Preparation and amino acid sequence of human kappa-casein

Investigating the genetic polymorphism of sheep milk proteins: a useful tool for dairy production

Sheep is the second most important dairy species after cow worldwide, and especially in the Mediterranean and Middle East regions. In some countries, the difficult environmental conditions require a peculiar adaptation and, in these contexts, sheep are able to provide higher quality protein than cattle. In the least-developed countries, the amount of dairy sheep and ovine milk production is progressively increasing. In order to improve dairy productions, in particular those with local connotations, it is necessary to obtain in-depth information regarding milk quality and rheological properties. The genetic polymorphisms of milk proteins are often associated with quantitative and qualitative parameters in milk and are potential candidate markers that should be included in breeding strategies similar to those already available for cattle. Due to the current and growing interest in this topic and considering the large amount of new information, the aim of this study was to review the literature on sheep milk protein polymorphisms with a particular emphasis on recent findings in order to give scientists useful support. Moreover, the effects of different protein variants on milk yield and composition are discussed.

Hot-spot mapping of the interactions between chymosin and bovine κ-casein

Chymosin is a commercially important enzyme in the manufacturing of cheese. Chymosin cleaves the milk protein κ-casein, which initiates the clotting process. Recently, it has been shown that camel chymosin has superior enzymatic properties toward cow's milk, compared to bovine chymosin. The two enzymes possess a high degree of homology. There are only minor differences in the binding cleft; hence, these must be important for binding the substrate. Models for the binding of a 16 amino acid fragment, consisting of the chymosin-sensitive region of bovine κ-casein (97-112), to both enzymes have previously been presented. Computational alanine scanning for mutating 39 residues in the substrate and the bovine enzyme are presented herein, and warm- (ΔΔG > 1 kcal/mol) and hot-spot (ΔΔG > 2 kcal/mol) residues in the bovine enzyme are identified. These residues are relevant for site-directed mutagenesis, with the aim of modifying the binding affinity and in turn affecting the catalytic efficacy of the enzyme.

Initial stage of cheese production: a molecular modeling study of bovine and camel chymosin complexed with peptides from the chymosin-sensitive region of κ-casein

Bovine chymosin has long been the preferred enzyme used to coagulate cow's milk, in the initial stage of cheese production, during which it cleaves a specific bond in the milk protein κ-casein. Recently, camel chymosin has been shown to have a 70% higher clotting activity toward cow's milk and, moreover, to cleave κ-casein more selectively. Bovine chymosin, on the other hand, is a poor clotting agent toward camel's milk. This paper reports a molecular modeling study aimed at understanding this disparity, based on homology modeling and molecular dynamics simulations using peptide fragments of κ-casein from cow and camel in both bovine and camel chymosin. The results show that the complex between bovine chymosin and the fragment of camel κ-casein is indeed less stable in the binding pocket. The results also indicate that this in part may be due to charge repulsion between a lysine residue in bovine chymosin and an arginine residue in the P4 position of camel κ-casein.

Purification of goats' milk casein by reversed-phase high-performance liquid chromatography and identification of αs1-casein

Goats’ milk caseins were separated into four components in a single run using reversed-phase gradient high-performance liquid chromatography. The purity of the isolated components was checked by sodium dodecyl sulphate polyacrylamide gel electrophoresis, amino acid analysis and determination of the N-terminal residue. By a comparison with previously published results for goats’ milk caseins the four peaks were identified as κ-, αs1-, αs2- and β-casein. In order to confirm the existence of αsl-casein in goats’ milk, this component was sequenced for 44 steps, revealing a sequence homologous to bovine αsl-casein and almost identical to the N-terminal sequence previously published by Boulangeret al.(1984).

Application of capillary zone electrophoresis to the characterisation of the human milk protein profile and its evolution throughout lactation

This work describes the use of capillary zone electrophoresis for the characterisation of human milk proteins. The major proteins were identified following different strategies, such as the treatment with enzymes for selective protein modification. Using this method we studied the proteins in human milk from different donors throughout lactation. Qualitative and quantitative differences in the composition of the individual proteins were observed. The different beta-casein phosphoforms were separated and quantified. The average proportion of the 0P:1P:2P:3P:4P:5P was, approximately, 3:6:9:4:10:2. The evolution of the ratio of the different beta-casein phosphoforms during lactation is reported.

Nutritional and physiologic significance of human milk proteins

Human milk contains a wide variety of proteins that contribute to its unique qualities. Many of these proteins are digested and provide a well-balanced source of amino acids to rapidly growing infants. Some proteins, such as bile salt-stimulated lipase, amylase, beta-casein, lactoferrin, haptocorrin, and alpha1-antitrypsin, assist in the digestion and utilization of micronutrients and macronutrients from the milk. Several proteins with antimicrobial activity, such as immunoglobulins, kappa-casein, lysozyme, lactoferrin, haptocorrin, alpha-lactalbumin, and lactoperoxidase, are relatively resistant against proteolysis in the gastrointestinal tract and may, in intact or partially digested form, contribute to the defense of breastfed infants against pathogenic bacteria and viruses. Prebiotic activity, such as the promotion of the growth of beneficial bacteria such as Lactobacilli and Bifidobacteria, may also be provided by human milk proteins. This type of activity can limit the growth of several pathogens by decreasing intestinal pH. Some proteins and peptides have immunomodulatory activities (eg, cytokines and lactoferrin), whereas others (eg, insulin-like growth factor, epidermal growth factor, and lactoferrin) are likely to be involved in the development of the intestinal mucosa and other organs of newborns. In combination, breast-milk proteins assist in providing adequate nutrition to breastfed infants while simultaneously aiding in the defense against infection and facilitating optimal development of important physiologic functions in newborns.

Formation of reconstituted casein micelles with human beta-caseins and bovine kappa-casein

Human beta-casein (CN) is the major protein of the human milk casein fraction (approximately 80%) and exists in six calcium-sensitive forms, having zero to five organic phosphates per molecule. The major forms are the doubly-phosphorylated (beta-CN-2P; approximately 30%) and the quadruply phosphorylated (beta-CN-4P; approximately 35%) forms. Although calcium-insensitive, kappa-CN is known for its role in preventing the precipitation of beta-CN in the presence of Ca+2, but it is not known how the different levels of phosphorylation may affect this. In the present investigation, turbidity, measured at 400 nm, was determined at increasing temperatures (4 up to 37 degrees C) for solutions of beta-CN-2P and beta-CN-4P (3 mg/ml in 0.02 M NaCl, 0.01 M imidazole, pH 7) individually and also mixed with bovine kappa-CN in 6/1 and 3/1 weight ratios of beta/kappa and containing 0, 5, and 10 mM Ca+2. The results indicate that the first step of micelle formation probably leads to polymers of limited size, the only complexes available to beta-CN-2P under most conditions. With beta-CN-4P, these polymers aggregate further to give reconstituted micelles, probably because of the ability to form crosslinks at this phosphorylation level. The formation of reconstituted micelles under various conditions of pH, Ca+2 concentration and kappa-CN content indicates that both hydrophobic interactions and Ca+2 bridges or crosslinks may contribute to protein aggregation and micelle building.

Association of mixtures of the two major forms of beta-casein from human milk

Human milk beta-casein (CN) is unique in that it may be phosphorylated at any level from zero (beta-CN-0P) to five (beta-CN-5P) organic phosphates per molecule. The 2P and 4P forms are the major components, with about 30 to 35% each. Here, we present the association properties of mixtures of these two moieties of human beta-CN. The aggregation patterns, as functions of temperature and ionic strength of these mixtures, generally follow those for the individual components. However, the mixtures yielded polymers with slightly different properties, which indicates extensive interaction between the two. Some properties of the mixtures were more like those for the 2P form, such as association in low salt buffer to give a peak with a sedimentation coefficient, s20,w, of approximately 11 S, in contrast to the 2P form alone with a peak of approximately 13 S and 4P alone with only a small amount of material with s20,w greater than 2 S at 27 degrees C. The solubility and interactions in the presence of Ca2+ ions were intermediate but more like the 4P form. A protein-concentration dependence for s20,w was seen, and laser light scattering indicated that there was an increase in size and/or a change in shape as the protein concentration increased. From the results, it is apparent that submicellar oligomers are probably formed by rapidly established equilibrium association reactions. The presence of an equal amount of the 2P form along with the 4P form does not appear to be a disadvantage in casein micelle formation and function.

Protective function of proteins and lipids in human milk

Human milk provides the infant with protection against infectious diseases. This protection is conferred through several mechanisms: specific antibody targeted protection against pathogens in the infant's environment (through milk IgA, IgG, and IgM) and broad-spectrum, nonspecific protection provided through several distinct mechanisms. These are: bactericidal effects (lactoferrin), bacteriostatic action (lactoferrin, lysozyme), lysis of microorganisms (lysozyme), antiviral effects (lactoferrin, products of milk fat digestion), antiprotozoan activity (free fatty acids produced during gastric and intestinal digestion of milk fat), and ligand action (inhibition of Helicobacter pylori adhesion to gastric mucosa by kappa-casein). In addition to these protective functions of the proteins and lipids of human milk, several enzymes present in human milk might provide protection by generating components that are bactericidal (bile salt dependent lipase, peroxidase), prevent inflammatory reactions (platelet-activating factor acetylhydrolase), or protect the integrity of milk proteins (antiproteases).

Comparative aspects of milk caseins

The caseins comprise the major protein component of milk of most mammals and are secreted as micelles that also carry high concentrations of calcium. They are phosphoproteins that represent the products of four genes, equivalent to those that encode the bovine alpha s1, alpha s2, beta, and kappa-caseins. There is considerable variation in the relative proportions of the particular caseins across species. The primary sequences of the alpha s1, alpha s2, and beta-caseins also show considerable species variation consistent with rapidly evolving genes that are proposed to have a common precursor. In contrast, the kappa-caseins exhibit features that demonstrate a separate origin and function where they are proposed to stabilise the micelle structure. This review focuses on comparative aspects of the caseins across a number of species for which information is now available.

Characterization of the particles of purified kappa-casein: trypsin as a probe of surface-accessible residues

kappa-Casein as purified from bovine milk exhibits a rather unique disulfide bonding pattern as revealed by SDS-PAGE. The disulfide-bonded caseins present range from dimer to octamer and above and preparations contain about 10% monomer. All of these heterogeneous polymers, however, self-associate into nearly spherical particles with an average diameter of 13 nm at pH 8.0, as revealed by negatively stained transmission electron micrographs and dynamic light scattering. The weight-average molecular weight of the aggregates at pH 8.0, as judged by analytical ultracentrifugation, is 648,000. Trypsin digestion at pH 8.0 was used to probe the surface groups of the kappa-casein A polymers. The reaction with trypsin was rapid and the peptides liberated were identified by separation with reverse-phase HPLC, amino acid analysis, and protein sequencing. The most rapidly released peptides (t1/2 < 30 sec) were from cleavage at Arg 97 and Lys residues 111 and 112. These results suggest a surface orientation for these residues, and the data are in accord with earlier proposed 3D predictive models for kappa-casein. It is speculated that Arg 97, together with adjacent His residues (98 and 100) and Lys residues 111 and 112, form two positively charged clusters on the surface of the otherwise negatively charged casein. These clusters bracket the neutral chymosin cleavage site (whose hydrolysis triggers a well-known digestive process) and so these clusters may facilitate docking of the substrate caseins with chymosin.

[Intestinal flora in the neonate: impact on morbidity and therapeutic perspectives]

Studies in recent years have focused on the role that intestinal flora plays in health and disease. At birth, infant gut colonization begins with bacteria which are derived from the mother during delivery. Environmental factors (hospital, hygiene, antibiotics administered to the mother or to the neonate) may contribute to modification of the type of primary colonizing germs. Afterwards, diet represents the most important variable by the end of the first postnatal week. Exclusive breast-feeding promotes growth of Bifidobacteria which have been associated with the healthy nature of stool flora in infants because of their potential role in resisting pathogen colonization. Clinical trials have been made to promote bifidobacteria growth in the feces of bottle-fed infants. In addition, administration of non-pathogenic micro-organisms (probiotics) has been claimed to exert a positive influence on host health or physiology, and is a new approach to the prevention or elimination of infection originating from gut.

Identification of a novel opioid peptide (Tyr-Val-Pro-Phe-Pro) derived from human alpha S1 casein (alpha S1-casomorphin, and alpha S1-casomorphin amide)

A new casomorphin pentapeptide (alpha S1-casomorphin) has been isolated from the sequence of human alpha S1-casein [alpha S1-casein-(158-162)], with the sequence Tyr-Val-Pro-Phe-Pro. This peptide was found to bind with high affinity to all three subtypes of the kappa-opioid receptor (kappa 1-kappa 2). When amidated at the C-terminus, alpha S1-casomorphin amide binds to the delta- and kappa 3-opioid sites. Both alpha S1-casomorphin and its amide inhibit in a dose-dependent and reversible manner the proliferation of T47D human breast cancer cells. This anti-proliferative activity was greater for alpha S1-casomorphin, which was the most potent opioid in inhibiting T47D cell proliferation. In T47D breast cancer cells, other casomorphins have been found to bind to somatostatin receptors in addition to opioid sites. In contrast, alpha S1-casomorphin and its amide do not interact with somatostatin receptors in our system.

Post X-ray crystallographic studies of chymosin: the existence of two structural forms and the regulation of activity by the interaction with the histidine-proline cluster of kappa-casein

Calf chymosin molecules exist in the two alternative structural forms: the first one has S1 and S3 binding pockets occluded by its own Tyr77 residue (the self-inhibited form); the second has these pockets free for a substrate binding (the active form). The preliminary incubation of the enzyme with a pentapeptide corresponding to the histidine-proline cluster of the specific substrate kappa-casein results in a 200-fold increase of the hydrolysis rate for the enzyme 'slow substrate'. The result suggests that the cluster is an allosteric effector that promotes the conversion of the enzyme into the active form. These data provide the experimental ground for the explanation of chymosin specificity towards kappa-casein.

Restrained molecular dynamics study of the interaction between bovine kappa-casein peptide 98-111 and bovine chymosin and porcine pepsin

The cleavage of bovine kappa-casein at the Phe105-Met106 bond by chymosin or pepsin is the first stage in casein micelle coagulation and casein digestion. The nature of the interaction of the peptide His98-Pro-His-Pro-His-Leu-Ser-Phe105-Met-Ala-Ile-Pro-Pro- Lys111 with chymosin and porcine pepsin was investigated using molecular modelling and energy minimization techniques. This study verified and extended a proposed model that electrostatic binding (involving His98, His100, His102 and Lys111 or Lys112) at either end of the active site cleft of chymosin is important for the positioning of residues 103-108 in the cleft. The peptide conformation remained unchanged in going from solution to binding into the active site cleft, with the exception that optimum binding of substrate to chymosin required the isomerization of the His98-Pro99 peptide bond from the trans to the cis conformation. The study also identified an acidic region in porcine pepsin that is in a position to form strong electrostatic interactions with the histidines at the N-terminus of the peptide.

Characterization of three types of human alpha s1-casein mRNA transcripts

Here we report the molecular cloning and sequencing of three types of human alpha s1-casein transcripts and present evidence indicating that exon skipping is responsible for deleted mRNA transcripts. The largest transcript comprised 981 bp encoding a signal peptide of 15 amino acids followed by the mature alpha s1-casein sequence of 170 amino acids. Human alpha s1-casein has been reported to exist naturally as a multimer in complex with kappa-casein in mature human milk, thereby being unique among alpha s1-caseins [Rasmussen, Due and Petersen (1995) Comp. Biochem. Physiol., in the press]. The present demonstration of three cysteines in the mature protein provides a molecular explanation of the interactions in this complex. Tissue-specific expression of human alpha s1-casein was indicated by Northern-blot analysis. In addition, two cryptic exons were localized in the bovine alpha s1-casein gene.

New data on the proteins of rabbit (Oryctolagus cuniculus) milk

The main rabbit milk proteins have previously been prepared by reversed-phase HPLC of the acid-precipitated material ('whole casein') and of its supernatant (acid whey). Most of them were nearly homogeneous on SDS-PAGE. Among those isolated from whole casein, alpha s1-, beta- and kappa-caseins, as well as whey acidic protein (WAP) were identified by N-terminal sequencing. After further internal sequencing, two unknown proteins were found to be the putative products, alpha s2a- and alpha s2b-caseins of two recently sequenced transcripts from rabbit mammary gland. Each whole casein component gave several bands on IEF. For kappa-casein, this was probably due to uneven glycosylation as in all kappa-caseins studied so far. For the other whole casein components, including WAP, the number of bands roughly reflected the number of potential phosphorylation sites predicted from the sequences. For alpha s1- and alpha s2-caseins polymorphism could be detected. From acid whey, in addition to WAP, which was a minor component, reversed phase HPLC separated three proteins. These were alpha-lactalbumin, transferrin and serum albumin, on the basis of their apparent molecular weights deduced from SDS-PAGE. WAP was a major component of the native whey obtained by ultracentrifugation of rabbit milk. It was found to consist of two identical subunits linked by at least one disulfide bridge.

Human alpha s1-casein: purification and characterization

The human counterpart of alpha s1-casein has been purified by a combination of gel-filtration and ion-exchange chromatography under denaturing conditions. SDS-PAGE analysis revealed the presence of a diffuse ladder with a high molecular mass which upon reduction was replaced by several closely spaced bands of lower molecular masses and a broad diffuse band corresponding to kappa-casein. Amino acid sequence analysis of the closely spaced bands all resulted in the same N-terminal sequence which was found to be homologous with alpha s1-casein from other species. Sequence analysis of a major radiolabelled tryptic peptide from purified 14C-carboxymethylated alpha s1-casein demonstrated that the protein contains at least two cysteine residues. As judged by SDS-PAGE in the presence or absence of a reducing agent, the molecular structure of the polymers constituting the ladder is composed of heteropolymers of alpha s1- and kappa-casein cross-linked by disulfide bonds.

Human alpha S1-casein like protein: purification and N-terminal sequence determination

Casein components of human milk are generally reported to belong to the beta- and kappa-groups. In this research human casein fraction was obtained from pooled mature milk, either by ultracentrifugation or by acid precipitation. In both cases a minor component with a slightly higher mobility in SDS-PAGE than beta-casein was identified. The protein was purified to homogeneity, the N-terminal sequence of the first 14 amino acid residues of this new human casein subunit shows a high degree of homology with the alpha s1-casein sequences from other species.

Three-dimensional molecular modeling of bovine caseins: a refined, energy-minimized kappa-casein structure

A refined three-dimensional molecular model of kappa-casein has been produced using energy minimization techniques and a Kollman force field on a previously reported predicted three-dimensional structure. This initial model was constructed via molecular modeling techniques from sequence-based secondary structural prediction algorithms. Both the initial and refined structures agreed with global secondary structure analysis from vibration spectroscopy. The refined structure contained many of the features of the initial model, including two sets of antiparallel beta-sheet structures containing predominantly hydrophobic side chains, which could form interaction sites with alpha s1-casein. Two types of energy-minimized dimer and tetramer models are presented: 1) using Cys as potential intermolecular disulfide binding sites and 2) using the two sheets as possible hydrophobic self-association sites, without Cys interactions. All structures yielded good stabilization energies and are in agreement with chemical, biochemical, and physical chemical results obtained for kappa-casein.

Characterization of human kappa-casein purified by FPLC

Because previous purification procedures for human kappa-casein may have caused the loss of some carbohydrate, relatively gentle methods were used. The protein was isolated by a four-step procedure which included isoelectric precipitation of whole casein, gel chromatography on Sephadex G-200 in the presence of SDS, removal of the SDS with Extracti-Gel D, and FPLC chromatography on Mono Q with buffers containing 6 M urea. The purified protein was nearly identical in amino acid composition to that found earlier by amino acid analysis and peptide sequencing and a molar extinction coefficient of 11.2 +/- 0.1 was determined on the basis of amino acid analysis with a norleucine internal standard. Hydrolysis, acylation, and methylsilylation of the carbohydrate, followed by gas chromatographic analysis on a fused silica column, yielded approximately 5% fucose, 17% galactose, 18% N-acetylglucosamine, 8% N-acetylgalactosamine and 7% sialic acid, totaling almost 55% by weight. The percentages from two different donors were almost the same. About 1 mole phosphorus per mole of kappa-casein was also detected. Using low-speed sedimentation equilibrium methods, a molecular weight of only 33,400 was obtained for human kappa-casein, suggesting carbohydrate lability. Human beta-casein with four phosphoryls was stabilized against precipitation by 10 mM Ca+2 ions at a level greater than 95% when the molar ratio of kappa/beta exceeded 0.15.

The complete sequence of the gene encoding bovine alpha s2-casein

From a bovine genomic library, five overlapping clones, spanning some 50 kb, have been isolated. These clones contain the complete alpha s2-casein-encoding gene (alpha s2ca) and its 5' and 3' flanking regions. The nucleotide (nt) sequence of the complete gene including 2510 bp of the 5' flanking region and 276 bp of the 3' region has been determined. The total length of alpha s2ca appears to be 18483 bp and, therefore, it is the longest of the four bovine casein-encoding genes. The alpha s2ca gene is comprised of 18 exons ranging in size from 21 to 266 nt. There are 16 Alu-like artiodactyla retroposons inserted at ten different locations within the gene. About 14% of the gene is composed of these repetitive sequences. Although the organization of alpha s2ca appears to be similar to that of the alpha s1-casein-encoding gene (alpha s1ca), sequence comparisons and the length of the exons indicate that it is more closely related to the beta-casein-encoding gene. Furthermore, it is shown that both genes could have evolved from a common ancestor by means of internal duplications.

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.

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.

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).

Cloning and sequencing of human kappa-casein cDNA

A cDNA encoding kappa-casein of human milk was cloned and sequenced. The kappa-casein cDNA was isolated from a lambda gt11 library generated from mRNA prepared from a mammary gland biopsy obtained from a lactating woman. The library was screened with polyclonal rabbit antibodies raised against purified native kappa-casein. The obtained nucleotide sequence contained an ORF sufficient to encode the entire amino acid sequence of a kappa-casein precursor protein consisting of 182 amino acids. This includes a tentative signal peptide of 20 amino acids and a processed protein of 162 amino acids.

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.

Opioid antagonist peptides derived from kappa-casein

Opioid antagonists were sought in the fragments of kappa-casein which were obtained by chemical synthesis and enzymic digestion. A synthetic bovine kappa-casein peptide (35-41), Tyr-Pro-Ser-Tyr-Gly-Leu-Asn (casoxin A) showed opioid antagonist activity at 200 microM in the guinea pig ileum assay. A synthetic peptide Tyr-Pro-Tyr-Tyr (casoxin B) which is found in bovine and human kappa-casein, also showed opioid antagonist activity at 100 microM. Another opioid antagonist peptide (casoxin C) was isolated from tryptic digests of bovine kappa-casein by reverse-phase HPLC. The structure of the peptide was Tyr-Ile-Pro-Ile-Gln-Tyr-Val-Leu-Ser-Arg, which corresponded to kappa-casein (25-34). Casoxin C was active at 5 microM in the guinea pig ileum assay. Thus, bovine kappa-casein contains three potential opioid antagonist sequences.

Isolation and characterization of the bovine kappa-casein gene

1. The bovine kappa-casein gene has been isolated as a series of overlapping lambda clones and shown to consist of five exons distributed over a total length of approximately 13 kb. Most of the mature protein-coding sequence is contained in a single large exon. 2. Approximately 65% of the gene has been sequenced together with portions of the 5'- and 3'-flanking sequences. The immediate 5'-flanking sequence contains several motifs which are characteristic of upstream regions including a TATA box, a CAAT box, a sequence similar to that recognized by transcription factor AP-1 and a purine-rich sequence resembling that found upstream in all other lactoprotein genes. Other possible regulatory sequences are found upstream of exon 4. 3. The organization of the kappa-casein gene, together with its upstream sequence, confirms previous conclusions that it is unrelated to the calcium-sensitive-casein gene family to which it is linked. Evidence is presented which supports a previous suggestion that kappa-casein and the fibrinogens are evolutionarily related. 4. Intron sequences contain several examples of the A family of the artiodactyl Alu-like repeated sequences, together with a single example of a C-family sequence. The remainders of the introns of the kappa-casein gene, compared with the repeat elements and exons, are A + T-rich. 5. Among the lambda clones isolated, representatives were found of the A and B genetic variants which can be distinguished by restriction-enzyme analysis. Several other examples of polymorphisms in the non-coding region were found.

Characterization of bovine kappa-casein fractions and the kinetics of chymosin-induced macropeptide release from carbohydrate-free and carbohydrate-containing fractions determined by high-performance gel-permeation chromatography

Bovine kappa-casein was fractionated at pH 8.0 on DEAE-Sepharose with an NaCl gradient, followed by DEAE-cellulose chromatography using a decreasing pH gradient from pH 6.0 to 4.5. At least ten components could be identified, each differing in N-acetylneuraminic acid (NeuAc) and/or phosphorus content. Two components appeared to be multiply-phosphorylated, but did not contain NeuAc. The possible significance of this finding in relation to the mode of phosphorylation and glycosylation in vivo is discussed. A carbohydrate-free fraction as well as two NeuAc-containing fractions were compared in their substrate behaviour towards the action of the milk-clotting enzyme chymosin at pH 6.6 and 30 degrees C. To this end the trichloroacetic acid-soluble reaction products were analysed by high-performance gel-permeation chromatography. In order of increasing carbohydrate content the kcat. values found ranged from 40 to 25 s-1 and the Km values from 9 to 3 microM; the overall substrate properties of these components as reflected by the kinetic parameter kcat./Km ranged from 5 to 8 microM-1 X S-1. Irreversible polymerization of the carbohydrate-free fraction brought about a more-than-2-fold increase in Km, the kcat. value remaining virtually constant. The kcat./Km found for the cleavage of whole kappa-casein at pH 6.6 was of the same magnitude as the kcat./Km found for the polymerized carbohydrate-free fraction (i.e. about 3 microM-1 X S-1). No indication of substrate inhibition was found for the carbohydrate-free fraction.