Transport of IRW, an ovotransferrin-derived antihypertensive peptide, in human intestinal epithelial Caco-2 cells

Hypotensive effect of potent angiotensin-I-converting enzyme inhibitory peptides from corn gluten meal hydrolysate: Gastrointestinal digestion and transepithelial transportation modifications

The study examined the antihypertensive effect of peptides derived from pepsin-hydrolyzed corn gluten meal, namely KQLLGY and PPYPW, and their in silico gastrointestinal tract digested fragments, KQL and PPY, respectively. KQLLGY and PPYPW showed higher angiotensin I-converting enzyme (ACE)-inhibitory activity and lower ACE inhibition constant (Ki) values when compared to KQL and PPY. Only KQL showed a mild antihypertensive effect in spontaneously hypertensive rats with -7.83 and - 5.71 mmHg systolic and diastolic blood pressure values, respectively, after 8 h oral administration. During passage through Caco-2 cells, KQL was further degraded to QL, which had reduced ACE inhibitory activity. In addition, molecular dynamics revealed that the QL-ACE complex was less stable compared to the KQL-ACE. This study reveals that structural transformation during peptide permeation plays a vital role in attenuating antihypertensive effect of the ACE inhibitor peptide.

Intestinal absorption of bioactive oligopeptides: paracellular transport and tight junction modulation

Bioactive oligopeptides have gained increasing attention due to their diverse physiological functions, and these can be transported into the vasculature via transcellular and paracellular pathways. Among these, paracellular transport through the intercellular space is a passive diffusion process without energy consumption. It is currently the most frequently reported absorption route for food-derived bioactive oligopeptides. Previous work has demonstrated that paracellular pathways are mainly controlled by tight junctions, but the mechanism by which they regulate paracellular absorption of bioactive oligopeptides remains unclear. In this review, we summarized the composition of paracellular pathways across the intercellular space and elaborated on the paracellular transport mechanism of bioactive oligopeptides in terms of the interaction between oligopeptides and tight junction proteins, the protein expression level of tight junctions, the signaling pathways regulating intestinal permeability, and the properties of oligopeptides themselves. These findings contribute to a more profound understanding of the paracellular absorption of bioactive oligopeptides.

Bioavailability and Metabolism of Bioactive Peptide IRW with Angiotensin-Converting Enzyme 2 (ACE2) Upregulatory Activity in Spontaneously Hypertensive Rats

Peptide IRW is the first food-derived angiotensin-converting enzyme 2 (ACE2) upregulator. This study aimed to investigate the pharmacokinetic characteristics of IRW and identify the metabolites contributing to its antihypertensive activity in spontaneously hypertensive rats (SHRs). Rats were administered 100 mg of IRW/kg of the body weight via an intragastric or intravenous route. The bioavailability (F %) was determined to be 11.7%, and the half-lives were 7.9 ± 0.5 and 28.5 ± 6.8 min for gavage and injection, respectively. Interestingly, significant blood pressure reduction was not observed until 1.5 h post oral administration, or 2 h post injection, indicating that the peptide's metabolites are likely responsible for the blood pressure-lowering activity. Time-course metabolomics revealed a significant increase in the level of kynurenine, a tryptophan metabolite, in blood after IRW administration. Kynurenine increased the level of ACE2 in cells. Oral administration of tryptophan (W), but not dipeptide IR, lowered the blood pressure and upregulated aortic ACE2 in SHRs. Our study supports the key role of tryptophan and its metabolite, kynurenine, in IRW's blood pressure-lowering effects.

Digestion, absorption, and transport properties of soy-fermented douchi hypoglycemic peptides VY and SFLLR under simulated gastrointestinal digestion and Caco-2 cell monolayers

Two novel hypoglycemic peptides VY and SFLLR were identified from douchi as the major peptides responsible for the glucose uptake activity. The present work aimed to elucidate their digestion, absorption and transport properties using simulated digestion and Caco-2 cell monolayers transport models. Besides, the effects of digestion and absorption on the structure and activity were also studied. The results showed that VY was resistant to gastrointestinal tract digestion and could cross Caco-2 cell monolayers intactly via both TJs-mediated passive paracellular pathway and PepT1-mediated active route. In comparison, SFLLR was partially degraded into small fragments of SFLL, SFL, and SF by the digestive system, leading to increased glucose uptake activity. Notably, SFLLR, SFLL, and SFL were partly hydrolyzed by aminopeptidase N or dipeptidyl peptidase IV during transport, but they were transported intact. SFL was transported via both paracellular diffusion and PepT1-mediated routes, while SFLLR and SFLL were via paracellular route only.

Peptides and protein hydrolysates exhibiting anti-inflammatory activity: sources, structural features and modulation mechanisms

Inflammation is the response of the immune system to harmful stimuli such as tissue injury, infection or toxic chemicals, which has the aim of eliminating irritants or pathogenic microorganisms and enhancing tissue repair. Uncontrolled long-lasting acute inflammation can gradually progress to chronic, causing a variety of chronic inflammatory diseases that are usually treated with anti-inflammatory drugs, but most of them are inadequate to control chronic responses and are also associated with adverse side effects. Thus, many efforts are being directed to develop alternative and more selective anti-inflammatory therapies from natural products. One main field of interest is the obtaining of bioactive peptides exhibiting anti-inflammatory activity from sustainable protein sources like edible insects or agroindustry and fishing by-products. This work highlighted the structure-activity relationship of anti-inflammatory peptides. Small peptides with molecular weight under 1 kDa and amino acid chain length between 2 to 20 residues are generally the most active because of the higher probability to be absorbed in the intestine and penetrate into cells when compared with the larger size peptides. The presence of hydrophobic (Val, Ile, Pro) and positively charged (His, Arg, Lys) amino acids is another common occurrence for anti-inflammatory peptides. Interestingly, a high percentage (77%) of these bioactive peptides can be found in alternative sustainable protein sources such as Tenebrio molitor or sunflower, apart from its original protein source. However, not all of these peptides with anti-inflammatory potential in vitro achieve good scores by the in silico bioactivity predictors studied. Therefore, it is essential to implement current bioinformatics tools, in order to complement in vitro experiments with prior prediction of potential bioactive peptides.

Current trends and perspectives on bioaccessibility and bioavailability of food bioactive peptides: in vitro and ex vivo studies

The bioaccessibility and bioavailability of food-derived bioactive compounds are important issues when assessing their in vivo physiological health-promoting effects. Food components such as proteins and peptides are exposed to different proteases and peptidases during gastrointestinal digestion and absorption. Different in vitro approaches have therefore been developed to evaluate the bioaccessibility and stability of bioactive peptides. The static simulated gastrointestinal digestion model (SGD) was widely reported to assess the bioaccessibility of bioactive peptides. On the other hand, although the dynamic SGD model may better simulate human digestion, it has rarely been explored in bioaccessibility studies of food bioactive peptides due to its high cost and lack of standardization. For bioavailability studies, the Caco-2 cell monolayer model has been used extensively for the assessment of food bioactive peptides. In fact, very few reports using alternative methods for determining transepithelial transport of bioactive peptides have been employed. In this sense, ex vivo tissue-based models such as the Ussing chamber and the everted sac gut have been used. Current evidence supports the fact that using SGD with cell-based models for evaluating the bioaccessibility, absorption, and bioavailability of food-derived bioactive peptides, is the most commonly used approach. Nevertheless, SGD with ex vivo tissue-based models such as the everted sac, remains to be further explored because it seems to be the model that better mimics the physiological process - it is also fast and inexpensive, and several compounds may be tested simultaneously. In the present review, we discuss information available on the different in vitro approaches for the determination of bioaccessibility and bioavailability of food-derived bioactive peptides with special emphasis on ex vivo tissue-based models such as the everted sac and the Ussing chamber models. © 2022 Society of Chemical Industry.

Egg White Protein Ovotransferrin-Derived IRW (Ile-Arg-Trp) Inhibits LPS-Induced Barrier Integrity Dysfunction and Inflammation in Caco-2 Cells

Tripeptide IRW derived from egg ovotransferrin was initially identified to be an inhibitor of angiotensin-converting enzyme. Later, IRW has been shown to possess various bioactivities, including anti-inflammatory activity and the ability to suppress colitis development. Nevertheless, its role in protecting intestinal barrier integrity has not been reported. This study aims to investigate the effect of IRW on inhibiting intestinal barrier dysfunction and inflammation in lipopolysaccharide (LPS)-treated Caco-2 cells. Pretreatment with IRW could mitigate the LPS-induced reduction of transepithelial electronic resistance values and decrease the paracellular permeation of differentiated Caco-2 cell monolayers. Meanwhile, IRW restored the expression level and cell surface distribution of the tight junction protein occludin. Furthermore, IRW showed LPS-neutralizing activity and could significantly inhibit LPS-induced activation of nuclear factor-κB (NF-κB) and mitogen-activated protein kinase (MAPK) signaling pathways. In conclusion, our study demonstrated the ability of IRW to prevent LPS-induced intestinal barrier dysfunction and prohibit inflammatory responses.

The Effect of Soybean Peptides on Improving Quality and the ACE Inhibitory Bioactivity of Extruded Rice

It is crucial to address the dietary problems of hypertensive patients. The effect and mechanism of different contents of soybean protein on cooking quality and angiotensin-converting enzyme (ACE) inhibitory action in the extruded rice were firstly investigated. The results showed that the extruded rice with soybean protein possessed the higher taste value (90.32 ± 2.31), hardness (2.65 ± 0.01 g), and good pasting quality (p ≤ 0.05). Meanwhile, the soybean protein notably retarded the starch digestibility; the sample with 6% soybean protein showed the fewest rapidly digestible starch (RDS) content (78.82 ± 0.01 mg g−1) and the most slowly digestible starch (SDS) content (8.97 ± 0.45 mg g−1). Importantly, the ACE inhibition rate improved from 17.09 ± 0.01% to 74.02 ± 0.65% in the 6% soybean protein sample because of the production of peptides. The peptide composition of samples were compared, which showed that the effective ACE-inhibitory peptides usually contain 2~20 amino acids, and Pro, Leu, Ile, Val, Phe, and Ala were the main components. Overall, moderate soybean protein would give a good quality and lower ACE activity in extruded food.

Comprehensive Review of γ-Glutamyl Peptides (γ-GPs) and Their Effect on Inflammation Concerning Cardiovascular Health

γ-Glutamyl peptides (γ-GPs) are a group of peptides naturally found in various food sources. The unique γ-bond potentially enables them to resist gastrointestinal digestion and offers high stability in vivo with a longer half-life. In recent years, these peptides have caught researchers' attention due to their ability to impart kokumi taste and elicit various physiological functions via the allosteric activation of the calcium-sensing receptor (CaSR). This review discusses the various food sources of γ-glutamyl peptides, different synthesis modes, allosteric activation of CaSR for taste perception, and associated multiple biological functions they can exhibit, with a special emphasis on their role in modulating chronic inflammation concerning cardiovascular health.

Corn peptides improved obesity-induced non-alcoholic fatty liver disease through relieving lipid metabolism, insulin resistance and oxidative stress

Non-alcoholic fatty liver disease (NAFLD) is increasingly threatening human health. The remarkable effects of corn peptides (CPs) as bioactive peptides on liver protection have attracted much attention. Nevertheless, the specific effect of CPs on NAFLD remains unclear. The present study was designed to investigate the efficacy of CPs in the prevention and auxiliary treatment of high-fat diet (HFD)-induced NAFLD in SD rats, and puerarin was used as the positive control. SD rats were fed a high-fat diet to establish the NAFLD rat model, and LO2 cells were treated with a high concentration of fructose to simulate the NAFLD cell model. NAFLD was comprehensively examined in terms of body weight, liver function markers, serum biochemistry and liver histology. Protein expression was determined using western blot analysis. The results of animal experiments showed that CPs could effectively inhibit the rate of weight gain, reduce the blood lipid level and liver index, and enhance glucose tolerance. The results of cell experiments showed that CPs could effectively reduce the accumulation of lipids in LO2 cells and inhibit the accumulation of reactive oxygen species (ROS). In addition, CPs could markedly reduce liver lipid accumulation in the liver cell and liver tissue, as further evidenced by the reduced expression of SREBP-1c in human non-tumour hepatic (LO2) cells. Meanwhile, the increased expression of SIRT1/PPAR-α and Nrf2/HO-1 pathways under the pretreatment of CPs in LO2 cells indicated that CPs could markedly relieve high fat-induced fatty liver injury, regulate insulin sensitivity, and reduce production of ROS. The results of in vivo and in vitro experiments demonstrated that CPs provided potential prevention and auxiliary treatment for NAFLD through reducing lipid accumulation, alleviating insulin resistance, and inhibiting oxidative stress. This study investigated the biological activity of CPs and laid the theoretical basis for the development of CP-based functional foods and dietary supplements.

Identification of zinc-chelating pumpkin seed (Cucurbita pepo L.) peptides and in vitro transport of peptide-zinc chelates

In this study, pumpkin seeds peptide was purified, characterized, and evaluated for their zinc-chelating capability, as well as in vitro bioaccessibility and transportation. Raw pumpkin seeds protein hydrolysate (PSPH) was produced by papain hydrolysis. The peptide fractions with the highest zinc-chelating abilities were purified using immobilized metal affinity chromatography (IMAC) followed by gel filtration chromatography (GF). Eight peptides were identified, two of which with the lowest molecular weights were synthesized (RPKHPLK and RPKHPLSHDL) for determining potential bioaccessibility and bioavailability. Our results showed that the gastrointestinal stability of RPKHPLK-Zn and RPKHPLSHDL-Zn was higher than that of inorganic zinc salts in the simulated gastrointestinal tract model. Furthermore, the influence of the peptide zinc chelates on zinc transport was explored in vitro using Caco-2 cell monolayer model. It was also shown that both RPKHPLK-Zn and RPKHPLSHDL-Zn could increase zinc transport rate and may be used to facilitate effective zinc absorption. The result of this study may provide important implications for developing plant protein foods with higher nutritional value. PRACTICAL APPLICATION: As a potential alternative protein source, pumpkin seeds may find promising applications in plant-based foods and drinks to meet the growing market for nonanimal foods. In this study, pumpkin seed protein peptides were prepared and purified, and the zinc-chelating peptides were identified and evaluated for the abilities to promote the uptake of zinc. This type of mineral peptide chelates could be incorporated into plant-based foods to increase mineral contents, which is significantly higher in foods originated from animals. The result of our study may provide important information for food industry to increase the nutritional value of plant-based foods.

Current findings support the potential use of bioactive peptides in enhancing zinc absorption in humans

More than two billion people around the world are affected by zinc deficiency, mainly due to the inadequate intake and absorption of zinc. Based on recent research findings, the bioactive peptides could potentially be used to combat zinc deficiency particularly due to their Zinc chelating ability. The main aim of this review was to present current findings, supporting the potential use of bioactive peptides based on their ability to enhance zinc absorption. In-vivo, in-vitro, and ex-vivo studies have demonstrated that zinc chelating peptides can enhance the retention, transportation, and absorption of zinc. Comparative studies on zinc bioavailability from protein hydrolysates and zinc salts have demonstrated that the protein hydrolysates-zinc complexes are more bioavailable than the zinc salts. Data from the structure-function relationship of zinc chelating peptides suggest that the zinc chelating capacities of peptides increase in the following order; the position of zinc chelator > zinc chelator strength > abundance of zinc chelators > net charge > molecular weight. In addition, the transport mechanism of peptide-zinc complex is hypothesized, and the potential use of bioactive peptides based on their safety and taste and limitations to their commercialization are also discussed.

Advancement and prospects of production, transport, functional activity and structure-activity relationship of food-derived angiotensin converting enzyme (ACE) inhibitory peptides

Food-derived antihypertensive peptides have attracted increasing attention in functional foods for health promotion, due to their high biological activity, low toxicity and easy metabolism in the human body. Angiotensin converting enzyme (ACE) is a key enzyme that causes the increase in blood pressure in mammals. However, few reviews have summarized the current understanding of ACE inhibitory peptides and their knowledge gaps. This paper focuses on the food origins and production methods of ACE inhibitory peptides. Compared with conventional methods, the advanced technologies and emerging bioinformatics approaches have recently been applied for efficient and targeted release of ACE inhibitory peptides from food proteins. Furthermore, the transport and underlying mechanisms of ACE inhibitory peptides are emphatically described. Molecular modeling and the Michaelis-Menten equation can provide information on how ACE inhibitors function. Finally, we discuss the structure-activity relationships and other bio-functional properties of ACE inhibitory peptides. Molecular weight, hydrophobic amino acid residues, charge, amino acid composition and sequence (especially at the C-terminal and N-terminal) have a significant influence on ACE inhibitory activity. Some studies are required to increase productivity, improve bioavailability of peptides, evaluate their bio-accessibility and efficiency on reducing blood pressure to provide a reference for the development and application of health products and auxiliary treatment drugs.

Natural Products Modulating Angiotensin Converting Enzyme 2 (ACE2) as Potential COVID-19 Therapies

The 2019 coronavirus disease (COVID-19) is a potentially fatal multisystemic infection caused by the severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2). Currently, viable therapeutic options that are cost effective, safe and readily available are desired, but lacking. Nevertheless, the pandemic is noticeably of lesser burden in African and Asian regions, where the use of traditional herbs predominates, with such relationship warranting a closer look at ethnomedicine. From a molecular viewpoint, the interaction of SARS-CoV-2 with angiotensin converting enzyme 2 (ACE2) is the crucial first phase of COVID-19 pathogenesis. Here, we review plants with medicinal properties which may be implicated in mitigation of viral invasion either via direct or indirect modulation of ACE2 activity to ameliorate COVID-19. Selected ethnomedicinal plants containing bioactive compounds which may prevent and mitigate the fusion and entry of the SARS-CoV-2 by modulating ACE2-associated up and downstream events are highlighted. Through further experimentation, these plants could be supported for ethnobotanical use and the phytomedicinal ligands could be potentially developed into single or combined preventive therapeutics for COVID-19. This will benefit researchers actively looking for solutions from plant bioresources and help lessen the burden of COVID-19 across the globe.

Novel Hyp-Gly-containing antiplatelet peptides from collagen hydrolysate after simulated gastrointestinal digestion and intestinal absorption

Bioactive components causing the antiplatelet activity upon collagen hydrolysate (CH) ingestion have not been clarified yet. This study aimed to identify antiplatelet peptides from CH after simulated gastrointestinal digestion and intestinal absorption. Four antiplatelet peptides containing the Hyp-Gly (OG) sequence including OG, Hyp-Gly-Glu (OGE), Pro-Gly-Glu-Hyp-Gly (PGEOG) and Val-Gly-Pro-Hyp-Gly-Pro-Ala (VGPOGPA) were successfully identified. All four peptides exhibited antiplatelet activity, but OGE and PGEOG exerted stronger activity than OG and VGPOGPA. The IC50 value of OGE and PGEOG was 1.076 mM and 1.167 mM, respectively. These four antiplatelet peptides could survive simulated gastrointestinal digestion and be absorbed intact by Caco-2 cells. Furthermore, plasma stability experiments showed that OG and OGE showed a good stability in human plasma, but PGEOG and VGPOGPA showed a relatively poor stability. In vivo studies indicated that OG and OGE were present in blood after the oral administration of CH. Meanwhile, OGE exerted significant in vivo anti-thrombotic activity after its ingestion. The present study clarifies the antiplatelet components causing the CH activity and highlights the potential application of CH or these four peptides as functional foods to combat thrombosis by inhibiting platelet aggregation.

Transport of Dietary Anti-Inflammatory Peptide, γ-Glutamyl Valine (γ-EV), across the Intestinal Caco-2 Monolayer

The present study analyzed the transepithelial transport of the dietary anti-inflammatory peptide, γ-glutamyl valine (γ-EV). γ-EV is naturally found in dry edible beans. Our previous study demonstrated the anti-inflammatory potency of γ-EV against vascular inflammation at a concentration of 1mM, and that it can transport with the apparent permeability coefficient (P_app) of 1.56 × 10^-6 ± 0.7 × 10^-6 cm/s across the intestinal Caco-2 cells. The purpose of the current study was to explore whether the permeability of the peptide could be enhanced and to elucidate the mechanism of transport of γ-EV across Caco-2 cells. The initial results indicated that γ-EV was nontoxic to the Caco-2 cells up to 5 mM concentration and could be transported across the intestinal cells intact. During apical-to-basolateral transport, a higher peptide dose (5 mM) significantly (p < 0.01) enhanced the transport rate to 2.5 × 10^-6 ± 0.6 × 10^-6 cm/s. Cytochalasin-D disintegrated the tight-junction proteins of the Caco-2 monolayer and increased the P_app of γ-EV to 4.36 × 10^-6 ± 0.16 × 10^-6 cm/s (p < 0.001), while theaflavin 3'-gallate and Gly-Sar significantly decreased the P_app (p < 0.05), with wortmannin having no effects on the peptide transport, indicating that the transport route of γ-EV could be via both PepT1-mediated and paracellular.

Transport, In Vivo Antihypertensive Effect, and Pharmacokinetics of an Angiotensin-Converting Enzyme (ACE) Inhibitory Peptide LVLPGE

The angiotensin-converting enzyme (ACE) inhibitory peptide LVLPGE provides outstanding antihypertensive effects in vivo, with a maximum systolic blood pressure (SBP) drop of 39 mmHg at a dose of 10 mg/kg. We evaluated the gastrointestinal digestion, transport, and in vivo antihypertensive effects of LVLPGE at different doses. LVLPGE was resistant to gastrointestinal enzymes with a stability of 97.8% and a permeability P_app of (5.09 ± 0.94) × 10^-7 cm/s. LVLPGE was mainly transported through the Caco-2 cell monolayer by the peptide transporter PepT 1 and passive-mediated transport. LVLPGE at doses of 30 and 50 mg/kg had a positive antihypertensive effect in vivo; 30 mg/kg had a more significant effect than 50 mg/kg. After oral administration, the pharmacokinetics of LVLPGE showed that the C_max was 4.65 ng/mL at 2 min. The blood pressure-lowering effect increased as the concentration of LVLPGE increased in the plasma of spontaneous hypertensive rats (SHRs).

Activity and bioavailability of food protein-derived angiotensin-I-converting enzyme-inhibitory peptides

Angiotensin-I-converting enzyme (ACE) inhibitory peptides are able to inhibit the activity of ACE, which is the key enzymatic factor mediating systemic hypertension. ACE-inhibitory peptides can be obtained from edible proteins and have the function of antihypertension. The amino acid sequences and the secondary structures of ACE-inhibitory peptides determine the inhibitory activities and stability. The resistance of ACE-inhibitory peptides to digestive enzymes and peptidase affect their antihypertensive bioactivity in vivo. In this paper, the mechanism of ACE-inhibition, sources of the inhibitory peptides, structure-activity relationships, stability during digestion, absorption and transportation of ACE-inhibitory peptides, and consumption of ACE-inhibitory peptides are reviewed, which provide guidance to the development of new functional foods and production of antihypertensive nutraceuticals and pharmaceuticals.

A Novel Angiotensin Converting Enzyme 2 (ACE2) Activating Peptide: A Reflection of 10 Years of Research on a Small Peptide Ile-Arg-Trp (IRW)

IRW (Ile-Arg-Trp) was identified as an inhibitor of angiotensin converting enzyme (ACE) from egg white protein ovotransferrin through an integrated in silico digestion and quantitative structure and activity relationship prediction in 2011. Oral administration of IRW to spontaneously hypertensive rats (SHRs) can significantly reduce blood pressure, via upregulation of ACE2, but not through the inhibition of ACE. ACE2 converts Ang II into Ang (1-7), thus lowering blood pressure via Mas receptor (MasR); coinfusion of Mas receptor antagonist A779 and IRW in SHRs abolished blood pressure-lowering effect of IRW, supporting a key role of ACE2/Ang (1-7)/MasR axis. Our ongoing study further established new roles of IRW as an antioxidant, an anti-inflammatory agent, an insulin sensitizer, and a bone cell anabolic. Future studies are warranted to understand the unique structure features of this peptide, its mechanisms of action at various targets, its bioavailability and metabolism, and its possible roles toward COVID-19.

Transport, metabolism and remedial potential of functional food extracts (FFEs) in Caco-2 cells monolayer: A review

Caco-2, a human intestinal carcinoma cell line, has been used to test the absorption and transport mechanism of functional foods and drugs across the intestinal epithelium in order to study their antioxidant, anticancer and anti-inflammatory activities. Caco-2 cells represent the morphological and functional characteristics of small intestinal cells and capable of expressing brush borders, tight junctions, intestinal efflux and uptake transporters which regulate permeation of drugs and functional food extracts from intestinal lumen to systemic circulation. The integrity of the Caco-2 monolayer is controlled by establishing the TEER between 200 and 1000 O per cm². FFEs affect intestinal permeability by adjusting the tight junction proteins between the cells in order to maintain the epithelial barrier function. Because of the side effects of medicines, there is an increased interest in functional food extracts (FFEs) as drug substitutes. Functional foods undergo intricate transport processes and biotransformation after oral administration. Metabolism and transport studies of FFEs in Caco-2 cells are very important for determining their bioavailability. Functional foods and their constituents produce anti-proliferative and anti-cancer effects through apoptosis, cell cycle arrest and inhibition of various signal transduction pathways across Caco-2 cell lines. The current review has summarized the anti-inflammation, anticancer, antioxidant and cholesterol lowering potential of FFEs using Caco-2 cells through reducing local inflammatory signals, production of ROS and lipid accumulation. The transport, bioavailability, metabolism, mechanisms of actions, cellular pathways adopted by FFEs across Caco-2 cell lines are predominantly affected by their molecular weight, structures and physicochemical properties. These studies are beneficial for investigating the different mechanisms of action of FFEs in the human body.

Regulatory Effects of a Pea-Derived Peptide Leu-Arg-Trp (LRW) on Dysfunction of Rat Aortic Vascular Smooth Muscle Cells against Angiotensin II Stimulation

Vascular oxidative stress, inflammatory response, and proliferation are crucial mediators of vascular dysfunction which contribute to the pathology of hypertension. A tripeptide, LRW (Leu-Arg-Trp), was characterized from pea protein legumin, and its previously studied isomer IRW (Ile-Arg-Trp) was reported to exhibit antihypertensive activity via activation of angiotensin-converting enzyme 2. The objective of the current study was to explore the effects of LRW on vascular stress in vascular smooth muscle cells (VSMCs) under angiotensin II (Ang II)-induced cellular stress. LRW treatment could decrease Ang II-triggered superoxide production, inflammation, and proliferation in VSMCs. The abovementioned advantageous effects appeared to involve the upregulation of the ACE2-Ang-(1-7)-MasR axis and modulation of the nuclear factor-κB pathway. These findings specified the prospective role of LRW as a functional food ingredient or nutraceutical in the prevention of cardiovascular diseases, particularly hypertension and vascular damage.

Structural Requirement of Casein Peptides for Transcytosis through the Caco-2 Cell Monolayer: Hydrophobicity and Charge Property Affect the Transport Pathway and Efficiency

Casein is a rich source of bioactive peptides with complete amino acid composition. In this study, the casein peptides identified in our previous study with different hydrophobicities and charge properties were employed to investigate the transport efficiency via the transcytosis pathway across Caco-2 cell monolayers. Results revealed that the apparent permeability coefficient (P_app) values of transcytosis exhibited a linear correlation with a pI of positively charged peptides during bidirectional transport. A similar law was found as for the peptides with different hydrophobicities. The transcytosis route of Pep-II to Pep-VII appears to be the clathrin- and caveolin-independent transcytosis pathway as well as caveolae-mediated transcytosis pathway, showing a linear correlation with P_app values, respectively. Additionally, no direct correlation was shown between the hydrophobicity of peptides and clathrin-mediated transcytosis. Our results help to increase the bioaccessibility of peptide drugs across intestinal mucosa by developing strategies to alter the physicochemical properties without changing bioactivity.

Transepithelial Transport Characteristics of the Cholesterol- Lowing Soybean Peptide, WGAPSL, in Caco-2 Cell Monolayers

Recent studies have shown that soybean protein and its peptides have cholesterol-lowering activities. However, it is not clear whether these peptides could overcome physiological barriers, such as phase II metabolism in gastrointestinal tract and poor permeability, to reach the blood stream in its intact form. Therefore, the transepithelial transport characteristics of soybean peptide Trp-Gly-Ala-Pro-Ser-Leu (WGAPSL) with cholesterol- lowering activity were investigated in Caco-2 cells. In this study; the transepithelial absorption of WGAPSL was studied using human intestinal Caco-2 cell monolayers. The results showed that WGAPSL had good stability (83.9% ±1.98%) after simulated gastric and intestinal digestion. During the apical (AP) side to basolateral (BL) side transport, WGAPSL was absorbed intact through Caco-2 cell monolayers with apparent permeability coefficient (Papp) values of 4.4 × 10-8 to 1.2 × 10-8 cm/s. Cytochalasin D loosened the tight junctions of Caco-2 cell monolayers and significantly (p < 0.05) improved the transport process. Sodium azide, wortmannin, and Gly-Pro had minimal effects on transport, demonstrating that the major transport route of WGAPVL was paracellular via tight junctions. Finally, LC-MS analysis showed that Gly-Ala-Pro (GAP) was the important part for the intact absorption of WGAPVL and Trp (W) was the most unstable amino acid residue.

Food-Derived Bioactive Peptides in Human Health: Challenges and Opportunities

Recent scientific evidence suggests that food proteins not only serve as nutrients, but can also modulate the body's physiological functions. These physiological functions are primarily regulated by some peptides that are encrypted in the native protein sequences. These bioactive peptides can exert health beneficial properties and thus are considered as a lead compound for the development of nutraceuticals or functional foods. In the past few decades, a wide range of food-derived bioactive peptide sequences have been identified, with multiple health beneficial activities. However, the commercial application of these bioactive peptides has been delayed because of the absence of appropriate and scalable production methods, proper exploration of the mechanisms of action, high gastro-intestinal digestibility, variable absorption rate, and the lack of well-designed clinical trials to provide the substantial evidence for potential health claims. This review article discusses the current techniques, challenges of the current bioactive peptide production techniques, the oral use and gastrointestinal bioavailability of these food-derived bioactive peptides, and the overall regulatory environment.

Stability and Transport of Spent Hen-Derived ACE-Inhibitory Peptides IWHHT, IWH, and IW in Human Intestinal Caco-2 Cell Monolayers

Ile-Trp-His-His-Thr (IWHHT), initially identified as an ACE inhibitory peptide, was shown to have antioxidant and anti-inflammatory activities in cells and blood pressure lowering activity in animals. IWHHT was degraded into IWH and IW during simulated gastrointestinal digestion. The purpose of this study was to investigate the stability, permeability, and transport pathways of IWHHT, IWH, and IW across intestinal epithelium using human intestinal Caco-2 cell monolayers. IWHHT, IWH, and IW were partly degraded by aminopeptidase N or dipeptidyl peptidase IV, but they were transported intact, with apparent permeability coefficients of (22.0 ± 1.42) × 10^-8, (37.5 ± 1.11) × 10^-8, and (19.6 ± 0.62) × 10^-8 cm s^-1, respectively. The results firstly evidenced an important role of aminopeptidase N in cleaving small ACE inhibitory peptides during transport. IWH was transported via both peptide transporter 1 (PepT1) and paracellular route, while IW was via PepT1 and IWHHT was via paracellular route only. Transport of IW implied that hydrophobic peptides (even with a small size), consisting of only highly hydrophobic amino acid residues, might not be transported via paracellular diffusion. This study suggested that all three peptides could pass through the intestinal epithelium and that the degraded IWH and IW might also contribute to the antihypertensive activity of IWHHT.

Transport of ACE Inhibitory Peptides Ile-Gln-Pro and Val-Glu-Pro Derived from Spirulina platensis Across Caco-2 Monolayers

This study evaluated transepithelial transport mechanisms of Ile-Gln-Pro (IQP) and Val-Glu-Pro (VEP), two ACE-inhibitory peptides derived from Spirulina platensis, using human intestinal Caco-2 cell monolayers. IQP and VEP were absorbed intact through Caco-2 cell monolayers with P_app values of 7.48 ± 0.58 × 10^-6 and 5.05 ± 0.74 × 10^-6 cm/s, respectively. The transport of IQP and VEP were affected neither by Gly-Pro nor by wortmannin, indicating that they were not PepT1-mediated and did not involve endocytosis. However, transport of IQP and VEP were increased significantly by sodium deoxycholate, suggesting that the major transport mechanism was paracellular. In addition, the increased transport of VEP and IQP were followed with the addition of sodium azide, suggesting influence of energy to the process. The transport of VEP was also increased by verapamil, indicating an apical-to-basolateral flux mediated by P-gp.

PRACTICAL APPLICATION

Bioactive peptides derived from food proteins have been considered as potentially ideal products to reduce hypertension because of their safety and positive impacts on health. IQP and VEP are the 2 ACE inhibitory peptides derived from Spirulina platensis, a kind of edible cyanobacteria with rich nutrition and multiple physiological functions, and were demonstrated to inhibit ACE and lower blood pressure in spontaneously hypertensive rats. However, it is prerequisite that such bioactive peptides must be absorbed intact across the intestinal epithelium, so as to exert antihypertensive effects in vivo. This study evaluated transepithelial transport mechanisms of IQP and VEP. It contributes to the study of Spirulina in lowering blood pressure and supports the development of bioactive peptide products.

Molecular interactions, bioavailability, and cellular mechanisms of angiotensin-converting enzyme inhibitory peptides

Food-derived angiotensin-converting enzyme inhibitory (ACEi) peptides have gained substantial interest as potential alternatives to synthetic drugs in the management of hypertension. Peptide size and sequence are two critical factors that determine their potency, bioavailability, and cellular mechanisms. Molecular interaction studies between ACE and ACEi peptides support that potent ACEi peptides are generally composed of hydrophobic, positively charged, and aromatic or cyclic amino acid residues at the third, second, and first position from the C-terminus, respectively. Small peptides containing N-terminal Tyr and/or C-terminal Pro could improve their stability against enterocyte peptidases, thus their bioavailability. Different ACEi peptides can reduce aberrant cellular proliferation, excessive inflammation, and oxidative stress but through different mechanisms. Further understanding the structure-activity-bioavailability relationships will help design novel potent ACEi peptides with improved bioavailability and in vivo efficacy. PRACTICAL APPLICATIONS: ACEi peptides have the potential for uses as functional food ingredients against hypertension.

Current knowledge of intestinal absorption of bioactive peptides

Peptides have been demonstrated as potentially beneficial compounds against several life-style related diseases such as hypertension, hypercholesterolemia, and atherosclerosis, among others. However, limited research has been carried out on peptide absorption, resulting in a lack of understanding and control of this process. Therefore, this review discusses the recent insights gathered on in vitro and in vivo absorption of peptides across intestinal membranes, into blood circulation. Briefly, some di-/tripeptides permeate through intestinal membranes in their intact forms via peptide transporter systems, while others are vulnerable to protease degradation. Oligopeptides (>tetrapeptides) show a lower transport ability than di-/tripeptides, possibly due to the presence of paracellular tight junctions. The hydrophobicity of peptides (log P) does not seem to influence absorption, while peptide length and degradation of peptides (and peptide sequences) by intestinal proteases may be determinant factors of the absorption process.

Egg Protein-Derived Bioactive Peptides: Preparation, Efficacy, and Absorption

The hen's egg is an important protein source of human diet. On average one large egg contains ~6g protein, which contributes to ~11% of daily protein intake. As a high-quality protein, egg proteins are well recognized as excellent sources of bioactive peptides. The objectives of this chapter are to introduce generation, bioactivities, and absorption of egg protein-derived bioactive peptides. Research on egg protein-derived bioactive peptides has been progressed during the past decades. Enzymatic hydrolysis is the major technique to prepare bioactive peptides from egg protein. Quantitative structure-activity relationships-aided in silico prediction is increasingly applied as a promising tool for efficient prediction of novel bioactive peptides. A number of bioactive peptides from egg proteins have been characterized for antioxidant, immunomodulatory, antihypertensive, antidiabetic, anticancer, and antimicrobial activities. Egg protein-derived peptides that can improve bone health have been reported as well. However, molecular mechanisms of many peptides are not fully understood. The stability and absorption routes, bioavailability, safety, and production of bioactive peptides await further investigation.

Identification of New Anti-inflammatory Peptides from Zein Hydrolysate after Simulated Gastrointestinal Digestion and Transport in Caco-2 Cells

Chronic inflammation is an underlying contributor to various chronic diseases. The objectives of this study were to investigate the anti-inflammatory activity of zein hydrolysate after simulated gastrointestinal digestion and Caco-2 cell absorption and to identify novel anti-inflammatory peptides after transport across Caco-2 cells. Three zein hydrolysates were prepared and further digested using gastrointestinal proteases; their transports were studied in Caco-2 cells. Anti-inflammatory activity was studied in endothelial EA.hy926 cells. Three zein hydrolysates and their digests significantly decreased the expression of tumor necrosis factor-α (TNF-α) induced pro-inflammatory vascular cell adhesion molecule-1 (VCAM-1) by 37.3-66.0%. Eleven novel peptides with 5-9 amino acid residues were sequenced; three peptides showed strong anti-inflammatory activity by inhibiting the VCAM-1 by 54-38.9% and intercellular cell adhesion molecule-1 (ICAM-1) by 36.5-28.6% at 0.2 mM. A new approach to identify novel anti-inflammatory peptides that could survive gastrointestinal digestion and absorption was developed.

Hydrolysis and transepithelial transport of two corn gluten derived bioactive peptides in human Caco-2 cell monolayers

The objective of this paper was to investigate the transepithelial transport of two novel corn gluten-derived antioxidant peptides, YFCLT and GLLLPH, using Caco-2 cell monolayers. Results showed that both of YFCLT and GLLLPH could transport in intact form across Caco-2 cell monolayers with apparent permeability coefficient (P_app) values of (1.10±0.16)×10^-7cm/s and (1.98±0.23)×10^-7cm/s, respectively. However, it was found that the two peptides were susceptible and easily hydrolyzed by brush border membrane peptidases. In the presence of diprotin A, an inhibitor of dipeptidyl peptidase IV (DPPIV), the hydrolysis of YFCLT and GLLLPH decreased and their permeabilities increased significantly compared to control group (P<0.05). The results of transport routes revealed that Gly-Sar, a peptide transporter 1 (PepT1) substrate, had little effects on the transepithelial permeability (P>0.05), suggesting that the transport of YFCLT and GLLLPH across Caco-2 cell monolayers was not mediated by PepT1. However, it was found that cytochalasin d, a tight junctions (TJs) disruptor, increased the permeability significantly (P<0.05). While wortmannin, a transcytosis inhibitor, and sodium azide, an ATP synthesis inhibitor, both decreased the permeability significantly (P<0.05). It indicated that the TJs-mediated paracellular pathway and energy-dependent transcytosis were involved in the transport of YFCLT and GLLLPH across Caco-2 cell monolayers.

Transport Study of Egg-Derived Antihypertensive Peptides (LKP and IQW) Using Caco-2 and HT29 Coculture Monolayers

The objective of this study was to investigate the mechanisms of the transport of antihypertensive tripeptides LKP (Leu-Lys-Pro) and IQW (Ile-Gln-Trp) derived from egg white using a coculture system of Caco-2 and HT29 cell monolayers. The results revealed that LKP and IQW have no cytotoxicity to the cell viability after 2 h incubation, could be transported intact across coculture monolayers (apparent permeability coefficient: (18.11 ± 1.57) × 10^-8 and (13.21 ± 1.12) × 10^-8 cm/s, respectively), and were resistant to peptidase secreted by enterocytes. In addition, the transports were significantly inhibited by dipeptide Gly-Pro (P < 0.05), a competitive substance of peptide transporter 1 (PepT1). The transports from apical to basolateral side were significantly higher than that of the reverse direction (P < 0.05). These results suggest that PepT1 is involved in LKP and IQW transports. The transports were also significantly decreased by theaflavin-3'-O-gallate (P < 0.05), an enhancer of tight junction (TJ) and increased by cytochalasin D (P < 0.05), a disruptor of TJ but not influenced by wortamanin, a transcytosis inhibitor, suggesting that passive paracellular route via TJs is also involved in LKP and IQW transports but not transcytosis. In addition, siRNA was also used to knockdown the expression of PepT1 and significantly inhibited the transport (P < 0.05), confirming that PepT1 is involved in transport process. Therefore, both passive paracellular route via TJ and active route via PepT1 coexist in the transport of antihypertensive LKP and IQW across Caco-2/HT29 coculture monolayers.

Egg-derived bioactive peptides with ACE-inhibitory properties: a literature update

Egg proteins contain a wide set of peptide sequences which have an impact on cardiovascular health.