METABOLISM 



human urine consisted of N^-methylnicotinamide and that only i to 

 1*5 % ^^'3.s excreted in the form of the free acid and 3-5 to 4-5 % as 

 nicotinamide. Thus, opinions vary greatly as to the amounts of 

 N^-methylnicotinamide excreted at different levels of nicotinic acid 

 intake. A possible reason for these discordant results was suggested 

 by W. A. Perlzweig and J. W. Huff,^^ who found that when nicotin- 

 amide was administered orally to man, 10 to 20 % was excreted in 

 the form of N^-methylnicotinamide, and that a similar amount was 

 excreted when N^-methylnicotinamide itself was administered orally. 

 When trigonelline was injected, no appreciable conversion into N^- 

 methylnicotinamide occurred, whilst when N^-methylnicotinamide was 

 injected intravenously, 67 % was recovered unchanged in the urine. 

 Perlzweig and Huff therefore suggested that the amount of N^-methyl- 

 nicotinamide excreted is the resultant of at least two processes, one 

 the conversion of nicotinic acid into N^-methylnicotinamide and the 

 other the conversion of N^-methylnicotinamide into other sub- 

 stances. This was confirmed by subsequent work. Thus when rats 

 were fed large amounts of nicotinic acid containing C^^ in the carboxyl 

 group, almost all of the isotopic carbon was recovered in the N^- 

 methylnicotinamide excreted in the urine,^^" although in the mouse 

 about 15 % of a dose of nicotinic acid or amide was lost by decar- 

 boxylation ; for on feeding nicotinic acid containing C^* in the car- 

 boxyl group, or the corresponding amide, 15 % of the C^* appeared as 

 exhaled carbon dioxide. ^^^ In man, the urinary excretion of N^- 

 methyl-6-pyridone-3-carboxylamide accounted for about 50 % of a 

 dose of nicotinamide and, when N^-methylnicotinamide was adminis- 

 tered, 20 % was excreted in the urine unchanged and 19 % as the 

 pyridone.2'^ Relatively less pyridone and more N^-methylnicotin- 

 amide was excreted by infants than by adults. ^'^ 



The most complete data on the urinary excretion of nicotinic acid 

 and its metabolites are those of P. Ellinger and M. M. Abdel Kader.^s 

 They found, contrary to the claims made by many earlier workers, 

 that no trigonelline or nicotinuric acid was excreted by man or by the 

 rat, cat, guinea-pig or rabbit on a normal diet. The following values 

 were obtained for the excretion of other metabolites (mg. per day) : 



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