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

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.

The specificity for κ-casein as the stabilizer of αs-casein and β-casein. I. Replacement of κ-casein by other proteins

The specificity of the interaction betweenκ-casein,αs-casein andβ-casein which forms the basis of micelle stabilization was studied by investigating the extent to whichκ-casein could be replaced by other proteins. Of those tested, only gelatin replacedκ-casein and even it was only 2·5% (w/v) as effective and required a long pre-incubation period. The micelles formed by each ofκ-casein and gelatin withαs-casein and Ca2+were of a similar size to the casein—Ca complexes which compose natural micelles. Gelatin also formed complexes withαs- and withβ-casein at 30°C in the absence of CaCl2. Evidence was obtained that the interactions between gelatin and the caseins had a much stronger ionic component than had those betweenκ-casein and the other caseins. It was concluded that the interactions betweenκ-casein andαs- andβ-caseins which lead to micelle formation are highly specific and probably involve definite sites in each molecule.

[Group separation of caseins from cow milk on hydroxyapatite]

Ion exchange-chromatography method with the use of hydroxy apatite is considered one of the methods used for the casein fractionation particularly those containing phosphate in its composition. Simplicity of hydroxyapatite-preparation and its easily use in casein fractionation in batches are advantages of this method. Sephadex G-25 or Agarose is used to easement the sample passage through the column. By using of hydroxy-apatite in batches, alpha s-casein can easily separated with the use of buffer solution 0.006 M phosphate at pH 6.6 and the elution is carried out by buffer solution containing 0.25 M KH2PO4, 0.2 M KCl, 4.5 M urea and 0.002 M di-thiotreitol. Before the elution of alpha s-casein, beta- and kappa-casein are separated using buffer solutions containing 0.005 M and 0.1 M KH2PO4 in addition to the previously mentioned components.

Fractionation of whole casein on hydroxyapatite. Application to a study of buffalo kappa-casein

When whole caseins from cow and Italian buffalo (Bubalus arnee) were fractionated by chromatography on a column of hydroxyapatite they behaved in a similar manner. kappa-Casein was eluted with 5 mM phosphate buffer, pH 6-8, containing 0-2 M-KCl, 4-5 M-urea and 2 mM-2-mercaptoethanol, but beta- and alphas-caseins were retained and could be eluted successively by a linear gradient from 5 mM to 250 mM-phosphate buffer. Buffalo kappa-casein preparations, obtained from bulk milk or from milks of individual animals by chromatography on hydroxyapatite, produced identical electrophoretic patterns at pH 8-6. By further fractionation of these kappa-caseins on DEAE-cellulose, in each case, at least 7 components were purified; they had different electrophoretic mobilities but were all sensitive towards chymosin. The major fraction migrated like component 1 of bovine kappa-casein B.

[Transparency of analytical results found by different methods of electrophoresis studies on casein (author's transl)]

The electrophoretic mobility of the major casein components has been studied on different carriers. Besides free electrophoresis and paper electrophoresis starch gel, polyacrylamide gel and cellulose acetate gel have compared. With urea buffer mixtures of neutral to alkaline pH a mobility scale on all gels was found corresponding the order alphas-, beta-, kappa-, gamma-casein in the sense of decreasing mobility. The application of commercially available cellulose acetate strips for the investigation of genetic material in large numbers has been found to be sufficient. The transparency of polyacrylamide gel, starch gel and cellulose acetate gel results is guaranteed.

[Purification of kappa-caseins from sheep. Analysis of the glycan and peptide components (author's transl)]

Starting from whole individual ovine casein prepared according to the method of Shahani, K. M. & Sommer, H. H. [J. Dairy Sci. 34, 1003-1009 (1951)], kappa-casein was isolated and purified by successive steps of chromatography on columns of dextran gel and hydroxyapatite. On filtration through Sephadex G-150 in a buffer containing urea, the bulk of the kappa-casein behaved as aggregates appearing in the void volume. Dissociation of these aggregates by reductive cleavage of disulfide bonds with 2-mercaptoethanol, followed by a second filtration step on Sephadex G-150 in the presence of both urea and 2-mercaptoethanol, resulted in retardation of the kappa-casein, with separation from a contaminant representing 10-12% of the material applied. Further purification was achieved by chromatography on hydroxyapatite which eliminated the alpha-s- and beta-caseins. The purified kappa-casein had a molecular weight of about 20000, an absorption coefficient (see journal for formula) at 280 nm of 10.85 and a sialic acid and phosphorous content of 0.3% (w/w) each. The sugar fraction liberated on acid hydrolysis of the caseinomacropeptide showed the presence of N-acetylgalactosamine, galactose and neuraminic acid in equimolar ratio. Neuraminic acid existed mainly as the N-glycolyl derivative. The polypeptide chain of the ovine kappa-casein was composed of about 170 amino-acids residues. Compared to bovine kappa-caseins, the most notable difference was the presence of one additional cysteinyl and four additional aspartyl residues. Starch-gel and polyacrylamide-gel electrophoresis clearly revealed the heterogeneity of ovine kappa-casein. Chromatographic fractionation of whole kappa-casein on DEAE-cellulose also led to the separation of several fractions, the main characteristics of which are presented. Analysis of these fractions indicated that only those components which were firmly bound to DEAE-cellulose were glycosylated.