Uptake of small neutral peptides by mixed rumen microorganisms in vitro

The metabolism of a range of neutral di- and oligopeptides and peptide p-nitroanilides was investigated in strained ruminal fluid diluted with anaerobic buffer. Peptide uptake was assayed by decrease in fluorescamine-reactive material in extracellular fluid, and peptide p-nitroanilide hydrolysis by diazotisation of the released p-nitroaniline. Addition of glucose and dithiothreitol did not alter uptake of di- or trialanine. Calculated Vmax for di- and trialanine uptake were 1.4 and 1.9 nmol min−1 mg DM−1, corresponding Km were 0.30 and 0.14 mmol litre−1. Dipeptide uptake was 0.72 to 0.90 nmol min−1 mg DM−1, except for glycylproline, which was taken up at 0.43 nmol min−1 mg DM−1. There was a wider range for tripeptide uptake (0.5 to 1.6 nmol min−1 mg DM−1), with trialanine being taken up most rapidly. Tetra- and pentaalanine were removed at 0.93 and 0.71 nmol min−1 mg DM−1. Uptake of amino acid residues as alanine oligopeptides was two to three times more rapid than uptake as dialanine. These data suggest that peptides would accumulate in rumen fluid during hydrolysis of rapidly degraded proteins, but peptide uptake would exceed rate of release from more slowly degraded proteins. Rates of hydrolysis of eight peptide p-nitroanilides were 0.56 to 1.53 nmol min−1 mg DM−1, and were of similar magnitude to uptake of di- and tripeptides; proline-containing p-nitroanilides were also more slowly metabolised.