Effect of the Plastein Reaction in Presence of Extrinsic Amino Acids on the Protective Activity of Casein Hydrolysate against Ethanol-Induced Damage in HHL-5 Cells

Bioactive peptides as a novel strategy to prevent alcoholic liver injury

Alcohol intake has become one of the leading risks to human health and wellness, among which acute and/or chronic alcohol-induced liver injury is a leading threaten, with few therapeutic options other than abstinence. In recent years, studies suggested that certain bioactive peptides from food sources could represent natural and safe alternatives for the prevention of alcoholic liver injury. Hence, this chapter focus on the advanced research on bioactive peptides exerting hepatoprotective activity against alcoholic liver injury. The main sources of protein, strategies for the preparation of hepatoprotective hydrolysates and peptides, underlying mechanisms of peptides on hepatoprotection, and possible structure-activity relationship between peptides and hepatoprotective activity were summarized and discussed, aiming to give a systematic insight into the research progress of hepatoprotective peptides. However, more efforts would be needed to give a clearer insight into the underlying mechanisms and structure-activity relationship before using hepatoprotective peptides as functional food ingredients or dietary supplements.

Feasibility of using a realistic food bolus for semi-dynamic in vitro gastric digestion of hard cheese with pH-stat monitoring of protein hydrolysis

Oral processing of solid foods leads to boluses made of a human saliva and particles distributed in the size range ∼ 0 to 5 mm. However, studies on the release of nutrients from realistic solid food boluses during digestion are scarce because such mechanisms are difficult to investigate in vivo, and in vitro experiments generally recommend to extensively mince solid foods during the oral stage. Similarly, it has previously been shown that the peptic hydrolysis of protein solutions during in vitro gastric digestion can be monitored by acid titration in both static and dynamic pH conditions, but such approach has never been evaluated in the presence of particles of several millimetres in size. The first objective of the study was therefore to test the feasibility of using a realistic food bolus for gastric digestion studies with a pH-stat monitoring of proteolysis, using Emmental cheese as a solid food and with consideration of gastric acidifying kinetics. Degree of hydrolysis (DH) of proteins was monitored from two series of experiments performed in the presence and absence of pepsin. Other DH measurements, estimated from an independent approach based on the amount of free NH₂ groups (OPA method) contained by peptides released in the supernatant (UV absorbance) validated the pH-stat results. A second objective of this work was to test the possible influence of human saliva on gastric proteolysis (in comparison with a water-based bolus). Results showed that saliva slightly delayed initiation of proteolysis, which could be explained by the slightly higher initial pH of the saliva-based bolus, but had no statistical effects on pepsin activity. We conclude that acid titration with a pH-stat system can be a valuable approach to monitor the gastric in vitro proteolysis of realistic solid food boluses in dynamic pH conditions.

Impact of the Plastein Reaction of Casein Hydrolysates in the Presence of Exogenous Amino Acids on Their Anti-Inflammatory Effect in the Lipopolysaccharide-Stimulated Macrophages

In this study, papain-generated casein hydrolysates (CH) with a degree of hydrolysis of 13.7% were subjected to a papain-mediated plastein reaction in the absence or presence of one of the exogenous amino acids-Gly, Pro, and Hyp-to prepare four plastein modifiers, or mixed with one of three amino acids to prepare three mixtures. The assay results confirmed that the reaction reduced free NH2 for the modifiers and caused amino acid incorporation and peptide condensation. When RAW264.7 macrophages were exposed to the CH, modifiers, and mixtures, these samples promoted macrophage growth and phagocytosis in a dose-dependent manner. In addition, the CH shared similar activity in the cells as the mixtures, while the modifiers (especially the PCH-Hyp prepared with Hyp addition) exerted higher potential than CH, the mixtures, and PCH (the modifier prepared without amino acid addition). The plastein reaction thus enhanced CH bioactivity in the cells. When RAW264.7 macrophages were stimulated with lipopolysaccharide (LPS), the inflammatory cells produced more lactate dehydrogenase (LDH) release and reactive oxygen species (ROS) formation, and caused more four inflammatory mediators (NO, PGE2, TNF-α, and IL-6) and two anti-inflammatory mediators (TGF-β1 and IL-10). However, the PCH-Hyp, PCH, and CH at dose levels of 100 μg/mL could combat against the LPS-induced inflammation. Overall, the PCH-Hyp was more active than the CH and PCH in reducing LDH release, ROS formation, and the secretion of these inflammatory mediators, or in increasing the secretion of the anti-inflammatory mediators. The qPCR and Western blot analysis results further confirmed that these samples had anti-inflammatory effects on the stimulated cells by suppressing the LPS-induced activation of the NF-κB signaling pathway, via regulating the mRNA/miRNA expression of iNOS, IL-6, TNF-α, IL-1β, COX-2, TLR4, IL-10, TGF-β1, miR-181a, miR-30d, miR-155, and miR-148, as well as the protein expression of MyD88, p-IKKα, p-IκBα, p-NF-κB p65, and iNOS, involved in this signaling pathway. In addition, the immunofluorescence assay results revealed that these samples could block the LPS-mediated nuclear translocation of the p65 protein and displayed the same function as the NF-κB inhibitor BAY 11-7082. It was concluded that CH could be endowed with higher anti-inflammatory activity to the macrophages by performing a plastein reaction, particularly that in the presence of exogenous Hyp.