Casein. IX. Carbohydrate moiety of k-casein

Micelas de caseína: dos monômeros à estrutura supramolecular

Resumo A importância primária das micelas de caseína reside no fato de que os processos empregados na transformação do leite em quaisquer de seus derivados dependem, direta ou indiretamente, de sua estabilidade ou de sua desestabilização controlada. Assim, o objetivo do presente trabalho é apresentar uma revisão atualizada sobre a organização estrutural das micelas de caseína. Em termos físico-químicos, as micelas de caseína podem ser definidas como agregados supramoleculares esféricos e porosos, altamente hidratados, carregados negativamente, com diâmetro médio de 200 nm, e que apresentam aproximadamente 104 cadeias polipeptídicas. Além de água, as micelas são constituídas por quatro tipos de caseínas, chamadas de αS1, αS2, β, e κ-caseínas, que estão unidas por meio de interações hidrofóbicas e eletrostáticas, e pela presença de minerais, sobretudo sais de fosfato de cálcio, os quais são os principais responsáveis pela manutenção da estrutura micelar. A estabilidade das micelas de caseína é atribuída à presença de uma camada externa difusa, formada basicamente por κ-caseína. Apesar de as propriedades coloidais das micelas de caseína serem conhecidas, ainda não há consenso sobre como as moléculas de caseína estão estruturadas em seu interior. Portanto, os principais modelos que descrevem a organização interna das micelas de caseína são apresentados na parte final do artigo.

Heat stability of milk: influence of modifying sulphydryl-disulphide interactions on the heat coagulation time–pH profile

Addition of reducing agents such as 2-mercaptoethanol (2-ME), dithio-threitol and Na sulphite to milk markedly reduced its heat stability at pH values below 7·1. 2-ME reversibly destabilized milk or serum protein-free casein micelle dispersions and promoted the release of κ-casein-rich protein from the micelles. Reduction of either casein micelles or β-lactoglobulin (β-lg) with 2-ME and subsequent blocking of the newly formed –SH groups with N-ethylmaleimide irreversibly reduced the maximum to minimum ratio in the heat stability profile. 2-ME disrupted κ-casein/β-lg complexes and stripped κ-casein from the micelles on heating. The milk or caseinate systems were thus destabilized. Addition of KBrO4or iodosobenzoate to milk at 5 HIM eliminated the minimum but destabilized milk in the region of the maximum. However, KIO3at 5 mm had a strong stabilizing effect throughout the pH range 6·5–7·3.

Nomenclature of the Proteins of Cows’ Milk—Sixth Revision

This report of the American Dairy Science Association Committee on the Nomenclature, Classification, and Methodology of Milk Proteins reviews changes in the nomenclature of milk proteins necessitated by recent advances of our knowledge of milk proteins. Identification of major caseins and whey proteins continues to be based upon their primary structures. Nomenclature of the immunoglobulins consistent with new international standards has been developed, and all bovine immunoglobulins have been characterized at the molecular level. Other significant findings related to nomenclature and protein methodology are elucidation of several new genetic variants of the major milk proteins, establishment by sequencing techniques and sequence alignment of the bovine caseins and whey proteins as the reference point for the nomenclature of all homologous milk proteins, completion of crystallographic studies for major whey proteins, and advances in the study of lactoferrin, allowing it to be added to the list of fully characterized milk proteins.

Biological activities of bovine glycomacropeptide

Biological activity of bovine kappa-caseino glycomacropeptide (GMP) has received much attention in recent years. Research has focused on the ability of GMP to bind cholera and Escherichia coli enterotoxins, inhibit bacterial and viral adhesion, suppress gastric secretions, promote bifidobacterial growth and modulate immune system responses. Of these, protection against toxins, bacteria, and viruses and modulation of the immune system are the most promising applications.

Proteolytic activity of proteinases on macropeptide isolated from kappa-casein

Proteolytic activities of chymosin, bovine pepsin, Mucor miehei rennet, Cryphonectria parasitica (formerly Endothia parasitica) rennet, trypsin, and chymotrypsin on kappa-casein macropeptide were measured. Macropeptide solutions (10 mg/ml of .05 M, pH 6.6 phosphate buffer) were incubated with the enzymes at 37 degrees C for various times, and their reactions were stopped by adding .025 ml of pepstatin (1 mg/ml of methanol). Peptides released from kappa-casein macropeptide were then fractionated using reverse-phase HPLC. At the pH of milk (pH 6.6), kappa-casein macropeptide was resistant to enzymic action by chymosin, bovine pepsin, and M. miehei and C. parasitica rennets. Bovine pepsin hydrolyzed kappa-casein macropeptide at pH 3. kappa-Casein macropeptide was readily hydrolyzed at pH 6.6 by trypsin and chymotrypsin. Possible physiological functions of the kappa-casein macropeptide are discussed in light of these findings.

Calcium and calmodulin-dependent phosphorylation of kappa-casein by a bovine mammary casein kinase

A calcium and calmodulin-dependent kappa-casein kinase activity has been described in the bovine mammary gland. This kinase required previously dephosphorylated kappa-casein for substrate, thus suggesting a physiological role for this enzyme. The kappa-casein kinase required magnesium and the presence of both calcium and calmodulin for full activity. Calmodulin concentrations of .32 microM achieved one-half maximal activation of this enzyme. The calcium and calmodulin-dependent kappa-casein kinase was found in preparations of mammary acini and could be localized in a membranous fraction by centrifugation. The particles containing this activity had a high density (1.309 g/cc) and cofractionated with caseins, suggesting this enzyme may be present in secretory granules.

Structure of the asialyl oligosaccharide chains of kappa-casein isolated from ovine colostrum

Three oligosaccharide alditols, a di (44-63%), tetra (32-46%), and a pentasacchariditol (5-10%) have been isolated from desialylated caseinomacropeptide obtained from kappa-casein of ewe colostrum. The disacchariditol is 2-acetamido-2-deoxy-3-O-(beta-D-galactopyranosyl)galactitol; in the tetrasaccharide this unit is substituted in position 6 of the galactosaminitol moiety by 2-acetamido-2-deoxy-4-O-(beta-D-galactopyranosyl)-beta-D-glucopyranose, while the pentasacchariditol is derived from the tetrasaccharide by addition of a D-galactopyranosyl unit to position 3 of the galactopyranose unit substituting the glucosamine unit.

Nomemclature of the proteins of cow's milk: fourth revision

This report reviews the nomenclature of the milk proteins of cow's milk in light of more recent advances in our knowledge. With the establishment of the primary structures of a number of these proteins, we now have a definite identification of alphas1-, kappa-, beta-, and the gamma-caseins as well as beta-lactoglobulin and alpha-lactalbumin. On the basis of new information on their primary structures and relationship to beta-casein polymorphs, changes in nomenclature have been recommended for proteins of the gamma-casein fraction. Although the primary structure serves as the unambiguous definition of proteins for which it is known, a more practical identification is necessary. We recommend that their behavior in gel electrophoresis under suitable conditions be employed for this purpose for all of the "major" milk proteins of raw skim milk except the immunoglobulins where, because of their heterogeneity and molecular genetics, physical parameters are less useful and their identification must be based upon antigenic determinants and their homology with their human counterparts. More work is needed and, with the accumulation of more information, additional changes in nomenclature can be expected for such proteins as the minor components of alphas- and kappa-caseins, alpha-lactalbumin, and the proteose-peptone fraction as well as further confirmation of the presence of immunoglobulins IgE and additional IgG subclasses. Additional components and genetic variants also can be expected.

The sugar part of kappa-caseins from cow milk and colostrum and its microheterogeneity

Cow kappa-casein contains only three different sugars (Gal, GalNAc, NeuNAc). However detailed analyses achieved mainly by gas liquid chromatography suggested a microheterogeneity at the sugar level. After alkaline borohydride treatment, filtration on Bio-Gel P4 and paper chromatography, different carbohydrate parts were obtained. The two main compounds had the following molar compositions: GalNAc (1), Gal(1) and NeuNAc (1) and GalNAc (1), Gal(1) and NeuNAc (2). From these data and our previous sequence studies, some formulae of the polysaccharide part were proposed. One of them was closely related to the sugar sequence of a glycopeptide with MN activity which was in agreement with our observation concerning a cross antigenic reactivity between the N blood group substances and the caseinoglycopeptides. All the polysaccharide parts isolated from colostrum caseinoglycopeptide were much more complex than those obtained from the normal glycopeptide, confirming an evolution of the sugar part as a function of time after parturition.

[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.