METABOLISM OF THE B VITAMINS 



357 



therefore now possible to account for a fair portion of the nicotinic acid 

 metabolized in the animal body (see p. 365). 



There are at present some eight known derivates of nicotinic acid which 

 are excreted by various animal species. These are: 



-COOH 



Nicotinic acid 



-COOH 



m 



CH 3 

 Trigonellin 

 -CO— NH— CH 2 COOH 



-CONH 2 

 Nicotinamide 



CH 3 



N'-Methylnicotinamide (F 2 ) 



■CO— NH 2 



X' 



Nicotinuric acid 



COOH 

 COOH 



Quinolinic acid 



i 



CH 3 

 N'-Methyl-6-pyridone-3-carboxylamide 



,-CO— NH— (CH 2 ) 3 — CH— NH— CO- 

 k N J ioOH 



Dinicotinylornithine 



In addition to these compounds, evidence has been presented to show 

 that many other nicotinic acid metabolites are formed in the body. 124-126 

 It seems well established that coenzymes I or II are not excreted, and 

 there is little evidence to suggest that riboside derivatives of niacin are 

 found in the excreta, so that these coenzymes are apparently hydrolyzed 

 at the nicotinamide bond as a general procedure in their metabolism 

 (see p. 352). 



Our knowledge of niacin metabolism in various species must be inter- 

 preted carefully, in view of the fact that much earlier work was done 

 with analytical procedures that were not specific or sensitive, and that 

 certain compounds closely related to those measured were not known 

 until recent years. Most species probably excrete at least some nicotinic 

 acid and nicotinamide as such, although there is some conflicting evidence 



