FACTORS INFLUENCING B VITAMIN REQUIREMENTS 287 



equivalent to 50 mg of tryptophan. For chicks, 5 mg of nicotinic acid are 

 equivalent to about 200 mg of DL-tryptophan, 84 and for dogs, 1 gm of 

 L-tryptophan is equivalent to 5.6-10 mg of nicotinic acid. lllb There are 

 considerable individual variations in this regard, but in general the overall 

 efficiency of the conversion seems to be of the order of 1 or 2 per cent. 

 Only L-tryptophan is effective. Fifty to 100 gm of protein per day would 

 be required to meet the niacin requirement of man solely through synthesis 

 from tryptophan on this basis. Since many animals excrete N'-methyl- 

 nicotinamide (F 2 ), it was thought that some might be able to demethylate 

 this product to nicotinamide, but to date only divergent and generally 

 questionable results have been obtained (p. 359). 



Table 14. Nicotinic Acid Activity of Some M etabolically Related Compounds. 



Compound Man Dog Chick Neurospora 



* Active for the rat in increasing F2 excretion and growth. llta 



It has been stated as a generalization that animal tissues contain a 

 preponderance of nicotinamide, and that plant tissues contain a smaller 

 and more variable amount in this form. 112 There is also evidence indicat- 

 ing that cereals contain a nicotinic acid precursor that is liberated only 

 upon alkaline extraction. This precursor is apparently unavailable to dogs 

 or chicks unless liberated by hydrolysis prior to feeding. 113-115 



The recent isolation of dinicotinylornithine from natural sources pro- 



