The absorption of glycine and alanine and their peptides by Lactobacillus casei.

A striking aspect of peptide utilization by bacteria auxotrophic for amino acids is provided by those cases in which peptides promote growth to a greater extent than an equivalent amount of the free essential amino acid that they supply. Kihara and Snell (1) recognized three sets of circumstances that lead to thii result. In Case 1 the free amino acid, but not its peptides, is partially destroyed by reactions competitive to protein synthesis. In Streptococcus faecalis, for example, tyrosine and its peptides are equally active in promoting growth under conditions in which tyrosine decarboxylase is inactive. However, when cultural conditions are changed to permit activity of this decarboxylase, tyrosine is partially destroyed by conversion to tyramine and becomes less active than tyrosine peptides in promoting growth (2). In Case 8 the assimilation of an essential free amino acid, but not that of its peptides, is inhibited by the presence of an antagonistic amino acid (3-11). For example, n-alanine prevents utilization of L-alanine but not of its peptides for growth of LacMmcillus casei (4). In Case S the absorption of a given essential amino acid may be less efficient than that of its peptides. Thus, a strain of Lactobadlus d&rueck-li appeared unable to absorb histidine from dilute solutions with the same ease as histidine peptides (7) ; a mutant culture of the same organism absorbed both the free amino acid and its peptides with equal facility. In Cases ,?? and S above, the differences between peptides and amino acids were attributed on indirect but experimental grounds to differences in properties of the systems responsible for their absorption into the cell; this explanation thus assumed both a mechanism for such absorption other than passive diffusion and the existence of separate mechanisms of absorption for free amino acids and for peptides. Subsequent to these experiments, experimental proof of the often postulated importance of permeability in bacterial nutrition has been provided by Rickenberg et al. (12, 13), and by Britten et al. (14) for amino acids and carbohydrates, and by Leach and Snell (15) for peptides. The latter study has been extended here, and has established that L. casei accumulates glycine and alanine from appropriate peptides that contain these amino acids at faster rates and by different pathways than it accumulates either free glycine or free alanine. In accordance with an earlier postulate based on growth data (4) these studies