XIII. REQUIREMENTS AND FACTORS 581 



acid deficiency on low casein diets but not if the casein level is increased 

 to 25 or26%.i2. ^i 



There is considerable evidence that tryptophan added to the diet as the 

 free amino acid results in considerably greater synthesis of nicotinic acid 

 than when an equivalent amount of tryptophan is added as protein.^-' ^^ 

 This interesting phenomenon might be explained by assuming that trypto- 

 phan presented to the tissues along with a well-balanced mixture of other 

 amino acids is used primarily in association with other amino acids in 

 processes involved in protein synthesis and replacement. However, when 

 tryptophan is presented to the tissues so that it is in excess of the available 

 amount of other amino acids, the excess tryptophan is diverted into the 

 channels which lead to nicotinic acid synthesis. 



b. Other Amino Acids 



Several groups of investigators have shown that proteins lacking or low 

 in tryptophan, amino acid mixtures simulating these proteins, and (certain 

 amino acids, singly or in combinations, have a marked ability to increase 

 nicotinic acid requirements when added to deficient rations in rats-' "■ *•*"" 

 and in chicks."- ^^-^^ Anderson et al.,^^ using chicks on a nicotinic acid 

 deficient diet, have recently shown that amino acids may be divided into 

 two groups, those which cause a growth depression correctable by nicotinic 

 acid and those which cause a growth depression not influenced by the 

 vitamin. 



In each of the experiments cited above, the basal ration to which the 

 various protein and amino acid supplements were added contained a limited 

 amount of tryptophan as well as being deficient in nicotinic acid. In general, 



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