Preparation and identification of dipeptidyl peptidase IV inhibitory peptides from quinoa protein

The objective of this study was to investigate the dipeptidyl peptidase IV (DPP-IV) inhibitory properties of quinoa protein-derived peptides. After germination, quinoa protein was extracted and then hydrolyzed by different enzymes such as papain, Alcalase, Neutrase, and Flavourzyme and pepsin-trypsin digestion. Results showed that the quinoa protein hydrolysates (QPH) by pepsin-trypsin digestion displayed the highest DPP-IV inhibitory activity. When ultrafiltrated, the fraction of quinoa protein hydrolysate with molecular weight less than 1 kDa (QPH1) exhibited a superior DPP-IV inhibitory activity with IC₅₀ of 3.40 ± 0.20 mg/mL in vitro and 2.20 ± 0.29 mg/mL in Caco-2 based in situ. Furthermore, the peptide sequences of QPH1 were identified by UPLC-MS/MS. Twenty quinoa-derived peptides were determined with in vitro DPP-IV inhibitory activities with IC₅₀ values less than 500 μM. The peptides IPI, IPV, VAYPL and IPIN showed the highest in vitro DPP-IV inhibitory activities with IC₅₀ of 5.25 ± 0.16, 26.15 ± 0.58, 42.93 ± 1.15, and 56.58 ± 3.36 μM, respectively. The in situ DPP-IV activities of Caco-2 cells were also attenuated by these four peptides with IC₅₀ of 10.75 ± 0.87, 29.11 ± 1.79, 61.9 ± 4.23, and 92.59 ± 12.89 µM, respectively. Moreover, these four peptides were identified as competitive inhibitors of DPP-IV. Molecular docking showed that quinoa peptides IPI and IPV were predicted to form multiple hydrogen bonds, attractive charge, and hydrophobic interactions with the residues of active site of DPP-IV. This study confirms that quinoa protein is a good source for DPP-IV inhibitory peptides and has potential as ingredients in functional foods for the prevention or management of type 2 diabetes.