Ruminal microbial metabolism of peptides and amino acids

Peptides and amino acids, either present in the diet or arising from proteolysis in the rumen, are potential nutrients for the growth of ruminal microorganisms but also are liable to be degraded to ammonia and lost from the rumen. In both cases, peptides generally are metabolized by the mixed population more rapidly than amino acids. The predominant mechanism of peptide degradation is biphasic, via dipeptidyl peptidases, which cleave dipeptides from larger peptides followed by the action of dipeptidases. Peptides blocked at the N-terminus, or with glycine or proline as the N-terminal or penultimate N-terminal residue, are more slowly degraded than others. Dipeptidyl peptidase activity occurs only in Prevotella ruminicola among the common ruminal microbial species. In contrast, dipeptidase is present in many species, including P. ruminicola, and is particularly high in ruminal protozoa. Deamination of amino acids is carried out by a combination of numerous low activity bacteria and protozoa and a much smaller number of high activity species. Ammonia production is probably carried out mainly by the low activity species, which again include P. ruminicola, but proliferation of the high activity species may be a problem when certain diets are fed. Excessive ammonia production can be controlled by ionophores, which slow peptide metabolism, suppress growth of the high activity deaminating bacteria and cause the residual ionophore-resistant population to break down amino acids more slowly.