III. PEPTIDES 587 



of dipeptides, thought to be the alanyl and tyrosyl peptides of vahiie, leu- 

 cine, and isoleucine. Also, Sloane and McKee^* have shown that the Slaphij- 

 lococcus albus factor of Hughes^^ is replaceable by an intact plasma protein 

 fraction, the activity of which may be due to special peptide structures, 

 especially those of cysteine. 



Several influences appear to be operative in determining the response of 

 organisms to peptides; the most obvious is the need for a particular unit 

 per se either because of a more rapid transfer into the cell or because of a 

 paucity of appropriate conjugating enzymes to bring about its biosynthesis 

 from the amino acids. Apart from this, it has been shown that in L. casei,^^ 

 when D-alanine was present in the medium it inhibited the utihzation of the 

 L isomer; however, D-alanine had no effect on L-alanine peptides. A similar 

 effect was then suggested for other systems, i.e., an antagonism among cer- 

 tain related amino acids that may not be experienced when peptides are 

 employed instead. The destructive action of tyrosine decarboxylase*'* upon 

 free tyrosine, but not on its peptides, was also noted and offered as an ex- 

 planation of the greater response of S. faecalis to tyrosine peptides. Finally, 

 it should be pointed out that many peptides have been shown to be less 

 active than their constituents.-^' ** These may simply become digested, as- 

 similated, and resynthesized into protein patterns in which the peptide 

 sequences in question may not appear at all. Some peptides (in addition 

 to the antibiotic polypeptides) actually delay or inhibit bacterial growth,*^ 

 perhaps by interference wdth the synthesis of peptides and proteins ^^^thin 

 the cells. 



The role of strepogeniii and other peptides in animal nutrition is doubt- 

 ful. Although Womack and Rose^° were able to produce more rapid weight 

 gains in rats fed intact protein (casein) than in those maintained on nineteen 

 amino acids, these differences have been eliminated by employing acid- 

 hydrolyzed casein supplemented with tryptophan and cystine (Ramasarma 

 et aZ.^0- The casein hydrolyzate was devoid of strepogenin activity. Other 

 workers^- • ^^ have also shown that properly balanced amino acid mixtures 

 supported good growth of mice and that these mixtures were not improved 



85 N. H. Sloane and R. W. McKee, J. Am. Chem. Soc. 74. 9S7 (1952). 



8« T. P. Hughes, ,/. Bacteriol. 23, 437 (19.32). 



*^ J. S. Fruton and S. Simmonds, Cold Spring Harbor Symposia Quant. Biol. 14, 55 



(1949). 

 8« V. Nurmikko and A. I. Virtanen, Acta Chem. Scand. 5, 97 (1951). 

 8' S. Simmonds, J. I. Harris, and J. S. Fruton, J. Biol. Chem. 188, 251 (1951). 

 "o M. Womack and W. C. Rose, /. Biol. Chem. 162, 735 (1946). 

 '^ G. B. Ramasarma, L. M. Henderson, and C. A. Elvehjom, J. A'ulritioti 38, 177 



(1949). 

 32 E. Brand and D. K. Bosshardt, Absir. 114lh Meeting, .im. Chem. Sac. p. 38C (1948). 

 " K. H. Maddy and C. A. Klvohjom, /. Biol. Chem. 177, 577 (1949). 



