Topography of the casein micelle surface by surface plasmon resonance (SPR) using a selection of specific monoclonal antibodies

Several theoretical models of the casein micelle structure have been proposed in the past, but the exact organization of the four individual caseins (α(s1), α(s2), β, and κ) within this supramolecular structure remains unknown. The present study aims at determining the topography of the casein micelle surface by following the interaction between 44 monoclonal antibodies specific for different epitopes of α(s1)-, α(s2)-, β-, and κ-casein and the casein micelle in real time and no labeling using a surface plasmon resonance (SPR)-based biosensor. Although the four individual caseins were found to be accessible for antibody binding, data confirmed that the C-terminal extremity of κ-casein was highly accessible and located at the periphery of the structure. When casein micelles were submitted to proteolysis, the C-terminal extremity of κ-casein was rapidly hydrolyzed. Disintegration of the micellar structure resulted in an increased access for antibodies to hydrophobic areas of α(s1)- and α(s2)-casein.

Disulphide arrangement in bovine caseins: localization of intrachain disulphide bridges in monomers of kappa- and alpha s2-casein from bovine milk

Naturally occurring monomeric kappa-casein and alpha s2-casein in bovine milk were purified by ion-exchange chromatography in order to localize potential intrachain disulphide bridges. Enzymic cleavage of the proteins followed by mass spectrometry and amino acid sequence analysis of cystine-containing peptides revealed the presence of an intrachain disulphide bond in both proteins.

The multimeric structure and disulfide-bonding pattern of bovine kappa-casein

Bovine kappa-casein was analyzed by SDS/PAGE, MS and amino acid sequence analysis in order to determine its multimeric composition and disulfide-bonding pattern. SDS/PAGE revealed that kappa-casein in the native state can range in size from a monomer to a multimeric structure larger than a decamer. Three types of interchain disulfide linkage, Cys11-Cys11, Cys11-Cys88 and Cys88-Cys88, were all assigned in multimers purified from [14C]carboxymethylated and untreated bulk milk, as well as a milk sample from a kappa-casein-variant-B homozygote Co20. These results indicate that multimerization occurs in a random or at present unpredictable disulfide-bonding pattern regardless of the size of the multimer or the genotype.

Reexamination of the polymeric distributions of kappa-casein isolated from bovine milk

kappa-Casein the stabilizing protein of the colloidal milk protein complex was purified from bovine skim milk by the method of McKenzie and Wake (Biochim, Biophys. Acta. 47, 240, 1961). The preparations were examined by sodium dodecyl sulfate gel electrophoresis in the presence and absence of a reducing agent. In the presence of a reducing agent, the kappa-casein migrates as a single low molecular weight band. However, in the absence of a reducing agent, a characteristic pattern of aggregates of varying molecular weight was observed with components ranging from monomer to octamer in integer steps. Densitometry of the Coomassie blue stained gels showed an almost equal distribution of components in each band; carbohydrate staining showed preferential location of sugar residues in lower molecular weight components. Treatment with chymosin (rennin) caused a downward shift in apparent molecular weight for each band with no change in the relative intensity of the Coomassie blue stained bands. Similar gel patterns were observed in whole caseins and partially purified kappa-caseins, indicating that this size distribution is a natural disulfide-linked reporter for the distribution of kappa-casein in casein colloids (micelles).

Protein-water interactions from 2H NMR relaxation studies: influence of hydrophilic, hydrophobic, and electrostatic interactions

The importance of water interactions with proteins in food systems is well documented. A controversy exists, however, as to the nature of these interactions and the effect of protein structural changes on them. To clarify these questions, a method has been developed for determining hydration from the protein concentration-dependence of deuteron resonance relaxation rates. Measurements were made in D2O on beta-lactoglobulin A to study effects of hydrophilic interactions, and on both casein micelles and submicelles to study hydrophobic and electrostatic effects. From the protein concentration-dependent relaxation rates, the second viral coefficients of the proteins were obtained by nonlinear regression analysis. Using either an isotropic tumbling or an intermediate asymmetry model, hydrations, upsilon, and correlation times, tau c, were calculated for the protein-associated water; from tau c, the Stokes radius, R, was obtained. Variations in upsilon and R were in accord with known structural changes in molecular states of the proteins. The NMR results are compared with hydrations and structural information derived independently from small-angle X-ray scattering.

Caseins of various origins and biologically active casein peptides and oligosaccharides: structural and physiological aspects

The first part of the present review is focused on structural aspects concerning the so far studied casein fractions of various origins: they are compared to the four classical major bovine caseins (alpha s1-, alpha s2-, beta- and kappa). The calcium-sensitive casein fractions are always phosphorylated whereas kappa-caseins are glycosylated. The study of the casein genes showed that the calcium-sensitive caseins diverged from a common ancestral gene and during the evolution, intergenic and intragenic duplications occurred. The considerable conservation of the phosphorylation sites emphasizes the importance of phosphorylated residues for the function of caseins, i.e. the formation of micelles and the binding of Ca2+. In kappa-caseins all the prosthetic sugar groups are linked by O-glycosidic linkages: their number varies from 0 to 5 in bovine kappa-casein and up to 10 in human kappa-casein. The structures of the known kappa-casein carbohydrate moieties are described. Finally the milk clotting process (interaction kappa-casein/chymosin) is compared to the blood clotting process (interaction fibrinogen/thrombin): a large number of similarities could be noted between both clotting phenomena. The second part of the review is devoted to the study of short casein peptides endowed with various biological activities. Some of them behaved as immunomodulators or casomorphins or angiotensin I converting enzyme inhibitors; others demonstrated an effect on platelet functions. A 'strategic zone' containing immunostimulating and opioid peptides could be located in cow and human beta-caseins. Furthermore bitter peptides, emulsifying peptides, calcium absorption enhancing peptides, chymosin-inhibiting peptides, have also been described and several further properties have been attributed to the kappa-caseinoglycopeptide; two tetrasaccharides isolated from the latter possess blood group activities. In conclusion caseins, the main milk proteins, should not only be considered as a nutriment but as a possible source of biologically active components. If, in the future, some of the discussed active peptides cannot be characterized in vivo, they can all, nevertheless, be synthesized and used either as food additives or in pharmacology.

Water interactions with varying molecular states of bovine casein: 2H NMR relaxation studies

The caseins occur in milk as spherical colloidal complexes of protein and salts with an average diameter of 1200 A, the casein micelles. Removal of Ca2+ is thought to result in their dissociation into smaller protein complexes stabilized by hydrophobic interactions and called submicelles. Whether these submicelles actually occur within the micelles as discrete particles interconnected by calcium phosphate salt bridges has been the subject of much controversy. A variety of physical measurements have shown that casein micelles contain an inordinately high amount of trapped water (2 to 7 g H2O/g protein). With this in mind it was of interest to determine if NMR relaxation measurements could detect the presence of this trapped water within the micelles, and to evaluate whether it is a continuum with picosecond correlation times or is associated in part with discrete submicellar structures with nanosecond motions. For this purpose the variations in 2H NMR longitudinal and transverse relaxation rates of water with protein concentration were determined for bovine casein at various temperatures, under both submicellar and micellar conditions. D2O was used instead of H2O to eliminate cross-relaxation effects. From the protein concentration dependence of the relaxation rates, the second virial coefficient of the protein was obtained by nonlinear regression analysis. Using either an isotropic tumbling or an intermediate asymmetry model, degrees of hydration, v, and correlation times, tau c, were calculated for the caseins; from the latter parameter the Stokes radius, r, was obtained. Next, estimates of molecular weights were obtained from r and the partial specific volume. Values were in the range of those published from other methodologies for the submicelles. Temperature dependences of the hydration and Stokes radius of the casein submicelles were consistent with the hypothesis that hydrophobic interactions represent the predominant forces responsible for the aggregation leading to a submicellar structure. The same temperature dependence of r and v was found for casein under micellar conditions; here, the absolute values of both the Stokes radii and hydrations were significantly greater than those obtained under submicellar conditions, even though tau c values corresponding to the great size of the entire micelle would result in relaxation rates too fast to be observed by these NMR measurements. The existence of a substantial amount of trapped water within the casein micelle is, therefore, corroborated, and the concept that this water is in part associated with submicelles of nanosecond motion is supported by the results of this study.

Analysis by fast protein liquid chromatography of variants of kappa-casein and their relevance to micellar structure and renneting

Fast protein liquid chromatography was used to study the kappa-casein fraction of casein micelles from bulk milk and from milk from individual animals homozygous for kappa-caseins A and B. The extent of glycosylation of the kappa-casein appeared to have no effect on its distribution in casein micelles of different sizes, nor did it affect the rate at which kappa-casein was destroyed during renneting. The rate of breakdown of kappa-casein during renneting was also almost independent of micellar size. The results may indicate a difference between methods which analyse for intact kappa-casein or for the product macropeptide during renneting.

Specific agglutination of Streptococcus agalactiae from bovine mastitis by casein components of bovine milk

Streptococcus agalactiae strains freshly isolated from bovine mastitis cases clumped within 15 min of addition of small amounts of bovine milk to a broth culture. This reaction was not observed with isolates from human infections or bovine strains that had been maintained in the laboratory for extensive periods. Intensity of the clumping response as measured by a microtiter dilution assay was highly variable. Analysis of several single colony isolates derived from one strain indicated that the clumping phenotype was genetically unstable. The clumping reaction took place in the presence of rifampicin or chloramphenicol. Milk components that caused aggregation were heat stable, relatively insensitive to proteases, and were larger than 10,000 daltons. Purified casein also induced clumping in these strains.

Localization of kappa-casein on thin sections of casein micelles by the gold method

The ultrastructural location of kappa-casein in bovine casein micelles was investigated by the protein A-gold method. Casein micelles, fixed in glutaraldehyde, were embedded at low temperature to enhance immunocytochemical marking of thin sections. kappa-Casein was found distributed throughout the micelles of all sizes with a higher concentration in the smaller micelles. No peripheral location of kappa-casein was observed, even in the larger micelles. These results do not agree with "coat-core" structures proposed for casein micelles. However they favor models where kappa-casein is distributed uniformly throughout the micelles.