Presence of aspartic dipeptides with β peptide bond and/or D-aspartyl residue in rat blood after ingestion of porcine liver protein hydrolysate

Background: Animal experiments have demonstrated that oral administration of a porcine liver protein hydrolysate (LPH) ameliorates alcohol-induced liver toxicity, as well as exercise- and concanavalin A-induced low locomotor activity in mice. The peptides responsible for the beneficial effect have not yet been identified. Recently, presence of food-derived peptides in human blood has been detected post ingestion of other food protein hydrolysates. These peptides are prolyl, hydroxyprolyl, or pyroglutamyl di- and tri-peptides, and resist exopeptidase digestion. Some of these peptides exert significant biological roles in vitro and in vivo, which have been associated with the biological response post ingestion. The objective of the present study was to identify the food-derived peptides in rat blood after ingestion of LPH.Results: In the in vitro exopeptidase digest of LPH, pyroglutamyl, prolyl, hydroxyprolyl, and aspartic dipeptides were identified. The aspartic peptides (Asp-Val, Asp-Ile, Asp-Leu, and Asp-Phe) showed multiple peaks by LC-MS/MS, indicating the presence of isomers. Four isomers with L- and D-aspartyl residues, and α and β peptide bonds were present in each sequence. After administration of LPH, the amounts of unusual aspartic dipeptides with β peptide bond and/or D-aspartyl residue significantly increased in rat plasma, while those of the other usual aspartic peptides did not.Conclusions: Racemization and isomerization of aspartyl residues in peptides occur during the preparation of LPH or following its digestion. The unusual aspartic peptides have a potential for carrying out diverse biological activities.Key words: Peptide, food protein hydrolysate, aspartic, isopeptide, bioavailability, D-amino acid