558 NIACIN 



urinary nicotinic acid excretion products and that it reflects the state of 

 nicotinic acid nourishment to some extent. Fi has never been identified 

 nor has it been studied in recent years. 



c. Nicotinic Acid and Its Derivatives in Urine 



It is now known that there are a number of forms in which nicotinic 

 acid is excreted (Fig. 11) and that different species vary markedly as to 

 which derivatives predominate in their urine (Tables IX and X). In man, 

 the principal derivatives are N^-methylnicotinamide and the 6-pyridone 

 of N^-methylnicotinamide. The dog, however, excretes none of the latter 



r^'cf r-r^cf r^?:^ ry-^"" 



^N^ ^N^ ^Nf O^N^ 



I I 



CH3 CH3 



Niacin -...,„^ Niacinamide N'-Methyl- 6-Pyridone of 



nicotinamide N^-Mcthyl- 



nicotinamide 



NH-CH2-COOH 



Nicotinuric acid 



^N' 



2,5-Dinicotinylornithine 

 (birds only) 



Fig. 11. Derivatives of nicotinic acid found in urine. The symbol (O) refers to 

 steps which have been proved by means of isotope tracer techniques. 



compound. There have been innumerable studies of the urinary excretion 

 of one or more of these nicotinic acid derivatives in relation to nicotinic 

 acid deficiency or nicotinic acid intake. Some of the early studies were 

 complicated, intentionally or unintentionally, by the fact that the assay 

 methods measured trigonelline as well as nicotinic acid derivatives. It is 

 now known that trigonelline is not a product of nicotinic acid metabolism 

 in mammals. 



In man, dog, and rat, the most studied species, it seems clear that urinary 

 N^-methylnicotinamide and the 6-pyridone of this compound (in man) re- 

 flect the amount of nicotinic acid available to the host. It also seems clear, 

 especially from the recent carefully controlled studies of Goldsmith and 

 her associates,^^ that the finding of low or even zero amounts of these 

 nicotinic acid metabolites in urine does not necessarily mean that the 

 patient is actually suffering from pellagra. This is true, since the excretion 

 of these substances drops rapidly to a low and constant level soon after 



