A preliminary study by gel filtration and ultracentrifugation of the interaction of bovine milk caseins with detergents

The detergents sodium dodecyl sulphate and octyl phenoxy polyethoxyethanol interact with casein and cause dissociation of the high-molecular-weight casein aggregates. It is presumed that the detergent binds with hydrophobic regions in the casein molecule. The size of the complexes formed between detergents and αs1-casein, β-casein and κ-casein, as estimated by gel filtration and sedimentation velocity experiments, suggests that the caseins were complexed as monomers.

During gel filtration under non-reducing conditions, detergent-κ-casein complexes were separated from other major components because of their conversion through formation of disulphide bonds into high-molecular-weight aggregates. This reaction, which did not occur in sedimentation velocity experiments, was presumably facilitated by the changes in the equilibrium between the individual caseins during gel filtration.

Sedimentation velocity experiments showed that a ratio of about 40 detergent molecules to 1 casein molecule was required to give the smallest casein-detergent complex.

Influence of sodium chloride on the proteolysis of casein by rennet and by pepsin

1. Proteolysis of β-casein by rennin and by pepsin was completely inhibited in the presence of 10% NaCl and was very significantly reduced by 5% NaCl. The rate of proteolysis of αs-casein was maximal in the presence of 5–10% NaCl. 2. The inhibitory effect of NaCl on the proteolysis of β-casein was independent of pH and incubation temperature. 3. The effectiveness of NaCl in controlling the development of bitter flavour in Cheddar cheese may be due to its inhibitory effect on the proteolysis of β-casein. 4. Rennin hydrolysates of β-casein were bitter in flavour whereas those of αs-casein were not.

On the mechanism of milk clotting by rennin

Two possible hypotheses for the mechanism of milk clotting were tested. The results obtained constituted strong evidence against one and suggested that the second is improbable. Milk was separated into a 5-fold-concentrated casein micelle suspension and milk serum. Pre-renneting of the serum did not reduce the rate of clotting on subsequent addition to the micelle suspension whether or not the conditions were such that the para-κ-casein became extensively aggregated. Washing of casein micelles up to 3 times with milk dialysate at 23°C extracted very little casein from the micelles and did not increase the clotting time of micelles resuspended to about the same concentration as in milk. The results appear to constitute decisive evidence against the hypothesis of milk clotting proposed by Parry & Carroll (1969). S-carboxymethyl-κ-casein, S-carboxymethyl-κ-casein containing 2·5 dimethylaminonaphthalene sulphonyl residues per mole, and rennin-treated dimethylaminonaphthalene sulphonated-S-carboxymethyl-κ-casein all bound Ca to the same extent at 30°C and pH 6·5, over the range 0·5–15·3 mM-CaCl2. This adds support to existing evidence that milk clotting does not involve formation of Ca bridges between casein micelles.

Kinetics of fibril formation of bovine kappa-casein indicate a conformational rearrangement as a critical step in the process

S-carboxymethylated (SCM) kappa-casein forms in vitro fibrils that display several characteristics of amyloid fibrils, although the protein is unrelated to amyloid diseases. In order to get insight into the processes that prevent the formation of amyloid fibrils made of kappa-caseins in milk, we have characterized in detail the reaction and the roles of its possible effectors: glycosylation and other caseins. Given that native kappa-casein occurs as a heterogeneous mixture of carbohydrate-free and carbohydrate-containing chains, kinetics of fibril formation were performed on purified glycosylated and unglycosylated SCM kappa-caseins using the fluorescent dye thioflavin T in conjunction with transmission electron microscopy and Fourier transform infrared spectroscopy for morphological and structural analyses. Both unglycosylated and glycosylated SCM kappa-caseins have the ability to fibrillate. Kinetic data indicate that the fibril formation rate increases with SCM kappa-casein concentration but reaches a plateau at high concentrations, for both the unglycosylated and glycosylated forms. Therefore, a conformational rearrangement is the rate-limiting step in fibril growth of SCM kappa-casein. Transmission electron microscopy images indicate the presence of 10- to 12-nm spherical particles prior to the appearance of amyloid structure. Fourier transform infrared spectroscopy spectra reveal a conformational change within these micellar aggregates during the fibrillation. Fibrils are helical ribbons with a pitch of about 120-130 nm and a width of 10-12 nm. Taken together, these findings suggest a model of aggregation during which the SCM kappa-casein monomer is in rapid equilibrium with a micellar aggregate that subsequently undergoes a conformational rearrangement into a more organized species. These micelles assemble and this leads to the growing of amyloid fibrils. Addition of alpha(s1)-and beta-caseins decreases the growth rate of fibrils. Their main effect was on the elongation rate, which became close to that of the limiting conformation change, leading to the appearance of a lag phase at the beginning of the kinetics.

Angiotensin I converting enzyme-inhibitory activity of bovine, ovine, and caprine kappa-casein macropeptides and their tryptic hydrolysates

This work evaluated the angiotensin-converting enzyme (ACE)-inhibitory activities of bovine, ovine, and caprine kappa-casein macropeptides (CMPs) and their tryptic hydrolysates. The results obtained indicate that bovine, ovine, and caprine CMPs exhibited moderate in vitro ACE-inhibitory activities that increased considerably after digestion under simulated gastrointestinal conditions. Active peptides could also be produced from CMPs via proteolysis with trypsin, with tryptic hydrolysates exhibiting a more extensive ACE-inhibitory activity than intact CMPs during simulated gastrointestinal digestion. Two active fractions were chromatographically separated from the tryptic hydrolysate of the bovine CMP, but their complexity hampered the assignment of the ACE-inhibitory activity to specific peptide sequences. Evidence for the release of the strong ACE-inhibitory tripeptide IPP was found upon simulation of the gastrointestinal digestion of peptides released by trypsin from the CMP sequence. These findings might help to promote further exploitation of cheese whey in the preparation of nutraceuticals for inclusion in the composition of functional food products with high added values.

Platelet aggregation inhibitory activity of bovine, ovine, and caprine kappa-casein macropeptides and their tryptic hydrolysates

Kappa-casein macropeptide (CMP) is one of the components of whey and is obtained as a by-product in cheesemaking. There has been increasing interest in research to find new uses of cheese industry by-products in order to improve their value and promote their use. Human platelet aggregation inhibitory activities of bovine, ovine, and caprine CMPs and their tryptic hydrolysates were studied. CMPs from the three species exhibited in vitro antithrombotic properties similar to the activity of the gamma-fibrinogen 400-411 peptide. Inhibitory activities increased following hydrolysis with trypsin. Active sequences were identified among the tryptic peptides by reversed-phase high-performance liquid chromatography with on-line mass spectrometry.

Suitability of animals' purified milk caseins and their subunit kappa-caseins as substrates for subtilisin and trypsin

Acid casein and kappa-casein were purified from different species of animal's milk, such as cow, sheep, goat, and water buffalo. These caseins were used as substrates for commercially available subtilisin and trypsin. It was established that, when acid caseins were used as a substrate for subtilisin, cow acid casein was found to be a better substrate for the enzymes, compared to other animals' milk casein. It was suggested that this acid casein has significantly more aromatic amino acids, as compared to arginine and lysine. K(M) and Vmax values, which were obtained for cow kappa-casein, showed that cow kappa-casein was a better susbstrate for trypsin than the others, suggesting that cow kappa-casein has a rich content of lysine, arginine, and aromatic amino acids by comparison with the others. The calculated C/N ratio also supports this suggestion.

Improved method for the simultaneous determination of whey proteins, caseins and para-κ-casein in milk and dairy products by capillary electrophoresis

A capillary electrophoresis method for the simultaneous determination of whey proteins, caseins and their degradation products, such as para-kappa-casein, was proposed. The effect of several parameters (pH, ionic strength and concentration of urea in the electrophoresis buffer and applied voltage) on the analysis time and on the separation efficiency of the major milk proteins was studied. Using a hydrophilically coated capillary, in combination with electrophoresis buffer 0.48 M citric acid-13.6 mM citrate-4.8 M urea at pH 2.3, and a separation voltage of 25 kV, a complete separation of beta-lactoglobulin and para-kappa-casein was achieved, permitting the quantification of both components.

Biological activities of bovine glycomacropeptide

Biological activity of bovine kappa-caseino glycomacropeptide (GMP) has received much attention in recent years. Research has focused on the ability of GMP to bind cholera and Escherichia coli enterotoxins, inhibit bacterial and viral adhesion, suppress gastric secretions, promote bifidobacterial growth and modulate immune system responses. Of these, protection against toxins, bacteria, and viruses and modulation of the immune system are the most promising applications.

Kinetics of beta-casein hydrolysis by wild-type and engineered trypsin

Apparent rate constants of tryptic hydrolysis of amide bonds containing Arg and Lys residues in beta-casein were determined by the analysis of kinetics of accumulation of 17 major peptide components revealed by high performance liquid chromatography. When studying pH influence on Arg/Lys bond cleavage preference, averaged rate constants over several Arg&bond;X and Lys&bond;X bonds were used for analysis of kinetics of wild-type trypsin, K188H, K188F, K188Y, K188W, and of K188D/D189K mutants. The pK(a1) value of 6.5 was found for all studied trypsins. For wild-type trypsin and its K188D/D189K mutant, pK(a2) was found to be 10. The lowest among studied engineered trypsins pK(a2) = 9.3 was determined for K188Y mutant. Considerable preference for the cleavage of Arg over Lys containing peptide bonds was demonstrated for all trypsins with engineered S2 site except for K188H and K188F. The comparison of individual rate constants for various bonds showed that during the hydrolysis by wild-type trypsin, the probabilities of splitting depend on secondary specificity and local hydrophobicity of amino acid residues, which are nearest to the hydrolyzed peptide bond (P2 site). The improvement of prediction of hydrolysis rates performed by the used program was achieved after considering the presence of hydrophobic neighborhood of Lys48--Ile49 and Arg202--Gly203 bonds.

Epitope analysis of the multiphosphorylated peptide alpha s1-casein (59-79)

The multiphosphorylated tryptic peptide alpha(s1)-casein(59-79) has been shown to be antigenic with anti-casein antibodies. In an approach to determine the amino acyl residues critical for antibody binding we undertook an epitope analysis of the peptide using overlapping synthetic peptides. With alpha(s1)-casein(59-79) as the adsorbed antigen in a competitive ELISA only two of five overlapping synthetic peptides at 1 mM significantly inhibited binding of the anti-casein antibodies. Peptides Glu-Ser(P)-Ile-Ser(P)-Ser(P)-Ser(P)-Glu-Glu and Ile-Val-Pro-Asn-Ser(P)-Val-Glu-Glu inhibited antibody binding by 20.0+/-3.6% and 60.3+/-7.9%, respectively. The epitope of Glu63-Ser(P)-Ile-Ser(P)-Ser(P)-Ser(P)-Glu-Glu70 was further localised to the phosphoseryl cluster as the peptide Ser(P)-Ser(P)-Ser(P) significantly inhibited binding of the anti-casein antibodies to alpha(s1)-casein(59-79) by 29.5+/-7.4%. Substitution of Ser(P)75 with Ser75 in the second inhibitory peptide Ile-Val-Pro-Asn-Ser(P)75-Val-Glu-Glu also abolished inhibition of antibody binding to x(s1)-casein (59-79) demonstrating that Ser(P)75 is also a critical residue for recognition by the antibodies. These data show that the phosphorylated residues in the cluster sequence -Ser(P)66-Ser(P)-Ser(P)68 and in the sequence -Pro73-Asn-Ser(P)-Val-Glu77- are critical for antibody binding to x(s1)-casein(59-79) and further demonstrate that a highly phosphorylated segment of a protein can be antigenic.

How the substitution of K188 of trypsin binding site by aromatic amino acids can influence the processing of beta-casein

Aspartyl 189 residue of trypsin is known to be essential for specific lysis of Arg-X and Lys-X bonds. Undertaking to modulate the catalytic properties of this protease, otherwise highly conserved K188 was replaced with aromatic amino acid residues aiming the perturbation of the electrostatics and the amplifying of hydrophobic interactions of the substrate binding site. The catalytic properties of the mutants K188F, K188Y, and K188W were measured at pH 7, 8, 9, and 10 using a pair of synthetic tetrapeptide p-nitroanilide substrates and beta-casein. The kinetic analysis reveals that all the mutants conserve the native trypsin capacity to split peptide bonds containing arginyl and lysyl residues. Surprisingly, however, depending on mutation, the optimum pH of activity changes. As demonstrated only by proteolysis of a natural substrate, all mutants cleave also peptide bonds involving asparagine and glutamine. These stuttered cleavage sites are close to the beta-casein fragments in beta-sheet according to Hydrophobic Cluster Analysis.

An ELISA for a multiphosphorylated peptide, alpha(s1)-casein(59-79)

Multiphosphorylated segments of proteins are predicted to be in or near epitopes. However, due to the very hydrophilic nature of multiphosphorylated peptides, epitope mapping by ELISA using conventional microtitre plates can produce false negatives due to poor antigen adsorption. We have developed a sensitive ELISA for a multiphosphorylated peptide alpha(s1)-casein(59-79) containing five phosphoseryl residues using Nunc-Immuno Maxisorp modules for antigen adsorption. The peptide alpha(s1)-casein(59-79) was detected in the ELISA at antigen coating concentrations of 1.0 microg/ml and above, using rabbit anti-casein antibodies at a dilution of 1/10,000 in Tris-buffered saline containing 0.05% (w/v) Tween 20 and 1.0% (v/v) normal goat serum. At an antigen coating concentration of 10 microg/ml, anti-casein antibodies bound to alpha(s1)-casein(59-79) and produced an absorbance of more than 100 times background. Using conventional polyvinyl chloride and polystyrene plates the peptide alpha(s1)-casein(59-79) could only be detected at very high antigen coating concentrations of 1-10 mg/ml. The addition of 0.05% (w/v) Tween 20 to the blocking, antibody diluting and wash buffers of the ELISA was shown to significantly reduce nonspecific binding of the primary antibody. Further, the inclusion of normal goat serum in the blocking and antibody diluting buffers resulted in a small but significant increase in absorbance. The ELISA developed in this study has been used successfully with a range of enzymatically derived and synthetic peptides containing one to five phosphorylated residues such that it should have general applicability to the study of antigenicity of multiphosphorylated peptides.

Isolation, purification, and alteration of some functional groups of major milk proteins: a review

This review covers selected methods of isolation and purification of mainly alpha s-casein, beta-casein, kappa-casein, beta-lactoglobulin, and alpha-lactalbumin. Selected methods of alteration of some functional groups of these proteins also were reviewed. Isolation and purification of milk proteins per se are methods of modifying the individual milk proteins. Gram quantities of these proteins can now be purified in a relatively short time using ion-exchange resins. Due to the prominent use of non-food-grade reagents in the procedures for preparation of these milk proteins, individual proteins are not maximally utilized for the manufacture of food/feed and pharmaceutical products. Therefore, intensive research efforts are needed to obviate the problems associated with underutilization of milk proteins.

Establishment and characterization of alpha s1-casein-specific T-cell lines from patients allergic to cow's milk: unexpected higher frequency of CD8+ T-cell lines

To study cow's milk allergy at the cellular level, we assessed the reactivity of peripheral blood mononuclear cells from patients allergic to cow's milk to alpha s1-casein, which is one of the major allergens in cow's milk. Proliferation of the cells to alpha s1-casein activation showed a rather weak response. Therefore to understand T-cell reactivity to alpha s1-casein in more detail, we prepared alpha s1-casein-specific T-cell lines from patients allergic to cow's milk and established 26 T-cell lines. These T-cell lines could be classified into three groups by analyzing their surface marker expression: those containing predominantly CD4+ CD8- T cells, those containing both CD4+CD8- and CD4-CD8+ T cells, and those containing predominantly CD4- CD8+ T cells. The CD8+ T cells were obtained at an unexpectedly higher frequency from the patients. These T-cell lines produced interferon-gamma and IL-4. These results suggest that CD8+ T cells specific for alpha s1-casein and CD4+ T cells were primed by the stimulation with alpha s1-casein in patients allergic to milk and that both T cells may play a key role in the onset, progression of, or recovery from cow's milk allergy.

Comparative study of methods for the isolation and purification of bovine kappa-casein and its hydrolysis by chymosin

kappa-Casein was purified from a single batch of whole acid casein (kappa-A variant) using different methods in order to compare their merits in producing a purified material with a carbohydrate and phosphate heterogeneity representative of the whole kappa-casein complement in milk. Ion-exchange methods of purification gave products of higher purity than precipitation techniques involving final purification by ethanol fractionation, but all methods resulted in kappa-caseins of apparently similar heterogeneity and chemical composition. The purified kappa-caseins were hydrolysed with chymosin and the derived macropeptides isolated. These were all virtually identical as determined by reversed-phase chromatography and gel electrophoresis. Some observations on chymosin hydrolysis of kappa-casein were made. In addition to formation of the major para-kappa-casein (Glu1-Phe105) and macropeptide (Met106-Val169), chymosin hydrolysis at pH 6.6 also resulted in two minor para-kappa-caseins with N-termini corresponding to Phe18 and Ser33 of kappa-casein. At pH 5.5 and 4.5 para-kappa-casein was rapidly hydrolysed into at least six fragments, one of which had an N-terminus corresponding to Trp76 of kappa-casein. At pH 6.6, 5.5 and 4.5 the kappa-casein macropeptide was stable to chymosin, but at pH 2.3 it was hydrolysed by chymosin into fragments with N-termini corresponding to Met106, Ile125, Ala138, Val139, Thr145 and Glu147 of kappa-casein.

Peptic proteolysis of esterified beta-casein and beta-lactoglobulin

Moderate esterification induces slight secondary structure changes in two major milk proteins, beta-lactoglobulin and beta-casein. Esterification of beta-lactoglobulin prompts its tertiary structure 'melting', opening it to peptic cleavage. Twenty-two new cleavage sites were characterised in beta-lactoglobulin and five in beta-casein. Some of them are due to esterification-improved peptide bond accessibility, some to the bias of pepsin specificity by glutamate and aspartate esters. The resulting fragmentation yields original and partially amphiphilic peptide populations.

Inhibition of proliferative responses of mouse spleen lymphocytes and rabbit Peyer's patch cells by bovine milk caseins and their digests

The modulating effect of bovine milk casein components and their digests on the proliferative responses of mouse spleen lymphocytes and rabbit Peyer's patch cells induced or not induced by mitogens has been studied with a colorimetric assay using 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide. All the casein components and their digests tested had little mitogenic effect on the proliferative responses of mouse spleen lymphocytes and rabbit Peyer's patch cells. Intact kappa-casein significantly inhibited the proliferative responses of mouse spleen lymphocytes and Peyer's patch cells induced by mitogens such as lipopolysaccharide from Salmonella typhimurium, concanavalin A, phytohaemagglutinin and pokeweed mitogen. In contrast, intact alpha s1-casein and beta-casein had little effect. kappa-Casein had an inhibitory effect after digestion by pancreatin or trypsin, but not after pepsin or chymotrypsin digestion. Both pancreatin and trypsin digests of alpha s1-casein and beta-casein significantly inhibited the proliferative responses of mouse spleen lymphocytes and rabbit Peyer's patch cells induced by mitogens, whereas pepsin and chymotrypsin digests of both caseins were without effect. Moreover, the trypsin digest of each casein component had an inhibitory effect on mouse spleen lymphocyte proliferation in the absence of mitogen. Since trypsin is a major proteinase in pancreatin, the substrate specificity of trypsin seems to be important for the formation of the inhibitory peptides from casein components. These observations suggest that intact kappa-casein and some peptides formed from milk casein components by the action of trypsin may suppress the immune responsiveness of neonates.

Specificity of hydrolysis of bovine kappa-casein by cell envelope-associated proteinases from Lactococcus lactis strains

The cell envelope-associated proteinases from Lactococcus lactis subsp. cremoris H2 (a PI-type proteinase-producing strain) and SK11 (a PIII-type proteinase-producing strain) both actively hydrolyze the kappa-casein component of bovine milk but with significant differences in the specificity of peptide bond hydrolysis. The peptide bonds Ala-23-Lys-24, Leu-32-Ser-33, Ala-71-Gln-72, Leu-79-Ser-80, Met-95-Ala-96, and Met-106-Ala-107 were cleaved by both proteinase types, although the relative rates of hydrolysis at some of these sites were quite different for the two proteinases. Small histidine-rich peptides were formed as early products of the action of the cell envelope-associated proteinases on kappa-casein, implicating this casein as a possible significant source of histidine, which is essential for starter growth. The major difference between the two proteinase types in their action on kappa-casein was in their ability to cleave bonds near the C-terminal end of the molecule. The bond Asn-160-Thr-161 and, to a lesser extent, the bond Glu-151-Val-152 were very rapidly cleaved by the PIII-type proteinase, whereas hydrolysis of these bonds by the PI-type proteinase was barely detectable (even after 24 h of digestion). Differential hydrolysis of kappa-casein at these sites by the two different proteinase types resulted in the formation of distinctive, high-M(r) products detectable by sodium dodecyl sulfate-polyacrylamide gel electrophoresis.(ABSTRACT TRUNCATED AT 250 WORDS)

Comparative study of the separation of casein from bovine, ovine and caprine milks using HPLC

The separation of cows', sheep's and goats' milk casein components by HPLC on a strong anion-exchange (P.L-SAX 8 mu 1000A) column is described. During HLPC, whole caseins of the three kinds of milk behaved differently from conventional separations. The casein components of the three kinds of milk were well resolved under the chromatographic conditions used. HPLC resolved the kappa-caseins better than did ion-exchange chromatography on DEAE cellulose, and was particularly efficient in the case of goats' milk. Goats' and sheep's milks had almost similar chromatographic profiles but these differed considerably from that of cows' milk. Caseins from the sheep and the goat were also similar in that a shallower NaCl gradient was required for the separation of casein components than for cows' milk.

Characterization of human kappa-casein purified by FPLC

Because previous purification procedures for human kappa-casein may have caused the loss of some carbohydrate, relatively gentle methods were used. The protein was isolated by a four-step procedure which included isoelectric precipitation of whole casein, gel chromatography on Sephadex G-200 in the presence of SDS, removal of the SDS with Extracti-Gel D, and FPLC chromatography on Mono Q with buffers containing 6 M urea. The purified protein was nearly identical in amino acid composition to that found earlier by amino acid analysis and peptide sequencing and a molar extinction coefficient of 11.2 +/- 0.1 was determined on the basis of amino acid analysis with a norleucine internal standard. Hydrolysis, acylation, and methylsilylation of the carbohydrate, followed by gas chromatographic analysis on a fused silica column, yielded approximately 5% fucose, 17% galactose, 18% N-acetylglucosamine, 8% N-acetylgalactosamine and 7% sialic acid, totaling almost 55% by weight. The percentages from two different donors were almost the same. About 1 mole phosphorus per mole of kappa-casein was also detected. Using low-speed sedimentation equilibrium methods, a molecular weight of only 33,400 was obtained for human kappa-casein, suggesting carbohydrate lability. Human beta-casein with four phosphoryls was stabilized against precipitation by 10 mM Ca+2 ions at a level greater than 95% when the molar ratio of kappa/beta exceeded 0.15.