Casein Fractions

The main constituents of this milk protein fraction have been fairly well investigated. 

Their amino acid sequences are summarized in Table 10.8. Data showing the genetic variations are provided in Table 10.9. Caseins are not denat-urable because of the lacking tertiary structure. 

Fraction Genetic variants Portion Isoionic point Molecular weight (kdal) Phosphorus content (%) 

Os2-Casein (Mr 25,000) consists of 207 amino acid residues, has a pronounced dipolar structure with a concentration of anionic groups in the region of the N-terminiis and cationic groups in the region of the C-terminus. It contains 11 phosphoserine and 2 cysteine residues and is even more easily precipitable with Ca than otsi-casein. Other proteins, previously known as ot 4- t s5-, and ttse-caseins, appear to be members of the aj2 family and to differ in the degree of phosphorylation. Dimers linked via disulfide bridges also appear to be present. 

K-Caseins. The B variant consists of a peptide chain with 169 residues and has a molecular weight of 18 kdal. The monomer, which contains 1 phosphoserine and 2 cysteine residues, is accessible only under reducing conditions. 

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In contrast to the caseins, the whey proteins do not precipitate from solution when the pH of milk is adjusted to 4.6. This characteristic is used as the usual operational definition of casein. This difference in the properties of the two milk protein groups is exploited in the preparation of industrial casein and certain varieties of cheese (e.g. cottage, quarg and cream cheese). Only the casein fraction of milk protein is normally incorporated into these products, the whey proteins being lost in the whey. 

In 1956, Waugh and von Hippel showed that the a-casein fraction of Hipp et al. contained two proteins, one of which was precipitated by low concentrations of Ca2 + and was called as-casein (s = sensitive) while the other, which was insensitive to Ca2+, was called /c-casein. as-Casein was later shown to contain two proteins which are now called a8l- and as2-caseins. Thus, bovine casein contains four distinct gene products, designated a5l-, x,2-, / - and /c-caseins which represent approximately 37, 10, 35 and 12% of whole casein, respectively. 

Hydrolysis of primary caseins by plasmin. In 1969, Groves and coworkers showed that the y-casein fraction, as isolated by Hipp et al., is very heterogeneous, containing at least four distinct proteins y-casein, temperature-sensitive casein (TS, which is soluble in the cold but precipitates above 20°C), R-casein and S-casein. These four proteins were shown to be C-terminal fragments of /3-casein. In 1976, the nomenclature of the y-casein group was revised, as shown in Figure 4.7 and Table 4.3. 

K-Caseins. The K-casein family comprises that portion of the a-casein fraction of Warner that is soluble in 0.4 M CaCl2 at pH 7.0 and 4°C and occurs as a mixture of polymers held together by disulfide bonds. An equilibrium is established between the polymers and monomers in a few hours (Vreeman et al. 1977). 

Tarrassuk, N. P., Yaguchi, M. and Callis, J. B. 1965. Effect of temperature on the composition of casein fractions eluted from DEAE-cellulose column. J. Dairy Sci. 48, 606-609. 

Wake, R. G. and Baldwin, R. L. 1961. Analysis of casein fractions by zone electrophoresis in concentrated urea. Biochim. Biophys. Acta 47, 225-239. 

Goranova, L. and Stefanova-Kondratenko, M. 1975. Effect of Bacillus mesentericus strain 76 clotting enzyme on casein fraction, relative to other enzymes of microbial or animal origin. Lait 55, 58-67. 

Higashio, K. and Yoshioka, Y. 1981C. Studies on milk clotting enzyme from microorganisms. III. Breakdown of casein fractions by milk clotting enzyme preparations of Mucor recemosus No. 50 and its mutants. J. Agric. Chem. Soc. Japan 55, 951-958. 

CM Hollar, AJR Law, DG Dagleish, RJ Brown. Separation of major casein fractions using cation-exchange fast protein liquid chromatography. J Dairy Sci 74 2403-2409, 1991. 

Veloso, A. C. A., Teixeira, N., and Ferreira, I. M. P. L. V. O. (2002). Separation and quantification of the major casein fractions by reverse-phase high performance liquid chromatography and urea-polyacrylamide gel electrophoresis Detection of milk adulterations.. Chromatogr. A 967, 209-218. 

The y-casein fractions and proteose peptone fractions 5, 8-slow, and 8-fast have amino acid sequences corresponding to parts of the sequence of p-casein (Fig. 1). Their occurrence in milk could be due to partial gene duplication but identical fractions can be produced... 

Heth and Swaisgood (1982) examined the preferential binding of caseins to thioester-derivatized glass beads. The micelles were first separated into micellar and serum casein fractions by chromatog-... 

Table I shows that the foaming properties of whole casein improved by slight phosphorylation. The lowest phosphorylated form of casein (4 mol P/mol protein) showed higher foam hydration and stability than the native whole casein. However, the highly phosphorylated whole casein (11 mol P/mol protein) showed poor foaming properties. The foam hydration of as-casein deteriorated while that of K-casein improved by phosphorylation. This discrepancy seemed to be caused by a different initial hydrophobic/ hydrophilic balance of the proteins in their native states. However, foam stabilities of all casein fractions were reduced by phosphorylation, with K-casein being only slightly affected.

Whole casein and individual casein fractions and their derivatives have been shown to modulate lymphocyte proliferation in vitro. Carr et al. (1990) found that asl-casein can enhance the mitogen-stimulated proliferation of murine splenic T-lymphocytes, when induced in in vitro cell culture at a concentration of 10 6 M. Wong et al. (1996b) showed that (3-casein significantly enhances the mitogen-induced proliferation of ovine T- and B-lymphocytes in a dose-dependent manner, when added to in vitro cell culture. With K-casein, on the other hand, the opposite effect was found, as K-casein was suppressive for murine... 

Casein fractions Lactobacillus GG asi-cn, (3-cn, K-cn X Immunomodulatory Siitas et al. (1996a, b)... 

The four major casein fractions contain subfractions and genetic polymorphs (Table 1), each of which possesses a unique primary structure and associated physico-chemical properties (1). 

UF permeate, in turn, is recirculated to the MF stage as a diafiltration medium to fully remove the whey proteins from the milk without changiug the outer environment for the casein fraction. 

Action of Prostate Phosphatase on Casein Fractions Each reaction mixture contained 0.6% protein and 0.006% enzyme in sodium cacodylate buffer of pH 6.1 and 0.1 T/2. 

Isolation of Caseins. Isoelectric caseins were obtained by precipitation from whole milk or from 5g of homogenized cheese in 30 mL of water by adding 2M FICl to pH 4.6 followed by centrifugation at 3500 rpm for 15 min. To isolate the casein fractions completely from whey and eliminate the remaining fat, it was washed once with 1M sodium acetate buffer (pH 4.6) and three times with dichloromethane/1 M sodium acetate buffer (pH 4.6) (1 1, v/v). The casein fractions obtained were lyophilized and stored at -20 °C. 

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