Proteolytic release of angiotensin converting enzyme inhibitory peptides from food proteins.

The potential of an Aspergillus niger derived prolyl endoproteinase (An-PEP) to produce potent angiotensin converting enzyme (ACE) inhibitory peptides was assessed. Bovine α- and β-casein were purified from acid caseinate using a protocol which exploited the differential solubility of the caseins in the presence of Ca2+ along with ion exchange fast protein liquid chromatography (IEX-FPLC). Sodium caseinate (NaCN), the purified α- and β-casein preparations and porcine skin gelatin were hydrolysed with An-PEP. Aliquots of the hydrolysate reactions were taken at different time points throughout hydrolysis. All the hydrolysates generated yielded potent ACE inhibitory activity. The most potent An-PEP hydrolysate was the 24 h β-casein digest which gave an ACE IC50 value of 16.41 ± 6.06 μg/ml. This IC50 value is among the most potent IC50s reported for food-derived hydrolysates to date. The NaCN An-PEP hydrolysates withdrawn at different time points were subjected to fractionation using a 3 kDa membrane. The subsequent permeates obtained had increased ACE inhibitory activity compared to the crude hydrolysates, with the 3 h hydrolysate possessing the most potent activity (IC50 = 2.9 ± 0.3 μg/ml). Furthermore, 13 synthetic peptides corresponding to regions in both α- and β-casein were assessed for their ACE inhibitory activity. Nine of the peptides produced highly potent or moderately potent IC50 values, the most potent of which was Trp-Ile-Gly-Pro (αs2-casein f193-196; 14.2 ± 2.9 μM). The ACE inhibitory activity of this peptide appears to not have been reported previously. The 1 and 24 h gelatin and the 24 h NaCN hydrolysates were subjected to a two-stage simulated gastrointestinal digestion (SGID) with pepsin and Corolase PP. The ACE inhibitory activity of the 1 h gelatin hydrolysate sigificantly decreased after SGID (P <0.05). However, the ACE inhibitory activity of the 24 h gelatin and NaCN hydrolysates were stable to SGID (P >0.05). The 24 h β-casein, α-casein and gelatin An-PEP hydrolysate were characterised using liquid chromatography mass spectrometry (LC-MS). In all, 29 β-casein peptides were identified in the β-casein 24 h hydrolysate. Preliminary analysis of the α-casein and gelatin digest detected 8 and 3 peptides, respectively. The results obtained confirmed a cleavage preference at the C-terminal side of Pro residues by An-PEP, but also observed cleavage at the C-terminal side of Glu, Ser, Lys, His, Ala and Leu residues. Finally, during an in silico analysis the ability of the substrate docking program AutoDock Vina to predict ACE inhibitory dipeptide sequences was assessed. Furthermore, peptide intestinal stability was assessed using a predictive amino acid clustering model. Selected dipeptides having AutoDock Vina scores ≤ -8.1 and predicted to be ‘stable’ intestinally were characterised using LIGPLOT and for ACE-inhibitory potency. While docking allowed identification of new ACE inhibitory dipeptides, it was concluded that it may not be a fully reliable predictive tool in all cases.