Structure of biomimetic casein micelles: Critical tests of the hydrophobic colloid and multivalent-binding models using recombinant deuterated and phosphorylated β-casein

Milk contains high concentrations of amyloidogenic casein proteins and is supersaturated with respect to crystalline calcium phosphates such as apatite. Nevertheless, the mammary gland normally remains unmineralized and free of amyloid. Unlike κ-casein, β- and αS-caseins are highly effective mineral chaperones that prevent ectopic and pathological calcification of the mammary gland. Milk invariably contains a mixture of two to five different caseins that act on each other as molecular chaperones. Instead of forming amyloid fibrils, several thousand caseins and hundreds of nanoclusters of amorphous calcium phosphate combine to form fuzzy complexes called casein micelles. To understand the biological functions of the casein micelle its structure needs to be understood better than at present. The location in micelles of the highly amyloidogenic κ-casein is disputed. In traditional hydrophobic colloid models, it, alone, forms a stabilizing surface coat that also determines the average size of the micelles. In the recent multivalent-binding model, κ-casein is present throughout the micelle, in intimate contact with the other caseins. To discriminate between these models, a range of biomimetic micelles was prepared using a fixed concentration of the mineral chaperone β-casein and nanoclusters of calcium phosphate, with variable concentrations of κ-casein. A biomimetic micelle was also prepared using a highly deuterated and in vivo phosphorylated recombinant β-casein with calcium phosphate and unlabelled κ-casein. Neutron and X-ray scattering experiments revealed that κ-casein is distributed throughout the micelle, in quantitative agreement with the multivalent-binding model but contrary to the hydrophobic colloid models.

Functional Role of circRNAs in the Regulation of Fetal Development, Muscle Development, and Lactation in Livestock

circRNAs are a class of endogenous noncoding RNA molecules with closed loop structures. They are mainly responsible for regulating gene expression in eukaryotic cells. With the emergence of high-throughput RNA sequencing (RNA-Seq) and new types of bioinformatics tools, thousands of circRNAs have been discovered, making circRNA one of the research hotspots. Recent studies have shown that circRNAs play an important regulatory role in the growth, reproduction, and formation of livestock products. They can not only regulate mammalian fetal growth and development but also have important regulatory effects on livestock muscle development and lactation. In this review, we briefly introduce the putative biogenic pathways and regulatory functions of circRNA and highlight our understanding of circRNA and its latest advances in fetal development, muscle development, and lactation biogenesis as well as expression in livestock. This review will provide a theoretical basis for the research and development of related industries.

Apical-to-basolateral transepithelial transport of cow's milk caseins by intestinal Caco-2 cell monolayers: MS-based quantitation of cellularly degraded α- and β-casein fragments

Casein (CN) is the major milk protein to nourish infants but, in certain population, it causes cow's milk allergy, indicating the uptake of antigenic CN and their peptides through the intestinal epithelium. Using human intestinal Caco-2 cell monolayers, the apical-to-basal transepithelial transport of CN was investigated. Confocal microscopy using component-specific antibodies showed that αs1-CN antigens became detectable as punctate signals at the apical-side cytoplasm and reached to the cytoplasm at a tight-junction level within a few hours. Such intracellular CN signals were more remarkable than those of the other antigens, β-lactoglobulin and ovalbumin, colocalized in part with an early endosome marker protein (EEA1) and decreased in the presence of cytochalasin D or sodium azide and also at lowered temperature at 4°C. Liquid chromatography coupled with mass spectroscopy analysis of the protein fraction in the basal-side medium identified the αs1-CB fragment including the N-terminal region and the αs2-CN fragment containing the central part of polypeptide at 100-1,000 fmol per well levels. Moreover, β-CN C-terminal overlapping peptides were identified in the peptide fraction below 10 kDa of the basal medium. These results suggest that CNs are partially degraded by cellular proteases and/or peptidases and immunologically active CN fragments are transported to basal side of the cell monolayers.

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.

Use of surface plasmon resonance (SPR) to study the dissociation and polysaccharide binding of casein micelles and caseins

Tests were made to determine whether surface plasmon resonance (SPR) could be used as a technique to study the dissociation properties of bovine casein micelles or of sodium caseinate and the interactions between these protein particles and different polysaccharides. Surfaces of bound micelles or caseinate were made, and the changes in refractive index of these layers were used to define changes in the structures of the chemisorbed material. The technique appears to have some potential for studying details of the dissociation of casein micelles and of the binding of different polysaccharides to caseins.

Simultaneous presence of PrtH and PrtH2 proteinases in Lactobacillus helveticus Strains improves breakdown of the pure alphas1-casein

Lactobacillus helveticus can possess one or two cell envelope proteinases (CEPs), called PrtH2 and PrtH. The aim of this work was to explore the diversity of 15 strains of L. helveticus, isolated from various origins, in terms of their proteolytic activities and specificities on pure caseins or on milk casein micelles. CEP activity differed 14-fold when the strains were assayed on a synthetic substrate, but no significant differences were detected between strains possessing one or two CEPs. No correlation was observed between the proteolytic activities of the strains and their rates of acidification in milk. The kinetics of hydrolysis of purified α(s1)- and β-casein by L. helveticus whole cells was monitored using Tris-Tricine sodium dodecyl sulfate (SDS) electrophoresis, and for four strains, the peptides released were identified using mass spectrometry. While rapid hydrolysis of pure β-casein was observed for all strains, the hydrolysis kinetics of α(s1)-casein was the only criterion capable of distinguishing between the strains based on the number of CEPs. Fifty-four to 74 peptides were identified for each strain. When only PrtH2 was present, 22 to 30% of the peptides originated from α(s1)-casein. The percentage increased to 41 to 49% for strains in which both CEPs were expressed. The peptide size ranged from 6 to 33 amino acids, revealing a broad range of cleavage specificities, involving all classes of amino acids (Leu, Val, Ala, Ile, Glu, Gln, Lys, Arg, Met, and Pro). Regions resistant to proteolysis were identified in both caseins. When strains were grown in milk, a drastic reduction in the number of peptides was observed, reflecting changes in accessibility and/or peptide assimilation during growth.

A procedure for the partial fractionation of the αs-casein complex

A method for the partial fractionation of the αs-casein complex is described. It involves removal of κ-casein from iso-electric casein by the method of Zittle & Custer (1963) followed by fractionating the residue (αs- and β-casein) by the urea procedure of Hipp et al. (1952). αs0- and αs1-caseins precipitate from 3·3 M urea at pH 4·6, but much of the αs2-, αs3- and αs4-casein remains in solution and co-precipitates with β-casein from 1·7 M urea at pH 4·9. Chromatography on DEAE cellulose clearly separates β-casein from the αs2-, αs3- and αs4-caseins, which are not resolved by the conditions employed. The method is suitable for the fractionation of iso-electric casein into all its major components (i) κ-casein; (ii) αs0- and αs1-caseins; (iii) αs2-, αs3- and αs4-caseins; (iv) β-casein; (v) γ-casein.

Ion-exchange fast protein liquid chromatography: optimization of the purification of caseins using a non-denaturing detergent

The separation of bovine milk proteins by fast protein liquid chromatography has been studied by ion-exchange chromatography on Mono S and Mono Q columns. The use of a non-ionic detergent, octyl-glucoside, made possible the separation of the four major casein components (αs1, αs2, β and κ) as well as minor caseins on the Mono S column using urea-containing buffers at pH 3·5. There was, however, considerable overlap between the αs1 and αs2-casein peaks. A good resolution was obtained with a low concentration of urea (3·5 M) and detergent (0·1%). These conditions allowed the separation of casein components in their native state in less than 50 min. The purity of isolated κ-casein was further improved by a second separation on a Mono Q column.

Separation of casein aggregates cross-linked by colloidal calcium phosphate from bovine casein micelles by high performance gel chromatography in the presence of urea

High performance gel chromatography of the casein micelles disaggregated by 6 m-urea was carried out on a TSK-GEL G4000SW column using 6 M-urea synthetic milk serum as the effluent. The eluate was divided into two fractions. Fast eluted fraction 1 decreased from 67·5 to 57·3% on reduction of casein micelles and was not observed in reduced colloidal phosphate-free casein micelles. Fraction 1 from reduced casein micelles contained 1·7 times as much Ca and Pibound to casein as did whole casein micelles, while fraction 2 essentially contained only bound Ca. These facts indicated that fraction 1 of reduced casein micelles consisted of the casein aggregates cross-linked by colloidal Ca phosphate (CCP). Polyacrylamide-gel electrophoresis showed that fraction 1 of reduced casein micelles contained more αs1- and αs2-caseins and less β-casein than whole micellar casein. No κ-casein was detected in fraction 1 of reduced casein micelles. It is suggested that the ester phosphate groups of casein are the sites for interaction with CCP.

Heat stability of milk: influence of colloidal calcium phosphate and β-lactoglobulin

Reduction of the level of colloidal calcium phosphate (CCP) progressively increased the heat stability of milk at pH values <~7·0 and increased the pH of maximum stability. Removal of 40% CCP also stabilized the system at the pH of minimum stability, but removal of ≥60% CCP rendered milk very unstable at pH values >7·2, an effect not offset by a 4-fold increase in κ-casein concentration. Doubling CCP had a slight destabilizing effect in the pH range 6·5–7·5.

Addition of β-lactoglobulin to serum protein-free casein micelles had a marked destabilizing effect at pH values > ~6·8, but increased stability in the pH range 6·4–6·8. β-Lactoglobulin had a similar and more apparent effect on the heat stability of Na caseinate dissolved in milk diffusate.

It is suggested that rather than being a stabilizing factor responsible for the maximum in the heat stability-pH curve, the true effect of β-lactoglobulin is to shift the curve to more acid pH values (reason unknown) and to sensitize the caseinate system to heat-induced Ca phosphate precipitation at pH values > ~7·0. Low stability at ~pH 7·0 introduces an apparent maximum in the heat stability-pH curve at ~pH 6·8, but this has no independent existence. At pH values >7·2, increased protein charge more than off-sets the influence of heat-precipitated CCP and stability again increases in micellar but not in soluble casein systems.

The interaction of bovine milk caseins with the detergent sodium dodecyl sulphate. I. The relationship between the composition and the size of the protein–detergent aggregate

Study of the dissociation of high-molecular-weight aggregates of preparations of αs1-, β-, κ-, and para-κ-casein by the detergent, sodium dodecyl sulphate (SDS), showed that there are differences in the aggregation properties of the individual caseins. Binding of detergent led first to the dissociation of casein aggregates and then to further interaction with the casein molecules. The amounts of detergent required to give the minimum sized protein-detergent aggregate when expressed as mg/mg casein were similar for κ-, para-κ- and αs1-casein but much less for β-casein. However, expressed as mole/mole the requirement for κ- and αs1-casein was similar but was twice that found for para-κ- and β-casein. The maximum amount of SDS bound was about twice that required for complete dissociation of the aggregates for κ-, para-κ- and αs1-casein but was 13 times greater for β-casein.

Complete dissociation of κ-casein aggregates by SDS alone was not possible due to the presence of aggregates formed by disulphide linkages. These aggregates, which consisted of 3±1 protein molecules, accounted for about one-third of the κ-casein in the preparations examined.

Binding of calcium ions to bovine αsl-casein and precipitability of the protein–calcium ion complexes

Binding isotherms for the calcium ion–αsl-casein system have been measured, as functions of ionic strength, temperature, and pH, and the isotherms have been analysed in terms of binding constants modified by substitution effects. The results demonstrate that the strength of binding is increased with increasing temperature and decreased by increasing ionic strength or decreasing pH, all of which may be explained semi-quantitatively. Parallel studies on the precipitability of the αsl-casein–Ca2+complexes showed that there is considerable variation in the extent of calcium binding required to initiate precipitation of the protein, and in the calcium concentration necessary to achieve the required extent of ligand binding.

A model for the assembly of bovine casein micelles from F2 and F3 subunits

Casein micelles were reassembled from mixtures of F2 and F3 subunits, the major subunits of bovine casein micelles, in varying ratios. The average radius of the reassembled micelles was inversely proportional to the F2 content. Based on this relationship, a model for casein micelle assembly is proposed in which the F3 subunit forms the core of the micelle with F2 subunit occurring on the surface. Micelle size can subsequently be defined by the F2 content.

Probing the location of casein fractions in the casein micelle using enzymes and enzyme–dextran conjugates

The rates of milk clotting and of formation of para-κ-casein in milk, colloidal phosphate-free milk and isolated κ-casein by pepsin and by its soluble size-fractionated conjugates with dextran were determined. Milk clotting with all the enzyme derivatives was dependent on the rate of the enzymic phase and required essentially complete κ-casein hydrolysis at 30 °C and throughout the range of pH 5·6–6·7. κ-Casein hydrolysis by pepsin at pH 6·6 was fastest in milk and slowest in isolated κ-casein, but the rate decreased as the enzyme size increased, especially with milk. When corrected for the changes in pepsin activity, the rates of κ-casein hydrolysis in all substrates were identical at 30 and 5 °C, but increased with decrease in pH, especially with the larger enzyme conjugates. Hydrolysis of the C-terminal bonds of β-and κ-casein in native and disrupted casein micelles by carboxypeptidase A and soluble conjugates of it were also investigated. κ-Casein was hydrolysed much faster, and β-casein slightly faster, in native than in disrupted micelles by the native enzyme. Increase in the size of carboxypeptidase A increased the rate of hydrolysis of κ-casein in disrupted micelles and also induced lag periods before hydrolysis commenced, especially with disrupted micelles. The results are compatible with a model for the casein micelle in which the κcasein is on the outside and the casein components are in a more ordered arrangement than in the casein complexes formed on micelle disruption. They also indicate that immobilized coagulants would be unable to clot milk.

Influence of aggregation on the susceptibility of casein to proteolysis

The susceptibility of the casein in milk to proteolysis was shown to be greatly influenced by its state of aggregation. In normal milk, where the casein is largely in micellar form, the αs1- and β-caseins are almost inaccessible to proteolysis. On removal of the colloidal phosphate, the casein micelles disintegrate, rendering the components, especially the αs1-casein, accessible to proteolysis. The role of colloidal calcium phosphate in the casein micelle is believed to be that of a non-specific aggregating agent which can be effectively replaced by calcium. Dissolved colloidal phosphate can be effectively reformed by elevation of the pH of colloidal phosphate-free (CPF) milk before equilibrium dialysis. Addition of κ-casein to CPF milk also causes aggregation of the component caseins but the micelles formed are smaller than those of normal milk.

The behaviour of micellar β-casein differs considerably from that of micellar αs1-casein. The evidence suggests that part of the β-casein freely dissociates either outside or within the micelle when the temperature is reduced. The temperature dependence of the susceptibility of β-casein to proteolysis was similar in skim-milk and in solutions of sodium caseinate, and increased as the temperature was reduced. αs1-Casein was quite resistant to proteolysis in normal milk but became susceptible when the micelle structure was disrupted on removal of colloidal phosphate.

It is concluded that limited proteolysis may prove a valuable technique in the study of casein micelle structure.

Simple methods for the purification of crude κ-casein and β-casein by treatment with calcium phosphate gel

Batch methods applicable on a large scale are described for the purification of crude κ- and βκ-Casein, dissolved in urea-containing buffer, was freed from αs- and β-caseins by treatment with calcium phosphate gel and recovered in about 60% yield. β-Casein was freed from most impurities by adsorption on to calcium phosphate gel at pH 7·8 in the presence of urea and elution with 6 M-urea–N-NH4OH at 4°C. The recovery was about 50%.

In vitro proteolysis and functional properties of reductively alkylated β-casein derivatives

β-Casein amino groups were modified with aldehydes and dialdehydes via reductive alkylation at pH 8·0. The degree of alkylation was controlled by the amount of the alkylating reagent applied. The initial rates of α-chymotrypsin-catalysed hydrolysis of alkylated β-casein were inversely related to the size of the modifying group. Proteolysis of modified β-casein with trypsin (18 h) or with α-chymotrypsin (48 h) depended on the nature and size of the substituent applied. The measurements of tryptophan fluorescence indicate that the modifications also induced conformation change. Solubilities of methyl-, ethyl- or benzyl-β-casein slightly increased; solubilities of β-casein dialdehyde derivatives were significantly lower than that of native β-casein. Emulsion stability of methyl- or ethyl- β-casein was higher than that of native β-casein in the acidic pH range. After modification with glyoxal, the emulsifying activity and the emulsion stability of β-casein decreased. The emulsifying activity of benzyl- β-casein was lower than that of native β-casein. Phthalylated β-casein displayed the poorest emulsion stability.

Heat stability of forewarmed milks: influence of κ-casein, serum proteins and divalent cations

Addition of isolated κ-casein to milk reduced the destabilizing influence of forewarming, while the addition of isolated β-lactoglobulin (β-lg) to the κ-casein-enriched systems resulted in a loss in stability on forewarming. This effect was ascribed mainly to a β-lg:κ-casein interaction. Some individual milks were unaffected by forewarming while others were markedly destabilized by the same treatment. Addition of Ca2++ Mg2+and interchanging of milk sera by dialysis influenced stability.

It is concluded that milks which are destabilized by forewarming generally give a type A response (minimum in the pH–heat stability curves) and that preheat-stable milks generally give a type B curve (no minimum).

Interaction between heated κ-casein and β-lactoglobulin: predominance of hydrophobic interactions in the initial stages of complex formation

Mixtures ofκ-casein and β-lactoglobulin (β-lg) were heated at 70 °C in 20 mM-imidazole buffer, pH 6·8 containing 20 mM-EGTA. Aggregation of theκ-casein/β-lg mixture occurred within 90 s and susceptibility to hydrolysis by chymosin decreased significantly within 180 s even though covalent interaction was not detected until after 4000 s. UV-absorbance indicated initial structural destabilization of the heatedκ-casein/β-lg mixture followed by a return ( > 350 s) to a spectrum comparable to the native state, indicating molecular rearrangement. Apparent hydrophobicity ofκ-casein decreased 80% within 250 s compared to a 38% decrease forβ-lg. Under similar conditions, theκ-casein/β-lg mixture (1:1) showed a faster (2·5 times) decrease in apparent hydrophobicity thanκ-casein alone with concomitant exposure of acidic (hydrophilic) groups. The results suggested that the driving force for the rearrangement was mainly hydrophobic, i.e. entropic in origin. The tendency of heated and subsequently cooledκ-casein/β-lg to aggregate reached a maximum after heating at 70 °C for 720 s.

Reductive methylation of lysine residues in casein

The effect of reductive methylation on the properties of αs-, β- and κ-caseins was studied using preparations of these proteins in which 20 or 60% of the lysyl residues had been reductively methylated. This modification appeared to affect the physicochemical properties of the protein to an insignificant extent and in particular, electrophoretic mobility, sensitivity to precipitation by Ca2+and solubility of Ca salts of individual proteins were only slightly altered. The use of caseins which had been subjected to methylation with radioactive formaldehyde is suggested as a potentially valuable method for the study of protein interactions.

Heat-induced interaction of β-lactoglobulin and κ-casein

The interaction of β-lactoglobulin and κ-casein at high temperature was studied polarimetrically. The rate of interaction was related to the genetic variant of β-lactoglobulin present. β-Lactoglobulin B, whose thermodenaturation was faster than that of A variant, interacted more rapidly with κ-casein than did the A variant. Velocity sedimentation studies indicated that characteristics of the complex were determined more by the β-lactoglobulin than by the κ-casein. The presence of κ-casein during the thermodenaturation of β-lactoglobulin appeared to prevent the aggregation of β-lactoglobulin from proceeding to completion, the κ-casein complexing with intermediate species and restricting the aggregation process.

Physico-chemical properties of casein micelles reformed from urea-treated milk

Micelles reconstituted from urea-treated milk by exhaustive dialysis against bulk milk were similar to native micelles with respect to colloidal phosphate: casein ratio, ethanol stability, heat stability and susceptibility to first-stage rennin action. Reconstituted micelles were considerably smaller than native micelles as indicated by turbidity, sedimentation and viscosity, had shorter second-stage rennet coagulation times, were unstable to [Ca2+] > 20 mM and had reduced base-binding capacity. It is suggested that the induced Ca sensitivity is due to unfavourable alterations in the micellar κ-casein coat arising from the decrease in average micelle size and can be offset by increasing the κ-casein complement of the system or by increasing the degree of aggregation by slowly raising the [Ca2+] level.

Composition of the bound lipids in caseins and in ripening cheeses

Neutral lipids and phospholipids, firmly bound to rennet- or acidprecipitated casein and also to α-, β- and κ-casein, have been detected by histochemical means. On sections of the caseins they appeared as areas of variable shape and size, uniformly distributed. In cheeses they partially disappeared, probably owing to the action of the milk and bacterial lipases. This led to new patterns of distribution of the lipids in the sections.

Lecithin, cephalin, sphingomyelin, cerebrosides (2 spots), ceramides (1 spot), phosphatidylserine and phosphatidylinositol were identified in the phospholipid fraction of the casein by 2-dimensional thin-layer chromatography. In the bound phospholipid fraction of 3 cheeses the same phospholipids were found together with 1 new unidentified spot.

The effect of the chemical structure of additives on the coagulation of casein micelle suspensions by rennet

Casein micelles in milk-salts solution adsorbed charged detergents and highly-charged polypeptides strongly, neutral detergents less strongly and low molecular-weight amines weakly. A tetra-amine was adsorbed more strongly than a tri-amine. The extent of adsorption of proteins tended to rise as the molecular weight increased. Glycerol and lactate were adsorbed to a limited extent but dextran and α-ketoglutarate were not adsorbed at all. Proline was partly adsorbed, indicating that hydrophobic binding sites were available, and caused some disruption of the casein micelles. Additives were bound to approximately the same extent by casein micelles and rennet coagula. The proportions adsorbed were constant over at least 10-fold ranges of concentration. Additives which increased the rennet clotting time (RCT) acted by binding Ca2+. Most additives decreased the RCT, the extent increasing with the amount adsorbed and the positive charge on the additive. The greatest reduction in RCT was observed with those additives which had positively-charged and hydrophobic moieties and bound most strongly to casein micelles. Of the additives tested, only sodium dodecyl sulphate affected the enzymic action of rennet. The reduction in RCT may have resulted from the neutralization of the negative charge of the micelles or enhancement of their hydrophobicity, favouring hydrophobic interactions between the particles.

Studies on the change in the properties of the fat globule membrane during the concentrating of milk

Previous work had shown that when milk is boiled in a climbing film evaporator a reaction occurs between the casein micelles and the fat globule membrane, probably at the liquid–gas interface. It now appears that the micelle–globule complex is formed by combination of the κ-casein components of the micelles and the membrane of the fat globule. The reaction is not significantly affected by increases in concentration of individual constituents, or by changes in temperature or pH.

The history of the milk before its concentration is an important factor in its subsequent behaviour. Thus, cooling and stirring of the milk during storage, as normally occurs in a bulk collection tank, promotes the reaction in the evaporator and, for some milks, is essential for starting the reaction.

A high retention of whey by curds made from the concentrated milks, diluted with water to their original volume before renneting, indicates that curd properties other than fat retention are also altered during concentration.

Carbohydrates of bovine κ-casein

Comparison of the carbohydrates of κ-casein with those of the glycopeptides released by the action of rennin on whole milk showed that there are consistent differences between them, which are due to the presence of carbohydrates in para-κ-casein. d-Mannose, d-galactose, 2-acetamido-2-deoxy-d-galactose and N-acetyl neuraminic acid were invariably present in κ-casein. N-acetyl neuraminic acid was located almost entirely on the glycopeptides whereas the greater proportion of d-mannose was attached to the para-κ-casein. d-Galactose and 2-acetamido-2-deoxy-d-galactose were detected in both para-κ-casein and the glycopeptides. In the glycopeptides there are usually about 2 molecules of d-galactose to 1 molecule of 2-acetamido-2-deoxy-d-galactose. Carbohydrates are attached to only a proportion of para-κ-casein molecules. Carbohydrates could not be detected in αs- and β-casein.

Rapid separation of bovine caseins by mass ion exchange chromatography

The rapid isolation of major bovine caseins in gram quantities was investigated. Whole casein was precipitated from individual cow's milk by adjusting the pH to 4·6 and the precipitated casein was suspended in 4·5 M urea (pH 8·0) containing 0·02 M imidazole and 0·03 M β-mercaptoethanol, and bound on a QAE Zeta Prep 250 cartridge. Stepwise elution with the urea/imidazole β-mercaptoethanol buffer and varying amounts of NaCl gave five well resolved peaks, which were identified by polyacrylamide gel electrophoresis and fast protein liquid chromatography to be pure γ-casein, κ-casein. β-casein, β-casein and αs-casein, respectively. The ion exchange cartridge was regenerated by flushing with buffer containing 0·50 Μ-NaCl followed by equilibration with starting buffer before separation of next sample. The time required to run each sample including cartridge regeneration and equilibration was 4 hours.

Properties of aseptically-packed UHT milk: casein modification during storage and studies with model systems

Storage of aseptically packed UHT milk produced changes in the electrophoretic pattern of the milk caseins when the milk was stored at ambient or higher temperatures. Lower temperature storage at 4°C did not give rise to these changes.

The alterations in electrophoretic properties of the caseins appeared to be due to the action of carbonyl compounds, produced by a Maillard type of reaction, which led to changes in the charge of the protein together with some degree of polymerization. These conclusions have been drawn from results obtained on model systems of casein-lactose subjected to various heat treatments, and on casein and milk treated with acetaldehyde.

Changes in the sensitivity to calcium ions of individual caseins, whole casein and milk that had been subjected to various heat treatments or to treatment with acetaldehyde showed that all these different treatments gave rise to modified casein which, in general, became less sensitive to calcium. κ-Casein when treated alone rapidly lost its ability to protect αs1-casein from precipitation by calcium ions, while αs1-casein treated alone only gradually became more soluble in the presence of calcium. Thus, on treating whole casein there was evidence for a stability minimum when the protective ability of the κ-casein has been destroyed without a compensating gain in the stability of the αs1-casein.

The importance of these changes in relation to the stability of UHT milks has not yet been elucidated but the results indicate that cross linking between protein chains and changes in calcium sensitivity occur during long-term storage.

The C-terminal sequence of amino acid residues of κ-casein isolated from buffalo's milk

When κ-casein from buffalo's milk was treated with carboxypeptidase A (EC 3. 4. 2. 1),4 amino acids, valine, threonine, serine and alanine were released from the protein in a manner consistent with the view that they originate in the C-terminal sequence of a single peptide chain. The amounts produced suggest a minimum molecular weight for buffalo κ-casein of approximately 17000, in agreement with the value calculated from the phosphorous content on the basis of the presence of 2 phosphorus atoms/molecule. A comparison is made with the C-terminal sequence reported for bovine κ-casein.

Separation of the submicelles from micellar casein by high performance gel chromatography on a TSK-GEL G4000SW column

Bovine caseins from skim-milk (micellar casein and acid precipitated casein) were separated into 4 discrete components by high performance gel chromatography on a TSK-GEL G4000SW columm. The resolution was better than that obtained by conventional gel chromatography which resulted in separation into 2 components at best. Our results showed that the second and third components consisted of αs1- and κ-caseins and αs1- and β-caseins respectively. They are presumed to be submicellar fractions. Samples prepared by removal of Ca phosphate from large and small micelles were rich in the third and the second components respectively. It is suggested that casein micelles consist of αs1- and κ-casein complexes and complexes and polymers of αs1- and β-casein which are situated preferentially on the periphery and in the interior of the micelles respectively.

Application of multiple regression analysis to sedimentation equilibrium data of αs1- and κ-casein interaction for calculation of molecular weight distributions

Multiple regression analysis was applied to sedimentation equilibrium data for determination of the molecular weight distribution (MWD) of model systems consisting of up to 3 components. Negative weight fractions which were frequently encountered during multiple regression analysis were forced to zero by sequentially eliminating from the regression matrix the corresponding molecular weights in order of the magnitude of negative t-values. The simplex optimization of Morgan & Deming (1974), modified by incorporating a prohibit–range– trespassing routine, was used to search for the best fit values for weight average molecular weights and relative concentrations of the components. This method almost quantitatively reproduced the molecular weights and concentrations of the original model systems. This quantitative information supplemented the multiple regression matrix to improve the resolution of MWD.

A direct comparison using model systems revealed that the multiple regression method in conjunction with the simplex optimization routine was more quantitative than the linear programming method of Scholte (1969). When applied to mixtures of standard proteins (ribonuclease A; ovalbumin; γ-globulin and ovalbumin; γ-globulin; apoferritin), the simplex optimization routine yielded values for average molecular weights and relative concentrations of the component proteins which were in good agreement with the known values in the original mixtures.

The MWD of αs1-k-casein mixtures at an ionic strength of 0.1 suggested that k-casein was readily dissociated to the monomer (or the dimer) and interacted with the monomer (or the dimer) of αs1-casein forming a complex of approximately 400000.

The acceleration by cationic materials of the coagulation of casein micelles by rennet

Three cationic materials markedly reduced the rennet clotting time of casein micelle suspensions, the efficacy of each being primarily dependent on the charge and the amount absorbed by the micelles. The reduction in coagulation time was unaffected by components of the milk serum other than salts. No enzymic action by lysozyme on casein micelles was detected. All materials acted by the same mechanism, increasing the affinity of rennet for the micelles and accelerating the aggregation phase. Coagulation did not occur until a minimum amount of κ-casein had been hydrolysed to para-κ-casein. All additives increased the proportion of added rennet retained by the casein in the coagulum. The results indicated that coagulation occurs by specific interactions between micelles modified by rennet.

The role of lysine residues in the coagulation of casein

Coagulation of rennin-treated casein was inhibited by treatment of the casein with dimethylaminonaphthalene sulphonyl chloride. The effect was caused by substitution on lysine side chains of theκ-casein fraction, and inhibition was complete when 2–3 lysine residues/molecule were blocked. This level of substitution did not affect other properties of theκ-casein, such as the release from it of non-protein nitrogen (NPN) by rennin and its ability to stabilizeαs-andβ-caseins in the presence of Ca++. The evidence suggests that lysine side chains onκ-casein take part in the coagulation of rennin-treated casein.

Kinetics of rennin action on casein prepared by ultracentrifugation

The kinetics of the primary phase of rennin action on casein micelles and on total casein prepared by ultracentrifugation have been studied. It was observed that the values of Km and V varied for the different casein samples. Addition of whey proteins to the reaction mixture or variation of the concentrations of Na+ and K+ had no effect on Km and V and V, but there was a consistent increase in the values of both these parameters when κ-casein was added. It was concluded that these changes were due to an increase in the accessibility of the κ-casein resulting from the rearrangement of the casein micelles. These results are in agreement with our earlier conclusion that the Km for rennin action on casein is dependent on the carbohydrate composition of the molecule.

Antimutagenic activity of whole casein on the pepper-induced mutagenicity to streptomycin-dependent strain SD 510 of Salmonella typhimurium TA 98

When the antimutagenic activity of milk or cultured milk components on the pepper-induced mutagenicity to streptomycin-dependent strain SD 510 of Salmonella typhimurium TA 98 was investigated, whole casein showed a significant antimutagenic activity. Heating at 121 °C for 15 min completely destroyed the antimutagenic activity of casein, which was temperature dependent. While acid or papain digestion resulted in the loss of antimutagenic activity of casein, trypsin digestion had little effect although the digestion of casein by each proteinase was over 75%. β-Casein, egg albumin, and bovine serum albumin also had a similar antimutagenic activity to that of whole casein.