VITAMIN AND GROWTH FACTOR RESEARCH 



acid, a, may be inhibited by a second amino acid, b. Under 

 these circumstances, peptides of a, perhaps because of their 

 greater structural dissimilarity to the inhibitor, may greatly sur- 

 pass the free amino acid in activity. For example, D-alanine 

 inhibits utilization of L-alanine by Lactobacillus casei, but not that 

 of peptides of L-alanine. Such peptides far surpass free L-alanine 

 in growth-promoting activity under these conditions and may 

 become obligatory requirements for growth (28). Many ex- 

 amples of this type of behavior are now known (28). (2) An 

 essential free amino acid may be subject to a degradation reaction 

 that rapidly destroys it before much of it can be used for growth, 

 i.e., protein synthesis. If its peptides are not subject to the 

 degradation reaction, they will greatly surpass the free amino 

 acid in growth-promoting activity. For example, tyrosine 

 peptides are much more active than free tyrosine in promoting 

 growth of S. faecalis under conditions such that free tyrosine can 

 be destroyed by tyrosine decarboxylase, but peptides and 

 free amino acid are equally active when conditions are so ar- 

 ranged as to make this enzyme nonfunctional, e.g., by growth in 

 the absence of vitamin Be (29). {3) For reasons still unknown 

 absorption of one free amino acid may take place with diffi- 

 culty or may require the presence of unusually high concentra- 

 tions of a second amino acid. Under these conditions, peptides 

 of the amino acid may be much more effective than the free 

 amino acid in promoting growth. For example, Lactobacillus 

 delbrueckii 730 requires relatively tremendous quantities o^ histi- 

 dine for growth, but very small amounts of histidine peptides. 

 In this same organism, tyrosine peptides are much more active 

 than free tyrosine in promoting growth when the histidine 

 concentration is low, but equally active when the histidine con- 

 centration is raised (42), and these phenomena cannot be ac- 

 counted for in terms of either mechanism (7) or (2). 



None of the instances cited above need be interpreted as 

 indicating direct utilization of the peptides without prior hydrol- 

 ysis; indeed some evidence suggests that hydrolysis occurs be- 

 fore utilization. In none of these experiments, further, is there 



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