Dynamic study of the release and the utilisation of N-labeled pea globulin peptides by mixed ruminal bacteria in vitro

There is no consensus about the effects of the size of peptides on their extracellular breakdown and their utilisation by rumen bacteria. This study was done to describe these effects for the peptides released during the first steps of the hydrolysis of a plant protein. The fates of five peptide fractions, characterised by their molecular weights (a > 10 000 Da, 5000 < b < 10 000 Da, 2000 < g < 5000 Da, 1000 < d < 2000 and e < 1000 Da) were monitored. These fractions were obtained by hydrolysing N-labelled pea globulins with pronase E and separation by HPLC. The utilisation of each of them as a part of a complex mixture of unlabelled globulin peptides by a goat mixed rumen bacteria inoculum was individually followed for 5 h. The excess N in each of the initially labelled fractions gradually decreased and labelled compounds were found in smaller peptides. Bacteria were labelled with N only after at least 30 min. This delay increased with the length of the incubated labelled peptide. Large peptides (a and b) were hydrolysed most rapidly and extensively. About 80% (SE 1.9) of the excess N coming from fraction a was found in smaller peptides in only 30 min. During the same time, only 45% (SE 3.3) of the excess N provided by fraction g was recovered in smaller fractions. Dipeptidylaminopeptidase type 1 and aminopeptidase activities combined with endopeptidase activities to produce nitrogenous compounds that could be absorbed by bacteria. The monitoring of N enabled us to obtain information on the effect of globulins peptides size on their extracellular degradation. # 1999 Elsevier Science B.V. All rights reserved.