Breakdown of peptides from a soya protein hydrolysate by rumen bacteria. Simultaneous study of enzyme activities and of two physico-chemical parameters : molecular weight and hydrophobicity

Breakdown of soya peptides by an inoculum of ruminal mixed bacteria was studied in vitro. Peptides were hydrolysed at 75% after 5 h. The dipeptidyl-aminopeptidase type 1 (DAP-1), exoaminopeptidase and leucine aminopeptidase (LAP) activities were measured using GlyArg-MNA, Ala-pNA and Leu-pNA as substrates, respectively. While the total proteolytic activity remained stable throughout peptide breakdown, the DAP-1 already present at the beginning of fermentation was multiplied by two. This peptidase activity is a major index for the presence of Prevotella ruminicola. The exoaminopeptidase activity increased during the exponential phase of bacterial growth. A peak of LAP activity was observed after 3 h of incubation. This activity which was associated to the production of lactate after 3 h suggested the Streptococcus bovis was present in the environment. Measuring by high-performance liquid chromatography (HPLC) gel filtration the size of peptides during their breakdown showed that high-molecular-weight peptides (4000–5000) were apparently more rapidly broken down relative to intermediate peptides (2000–3000) and to small assimilable peptides (1000–500). Separation of soya peptides by reverse-phase HPLC showed a disappearance of hydrophilic and hydrophobic peptides. This study showed that there is no preferential breakdown of hydrophilic peptides relative to hydrophobic peptides in vitro.