Casein, Alpha

Casein is the major protein of bovine milk. A phosphoprotein, it exists as an equilibrium of soluble and complex colloidal aggregates (micelles). The casein is solubilized by dialyzing skim milk against phosphate buffer or by dilution. Its heterogeneity has long been recognized. The α-, β-, γ-caseins (75%, 22% and 3% respectively) are electrophoretic components of unfractionated casein at pH 7. They may be prepared by acid precipitation. β-Casein can be separated from α-casein by utilizing its greater solubility at pH 4.4 and 2°C. von Hippel and Waugh (1955) have reported that soluble casein is completely and reversibly dissociated into monomers at pH 12. Repolymerization of α and β components occurs under different conditions of pH and temperatures.

Casein in solution is not heat coagulable under ordinary conditions. α-Casein consists of three components: α1 which is insoluble with CaCl₂ and is most abundant; and α2 and α3 which differ in mobilities and CaCl₂ solubility. α-3-Casein has a sialic acid moiety similar to κcasein (Hipp et al. 1961), (McMeekin et al. 1959). See also Groves and Townend (1970), Ho and Kurland (1966), Schmidt and Payens (1963), Groves et al. (1962) and the review by McMeekin (1954).

von Hippel and Waugh (1955) and Waugh and von Hippel (1956) describe a preparation of casein at constant pH. (The method involves shifting the casein into micelles by adding Ca²⁺ and isolating them centrifugally. By removing calcium from the micelles the casein is released into solution.) They showed that the classical α-casein is composed of two major components: αs-casein, precipitable with Ca²⁺ and κ-casein which is calcium insensitive.

The heterogeneity of casein manifest in different "purification" procedures is further complicated by genetic variations. Aschaffenburg (1961) indicates several independent genes are involved in milk secretion. Each of the casein fractions exists in genetically variable forms (Groves and Kiddy 1968), (El-Negoumy 1967). Genetic variants are indicated by A, B, C, etc. Schmidt (1970) indicates them to have slightly differing amino acids. See also Hill et al. (1970), DeKoning and van Rooijen (1967), Thompson et al. (1962), McMeekin et al. (1959).

αs-Casein has been electrophoretically separated into αs1, αs2 and αs3 fractions. A characteristic of α-s1, the dominant component, is insolubility in the presence of Ca²⁺ or other divalent cations (Bingham et al. 1972).

The primary structure of α-s1-casein has been reported by Grosclaude et al. (1970). Its molecular weight is indicated to be 23,600 (Mercier et al. 1971). See also Noelkem (1967). Polymerization of subunits of αs1 have been reported by Schmidt and VanMarkwijk (1968), Schmidt et al. (1967), and Payens and Schmidt (1966). Holt et al. (1975) have studied the reaction of α-s1-casein with CaCl₂. It can be protected against precipitation by Ca²⁺ with κ-casein.

κ-Casein has been reported on by Creamer (1972), Evan et al. (1971) and Hoagland (1968). Its primary structure has been determined by Mercier et al. (1973) and Ribadeau-Dumar et al. (1971).

κ-Casein is a glycoprotein (Wheelock and Sinkinson, 1969) containing S-S bonds (MacKinlay and Wake 1964) and accounts for approximately 15% of whole bovine casein (MacKinlay and Wake 1965). Its monomer molecular weight is approximately 20,000 (Talbot and Waugh 1970). κ-Casein was reported by Waugh and von Hippel (1956) as the factor responsible for micelle stabilization (Nakai and Kason 1974), (Hill and Wake 1969), (MacKinlay and Wake 1965). In the presence of Ca²⁺ it will form micelles with either alpha-s1 or beta caseins (Dickson and Perkins 1971). κ-Casein is split by rennin to produce para-κ-casein and polypeptide (MacDonald and Thomas 1970), (Wheelock and Sinkinson 1969), (Jolles et al. 1968), (MacKinlay et al. 1966), (McKenzie and Wake 1961). Mercier et al. (1972) have reported on the primary structure of the beta variant of bovine kappa-casein. Brignon et al. (1972) and Fiat et al. (1972) have also reported on the structure.

Addition of Ca²⁺ to soluble casein lacking the κ-component causes a heavy precipitate. κ-Casein is more plentiful in colostrum (Guerin et al. 1974).

Casein Micelles are colloidal complexes consisting of cores of insoluble αs and β-caseinates stabilized by kappa-casein (Holt et al. 1975), (Richardson and Creamer 1975), (Nakai and Kason 1974), (Payens and Nijhuis 1974), (Creamer et al. 1973), (Slattery and Evard 1973), (Bingham et al. 1972), (Creamer 1972), (Lin et al. 1972), (Waugh and Talbot 1971), (Talbot and Waugh 1970), ( Waugh et al. 1970 and 1971). According to Bakri and Wolfe (1971) the stability of casein micelles is dependent upon the sialic acid moiety of κ-casein but calcium plays a dominant stabilizing role. In the absence of calcium at pH 7 and 2°C, α-s and κ-caseins show little tendency to interact (Noble and Waugh 1965). On the addition of certain proteases like rennin, chymotrypsin or chymopapain, the relatively stable colloidal particles coalesce to form a gel.

Leslie et al. (1969) report on NMR studies on α-s1, β-, and κ-caseins and Dumas (1968) reports on their determination using carboxypeptidase A.