Calcium binding of phosphopeptides derived from hydrolysis of alpha s-casein or beta-casein using immobilized trypsin

Application of small angle scattering (SAS) in structural characterisation of casein and casein-based products during digestion

In recent years, small and ultra-small angle scattering techniques, collectively known as small angle scattering (SAS) have been used to study various food structures during the digestion process. These techniques play an important role in structural characterisation due to the non-destructive nature (especially when using neutrons), various in situ capabilities and a large length scale (of 1 nm to ∼20 μm) they cover. The application of these techniques in the structural characterisation of dairy products has expanded significantly in recent years. Casein, a major dairy protein, forms the basis of a wide range of gel structures at different length scales. These gel structures have been extensively researched utilising scattering techniques to obtain structural information at the nano and micron scale that complements electron and confocal microscopy. Especially, neutrons have provided opportunity to study these gels in their natural environment by using various in situ options. One such example is understanding changes in casein gel structures during digestion in the gastrointestinal tract, which is essential for designing personalised food structures for a wide range of food-related diseases and improve health outcomes. In this review, we present an overview of casein gels investigated using small angle and ultra-small angle scattering techniques. We also reviewed their digestion using newly built setups recently employed in various research. To gain a greater understanding of micro and nano-scale structural changes during digestion, such as the effect of digestive juices and mechanical breakdown on structure, new setups for semi-solid food materials are needed to be optimised.

A comprehensive review on preparation, structure-activities relationship, and calcium bioavailability of casein phosphopeptides

Calcium is one of the important elements for human health. Calcium deficiencies can lead to numerous diseases. Calcium chelating peptides have shown potential application in the management of calcium deficiencies. Casein phosphopeptides (CPP) are phosphoseryl-containing fragments of casein by enzymatic hydrolysis or fermentation during manufacture of milk products as well as during intestinal digestion. An increasing number of CPP with the ability to facilitate and enhance the bioavailability of calcium are being discovered and identified. In this review, 249 reported CPP derived from four types of bovine casein (αs1, αs2, β and κ) were collected, and the amino acid sequence and phosphoserine group information were sorted out. This review outlines the current enzyme hydrolysis, detection methods, purification, structure-activity relationship and mechanism of intestinal calcium absorption in vitro and in vivo as well as application of CPP.

Fabrication of a ferritin-casein phosphopeptide-calcium shell-core composite as a novel calcium delivery strategy

Plant ferritin has a natural cage-like nanospace for carrying bioactive ingredients. By taking advantage of the calcium binding ability of casein phosphopeptide (CPP) and the cage-like conformation of plant ferritin, a ferritin-CPP shell-core complex (FC) was fabricated with the reversible self-assembly character of ferritin induced by a pH 2.0/7.0 transition strategy. The FC-calcium composite (FCC) was further fabricated by binding of the FC with calcium ions. When the same amount of calcium was loaded, the calcium binding capacity of the FCC was 28.13 ± 1.65%, which was significantly higher than that of ferritin and CPP alone. Fluorescence and Fourier transform infrared analysis indicated that the CPP encapsulation and the calcium binding in the FCC influenced the ferritin structure. Transmission electron microscopy (TEM) and dynamic light scattering (DLS) results showed that the spherical morphology and the 12 nm-diameter size were sustained in the FC and FCC. Moreover, the FCC as a transport carrier could increase the precipitation time of calcium phosphate, and the encapsulated calcium could be released in a more sustained manner as compared with ferritin and CPP under simulated in vitro gastrointestinal conditions. This study presents a novel calcium delivery strategy based on the ferritin cage and CPP, which will improve the applicability of ferritin and CPP and enhance the bioavailability of calcium ions.

Advanced non-fluoride approaches to dental enamel remineralization: The next level in enamel repair management

In modern dentistry, a minimally invasive management of early caries lesions or early-stage erosive tooth wear (ETW) with synthetic remineralization systems has become indispensable. In addition to fluoride, which is still the non-plus-ultra in these early caries/ETW treatments, a number of new developments are in the test phase or have already been commercialized. Some of these systems claim that they are comparable or even superior to fluoride in terms of their ability to remineralize enamel. Besides, their use can help avoid some of the risks associated with fluoride and support treatments of patients with a high risk of caries. Two individual non-fluoride systems can be distinguished; intrinsic and extrinsic remineralization approaches. Intrinsic (protein/peptide) systems adsorb to hydroxyapatite crystals/organics located within enamel prisms and accumulate endogenous calcium and phosphate ions from saliva, which ultimately leads to the re-growth of enamel crystals. Extrinsic remineralization systems function on the basis of the external (non-saliva) supply of calcium and phosphate to the crystals to be re-grown. This article, following an introduction into enamel (re)mineralization and fluoride-assisted remineralization, discusses the requirements for non-fluoride remineralization systems, particularly their mechanisms and challenges, and summarizes the findings that underpin the most promising advances in enamel remineralization therapy.

KFP-1, a Novel Calcium-Binding Peptide Isolated from Kefir, Promotes Calcium Influx Through TRPV6 Channels

INTRODUCTION

Kefir is an acidic and alcoholic fermented milk product with multiple health-promoting benefits. A previous study demonstrated that kefir enhanced calcium absorption in intestinal Caco-2 cells. In this study, kefir-fermented peptide-1 (KFP-1) is isolated from the kefir peptide fraction, and its function as a calcium-binding peptide is characterized.

METHODS AND RESULTS

KFP-1 was identified as a 17-residue peptide with a sequence identical to that of κ-casein (residues 138-154) in milk protein. KFP-1 is demonstrated to promote calcium influx in Caco-2 and IEC-6 small intestinal cells in a concentration-dependent manner. TRPV6, but not L-type voltage-gated calcium channels, is associated with the calcium influx induced by KFP-1. An in vitro calcium binding assay indicates that the full-length KFP-1 peptide has a higher calcium-binding capacity than the two truncated KFP-1 peptides, KFP-1∆C5 and KFP-1C5. Alexa Fluor 594 labeling shows that KFP-1 is taken up by Caco-2 cells and interacts with calcium ions and TRPV6 protein. Moreover, KFP-1 is found moderately resistant to pepsin and pancreatin digestions and enhanced calcium uptake by intestinal enterocytes in vivo.

CONCLUSION

These data suggest that KFP-1, a novel calcium-binding peptide, binds extracellular calcium ions and enters Caco-2 and IEC-6 cells, and promotes calcium uptake through TRPV6 calcium channels. The present study is of great importance for developing kefir-derived metal ion-binding peptides as functional nutraceutical additives.

Potential of Food Hydrolyzed Proteins and Peptides to Chelate Iron or Calcium and Enhance their Absorption

Iron and calcium are two essential micronutrients that have strong effects on nutrition and human health because of their involvement in several biological and redox processes. Iron is responsible for electron and oxygen transport, cell respiration, and gene expression, whereas calcium is responsible for intracellular metabolism, muscle contraction, cardiac function, and cell proliferation. The bioavailability of these nutrients in the body is dependent on enhancers and inhibitors, some of which are found in consumed foods. Hydrolyzed proteins and peptides from food proteins can bind these essential minerals in the body and facilitate their absorption and bioavailability. The binding is also important because excess free iron will increase oxidative stress and the risks of developing chronic diseases. This paper provides an overview of the function of calcium and iron, and strategies to enhance their absorption with an emphasis on hydrolyzed proteins and peptides from foods. It also discusses the relationship between the structure of peptides and their potential to act as transition metal ligands.

Selection of marker peptides from casein phosphopeptide and application for quantification in infant formula

Casein phosphopeptides (CPPs) have been used worldwide as a nutritional supplement. However, the peptide components have been unknown; as a consequence, few quantification methods of CPP in infant formula were reported. This study introduced a quantification method based on peptide marker and corresponding peptide selection strategy using a simplified model with four commercial types of CPP. The peptides from four commercial CPPs were first identified. Due to the great variety of CPPs, two marker selection strategies were adopted: on one hand, universal marker peptide VLPVPQK can be used for the quantification of all four commercial CPPs, if the CPP can be obtained as a standard. On the other hand, the specific marker peptide LYQEPVLGPV can be used for identification and quantification of commercial CPP type K content in infant formula with a fixed calculation factor. In the simplified model, the combination use of the two markers can meet most of the requirements of CPP analysis in infant formula. The method validation revealed that this was suitable for the routine analysis laboratories without proteomics backgrounds. This selection strategy was suggested for the large-scale marker peptide selection with all commercial CPPs, which can give a comprehensive solution of CPP quantification in infant formula. Abbreviations: CPP: Casein phosphopeptides; LC: Liquid chromatography; TQMS: Triple quadrupole mass spectrometry; MRM: Multiple reaction monitoring; RSD: Relative standard deviation; L*: [¹³C₆, ¹⁵N]-leucine; SSSEE: Peptides sequence of serine-serine-serine-glutamic acid-glutamic acid.

Activation of Src family tyrosine kinases by ferric ions

The Src-family tyrosine kinases (SFKs) are oncogenic enzymes that contribute to the initiation and progression of many types of cancer. In normal cells, SFKs are kept in an inactive state mainly by phosphorylation of a consensus regulatory tyrosine near the C-terminus (Tyr(530) in the SFK c-Src). As recent data indicate that tyrosine modification enhances binding of metal ions, the hypothesis that SFKs might be regulated by metal ions was investigated. The c-Src C-terminal peptide bound two Fe(3+) ions with affinities at pH4.0 of 33 and 252μM, and phosphorylation increased the affinities at least 10-fold to 1.4 and 23μM, as measured by absorbance spectroscopy. The corresponding phosphorylated peptide from the SFK Lyn bound two Fe(3+) ions with much higher affinities (1.2pM and 160nM) than the Src C-terminal peptide. Furthermore, when Lyn or Hck kinases, which had been stabilised in the inactive state by phosphorylation of the C-terminal regulatory tyrosine, were incubated with Fe(3+) ions, a significant enhancement of kinase activity was observed. In contrast Lyn or Hck kinases in the unphosphorylated active state were significantly inhibited by Fe(3+) ions. These results suggest that Fe(3+) ions can regulate SFK activity by binding to the phosphorylated C-terminal regulatory tyrosine.

Determination of calcium-binding constants of caseins, phosphoserine, citrate and pyrophosphate: A modelling approach using free calcium measurement

Dairy products contain large amount of calcium which is bound to caseins and different chelating agents like citrate and polyphosphates. The present study aimed to determine the calcium-binding capacities of phosphoserine (SerP), caseinophosphopeptide (CPP), β-casein, caseinate, citrate and pyrophosphate in the same conditions of temperature, pH and ionic strength. The free calcium (Ca(2+)) was measured using a calcium ion-selective electrode and plotted as a function of total calcium concentration. The association constants and the number of calcium-binding sites were determined by fitting the experimental data to a theoretical model. The phosphate groups of caseins were the main binding sites with evidence for participation of carboxylate groups. The intrinsic association constants determined by the best fit of the data were in the order: pyrophosphate (557×10(3)M(-1))>citrate (20×10(3)M(-1))>β-casein (5×10(3)M(-1))>caseinate, CPP and SerP (∼10(3)M(-1)). These findings may be of interest for the development of calcium-enriched products to overcome calcium deficiency in specific populations.

New Approaches to Enhanced Remineralization of Tooth Enamel

Dental caries is a highly prevalent diet-related disease and is a major public health problem. A goal of modern dentistry is to manage non-cavitated caries lesions non-invasively through remineralization in an attempt to prevent disease progression and improve aesthetics, strength, and function. Remineralization is defined as the process whereby calcium and phosphate ions are supplied from a source external to the tooth to promote ion deposition into crystal voids in demineralized enamel, to produce net mineral gain. Recently, a range of novel calcium-phosphate-based remineralization delivery systems has been developed for clinical application. These delivery systems include crystalline, unstabilized amorphous, or stabilized amorphous formulations of calcium phosphate. These systems are reviewed, and the technology with the most scientific evidence to support its clinical use is the remineralizing system utilizing casein phosphopeptides to stabilize and deliver bioavailable calcium, phosphate, and fluoride ions. The recent clinical evidence for this technology is presented and the mechanism of action discussed. Biomimetic approaches to stabilization of bioavailable calcium, phosphate, and fluoride ions and the localization of these ions to non-cavitated caries lesions for controlled remineralization show promise for the non-invasive management of dental caries.

Phosphorylation of cyclin-dependent kinase 2 peptides enhances metal binding

The cyclin-dependent kinase CDK2 is inactivated by phosphorylation on either of the two neighbouring residues Thr14 or Tyr15. The effect of phosphorylation on metal ion binding has been investigated with peptides incorporating residues 6-20 of CDK2. The stoichiometry of Ca(2+) binding increased from 1 in the un- and singly-phosphorylated peptides to 2 in the doubly phosphorylated peptide, without large changes in the affinity (75-250 microM). In contrast although binding of ferric ions to the un-phosphorylated peptide was not detected, both singly- and doubly-phosphorylated peptides bound two Fe(3+) ions. Binding of Ca(2+) or Zn(2+) ions to the doubly phosphorylated CDK2 peptide did not cause any change in absorbance, but increased the affinity of the peptide for Fe(3+) ions. These results demonstrate that double phosphorylation of CDK2 peptides increases the stoichiometry of metal ion binding, and hence may contribute to the previously observed regulation of CDK2 activity by metal ions.

Tyrosine modification enhances metal-ion binding

Tyrosine sulfation is a common modification of many proteins, and the ability to phosphorylate tyrosine residues is an intrinsic property of many growth-factor receptors. In the present study, we have utilized the peptide hormone CCK(8) (cholecystokinin), which occurs naturally in both sulfated and unsulfated forms, as a model to investigate the effect of tyrosine modification on metal-ion binding. The changes in absorbance and fluorescence emission on Fe(3+) binding indicated that tyrosine sulfation or phosphorylation increased the stoichiometry from 1 to 2, without greatly affecting the affinity (0.6-2.8 microM at pH 6.5). Measurement of Ca(2+) binding with a Ca(2+)-selective electrode revealed that phosphorylated CCK(8) bound two Ca(2+) ions. CCK(8) and sulfated CCK(8) each bound only one Ca(2+) ion with lower affinity. Binding of Ca(2+), Zn(2+) or Bi(3+) to phosphorylated CCK(8) did not cause any change in absorbance, but substantially increased the change in absorbance on subsequent addition of Fe(3+). The results of the present study demonstrate that tyrosine modification may increase the affinity of metal-ion binding to peptides, and imply that metal ions may directly regulate many signalling pathways.

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.

Expression and bioactivity analysis of recombinant beta-CPP dimer

Beta-casein phosphopeptide (beta-CPP) is a bioactive peptide that carries different minerals, especially calcium. To investigate more effects of beta-CPP, eukaryotic expression vector of beta-CPP dimer was constructed and transfected into Chinese hamster ovary (CHO) cells. After selection, the cell lines stably expressing beta-CPP dimer were obtained, and the recombinant product was identified and purified. Activity assay of recombinant protein indicated that the recombinant beta-CPP dimer could improve Ca(2+) uptake of sperm, stimulate the proliferation of spleen cells, and induce apoptosis of some malignant tumor cells.

Secretagogue activities in cod (Gadus morhua) and shrimp (Penaeus aztecus) extracts and alcalase hydrolysates determined in AR4-2J pancreatic tumour cells

Peptides with gastrin immunoreactivity were measured in cod muscle (Gadus morhua) and shrimp heads (Penaeus aztecus) extracts and alcalase hydrolysates and separated by two chromatographic steps. Secretagogue activities present in crude extracts fractions were examined with or without specific antagonists of CCK receptors in AR4-2J cells. Several sub-fractions significantly stimulate amylase release, up to 110%. These stimulatory effects could be completely inhibited by the presence of L 365, 260 specific antagonist of CCKB receptors. After hydrolysis of the raw material, the samples were partially fractionated by two chromatographic steps and potential active fractions detected by a gastrin-CCK radioimmunoassay. The molecular masses of the active fractions were lower than for the extracts. Stimulation of amylase release was higher than with extracts, and the inhibition by L 365, 260, less pronounced. These results show that some peptides remaining after hydrolysis or extraction still exert biological activities and have to be tested in nutritional studies.

Effects of an anticariogenic casein phosphopeptide on calcium diffusion in streptococcal model dental plaques

Casein phosphopeptides (CPP) stabilize amorphous calcium phosphate (ACP) and may be used to localize ACP in dental plaque, maintaining a state of supersaturation with respect to tooth enamel, reducing demineralization and enhancing remineralization. The aim here was to investigate these effects by measuring the effect of CPP-ACP on calcium diffusion in plaque. Using Dibdin's effusion system, calcium diffusion was measured in streptococcal model plaques. This demonstrated that by providing a large number of possible binding sites for calcium, 0.1% CPP-ACP reduces the calcium diffusion coefficient by about 65% at pH 7 and 35% at pH 5. Hence, CPP-ACP binds well to plaque, providing a large calcium reservoir within the plaque and slowing diffusion of free calcium. This is likely to restrict mineral loss during a cariogenic episode and provide a potential source of calcium for subsequent remineralization. Overall, once in place, CPP-ACP will restrict the caries process.

Preparation of phosphopeptides derived from alpha s-casein and beta-casein using immobilized glutamic acid-specific endopeptidase and characterization of their calcium binding

Phosphopeptides that were derived from alpha s-CN or beta-CN were prepared with immobilized glutamic acid-specific endopeptidase, and their Ca2+ binding was characterized. alpha s-Casein or beta-CN was hydrolyzed in a fluidized bed bioreactor containing 2 ml of immobilized glutamic acid-specific endopeptidase by recirculating 20 ml of alpha s-CN or beta-CN solution (10 mg/ml in 50 mM Tris.HCl and 0.02% NaN3, pH 8.0) for 3 h at 20 degrees C. The molecular masses of casein peptides were monitored by SDS-PAGE. Each hydrolysate was applied to an anion-exchange column using stepwise elution with various concentrations of KCl to separate peptides. The casein phosphopeptide content of the elution profile was monitored by analysis of protein and P concentrations. Calcium binding in phosphopeptide-enriched fractions was determined by CaCl2 titration and measurement of free Ca2+ with a Ca-selective electrode. The electrophoresis patterns showed four major peptides having molecular masses of 10.8, 9.0, 6.6, and 3.6 kDa in the alpha s-CN hydrolysate and 9.3, 8.2, and 6.2 kDa in the beta-CN hydrolysate. The highest concentrations of P were detected in the fractions that eluted with 0.4 and 0.5 M KCl for the alpha s-CN hydrolysate and with 0.4 M KCl for the beta-CN hydrolysate. The calcium-binding ability was found only in the fraction that was eluted with 0.4 M KCl; the maximum Ca2+ binding and the apparent binding constant were 0.24 mmol/mg of protein and 75 M-1, and 0.14 mmol/mg of protein and 148 M-1, respectively. alpha s-Casein phosphopeptides had different patterns for Ca2+ binding than did beta-CN phosphopeptides as the total Ca concentration was increased. Calcium binding to these casein phosphopeptides differed from that previously characterized for the tryptic peptides.