ESMOND E. SNELL 



evidence for a special catalytic role of the stimulatory peptides 

 in metabolism, although the possibility of such a role for other 

 peptides is not, of course, excluded. 



Variability in Nutritional Requirements with Conditions 



Cursory reading of the nutritional literature, especially that 

 dealing with animal nutrition, leaves the impression that the 

 nutritional requirements of a given species for vitamins, etc., 

 is a rather constant thing, that these requirements can be deter- 

 mined and expressed in terms of a quantitative figure for the 

 average daily requirement which has validity quite apart from 

 the conditions used in assessing the requirement. 



From parts of the foregoing discussion it should be clear 

 that this is far from the case in microbial nutrition. Lactobacillus 

 casei grows well in the absence of both D-alanine and L-alanine 

 peptides if pyridoxal is supplied; in the absence of pyridoxal, 

 both D-alanine and such peptides are required, the former be- 

 cause it cannot be synthesized in the absence of vitamin Bg, the 

 latter because D-alanine inhibits utilization of free L-alanine 

 (which also cannot be synthesized in the absence of vitamin Be) 

 but not that of L-alanine peptides. Again, L. casei requires 

 relatively large amounts of biotin if aspartate and unsaturated 

 fatty acids are omitted from the medium, much less if aspartate 

 (but not the fatty acid) is supplied, and none at all if both prod- 

 ucts are supplied. Such sparing effects of one nutrient on the 

 requirement for another frequently indicate important metabolic 

 roles played by the vitamins. In the instances cited above, a 

 role for vitamin Be in the synthesis of d- and L-alanine is indicated, 

 and the necessity for biotin in the synthesis of aspartate and un- 

 saturated fatty acids likewise appears highly probable. Similarly, 

 the necessity for folic acid in the synthesis of the purine and 

 pyrimidine bases and serine early became evident from the fact 

 that in appropriate media these substances greatly reduced or 

 eliminated completely the requirement of L. casei and S. faecalis 

 for folic acid. 



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