lOO NITROGEN METABOLISM ■ 



i.e. the carboxyl moiety of a peptide bond can be transferred 

 to a suitable amino acceptor. Thus incubation of GSH with 

 phenylalanine or tyrosine resulted in the formation of y- 

 glutamylphenylalanine and y-glutamyltyrosine respectively. 

 Evidence that new peptides had been synthesized was first 

 obtained by paper chromatography, and some of these com- 

 pounds have now been isolated and characterized. The term 

 transpeptidation has been applied to such transfer reactions 

 and similar results were later obtained with Pr. vulgaris [37], 

 Hanes et al. noted that prolonged incubation tended to pro- 

 duce complete hydrolysis of all the peptides in the experi- 

 mental system: it is therefore possible that these transpep- 

 tidation reactions are catalysed by the intracellular proteases 

 and indeed various proteases, like several other hydrol)rtic 

 enzymes, are known to be capable of performing transfer 

 reactions (pp. 104-5). 



Utilization of peptides by micro-organisms 



Information concerning the utilization of peptides comes 

 mainly from the response of nutritionally exacting organisms 

 to peptides containing an amino-acid essential for growth. 

 The majority of these studies have been performed with 

 non-proteolytic species and provide evidence that hydro- 

 lysis by extracellular proteases is not an obligatory step in 

 the utilization of simple peptides. Using four mutants of 

 Esch. coli, exacting towards phenylalanine, tyrosine, proline 

 and leucine respectively, Fruton and Simmonds compared 

 growth in the presence of simple dipeptides containing the 

 required amino-acid with that in the presence of the free 

 acid [12]. Peptides of phenylalanine or tyrosine were as 

 eifective as equimolecular amounts of the uncombined acids 

 and there was little or no difference in the growth curves. It 

 was concluded that prior to utilization, these peptides were 

 hydrolysed by intracellular peptidases at a rate which did 

 not limit growth. Lactobacillus arabinosus and Ln. mesen- 

 teroides likewise utilize dipeptides of glutamine as readily 

 as free glutamine or glutamic acid [45]. The leucineless 

 Esch. coli mutant grew at the expense of peptides containing 

 leucine, but although the rate of grov^^h in the log phase and 



