522 NIACIN 



onstrate no nicotinic acid requirement for the rat.^-^ Shourie and Swami- 

 nathan,^ Dann and Kohn,'" and Dann" showed that rats accumulated in 

 their tissues and excreted more nicotinic acid than they ingested, unequiv- 

 ocal evidence of synthesis. Similar findings have been recorded for 

 calves,'2-i4 horses,'^' ^^ sheep, '^' ^^ and man.^^-i 



B. MECHANISM OF BIOSYNTHESIS 



Although these data certainly established the fact that nicotinic acid 

 could be synthesized in vivo, the mechanism of this synthesis was obscure 

 for many years. 



1. The Ornithine Scheme 



Earlier workers suggested that nicotinic acid might be synthesized ac- 

 cording to the scheme shown in Fig. 9. However, the evidence that this 

 mechanism provides any substantial amount of nicotinic acid in mammals 

 is very shaky. Neither proline, glutamic acid, nor ornithine will substitute 

 for nicotinic acid in diets; indeed proline may, along with other amino 

 acids, increase nicotinic acid requirements.- ■ -^ However, Bovarnick^* • ^^ has 



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