Dynamic study of the release and the utilisation of N-15-labeled pea globulin peptides by mixed ruminal bacteria in vitro

There is no consensus about the effects of the size of peptides on their ex tracellular breakdown and their utilisation by rumen bacteria. This study w as done to describe these effects for the peptides released during the firs t steps of the hydrolysis of a plant protein. The fates of five peptide fractions, characterised by their molecular weigh ts (alpha > 10 000 Da, 5000 < beta < 10 000 Da, 2000 < gamma < 5000 Da, 100 0 < delta < 2000 and epsilon < 1000 Da) were monitored. These fractions wer e obtained by hydrolysing N-15-labelled pea globulins with pronase E and se paration by HPLC. The utilisation of each of them as a part of a complex mi xture of unlabelled globulin peptides by a goat mixed rumen bacteria inocul um was individually followed for 5 h. The excess N-15 in each of the initia lly labelled fractions gradually decreased and labelled compounds were foun d in smaller peptides. Bacteria were labelled with N-15 only after at least 30 min. This delay increased with the length of the incubated labelled pep tide. Large peptides (alpha and beta) were hydrolysed most rapidly and exte nsively. About 80% (SE 1.9) of the excess N-15 coming from fraction a was f ound in smaller peptides in only 30 min. During the same time, only 45% (SE 3.3) of the excess N-15 provided by fraction gamma was recovered in smalle r fractions. Dipeptidyl-aminopeptidase type 1 and aminopeptidase activities combined with endopeptidase activities to produce nitrogenous compounds th at could be absorbed by bacteria. The monitoring of N-15 enabled us to obta in information on the effect of globulins peptides size on their extracellu lar degradation. (C) 1999 Elsevier Science B.V. All rights reserved.