PEPTIDES AND PROTEINS lOI 



the total amount of growth were equivalent to those on un- 

 combined leucine, the length of the lag period was increased 

 in proportion to the concentration of the peptide in the 

 medium. This effect was not due to time being required for 

 the formation of an adaptive enzyme. In experiments with 

 Lactobacillus delbruckii, Lb. casei and Strep, faecalis, other 

 workers have observed that the utilization of di- and tri- 

 peptides serving as sources of valine or leucine was affected 

 by the position of the amino-acid in the peptide, the nature 

 of adjacent amino-acids, and in some instances the composi- 

 tion of the medium [i, 23]. Unlike the examples described 

 so far, the total amount of growth of the proline requiring 

 Esch. coll mutant was greater when the amino-acid was sup- 

 plied in the form of a dipeptide, yet the rate of growth was 

 not affected. A possible explanation is that enzymes which 

 decompose amino-acids are unable to attack those bound in 

 peptides, consequently if as a result of peptidase activity an 

 amino-acid gradually becomes available over a period of 

 time, a greater proportion will be used in anabolic systems 

 than if it is all initially present in the free state. Two other 

 observations support such a conclusion. With Strep, faecalis , 

 an organism with an active arginine dihydrolase system 

 (p. 26), the same amount of growth was produced by appre- 

 ciably less arginine when it was supplied in the form of small 

 peptides. Similarly if the organism developed an active tyro- 

 sine decarboxylase, dipeptides of tyrosine evoked greater 

 growth than an equivalent amount of free tyrosine [20]. In 

 a medium containing D-alanine in place of pyridoxin. Lb. 

 casei becomes exacting towards dipeptides containing L- 

 alanine, because the D-isomer inhibits the normal utilization 

 of uncombined L-alanine, another essential nutrilite. High 

 concentrations of glycine prevent the utilization of D-alanine, 

 but in the presence of pyridoxin, neither glycine nor D-ala- 

 nine is inhibitory and there is no requirement for L-alanyl 

 peptides [21]. 



Unidentified growth factors believed to be peptides 



The growth of several nutritionally exacting bacteria 

 appears to be dependent on, or is stimulated by, unidentified 



