Peptides and Bacterial Growth

Previous papers from this (l-5) and other (6-12) laboratories have established several conditions under which appropriate peptides may surpass their component amino acids in growth-promoting activity for bacteria.(a) Assimilation of a free amino acid, but not that of its peptides, may be inhibited by the presence of an antagonistic amino acid (1, 4-8, 11, 12); (b) a free essential amino acid, but not appropriate peptides of it, may be partially destroyed during growth, thus decreasing the amount of the free amino acid available for protein synthesis (2, 9, 10); and (c), for reasons other than the presence of antagonistic amino acids, absorption of a given essential amino acid may be less efficient than that of its peptides (3, 11, 12); e.g., the efficient absorption of the free amino acid but not of appropriate peptides may require heightened levels of structurally unrelated amino acids (or other nutrients) in the medium (3, 12).In none of these studies has evidence favoring direct incorporation of peptides into protein without prior hydrolysis (or "transpeptidation") appeared.Few studies of the effectiveness of peptides in counteracting inhibitory effects of synthetic antimetabolites of amino acids have been made, and these have yielded conflicting results.In line with results obtained with naturally occurring antagonistic amino acid pairs cited in category (a) above, appropriate peptides of tryptophan, in contrast to the free amino acid, non-competitively prevent the inhibitory effects of 4-methyltryptophan for Streptococcus jaecalis and Staphylococcus aureus.At high concentrations of the inhibitor, therefore, they are much more active than free tryptophan in supporting growth (13).However, the same peptides were no better than free tryptophan in preventing the inhibitory effects of 4methyltryptophan for Escherichiu coli (13).Similarly, peptides of phenylalanine were no more active than phenylalanine in counteracting the inhibitory effects of p-2-thienylalanine for E. coli, and were less active for Xaccharomyces cerevisiae (14).It appeared worth while, therefore, to examine available inhibitors and peptides to clarify this relationship further.