A Study of β-Casein Tertiary Structure by Intramolecular Crosslinking and Mass Spectrometry

Cactus Pear Mucilage (Opuntia spp.) as a Novel Functional Biopolymer: Mucilage Extraction, Rheology and Biofilm Development

The investigation of novel, natural polymers has gained considerably more exposure for their desirable, often specific, functional properties. Multiple researchers have explored these biopolymers to determine their potential to address many food processing, packaging and environmental concerns. Mucilage from the cactus pear (Opuntia ficus-indica) is one such biopolymer that has been identified as possessing a functional potential that can be used in an attempt to enhance food properties and reduce the usage of non-biodegradable, petroleum-based packaging in the food industry. However, variations in the structural composition of mucilage and the different extraction methods that have been reported by researchers have considerably impacted mucilage's functional potential. Although not comparable, these factors have been investigated, with a specific focus on mucilage applications. The natural ability of mucilage to bind water, alter the rheology of a food system and develop biofilms are considered the major applications of mucilage's functional properties. Due to the variations that have been reported in mucilage's chemical composition, specifically concerning the proportions of uronic acids, mucilage's rheological and biofilm properties are influenced differently by changes in pH and a cross-linker. Exploring the factors influencing mucilage's chemical composition, while co-currently discussing mucilage functional applications, will prove valuable when evaluating mucilage's potential to be considered for future commercial applications. This review article, therefore, discusses and highlights the key factors responsible for mucilage's specific functional potential, while exploring important potential food processing and packaging applications.

An in vitro protocol to characterise the resistance of food proteins to intestinal digestion

In vitro digestion tests provide data on the form in which dietary proteins maybe presented to the gut mucosal immune system, one of many strands of evidence used in allergenicity risk assessment. A 96-well plate format in vitro intestinal digestion protocol has been developed with a high and low enzyme activity test executed at pH 6.5 and 8.0. It was applied to the systematic analysis of test proteins (including six allergens and one non-allergenic comparator) which were either completely resistant to pepsinolysis or gave rise to large persistent fragments following in vitro gastric digestion. Digestion was monitored using SDS-PAGE and densitometry. Proteins resistant to pepsin were also resistant to intestinal digestion irrespective of the protocol applied and gave rise to large persistent digestion fragments. In contrast persistent fragments from pepsin digestion were readily digested. Bile salts enhanced the digestibility of two highly resistant proteins, lysozyme ad β-lactoglobulin, changing the rank order of protein digestibility. Intestinal digestion tests that include bile salts provide a more physiologically relevant system for future investigation into how digestion products may influence the balance between tolerance and sensitization - and hence contribute to future development of a more effective allergenicity risk assessment process.

Label-Free Higher Order Structure and Dynamic Investigation Method of Proteins in Solution Using an Enzymatic Reactor Coupled to Electrospray High-Resolution Mass Spectrometry Detection

For decades, structural analysis of proteins have received considerable attention, from their sequencing to the determination of their 3D structures either in the free state (e.g., no host-guest system, apoproteins) or (non)covalently bound complexes. The elucidation of the 3D structures and the mapping of intra- and intermolecular interactions are valuable sources of information to understand the physicochemical properties of such systems. X-ray crystallography and nuclear magnetic resonance are methods of choice for obtaining structures at the atomic level. Nonetheless, they still present drawbacks which limit their use to highly purified systems in a relatively high amount. On the contrary, mass spectrometry (MS) has become a powerful tool thanks to its selectivity, sensitivity, and the development of structural methods both at the global shape and the residue level. The combination of several MS-based methods is mandatory to fully assign a putative structure in combination with computational chemistry and bioinformatics. In that context, we propose a strategy which complements the existing methods of structural studies (e.g., circular dichroism, hydrogen/deuterium exchange and cross-links experiments, nuclear magnetic resonance). The workflow is based on the collection of structural information on proteins from the apparition rates and the time of appearance of released peptides generated by a protease in controlled experimental conditions with online detection by electrospray high-resolution mass spectrometry. Nondenaturing, partially or fully denatured proteins were digested by the enzymatic reactor, i.e., β-lactoglobulin, cytochrome c, and β-casein. The collected data are interpreted with regard to the kinetic schemes with time-dependent rates of the enzymatic digestion established beforehand, considering kinetics parameters in the Michaelis-Menten formalism including k_cat (the turnover number), k₁ (formation of the enzyme-substrate complex), k_-1 (dissociation of the enzyme-substrate complex), k_off (local refolding of the protein around the cleavage site), and k_on (local unfolding of the protein around the cleavage site). Solvent-accessible surface analysis through digestion kinetics was also investigated. The initial apparition rates of released peptides varied according to the protein state (folded vs denatured) and informs the k_off/k_on ratio around the cleavage site. On the other hand, the time of appearance of a given peptide is related to its solvent accessibility and to the resilience of the residual protein structure in solution. Temperature-dependent digestion experiments allowed estimation of the type of secondary structures around the cleavage site.

Intracellular Staphylococcus aureus Infection Decreases Milk Protein Synthesis by Preventing Amino Acid Uptake in Bovine Mammary Epithelial Cells

Staphylococcus aureus (S. aureus) is one of the main pathogens in cow mastitis, colonizing mammary tissues and being internalized into mammary epithelial cells, causing intracellular infection in the udder. Milk that is produced by cows that suffer from mastitis due to S. aureus is associated with decreased production and changes in protein composition. However, there is limited information on how mastitis-inducing bacteria affect raw milk, particularly with regard to protein content and protein composition. The main purpose of this work was to examine how S. aureus infection affects milk protein synthesis in bovine mammary epithelial cells (BMECs). BMECs were infected with S. aureus, and milk protein and amino acid levels were determined by ELISA after S. aureus invasion. The activity of mTORC1 signaling and the transcription factors NF-κB and STAT5 and the expression of the amino acid transporters SLC1A3 and SLC7A5 were measured by western blot or immunofluorescence and RT-qPCR. S. aureus was internalized by BMECs in vitro, and the internalized bacteria underwent intracellular proliferation. Eight hours after S. aureus invasion, milk proteins were downregulated, and the level of BMECs that absorbed Glu, Asp, and Leu from the culture medium and the exogenous amino acids induced β-casein synthesis declined. Further, the activity of mTORC1 signaling, NF-κB, and STAT5 was impaired, and SLC1A3 and SLC7A5 were downregulated. Eight hours of treatment with 100 nM rapamycin inhibited NF-κB and STAT5 activity, SLC1A3 and SLC7A5 expression, and milk protein synthesis in BMECs. Thus mTORC1 regulates the expression of SLC1A3 and SLC7A5 through NF-κB and STAT5. These findings constitute a model by which S. aureus infection suppresses milk protein synthesis by decreasing amino acids uptake in BMECs.

The effects of pH and NaCl concentration on the structure of β-casein from buffalo milk

In the present study, we aimed to investigate the effects of pH and sodium chloride (NaCl) concentration on the structure of β-casein (β-CN) purified from buffalo milk using circular dichroism (CD), intrinsic tryptophan, and anilino-8-naphthalene sulfonate (ANS) fluorescence spectroscopy. We found that NaCl concentration played a critical role in the stability of the secondary structure of β-CN. The CD negative peak had a redshift as the NaCl concentration was increased and accompanied by a decrease of β-sheet content and an increase of α-helix content. ANS fluorescence spectroscopy also indicated that higher NaCl concentration and lower pH significantly affected the tertiary structure of β-CN. Dynamic light scattering (DLS) results showed that the particle size of buffalo β-CN had a blueshift, and then a redshift within the pH range of 5.0-7.5, and it showed a redshift when the NaCl concentration was increased.

Self-Assembled Casein Nanoparticles Loading Triptolide for the Enhancement of Oral Bioavailability

Triptolide (TP), a broad-spectrum antitumor drug, has very poor solubility and oral bioavailability, which limits its clinical use. Compared with conventional formulations of TP, a casein (Cas)-based drug delivery system has been reported to have significant advantages for the improvement of solubility and bioavailability of insoluble drugs. In this paper, we report the successful preparation of TP-loaded Cas nanoparticles (TP-Cas) using the self-assembly characteristics of Cas in water and the optimization of the formulation by evaluation of entrapment efficiency (EE) and loading efficiency (LE). Dynamic light scattering, transmission electron microscopy, Fourier-transform infrared spectrometry, X-ray diffractometry (XRD), and differential scanning calorimetry (DSC) was adopted to characterize the TP-Cas. Results showed that the obtained TP-Cas were approximately spherical with a particle size of 128.7 ± 11.5 nm, EE of 72.7 ± 4.7 %, and LE of 8.0% ± 0.5%. Furthermore, in vitro release behavior of TP-Cas in PBS (pH = 7.4) was also evaluated, showing a sustained-release profile. Additionally, an in vivo study in rats displayed that the mean plasma concentration of TP after oral administration of TP-Cas was significantly higher than that treated with TP oral suspension. The C max value for TP-Cas (8.0 ± 4.4 μg/mL) was significantly increased compared with the free TP (0.9 ± 0.3 μg/mL). Accordingly, the area under the curve (AUC0-8) of TP-Cas was 2.8 ± 0.8 mg/L·h, 4.3-fold higher than that of TP suspension (0.6 ± 0.1 mg/L·h). Therefore, it can be concluded that TP-Cas enhanced the absorption and improved oral bioavailability of TP. Taking the good oral safety of Cas into consideration, TP-Cas should be a more promising preparation of TP for clinical application.

Casein nanoparticles as oral delivery carriers of mequindox for the improved bioavailability

Mequindox (Meq) is a promising broad-spectrum antibacterial agent, but the clinical application of Meq has been hampered by its low oral bioavailability. Casein (Cas) can bind to a variety of poorly water-soluble drugs to improve their water solubility through a micellar solubilization mechanism. Here, a low-cost and convenient method was introduced to prepare mequindox-loaded casein nanoparticles (Meq-Cas). Meq-Cas was characterized by several methods including differential scanning calorimetry (DSC), X-ray diffraction (XRD), and fourier transform infrared (FTIR) to illuminate the mutual effect between the drug and carriers. Meq-Cas presented nearly spherical nanoparticles with smooth surfaces and its mean particle size was lower than untreated Cas. Meq-Cas showed a nearly complete release of Meq, which displayed a biphasic drug release pattern in both phosphate-buffered solution (PBS) and simulated gastric fluid (SGF). The relative oral bioavailability of Meq-Cas was found to be about 1.20 times higher than that of the animals treated with Meq suspension (control). These results suggest that Cas is a good candidate to load in Meq for pharmaceutical purposes.

Polyphenol-oxidase-catalyzed cross-linking of Ara h 2: reaction sites and effect on structure and allergenicity

BACKGROUND

Peanut is among the most common of food allergies, and one of its allergens is Ara h 2. A previous study revealed that this allergen was recognized by serum immunoglobulin E (IgE) in over 90% of a peanut-allergic patient population. Enzymatic cross-linking is a popular processing method used to tailor food functionality, such as antigenicity.

RESULT

The cross-linking reactions of Ara h 2 were catalyzed by polyphenol oxidase (PPO), and the relevant reaction sites were identified using mass spectrometry and StavroX software. Two pairs of intramolecular cross-linking peptides and two intermolecular cross-linking peptides were found. Intramolecular cross-linking was speculated to occur between ARG¹³¹ (amino acids 116-131) and TYR⁶⁵ (amino acids 63-80) and between TYR⁶⁰ (amino acids 56-62) and ARG⁹² (amino acids 92-102); the intermolecular cross-linking sites were ARG³¹ with TYR⁸⁴ or TYR⁸⁹ and TYR⁶⁵ or TYR⁷² with ARG⁹² or ARG¹⁰² . Three out of four cross-linking peptides were found in α-helices, and destruction of this secondary structure resulted in a loose tertiary structure. Although seven linear allergen epitopes were involved in cross-linking, the IgE binding capacity of protein changed slightly, while its sensitization potential decreased in mouse model.

CONCLUSION

Exploring the structural change of Ara h 2 after cross-linking is beneficial in further understanding the influence of structure on sensitization. This result indicated the future possibility of precision processing on structure of proteins to improve their properties. © 2019 Society of Chemical Industry.

Soft-Matter Approaches for Controlling Food Protein Interactions and Assembly

Animal- and plant-based proteins are present in a wide variety of raw and processed foods. They play an important role in determining the final structure of food matrices. Food proteins are diverse in terms of their biological origin, molecular structure, and supramolecular assembly. This diversity has led to segmented experimental studies that typically focus on one or two proteins but hinder a more general understanding of food protein structuring as a whole. In this review, we propose a unified view of how soft-matter physics can be used to control food protein assembly. We discuss physical models from polymer and colloidal science that best describe and predict the phase behavior of proteins. We explore the occurrence of phase transitions along two axes: increasing protein concentration and increasing molecular attraction. This review provides new perspectives on the link between the interactions, phase transitions, and assembly of proteins that can help in designing new food products and innovative food processing operations.

Modification of IgE binding to αS1-casein by proteolytic activity of Enterococcus faecium isolated from Iranian camel milk samples

Milk is a perfect source of nutrients for neonates. When breast feeding cannot be done, an infant's alimentation is usually initiated to cow's milk, among the primary foods. It has been reported that about 2.5% of juveniles under the age of 3 years manifest allergic reactions to cow's milk proteins. Among the cow's milk proteins, casein fractions are considered as the strongest allergenic proteins. The proteolytic enzymes of lactic acid bacteria (LAB), during fermentation of dairy products, can break down milk proteins especially caseins and subsequently reduce the immune reactivity of allergenic proteins. In this research, raw bovine and camel milk samples were screened for cocci LAB strains and after isolation, their proteolytic activity against bovine milk caseins were evaluated by SDS-PAGE and RP-HPLC. The potential of cocci LAB strains on α_S1-casein degradation and their potential to break down the principle allergenic epitopes of this protein was detected using indirect competitive ELISA. Molecular identification of the best proteolytic strain was fulfilled by 16S rDNA fragment sequencing with universal primers. The obtained results demonstrated that Enterococcus faecium isolated from raw camel milk samples was the most efficient isolate in hydrolyzing Na-caseinate and α_S1-casein. Hydrolysated α_S1-casein by Enterococcus faecium was also less recognized by IgE of bovine milk allergic patients' sera in comparison with native α_S1-casein. It has been proposed that Enterococcus faecium could be an efficient strain in allergenicity reduction of cow's milk proteins. So it could be an excellent candidate to be potentially used in dairy industries.

β-casein nanovehicles for oral delivery of chemotherapeutic Drug combinations overcoming P-glycoprotein-mediated multidrug resistance in human gastric cancer cells

Multidrug resistance (MDR) is a primary obstacle to curative cancer therapy. We have previously demonstrated that β-casein (β-CN) micelles (β-CM) can serve as nanovehicles for oral delivery and target-activated release of hydrophobic drugs in the stomach. Herein we introduce a novel nanosystem based on β-CM, to orally deliver a synergistic combination of a chemotherapeutic drug (Paclitaxel) and a P-glycoprotein-specific transport inhibitor (Tariquidar) individually encapsulated within β-CM, for overcoming MDR in gastric cancer. Light microscopy, dynamic light scattering and zeta potential analyses revealed solubilization of these drugs by β-CN, suppressing drug crystallization. Spectrophotometry demonstrated high loading capacity and good encapsulation efficiency, whereas spectrofluorometry revealed high affinity of these drugs to β-CN. In vitro cytotoxicity assays exhibited remarkable synergistic efficacy against human MDR gastric carcinoma cells with P-glycoprotein overexpression. Oral delivery of β-CN - based nanovehicles carrying synergistic drug combinations to the stomach constitutes a novel efficacious therapeutic system that may overcome MDR in gastric cancer.

Milk Protein Polymer and Its Application in Environmentally Safe Adhesives

Milk proteins (caseins and whey proteins) are important protein sources for human nutrition; in addition, they possess important natural polymers. These protein molecules can be modified by physical, chemical, and/or enzymatic means. Casein is one of the oldest natural polymers, used for adhesives, dating back to thousands years ago. Research on milk-protein-based adhesives is still ongoing. This article deals with the chemistry and structure of milk protein polymers, and examples of uses in environmentally-safe adhesives. These are promising routes in the exploration of the broad application of milk proteins.

Advances in oral delivery of anti-cancer prodrugs

INTRODUCTION

Most anticancer drugs have poor aqueous solubility and low permeability across the gastrointestinal tract. Furthermore, extensive efflux by P-glycoproteins (P-gp) in the small intestine also limits the efficient delivery of anticancer drugs via oral route. Area covered: This review explores the prodrug strategy for oral delivery of anticancer drugs. Different categories of oral anticancer prodrugs along with recent clinical studies have been comprehensively reviewed here. Furthermore, novel anticancer prodrugs such as polymer-prodrugs and lipid-prodrugs have been discussed in detail. Finally, various nanocarrier-based approaches employed for oral delivery of anticancer prodrugs have also been discussed. Expert opinion: Premature degradation of anticancer prodrugs in the gastrointestinal tract could lead to variable pharmacokinetics and undesired toxicity. Despite their increased aqueous solubility, the oral bioavailability of several anticancer prodrugs are limited by their poor permeability across the gastrointestinal tract. These limitations can be overcome by the use of functional excipients (polymers, lipids, amino acids/dipeptides), which are specifically absorbed via transporters and receptor-mediated endocytosis. Oral delivery of anticancer prodrugs using nanocarrier-based drug delivery system is a recent development; however it should be justified based on the comparative advantages of encapsulating prodrug in a nanocarrier versus the use of anticancer prodrug molecule itself.

Relating gas phase to solution conformations: Lessons from disordered proteins

In recent years both mass spectrometry (MS) and ion mobility mass spectrometry (IM‐MS) have been developed as techniques with which to study proteins that lack a fixed tertiary structure but may contain regions that form secondary structure elements transiently, namely intrinsically disordered proteins (IDPs). IM‐MS is a suitable method for the study of IDPs which provides an insight to conformations that are present in solution, potentially enabling the analysis of lowly populated structural forms. Here, we describe the IM‐MS data of two IDPs; α‐Synuclein (α‐Syn) which is implicated in Parkinson's disease, and Apolipoprotein C‐II (ApoC‐II) which is involved in cardiovascular diseases. We report an apparent discrepancy in the way that ApoC‐II behaves in the gas phase. While most IDPs, including α‐Syn, present in many charge states and a wide range of rotationally averaged collision cross sections (CCSs), ApoC‐II presents in just four charge states and a very narrow range of CCSs, independent of solution conditions. Here, we compare MS and IM‐MS data of both proteins, and rationalise the differences between the proteins in terms of different ionisation processes which they may adhere to.

The effect of sugar stereochemistry on protein self-assembly: the case of β-casein micellization in different aldohexose solutions

Sugars promote self-assembly of amphiphilic proteins. The effectiveness of sugar stereoisomers correlates positively with their dynamic hydration number and negatively with their hydrophobic-to-hydrophilic molecular surface ratio.

Comprehensive peptidomic and glycomic evaluation reveals that sweet whey permeate from colostrum is a source of milk protein-derived peptides and oligosaccharides

Whey permeate is a co-product obtained when cheese whey is passed through an ultrafiltration membrane to concentrate whey proteins. Whey proteins are retained by the membrane, whereas the low-molecular weight compounds such as lactose, salts, oligosaccharides and peptides pass through the membrane yielding whey permeate. Research shows that bovine milk from healthy cows contains hundreds of naturally occurring peptides - many of which are homologous with known antimicrobial and immunomodulatory peptides - and nearly 50 oligosaccharide compositions (not including structural isomers). As these endogenous peptides and oligosaccharides have low-molecular weight and whey permeate is currently an under-utilized product stream of the dairy industry, we hypothesized that whey permeate may serve as an inexpensive source of naturally occurring functional peptides and oligosaccharides. Laboratory fractionation of endogenous peptides and oligosaccharides from bovine colostrum sweet whey was expanded to pilot-scale. The membrane fractionation methodology used was similar to the methods commonly used industrially to produce whey protein concentrate and whey permeate. Pilot-scale fractionation was compared to laboratory-scale fractionation with regard to the identified peptides and oligosaccharide compositions. Results were interpreted on the basis of whether industrial whey permeate could eventually serve as a source of functional peptides and oligosaccharides. The majority (96%) of peptide sequences and the majority (96%) of oligosaccharide compositions found in the laboratory-scale process were mirrored in the pilot-scale process. Moreover, the pilot-scale process recovered an additional 33 peptides and 1 oligosaccharide not identified from the laboratory-scale extraction. Both laboratory- and pilot-scale processes yielded peptides deriving primarily from the protein β-casein. The similarity of the laboratory-and pilot-scale's resulting peptide and oligosaccharide profiles demonstrates that whey permeate can serve as an industrial-scale source of bovine milk peptides and oligosaccharides.

Casein breakdown in bovine milk by a field strain of Staphylococcus aureus

The objectives of this study were to establish the proteolytic effects of Staphylococcus aureus during mastitis on economically important milk proteins. Concentrations of milk proteins were determined by capillary electrophoresis in an experimental model using a field strain of S. aureus. The pathogen was inoculated into bacteria-free control milk to imitate proteolysis caused by the pathogen in the mammary gland between milkings. Milk content of caseins (CN) α(S1), α(S2), κ, β(A1), and β(A2) and whey proteins α-lactalbumin and β-lactoglobulin were analyzed initially and after 6 h of incubation. After 6 h, the overall CN content was significantly reduced (21%) in milk inoculated with S. aureus compared with the bacteria-free control milk. S. aureus significantly lowered concentration of α(S1)-CN (2.5%), β(A1)-CN (3%), and β(A2)-CN (5%). S. aureus also hydrolyzed κ-CN into para-κ-CN, with significant reduction of κ-CN (7.4%) as a consequence.

Crosslinking and mass spectrometry suggest that the isolated NTD domain dimer of Moloney murine leukemia virus integrase adopts a parallel arrangement in solution

Background

Retroviral integrases (INs) catalyze the integration of viral DNA in the chromosomal DNA of the infected cell. This reaction requires the multimerization of IN to coordinate a nucleophilic attack of the 3’ ends of viral DNA at two staggered phosphodiester bonds on the recipient DNA. Several models indicate that a tetramer of IN would be required for two-end concerted integration. Complementation assays have shown that the N-terminal domain (NTD) of integrase is essential for concerted integration, contributing to the formation of a multimer through protein-protein interaction. The isolated NTD of Mo-MLV integrase behave as a dimer in solution however the structure of the dimer in solution is not known.

Results

In this work, crosslinking and mass spectrometry were used to identify regions involved in the dimerization of the isolated Mo-MLV NTD. The distances between the crosslinked lysines within the monomer are in agreement with the structure of the NTD monomer found in 3NNQ. The intermolecular crosslinked peptides corresponding to Lys 20-Lys 31, Lys 24-Lys 24 and Lys 68-Lys 88 were identified. The 3D coordinates of 3NNQ were used to derive a theoretical structure of the NTD dimer with the suite 3D-Dock, based on shape and electrostatics complementarity, and filtered with the distance restraints determined in the crosslinking experiments.

Conclusions

The crosslinking results are consistent with the monomeric structure of NTD in 3NNQ, but for the dimer, in our model both polypeptides are oriented in parallel with each other and the contacting areas between the monomers would involve the interactions between helices 1 and helices 3 and 4.

Extensive in vivo human milk peptidomics reveals specific proteolysis yielding protective antimicrobial peptides

Milk is traditionally considered an ideal source of the basic elemental nutrients required by infants. More detailed examination is revealing that milk represents a more functional ensemble of components with benefits to both infants and mothers. A comprehensive peptidomics method was developed and used to analyze human milk yielding an extensive array of protein products present in the fluid. Over 300 milk peptides were identified originating from major and many minor protein components of milk. As expected, the majority of peptides derived from β-casein, however no peptide fragments from the major milk proteins lactoferrin, α-lactalbumin, and secretory immunoglobulin A were identified. Proteolysis in the mammary gland is selective-released peptides were drawn only from specific proteins and typically from only select parts of the parent sequence. A large number of the peptides showed significant sequence overlap with peptides with known antimicrobial or immunomodulatory functions. Antibacterial assays showed the milk peptide mixtures inhibited the growth of Escherichia coli and Staphylococcus aureus . The predigestion of milk proteins and the consequent release of antibacterial peptides may provide a selective advantage through evolution by protecting both the mother's mammary gland and her nursing offspring from infection.

The HtrA protease from Streptococcus pneumoniae digests both denatured proteins and the competence-stimulating peptide

The HtrA protease of Streptococcus pneumoniae functions both in a general stress response role and as an error sensor that specifically represses genetic competence when the overall level of biosynthetic errors in cellular proteins is low. However, the mechanism through which HtrA inhibits development of competence has been unknown. We found that HtrA digested the pneumococcal competence-stimulating peptide (CSP) and constituted the primary extracytoplasmic CSP-degrading activity in cultures of S. pneumoniae. Mass spectrometry demonstrated that cleavage predominantly followed residue Phe-8 of the CSP-1 isoform of the peptide within its central hydrophobic patch, and in competition assays, both CSP-1 and CSP-2 interacted with HtrA with similar efficiencies. More generally, analysis of β-casein digestion and of digestion within HtrA itself revealed a preference for substrates with non-polar residues at the P1 site. Consistent with a specificity for exposed hydrophobic residues, competition from native BSA only weakly inhibited digestion of CSP, but denaturation converted BSA into a strong competitive inhibitor of such proteolysis. Together these findings support a model in which digestion of CSP by HtrA is reduced in the presence of other unfolded proteins that serve as alternative targets for degradation. Such competition may provide a mechanism by which HtrA functions in a quality control capacity to monitor the frequency of biosynthetic errors that result in protein misfolding.

β-Casein nanoparticle-based oral drug delivery system for potential treatment of gastric carcinoma: stability, target-activated release and cytotoxicity

We studied a potential drug delivery system comprising the hydrophobic anticancer drug paclitaxel entrapped within β-casein (β-CN) nanoparticles and its cytotoxicity to human gastric carcinoma cells. Paclitaxel was entrapped by stirring its dimethyl sulfoxide (DMSO) solution into PBS containing β-CN. Cryo-TEM analysis revealed drug nanocrystals, the growth of which was blocked by β-CN. Entrapment efficiency was nearly 100%, and the nanovehicles formed were colloidally stable. Following encapsulation and simulated digestion with pepsin (2 hours at pH=2, 37 °C), paclitaxel retained its cytotoxic activity to human N-87 gastric cancer cells; the IC(50) value (32.5 ± 6.2 nM) was similar to that of non-encapsulated paclitaxel (25.4 ± 2.6 nM). Without prior simulated gastric digestion, β-CN-paclitaxel nanoparticles were non-cytotoxic, suggesting the lack of untoward toxicity to bucal and esophageal epithelia. We conclude that β-CN shows promise to be useful for target-activated oral delivery of hydrophobic chemotherapeutics in the treatment of gastric carcinoma, one of the leading causes of cancer mortality worldwide.

Mesoscale crystallization of calcium phosphate nanostructures in protein (casein) micelles

Aqueous micelles of the multi-protein calcium phosphate complex, casein, were treated at 60°C and pH 7 over several months. Although partial dissociation of the micelles into 12 nm sized amorphous calcium phosphate (ACP)/protein nanoparticles occurred within a period of 14 days, crystallization of the ACP nanoclusters into bundles of hydroxyapatite (HAP) nanofilaments was not observed until after 12 weeks. The HAP nanofilaments were formed specifically within the partially disrupted protein micelles suggesting a micelle-mediated pathway of mesoscale crystallization. Similar experiments using ACP-containing synthetic micelles prepared from ß-casein protein alone indicated that co-aligned bundles of HAP nanofilaments were produced within the protein micelle interior after 24 hours at temperatures as low as 35°C. The presence of Mg²(+) ions in the casein micelles, as well as a possible synergistic effect associated with the multi-protein nature of the native aggregates, could account for the marked inhibition in mesoscale crystallization observed in the casein micelles compared with the single-component b-casein constructs.

Chaperone-like activities of different molecular forms of beta-casein. Importance of polarity of N-terminal hydrophilic domain

As a member of intrinsically unstructured protein family, beta-casein (beta-CN) contains relatively high amount of prolyl residues, adopts noncompact and flexible structure and exhibits chaperone-like activity in vitro. Like many chaperones, native beta-CN does not contain cysteinyl residues and exhibits strong tendencies for self-association. The chaperone-like activities of three recombinant beta-CNs wild type (WT) beta-CN, C4 beta-CN (with cysteinyl residue in position 4) and C208 beta-CN (with cysteinyl residue in position 208), expressed and purified from E. coli, which, consequently, lack the phosphorylated residues, were examined and compared with that of native beta-CN using insulin and alcohol dehydrogenase as target/substrate proteins. The dimers (beta-CND) of C4-beta-CN and C208 beta-CN were also studied and their chaperone-like activities were compared with those of their monomeric forms. Lacking phosphorylation, WT beta-CN, C208 beta-CN, C4 beta-CN and C4 beta-CND exhibited significantly lower chaperone-like activities than native beta-CN. Dimerization of C208 beta-CN with two distal hydrophilic domains considerably improved its chaperone-like activity in comparison with its monomeric form. The obtained results demonstrate the significant role played by the polar contributions of phosphorylated residues and N-terminal hydrophilic domain as important functional elements in enhancing the chaperone-like activity of native beta-CN. (c) 2009 Wiley Periodicals, Inc. Biopolymers 91: 623-632, 2009.This article was originally published online as an accepted preprint. The "Published Online" date corresponds to the preprint version. You can request a copy of the preprint by emailing the Biopolymers editorial office at biopolymers@wiley.com.

Affinity chromatography of GroEL chaperonin based on denatured proteins: role of electrostatic interactions in regulation of GroEL affinity for protein substrates

The chaperonin GroEL of the heat shock protein family from Escherichia coli cells can bind various polypeptides lacking rigid tertiary structure and thus prevent their nonspecific association and provide for acquisition of native conformation. In the present work we studied the interaction of GroEL with six denatured proteins (alpha-lactalbumin, ribonuclease A, egg lysozyme in the presence of dithiothreitol, pepsin, beta-casein, and apocytochrome c) possessing negative or positive total charge at neutral pH values and different in hydrophobicity (affinity for a hydrophobic probe ANS). To prevent the influence of nonspecific association of non-native proteins on their interaction with GroEL and make easier the recording of the complexing, the proteins were covalently attached to BrCN-activated Sepharose. At low ionic strength (lower than 60 mM), tight binding of the negatively charged denatured proteins with GroEL (which is also negatively charged) needed relatively low concentrations (approximately 10 mM) of bivalent cations Mg2+ or Ca2+. At the high ionic strength (approximately 600 mM), a tight complex was produced also in the absence of bivalent cations. In contrast, positively charged denatured proteins tightly interacted with GroEL irrespectively of the presence of bivalent cations and ionic strength of the solution (from 20 to 600 mM). These features of GroEL interaction with positively and negatively charged denatured proteins were confirmed by polarized fluorescence (fluorescence anisotropy). The findings suggest that the affinity of GroEL for denatured proteins can be determined by the balance of hydrophobic and electrostatic interactions.

Micellization of bovine beta-casein studied by isothermal titration microcalorimetry and cryogenic transmission electron microscopy

The association behavior, critical micellization concentration (CMC), and enthalpy of demicellization (DeltaHdemic) of bovine beta-casein were studied, for the first time by isothermal titration calorimetry, in a pH 7.0 phosphate buffer with 0.1 ionic strength and in pure water. In the buffer solutions, the CMC decreased asymptotically from 0.15 to 0.006 mM as the temperature was raised from 16 to 45 degrees C. DeltaHdemic decreased with increasing temperature between 16 and 28 degrees C but increased from 28 to 45 degrees C. Thermodynamic analysis below 30 degrees C is consistent with the Kegeles shell model, which suggests a stepwise association process. At higher temperatures, this model exhibits limitations, and the micellization becomes much more cooperative. The CMC values in water, measured between 17 and 28 degrees C, decreased with increasing temperature and, expectedly, were higher than those found in the buffer solutions. beta-Casein micelles were visualized and characterized, for the first time in their hydrated state, using advanced digital-imaging cryogenic transmission electron microscopy. The images revealed small, oblate micelles, about approximately 13 nm in diameter. The micelles shape and dimensions remained nearly constant in the temperature range of 24-35 degrees C.

Colloid science of mixed ingredients

Some recent advances in the colloid science of heterogeneous systems containing food biopolymer ingredients are reviewed. Understanding the instability processes controlling the shelf-life and rheology of food colloids requires a detailed knowledge of the factors affecting the nature of the interactions in emulsions and gels containing mixtures of protein + protein, protein + surfactant and protein + polysaccharide. Against this background, theoretical modelling and computer simulation are useful tools for predicting effects of system composition on stability mechanisms. Confocal microscopy combined with image analysis is providing new experimental insight into the microstructural origins of changes in macroscopic properties during processing and storage.

Influence of crosslinker identity and position on gas-phase dissociation of Lys-Lys crosslinked peptides

A systematic study of the dissociation patterns of crosslinked peptides analyzed by tandem mass spectrometry is reported. A series of 11-mer peptides was designed around either a polyalanine or polyglycine scaffold with arginine at the C terminus. One or two lysine residues were included at various locations within the peptides to effect inter- or intra-molecular crosslinking, respectively. Crosslinked species were generated with four commonly used amine-specific chemical crosslinking reagents: disuccinimidyl suberate (DSS), disuccinimidyl tartarate (DST), dithiobis(succinimidylpropionate) (DSP), and disuccinimidyl glutarate (DSG). The influence of precursor charge state, location of crosslink, and specific crosslinking reagent on the MS/MS dissociation pattern was examined. Observed trends in the dissociation patterns obtained for these species will allow for improvements to software used in the automated interpretation of crosslinked peptide MS/MS data.

Using Self-Consistent-Field Theory to Understand Enhanced Steric Stabilization by Casein-Like Copolymers at Low Surface Coverage in Mixed Protein Layers

We present a statistical mechanical approach to predicting the properties of mixed copolymer layers using the Scheutjens-Fleer self-consistent-field theory. Our model copolymers are based on the primary structures of the major bovine casein monomers, alpha(s1)-casein and beta-casein. Numerical calculations have been carried out to determine the polymer segment density profiles at an isolated hydrophobic surface and the interaction forces as a pair of polymer-coated surfaces is brought to close interlayer separation. For a copolymer model containing hydrophilic and hydrophobic segments, we show how the steric stabilizing capacity of a casein-like macromolecule at very low surface coverage is enhanced in the presence of a thin dense layer of shorter tethered amphiphilic chains. Using a more refined protein model, which also distinguishes between the charged and uncharged hydrophilic segments along the chain, we clearly demonstrate that the enhanced steric repulsion from beta-casein exceeds that from alpha(s1)-casein. These calculations explain how the replacement of just a few percent of beta-lactoglobulin by casein can inhibit the heat-induced thickening and flocculation behavior observed experimentally with some whey protein-stabilized oil-in-water emulsions.