomeric 2-pyridone may well be formed in small amount (542), but most methods for determination do not distinguish between the isomers (e.g., 397). The 6-pyridone is not hydrolyzed to the corresponding acid (398 but see 1012). The enzyme of rat liver converting nicotinic acid to nicotinuric acid occurs in the mitochondria (454). Methylation of nicotinamide is carried out by a soluble enzyme, nicotinamide methylkinase, which has been studied by Cantoni (135, 136). The methyl group is derived from methionine. 

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