Tryptic hydrolysis of bovine αS2-casein: identification and release kinetics of peptides

Identification and release kinetics of peptides from tilapia skin collagen during alcalase hydrolysis

Collagen from tilapia skin was extracted and confirmed as type I collagen. Collagen was then hydrolyzed with alcalase for 4 h and the released peptides were identified. The structure-activity relationship of collagen-released peptides showed that proline at position C3 played a key role in improving ACE inhibitory activity, while proline at position C2 had a negative effect. Collagen peptide release kinetics showed that with the extension of time, the number of peptides increased dramatically at first, decreased, and then tended to be stable. This indicated that collagen peptides mainly originated from primary enzymolysis at the first stage and began to undergo secondary hydrolysis in the second stage. Afterwards, secondary enzymolysis was dominant at the third stage and finally remained stable at final two stages. Understanding the pattern of collagen peptide release kinetics might offer a powerful approach in the collagen-peptide food processing industry to better control food safety and quality.

Optical chemosensors for the detection of proximally phosphorylated peptides and proteins

Proximal multi-site phosphorylation is a critical post-translational modification in protein biology. The additive effects of multiple phosphosite clusters in close spatial proximity triggers integrative and cooperative effects on protein conformation and activity. Proximal phosphorylation has been shown to modulate signal transduction pathways and gene expression, and as a result, is implicated in a broad range of disease states through altered protein function and/or localization including enzyme overactivation or protein aggregation. The role of proximal multi-phosphorylation events is becoming increasingly recognized as mechanistically important, although breakthroughs are limited due to a lack of detection technologies. To date, there is a limited selection of facile and robust sensing tools for proximal phosphorylation. Nonetheless, there have been considerable efforts in developing optical chemosensors for the detection of proximal phosphorylation motifs on peptides and proteins in recent years. This review provides a comprehensive overview of optical chemosensors for proximal phosphorylation, with the majority of work being reported in the past two decades. Optical sensors, in the form of fluorescent and luminescent chemosensors, hybrid biosensors, and inorganic nanoparticles, are described. Emphasis is placed on the rationale behind sensor scaffolds, relevant protein motifs, and applications in protein biology.

Native disulphide-linked dimers facilitate amyloid fibril formation by bovine milk αS2-casein

Bovine milk α_S2-casein, an intrinsically disordered protein, readily forms amyloid fibrils in vitro and is implicated in the formation of amyloid fibril deposits in mammary tissue. Its two cysteine residues participate in the formation of either intra- or intermolecular disulphide bonds, generating monomer and dimer species. X-ray solution scattering measurements indicated that both forms of the protein adopt large, spherical oligomers at 20 °C. Upon incubation at 37 °C, the disulphide-linked dimer showed a significantly greater propensity to form amyloid fibrils than its monomeric counterpart. Thioflavin T fluorescence, circular dichroism and infrared spectra were consistent with one or both of the dimer isomers (in a parallel or antiparallel arrangement) being predisposed toward an ordered, amyloid-like structure. Limited proteolysis experiments indicated that the region from Ala⁸¹ to Lys¹¹³ is incorporated into the fibril core, implying that this region, which is predicted by several algorithms to be amyloidogenic, initiates fibril formation of α_S2-casein. The partial conservation of the cysteine motif and the frequent occurrence of disulphide-linked dimers in mammalian milks despite the associated risk of mammary amyloidosis, suggest that the dimeric conformation of α_S2-casein is a functional, yet amyloidogenic, structure.

Impact of sequential enzymatic hydrolysis on antioxidant activity and peptide profile of casein hydrolysate

Abstract

This paper shows the potential of dual enzyme approach on antioxidant activity of casein hydrolysates. Casein was hydrolysed using the proteolytic enzymes alcalase, flavourzyme in isolation and in sequential order. Casein hydrolysates were evaluated for the degree of hydrolysis, antioxidant activity, molecular weight distribution patterns and peptide sequence. Casein hydrolysate produced by the sequential hydrolysis of alcalase and flavourzyme showed higher degree of hydrolysis and antioxidant activity as compared to hydrolysate obtained by individual enzymes. In size exclusion chromatograph of casein hydrolysate S3, peptides with molecular weight of 0.57 kDa share 12% area in total area of chromatogram which was 10 times higher than that of hydrolysate S1 and nearly half of that of hydrolysate S2. On subjecting to HPLC-TOF-ESI separation potential antioxidant peptides were identified. The peptide sequence VLPVPQ along with potential fragments was identified in hydrolysate S1 and S2 and HPHPHLS along with its potential sequence was identified in hydrolysate S1, S2 and S3. Sequential hydrolysis of casein showed better antioxidant activity and peptide profile in less duration as compared to the casein hydrolysate obtained by individual enzyme.

Graphic abstract

Real-Time Label-Free Kinetics Monitoring of Trypsin-Catalyzed Ester Hydrolysis by a Nanopore Sensor

Trypsin is an important proteolytic enzyme in the digestive system and its activity is a major indicator for evaluating diseases such as chronic pancreatitis. Here, we present a novel label-free method to detect trypsin kinetics using a nanopore technique. A mutant α-hemolysin (M113R)₇ protein nanopore equipped with a polyamine decorated β-cyclodextrin (am₇β-CD) was employed as a sensing platform for the real-time monitoring of the process of trypsin enzymatic cleavage of a substrate Nα-benzoyl-l-arginine ethyl ester (BAEE) at the single molecule level. Significantly, this sensor can exclusively respond to the current modulation caused by the product and prevent interference from the substrate, thus improving detection sensitivity, and it provides a new scheme to detect enzyme activity for cleaving small molecules.

Alternative splicing events expand molecular diversity of camel CSN1S2 increasing its ability to generate potentially bioactive peptides

In a previous study on camel milk from Kazakhstan, we reported the occurrence of two unknown proteins (UP1 and UP2) with different levels of phosphorylation. Here we show that UP1 and UP2 are isoforms of camel αs2-CN (αs2-CNsv1 and αs2-CNsv2, respectively) arising from alternative splicing events. First described as a 178 amino-acids long protein carrying eight phosphate groups, the major camel αs2-CN isoform (called here αs2-CN) has a molecular mass of 21,906 Da. αs2-CNsv1, a rather frequent (35%) isoform displaying a higher molecular mass (+1,033 Da), is present at four phosphorylation levels (8P to 11P). Using cDNA-sequencing, αs2-CNsv1 was shown to be a variant arising from the splicing-in of an in-frame 27-nucleotide sequence encoding the nonapeptide ENSKKTVDM, for which the presence at the genome level was confirmed. αs2-CNsv2, which appeared to be present at 8P to 12P, was shown to include an additional decapeptide (VKAYQIIPNL) revealed by LC-MS/MS, encoded by a 3'-extension of exon 16. Since milk proteins represent a reservoir of biologically active peptides, the molecular diversity generated by differential splicing might increase its content. To evaluate this possibility, we searched for bioactive peptides encrypted in the different camel αs2-CN isoforms, using an in silico approach. Several peptides, putatively released from the C-terminal part of camel αs2-CN isoforms after in silico digestion by proteases from the digestive tract, were predicted to display anti-bacterial and antihypertensive activities.

Impact of the environmental conditions and substrate pre-treatment on whey protein hydrolysis: A review

Proteins in solution are subject to myriad forces stemming from interactions with each other as well as with the solvent media. The role of the environmental conditions, namely pH, temperature, ionic strength remains under-estimated yet it impacts protein conformations and consequently its interaction with, and susceptibility to, the enzyme. Enzymes, being proteins are also amenable to the environmental conditions because they are either activated or denatured depending on the choice of the conditions. Furthermore, enzyme specificity is restricted to a narrow regime of optimal conditions while opportunities outside the optimum conditions remain untapped. In addition, the composition of protein substrate (whether mixed or single purified) have been underestimated in previous studies. In addition, protein pre-treatment methods like heat denaturation prior to hydrolysis is a complex phenomenon whose progression is influenced by the environmental conditions including the presence or absence of sugars like lactose, ionic strength, purity of the protein, and the molecular structure of the mixed proteins particularly presence of free thiol groups. In this review, we revisit protein hydrolysis with a focus on the impact of the hydrolysis environment and show that preference of peptide bonds and/or one protein over another during hydrolysis is driven by the environmental conditions. Likewise, heat-denaturing is a process which is dependent on not only the environment but the presence or absence of other proteins.

Encapsulation of antioxidant peptide enriched casein hydrolysate using maltodextrin-gum arabic blend

Antioxidant peptide enriched casein hydrolysate (AO-CH) are receiving increasing attention due to their potential as functional ingredient. Encapsulation of AO-CH using maltodextrin-gum arabic (MD/GA) as wall material could represent an attractive approach to overcome the problems related to their direct application. Encapsulation parameter were optimized using different ratio of core to coat and proportion of coating material (10:0, 8:2, 6:4) under varying pH (2-8) for encapsulation efficiency (EE).The preparation P3 resulted in maximum EE (87%) using core to coat ratio 1:20, at pH 6.0 with 8:2 MD/GA ratio. The encapsulated preparation showed reduced bitterness (p < 0.05) compared to the casein hydrolysate together with maximum retention of antioxidant activity (93%). Further, the narrow range of particle size, indicates their better stability and represents a promising food additive for incorporation in food.

Identification of Novel Phosphopeptides After Simulated Digestion of αs2-casein by Tandem Mass Spectrometry

Casein phosphopeptides (CPPs) are encrypted in αs1-, αs2-and β-casein (CN) and can be released by in vitro, in vivohydrolysis or food processing of dairy foods. Bovine αs2-CN contains two cluster sequences of anionic phosphoseryl and glutamyl residues SpSpSpEE in its structure (residues 8–12 and 56–63), which can modulate mineral bioavailability. In this study αs2-casein (αs2-CN) was subjected to simulated gastrointestinal digestion. CPPs released were sequenced by on-line reversed-phase high performance liquid chromatography coupled to electrospray ionisation tandem mass spectrometry (RP-HPLC-ESIMS/MS). Six novel αs2-CN derived CPPs, Three of them (αs2-CN(54–87)4P,αs2-CN(24–70)4P and αs2-CN(14–73)5P) with the mineral binding cluster sequence SpSpSpEE were identified and characterised. CPPs from αs2-CN identified in this study resist simulated gastrointestinal digestion. As the molecular weights of these CPPs are approx. 2,165–7,112Da, they could be absorbed by intestinal cells. Consequently, these αs2-CN derived CPPs could be promising candidates for incorporation to mineral fortified foods as functional ingredients.

Effect of Caseinophosphopeptides from αs- and β-Casein on Iron Bioavailability in HuH7 Cells

Two pools of caseinophosphopeptides (CPPs) obtained from αs- and β-casein fractions (α-CPPs and β-CPPs) were characterized. A total of 16 CPPs were identified in the α-CPPs pool, 9 of them derived from αs1-casein and 7 from αs2-casein. A total of 18 CPPs were identified in the β-CPPs pool. Four of the identified CPPs contained the characteristic phosphoseryl-glutamic acid cluster SpSpSpEE. Calcein assay was used to compare the iron-binding capacity of the α- and β-CPPs pools. At the concentration of 12.5 μM CPPs used in the iron bioavailability assays, β-CPPs pools show greater iron-binding capacity than α-CPPs pools. HuH7 human hepatoma cells show many differentiated functions of liver cells in vivo and can be used to evaluate iron bioavailability (ferritin content and soluble transferrin receptor) from Fe-α-CPPs and Fe-β-CPPs complexes. The α-CPPs and β-CPPs pools did not improve ferritin content or soluble transferrin receptor in HuH7 cells.

Iron (II)-chelating activity of buffalo αS-casein hydrolysed by corolase PP, alcalase and flavourzyme

Iron is a vital substance for human health which participates in many biochemical reactions. It also act as initiator for many harmful oxidative process. Buffalo αS-casein enriched fraction (80 %) was hydrolysed independently by corolase PP (H1), alcalase (H2), flavourzyme (H3) and sequentially by alcalase-flavourzyme (H4). After ultrafiltration (10 and 3 kDa) hydrolysates were analysed for their iron chelation activity using ferrozine. For H1 group of hydrolysates highest iron (II)-chelation activity (265.58 μM Fe(2+/)mg protein) was found after 8 h of hydrolysis for H2 (267.56 μM Fe(2+/)mg protein) and H3 group of hydrolysates (380.68 μM Fe(2+/)mg protein) after 6 h of hydrolysis. Sequential hydrolysis was not effective for iron (II)-chelation activity. 3 kDa fractions show higher iron (II)-chelation activity than 10 kDa fraction. Flavourzyme was more effective for generation of iron (II)-chelating peptides from buffalo αS-casein.

Plasmin activity in UHT milk: relationship between proteolysis, age gelation, and bitterness

Plasmin, the major indigenous protease in milk, is linked to quality defects in dairy products. The specificity of plasmin on caseins has previously been studied using purified caseins and in the indigenous peptide profile of milk. We investigated the specificity and proteolytic pathway of plasmin in directly heated UHT milk (>150 °C for <0.2 s) during 14 weeks of storage at 20 °C in relation to age gelation and bitter peptides. Sixty-six peptides from αS- and β-caseins could be attributed to plasmin activity during the storage period, of which 23 were potentially bitter. Plasmin exhibited the highest affinity for the hydrophilic regions in the caseins that most probably were exposed to the serum phase and the least affinity for hydrophobic or phosphorylated regions. The proteolytic pattern observed suggests that plasmin destabilizes the casein micelle by hydrolyzing casein-casein and casein-calcium phosphate interaction sites, which may subsequently cause age gelation in UHT milk.

Casein hydrolysis by Bifidobacterium longum KACC91563 and antioxidant activities of peptides derived therefrom

Milk protein is a well-known precursor protein for the generation of bioactive peptides using lactic acid bacteria. This study investigated the antioxidant activity of bovine casein hydrolysate after fermentation with Bifidobacterium longum KACC91563 using the 2,2'-azinobis-(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) assay and total phenolic content (TPC). The antioxidant activities of the 24-h and 48-h hydrolysates were higher than that of the 4-h hydrolysate (2,045.5 and 1,629.3 μM gallic acid equivalents, respectively, vs. 40.3 μM) in the ABTS assay. In contrast, TPC values showed activities of 43.2 and 52.4 μM gallic acid equivalents for the 4-h and 24-h hydrolysates, respectively. Three fractions (≥10 kDa, ≥3 but <10 kDa, and <3 kDa) were separated from the 24-h hydrolysate by ultrafiltration. Among these fractions, the <3 kDa fraction exhibited the highest antioxidant activity (936.7 μM) compared with the other fractions (42.1 and 34.2 μM for >10 kDa and 3-10 kDa fractions, respectively). Through liquid chromatography-electrospray ionization-tandem mass spectrometry analysis, 2 peptides, VLSLSQSKVLPVPQK and VLSLSQSKVLPVPQKAVPYPQRDMPIQA, containing the fragment VLPVPQ that has antioxidant properties, were identified in the <3kDa fraction after 24h of hydrolysis. The present study demonstrates the possibility of antioxidant peptide production from bovine casein using Bifidobacterium longum.

Alkaline hydrolysis of porcine blood haemoglobin: applications for peptide and amino acid production

Alkaline hydrolysis of proteins recovered from slaughterhouse blood is a method to obtain profitable peptides and free amino acids for animal feed, besides decreasing the waste produced by this industry. The current trend to use enzymatic hydrolysis may need reconsidering due to its high cost in materials and the need for control processes that are both complex and expensive. The use of caustic soda (NaOH), which is a low-cost product, to obtain useful peptides from porcine haemoglobin is studied in this paper. Concentrations of 6 M NaOH at 50°C for 24 h afforded an 80% peptide recovery yield with an average peptide size of 13 kDa. Product obtained at 24 h was composed of soluble haemoglobin (7%), peptides larger than 10 kDa (63%), peptides between 6 and 10 kDa (16%), peptides between 1 and 6 kDa (1%), free amino acids (4%) and non-soluble compounds (8%). A kinetic model was subsequently developed. It is proposed that neutralising the alkaline product using acid products allows the processing of a higher amount of protein while employing the same amounts of reagents, although this topic requires further research.

Variability of hydrolysis of β-, αs1-, and αs2-caseins by 10 strains of Streptococcus thermophilus and resulting bioactive peptides

Milk proteins contain numerous potential bioactive peptides, which may be released by digestive proteases or by the proteolytic system of lactic acid bacteria during food processing. The capacity of Streptococcus thermophilus to generate peptides, especially bioactive peptides, from bovine caseins was investigated. Strains expressing various levels of the cell envelope proteinase, PrtS, were incubated with α(s1)-, α(s2)-, or β-casein. Analysis of the supernatants by LC-ESI-MS/MS showed that the β-casein was preferentially hydrolyzed, followed by α(s2)-casein and then α(s1)-casein. Numbers and types of peptides released were strain-dependent. Hydrolysis appeared to be linked with the accessibility of different casein regions by protease. Analysis of bonds hydrolyzed in the region 1-23 of α(s1)-casein suggests that PrtS is at least in part responsible for the peptide production. Finally, among the generated peptides, 13 peptides from β-casein, 5 from α(s2)-casein, and 2 from α(s1)-casein have been reported as bioactive, 15 of them being angiotensin-converting enzyme inhibitors.

The yield of peptides and amino acids following acid hydrolysis of haemoglobin from porcine blood

Animal blood is the most important waste product from the meat industry due to the huge volumes produced and its pollutant power. Different methods are currently employed to process this by-product, such as drying, incineration or enzymatic hydrolysis. All these techniques are expensive, do not result in revalorisation or are not applicable at an industrial scale. In this paper, chemical hydrolysis is presented as an alternative to recover and increase the value of purified haemoglobin, the most abundant protein in blood. Non-enzymatic hydrolysis of haemoglobin is a good method for obtaining peptides due to its low cost, ease of control and the large amount of peptides produced, as well as being suitable for industrial applications. This paper presents a study of the use of two acids (sulfuric and hydrochloric) for this purpose under different experimental conditions. From the analysis of the kinetics of the hydrolysis process, four fractions can be defined: unbroken haemoglobin, soluble peptides, non-soluble peptides and free amino acids. A kinetic model was developed to simulate the hydrolysis mechanisms, providing a good fit to the experimental results. Both sulfuric and hydrochloric acid at concentrations of 6 M can hydrolyse the haemoglobin completely, but the average peptide size is lower for sulfuric than for hydrochloric acid.

Milk versus caseinophosphopeptides added to fruit beverage: resistance and release from simulated gastrointestinal digestion

The influence of simulated gastrointestinal digestion on caseinophosphopeptides (CPPs) formation in milk-based fruit beverage was evaluated, together with resistance of a pool of CPPs added to fruit beverage. In milk-based fruit beverage, four CPPs were identified that can be justified by their presence in raw milk or due to processing. When it was subjected to simulated gastrointestinal digestion, 10 CPPs were identified, and only 1 presented the cluster (SpSpSpEE) (3 phosphoseryl group followed by 2 glutamic acid residues), which corresponded to alpha(s2)-CN(1-19)4P. CPPs added to fruit beverage are resistant to simulated gastrointestinal digestion, and 16 CPPs were identified originating from the fragmentation of added CPPs, and with a greater presence of the cluster compared with CPPs originating from milk-based fruit beverage. This could justify the use of CPPs as functional ingredients, and offer a good alternative to milk-based fruit beverage for improving mineral bioavailability.

Epitope characterization of a supramolecular protein assembly with a collection of monoclonal antibodies: the case of casein micelle

In milk, kappa-, beta-, alphas(1)- and alphas(2)-casein (CN) are associated into a supramolecular assembly, the micelle. In this work, CN micelles contained in fresh skim milk were used to produce over 100 monoclonal antibodies. The specificity of these probes was determined using libraries of synthetic peptides and peptides fractionated from tryptic hydrolysis of purified CNs. Although kappa-CN and alphas(2)-CN are minor proteins in the micelle (ratio 1:1:4:4 for kappa, alphas(2), alphas(1), beta) a proportionally high number of clones were produced towards these two proteins (32 for each), compared to 9 and 29 for alphas(1)-CN and beta-CN, respectively. Most of the beta-CN and kappa-CN epitopes were identified, while about 50% of alphas(1)-CN and alphas(2)-CN antibodies were suspected to react to conformational linear or discontinuous epitopes, since no peptide binding could be identified. Antibody binding to the phosphoserine rich regions of the three calcium sensitive CNs was weak or non-existing, suggesting them to be hidden in the micelle structure together with alphas(1)-CN. The C-terminal glycomacropeptide of kappa-CN and the C-terminal moiety of beta-CN were well exposed generating the majority of the antibodies specific for these two proteins. The two major antigenic sites of alphas(2) were alphas(2)-CN (f96-114) and (f16-35). Cross-reaction between alphas(2)-CN specific antibodies with alphas(1)-CN illustrated the tangled structure between the two proteins. Immuno-dominant epitopes identified in the present study totally differ from those known for the purified caseins suggesting they were specific for the micelle supramolecular structure.

Identification of casein phosphopeptides released after simulated digestion of milk-based infant formulas

Adapted, follow-up, probiotic follow-up, toddler, and probiotic toddler infant formulas were subjected to an in vitro enzymatic procedure simulating physiological digestion. The formation and identification of casein phosphopeptides (CPPs) in the milk-based infant formulas were studied using reversed phase high-performance liquid chromatography coupled on line to an ion trap mass spectrometer. Most CPPs formed contained the cluster sequence SpSpSpEE, a mineral binding site. Phosphopeptide alpha(s2)-CN(1-19)4P was present in all formulas analyzed. Probiotic formulas released CPPs not detected in nonprobiotic formulas and probably formed by bifidobacteria action. These observations suggest that physiological digestion of these products promotes the formation of bioactive peptides with mineral carrier properties in the gastrointestinal tract, which resist further proteolysis.

Speciation analysis of calcium, iron, and zinc in casein phosphopeptide fractions from toddler milk-based formula by anion exchange and reversed-phase high-performance liquid chromatography?mass spectrometry/flame atomic-absorption spectroscopy

Casein phosphopeptides (CPP) are phosphorylated casein-derived peptides that can be released by in-vitro or in-vivo enzymatic hydrolysis of alpha(s1)-casein, alpha(s2)-casein, and beta-casein (CN). Many of these peptides contain a highly polar acidic sequence of three phosphoseryl groups followed by two glutamic acid residues. These domains are binding sites for minerals such as calcium, iron, and zinc and play an important role in mineral bioavailability. The aim of this study was speciation analysis of calcium, iron, and zinc in CPP fractions from the soluble fraction of a toddler milk-based formula. Methods for CPP separation by anion-exchange high-performance liquid chromatography (AE-HPLC) were combined with CPP identification by reversed-phase high performance liquid chromatography-electrospray ionization mass spectrometry and determination of the calcium, iron, zinc, and phosphorus content of the fractions obtained by AE-HPLC. Calcium and phosphorus were detected in all the analyzed AE-HPLC fractions. Calcium and zinc could be bound to CPP derived from alpha(s1)-CN and alpha(s2)-CN in fraction 3. Iron could be bound to CPP in fraction 4 in which beta-CN(15-34)4P was present with the cluster sequence S(P)S(P)S(P)EE. The results obtained prove the different distribution of calcium, iron, and zinc in heterogeneous CPP fractions.

Mass spectrometric characterization of peptides derived by peptic cleavage of bovine β-casein

This study investigated the digestion of the milk protein beta-casein with pepsin under gastro-analogous conditions. Peptide sequences were identified using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry with post-source decay as well as liquid chromatography-tandem mass spectrometry by means of database searching. The new software tool, Mascot Distiller, improved the identification rate remarkably. In the case of small peptides, such as di- and tri-peptides, which are promising candidates for intestinal absorption and possible biological effects, identification was possible only after spectrum simulation and manual matching. A list of 41 identified peptides having 2-36 amino acids is given, and unexpected cleavage sites for pepsin are reported. Sequence coverage was 75%.