Presence of indigestible peptide aggregates of soybean meal in pig ileal digesta residue

With the purpose of analysing the molecular size and composition, proteinaceous material was extracted from the insoluble components of a digesta sample obtained from pigs fed a feed consisting of only soybean meal. Gel permeation chromatography indicated that the alkali-extractable fraction of the proteinaceous material from the residue was of relatively high apparent molecular weight. However, the combined results from gel electrophoresis, RPLC-MS, and MALDI-ToF MS showed that the extracted protein material was in fact, to a high extent, composed of aggregated peptides. To our knowledge this has not previously been described. Aggregates extracted by dilute alkali were fully degraded upon subsequent proteolytic treatment. N-terminal sequencing of selected protein bands from SDS-PAGE gels indicated the presence of partly degraded -conglycinin alpha subunits in the residue