Manufacture of recombinant polyclonal antibodies

Polyclonal antibody therapy in the form of hyper-immune serum has for more than a century been used for treatment of many infectious diseases. However, with the emergence of first antibiotics and later recombinant monoclonal antibody therapy, the use of hyper-immune serum has declined. The main reason for this is that methods for consistent manufacturing of safe hyper immune immunoglobulin products have been lacking. In contrast, manufacturing processes of recombinant monoclonal antibodies follow a well established schedule and it appears obvious to use similar methods to produce recombinant polyclonal products. However, the methods for monoclonal antibody manufacturing are, for several reasons, not directly applicable to generation and manufacture of polyclonal recombinant antibodies. A new production strategy based on recombinant mammalian producer cells has recently been developed to support consistent generation of recombinant polyclonal antibodies for therapeutic use. This review describes aspects of this novel technology with emphasis on the generation, production and characterization procedures employed, and provides comparison with alternative polyclonal and monoclonal antibody manufacturing strategies.

Purification of MUC1 from bovine milk-fat globules and characterization of a corresponding full-length cDNA clone

The highly glycosylated protein MUC1 was purified from bovine milk-fat globule membranes by a procedure involving detergent extraction, ion-exchange chromatography and reverse-phase chromatography. The identity of the purified mucin protein was confirmed by N-terminal sequencing and partial amino acid sequences obtained by peptide mapping. The complete amino acid sequence of MUC1 was determined by cloning and sequencing the corresponding bovine mammary gland cDNA, which was shown to encode a protein of 580 amino acid residues comprising a cleavable signal peptide of 22 residues. The deduced amino acid sequence demonstrated structural features characteristic for mucins, including an extracellular tandem repeat region with 11 partially conserved repeats (20 amino acids each), a membrane-proximal SEA module, a transmembrane domain, and a cytoplasmic C-terminal region. Monosaccharide composition determinations suggested significant structural differences between O-linked glycans of MUC1 originating from either bovine or human milk. Interspecies differences of the consensus repeat sequence in MUC1 and the physiological functions are discussed.

Colloidal calcium phosphates in casein micelles studied by slow-speed-spinning 31P magic angle spinning solid-state nuclear magnetic resonance

The composition of bovine casein micelles was analyzed by 31P magic angle spinning solid-state nuclear magnetic resonance spectroscopy. By looking at isotropic and anisotropic 31P chemical shift parameters, resonance line shapes, the combination of single-pulse and 1H to 31P cross-polarization spectra, and comparison with spectra for various model compounds combined with multiple-component simulation and iterative fitting procedures, we were able to identify and quantify a variety of inorganic and organic phosphates in the micelles. These include phosphates from mobile and immobile inorganic hydroxyapatite-type phosphates as well as phosphates from kappa-casein and the Ca2+-binding phosphoserines from alphas1-, alphas2-, and beta-casein. This information is discussed in relation to previous knowledge and various models for the colloid formation.

Kinetic and structural characterization of a two-domain streptokinase: dissection of domain functionality

The mammalian protease plasminogen can be activated by bacterial activators, the three-domain (alpha, beta, gamma) streptokinases and the one-domain (alpha) staphylokinases. These activators act as plasmin(ogen) cofactors, and the resulting complexes initiate proteolytic activity of host plasminogen which facilitates bacterial colonization of the host organism. We have investigated the kinetic mechanism of the plasminogen activation mediated by a novel two-domain (alpha, beta) streptokinase isolated from Streptococcus uberis (Sk(U)) with specificity toward bovine plasminogen. The interaction between Sk(U) and plasminogen occurred in two steps: (1) rapid association of the proteins and (2) slow transition to the active complex Sk(U)-PgA. The complex Sk(U)-PgA converted plasminogen to plasmin with the following parameters: K(m) < or = 1.5 microM and k(cat) = 0.55 s(-)(1). The ability of proteolytic fragments of Sk(U) to activate plasminogen was investigated. Only two C-terminal segments (97-261 and 123-261), which both contain the beta-domain (126-261), were shown to be active. They initiated plasminogen activation in complex with plasmin, but not with plasminogen, and thereby exhibited functional similarity to the staphylokinase. The fusion protein His(6)-Sk(U) (i.e., Sk(U) with a small N-terminal tag) acted exclusively in complex with plasmin as well. These observations demonstrate that (1) the N-terminal alpha-domain, including a native N-terminus, was necessary for "virgin" activation of the associated plasminogen in the Sk(U)-PgA complex and (2) the C-terminal beta-domain of Sk(U) is important for recognition of the substrate in the Sk(U)-PgA complex.

The structure of the membrane-binding 38 C-terminal residues from bovine PP3 determined by liquid- and solid-state NMR spectroscopy

The secondary structure and membrane-associated conformation of a synthetic peptide corresponding to the putative membrane-binding C-terminal 38 residues of the bovine milk component PP3 was determined using 1H NMR in methanol, CD in methanol and SDS micelles, and 15N solid-state NMR in planar phospholipid bilayers. The solution NMR and CD spectra reveal that the PP3 peptide in methanol and SDS predominantly adopts an alpha-helical conformation extending over its entire length with a potential bend around residue 19. 15N solid-state NMR of two PP3 peptides 15N-labelled at the Gly7 and Ala32 positions, respectively, and dissolved in dimyristoylphosphatidylcholine/dimyristoylphosphatidylglycerol phospholipid bilayers shows that the peptide is associated to the membrane surface with the amphipathic helix axis oriented parallel to the bilayer surface.

Component PP3 from bovine milk is a substrate for transglutaminase. Sequence location of putative crosslinking sites

As the demands for food products of high quality increase, it will become more important to be able to modify the properties of food proteins. One possibility is to use enzymic modifications to improve functional properties and nutritional values. A highly efficient class of enzymic crosslinkers are transglutaminases (TG, EC 2.3.2.13), calcium-dependent enzymes that catalyse an acyl transfer reaction between protein-bound glutaminyl residues and primary amines (for review, see Aeschlimann & Paulsson, 1994). When protein-bound lysyl residues act as acyl acceptors, the reaction leads to the formation of intramolecular and/or intermolecular isopeptide bonds. Although a wide variety of amines such as putrescine, cadaverine or lysyl residues may act as amine donors in this reaction, only a limited number of glutamine residues in certain proteins will act as amine acceptors (Gorman & Folk, 1980).

Ikura et al. (1981, 1985) reported that TG can be used to introduce methionine into casein and soyabean proteins and lysine into wheat gluten, thereby showing that TG can also be utilized to improve the nutritional value of food proteins. There have been a number of reports concerning the TG-mediated polymerization of food proteins such as α-lactalbumin and β-lactoglobulin (Aboumahmoud & Savello, 1990; Færgemand et al. 1997), soyabean proteins and casein components (Ikura et al. 1980a, b) and pea legumin (Larré et al. 1993). Recently, improved protein foaming capacity and stability have been demonstrated in TG-catalysed polymers of soyabean protein and whey protein isolate (Yildirim et al. 1996).

Bovine PP3 (for review, see Girardet & Linden, 1996) is a phosphorylated glycoprotein isolated from the proteose peptone fraction of milk (Sørensen & Petersen, 1993a). The primary structure of PP3 has been determined (Sørensen & Petersen, 1993b) and comprises a polypeptide backbone of 135 amino acid residues containing five phosphorylated serines, two threonine-linked O-glycosylations, and one N-glycosylation. Immunological studies have shown that PP3 is present in the milk fat globule membrane, and that PP3 forms multimeric aggregates in bovine milk (Sørensen et al. 1997). Several functions have been suggested and investigated for bovine PP3 and fractions enriched with PP3, including emulsification (Shimizu et al. 1989), inhibition of lipolysis (Girardet et al. 1993) and mitogenesis (Mati et al. 1993).

Previously, we have localized the potential TG-reactive glutamines in the four bovine caseins (Christensen et al. 1996) and in milk osteopontin (Sørensen et al. 1994). In the present study we have shown that component PP3 is a substrate for guinea-pig liver TG containing both reactive glutamine and lysine residues. In addition, we have localized the glutamine residues that act as amine acceptors.

Localization of a single transglutaminase-reactive glutamine in the third domain of RAP, the alpha2-macroglobulin receptor-associated protein

The 39-kDa receptor-associated protein (RAP) is an intracellular glycoprotein that interacts with hitherto unknown sites in several members of the low-density-lipoprotein receptor gene family. Upon binding to these receptors, RAP inhibits all ligand interactions with the receptors. In the present study, the transglutaminase-catalyzed incorporation of radioactively labeled putrescine and a dansylated glutamine-containing peptide into human RAP has been studied. The results indicate the presence of both glutamine and lysine residues in RAP, accessible for transglutaminase cross-linking. Moreover, enzymatic digestion followed by sequence analysis of radiolabeled fractions demonstrated that Gln261 acts as the amine acceptor site. This residue is located in the third domain of RAP and is conserved among the RAP interspecies homologues. Insertion of a reporter group into the protein could prove useful to assess ligand/receptor interactions.

A refined kinetic analysis of plasminogen activation by recombinant bovine tissue-type plasminogen activator indicates two interconvertible activator forms

Bovine tissue-type plasminogen activator (tPA) was heterologously expressed in the methylotrophic yeast Pichia pastoris and characterized structurally and kinetically. The bovine single-chain tPA-mediated activation of bovine plasminogen was studied in the presence and absence of fibrinogen fragments. We have proposed a refined new method of kinetic analysis which allows examination of both stationary and prestationary phases of this process. The investigation revealed the presence of two interconvertible forms of the recombinant bovine tPA being in equilibrium at a 1 to 50 ratio. Only the minor form was able to bind and activate plasminogen. Saturation of the whole pool of tPA required high plasminogen concentration (Km >/= 5 microM) in order to reverse the equilibrium between the two forms. Fibrinogen fragments activated the single-chain tPA due to preferential binding and stabilization of the minor "active" form of the enzyme until all the molecules of tPA were converted. The same mechanism could be applied to human tPA as well. The Km values, obtained for recombinant bovine and human tPA in the presence of fibrinogen fragments, were found to be similar (Km = 0.1 microM) while kcat of human tPA was 5-10 times higher.

The primary structure of caprine PP3: amino acid sequence, phosphorylation, and glycosylation of component PP3 from the proteose-peptone fraction of caprine milk

Proteose-peptone component 3 is a phosphorylated glycoprotein that was isolated from the proteose-peptone fraction of caprine milk. By mass spectrometric analysis, amino acid sequencing, and polymerase chain reaction analysis, the primary structure has been determined and has been shown to contain 136 amino acids. Phosphorylations were identified at Ser30 and Ser41. A partial glycosylation was present at Thr16, and a N-linked glycosylation was present at Asn78. Galactosamine was the amino sugar detected at Thr16. Glucosamine and galactosamine were the amino sugars found in the carbohydrate group linked to Asn78. The caprine amino acid sequence exhibits 88% identity with the bovine proteose-peptone component 3 sequence. However, when compared with the bovine sequence, the caprine sequence contains an insertion of a serine residue at position 25.

The localization and multimeric nature of component PP3 in bovine milk: purification and characterization of PP3 from caprine and ovine milks

The distribution of proteose-peptone component PP3 in bovine whey, milk fat globule membrane, and casein has been investigated with antibodies raised against highly purified PP3. Using Western blot analysis, we show that PP3 is present in the milk fat globule membrane and in whey but is absent in the casein fraction. The proposed multimeric structure of bovine PP3 was analyzed by mass spectrometry and gel filtration. Calibrated gel filtration of acidic whey showed that PP3 eluted at a volume corresponding to 190 kDa, indicating that PP3 exists as a multimeric aggregate in bovine milk. Western blot analysis with anti-bovine PP3 immunoglobulins was used to analyze caprine, ovine, and human milks, and immunoreactive proteins were detected in caprine and ovine milks. Finally, the immunoreactive proteins from caprine and ovine milks were purified and characterized as PP3 analogues by amino acid analysis and N-terminal sequence analysis.

Plasminogen activation system in human milk

BACKGROUND

Plasmin is the major endogenous protease present in milk. The level of plasmin activity is controlled by the availability of the precursor plasminogen and by the levels of plasminogen activators and inhibitors. Recently, a differential distribution of tissue-type plasminogen activator (t-PA) and urokinase-type plasminogen activator (u-PA) has been demonstrated in bovine milk. To assess whether this distribution pattern is a general feature, the occurrence of components of the plasminogen activation system in different fractions of human milk was investigated.

METHODS

Milk samples were separated into the following fractions; milk fat, skim milk, and milk cells by centrifugation. The different fractions were detected for the presence of plasminogen and plasminogen activators by immunoblotting and zymography. The distribution of t-PA and u-PA was investigated by ligand binding analysis. t-PA-catalyzed plasminogen activation was examined by a coupled chromogenic assay.

RESULTS

A differential distribution of plasminogen, t-PA, and u-PA was found. Casein micelles were found to exhibit t-PA and plasminogen binding activity, whereas the u-PA receptor was identified as the u-PA binding component in the cell fraction. Furthermore, human casein enhanced t-PA-catalyzed plasminogen activation, comparable to the enhancing effect obtained with fibrinogen fragments.

CONCLUSION

The finding of a differential distribution of u-PA and t-PA in milk suggests that the two activators may have different physiological functions, which involve protection against invading microorganisms and maintenance of patency and fluidity in the ducts of mammary gland, respectively.

Characterization of phosphate sites in native ovine, caprine, and bovine casein micelles and their caseinomacropeptides: a solid-state phosphorus-31 nuclear magnetic resonance and sequence and mass spectrometric study

The phosphate sites in native ovine, caprine, and bovine casein micelles have been analyzed using sequence analysis, mass spectrometric analysis, and solid-state 31P nuclear magnetic resonance spectroscopy. Using a combination of S-ethylcysteine derivatization, sequence analysis, and mass spectrometric analysis, the phosphorylation sites of ovine (SerP151 and SerP168), caprine (SerP151 and SerP168), and bovine (SerP149) caseinomacropeptides have been localized. Various solid-state 31P methods using magic angle spinning have been applied to ascertain the local structure and dynamics of the phosphorylated serine residues and the inorganic calcium phosphates within the micelles. Contributions from the phosphorylated serine residues of kappa-CN, located in the C-terminal portion of the molecule, to the mobile constituents of the micelles were assigned by comparison with 31P nuclear magnetic resonance spectra of purified caseinomacropeptides from the various species in the dissolved state. Comparison of the 31P magic angle spinning nuclear magnetic resonance spectra of ovine, caprine, and bovine casein micelles indicates that the micelles from these species are very similar but not identical.

Bovine milk procathepsin D and cathepsin D: coagulation and milk protein degradation

Cathepsin D is an indigenous aspartic proteinase in bovine milk. By competitive enzyme-linked immunosorbent assay the amount of immunoreactive cathepsin D and procathepsin D in bovine skim milk was estimated to be 0.4 microgram/ml. Immunoreactive cathepsin D purified from whey consisted of a small fraction of mature cathepsin D, but the major form was the proenzyme procathepsin D. A preparation of bovine milk procathepsin D was, like mature cathepsin D, able to degrade purified alpha s1-, alpha s2-, beta- and kappa-casein and alpha-lactalbumin, while beta-lactoglobulin was resistant to cleavage. The cleavage sites in these proteins were determined and compared with those of chymosin. Cathepsin D was capable of generating the alpha s1-I, beta-I, beta-II and beta-III fragments originally described from the action of chymosin on the respective caseins, and these fragments were subjected to further proteolysis. Cathepsin D was also able to liberate the caseinomacropeptide from purified kappa-casein, and to coagulate bovine skim milk. This demonstrated that milk contains an indigenous coagulation enzyme present mainly in the whey fraction.

Residues in the synuclein consensus motif of the alpha-synuclein fragment, NAC, participate in transglutaminase-catalysed cross-linking to Alzheimer-disease amyloid beta A4 peptide

The widespread deposition of amyloid plaques is one of the hallmarks of Alzheimer disease (AD). A recently described component of amyloid plaques is the 35-residue peptide, non-A beta component of AD amyloid, which is derived from a larger intracellular neuronal constituent, alpha-synuclein. We demonstrate that transglutaminase catalyses the formation of the covalent non-A beta component of AD amyloid polymers in vitro as well as polymers with beta-amyloid peptide, the major constituent of AD plaques. The transglutaminase-reactive amino acid residues in the non-A beta component of AD amyloid were identified as Gln79 and Lys80. Lys80 is localized in a consensus motif Lys-Thr-Lys-Glu-Gly-Val, which is conserved in the synuclein gene family. Thus transglutaminase might be involved in the formation of insoluble amyloid deposits and participate in the modification of other members of the synuclein family.

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.

Solid-state magic-angle spinning 31P-NMR studies of native casein micelles

Solid-state magic-angle spinning 31P-NMR spectroscopy was used to characterize the structure and composition of native casein micelles. The features of the magic-angle spinning 31P-NMR spectra, including overlapping resonances from mobile/immobile phosphorylated serine residues and inorganic calcium phosphates, have been determined using different experimental techniques and assigned by comparison with spectra of the presumed constituents within the casein micelle. Comparison with 31P-NMR spectra of alpha s1-, alpha s2-, and beta-caseins in dissolved and freeze-dried forms demonstrated that a major fraction of the phosphoserines in these proteins was in an immobilized state within the micelle. Likewise, from 31P-NMR spectra of the C-terminal part of kappa-casein, it was shown that this region of the micelle has a considerable conformational mobility. Finally, magic-angle spinning 31P-NMR spectra for a series of inorganic calcium phosphates and mineralized bone tissue revealed that the micellar inorganic calcium phosphates exhibit structural similarities to hydroxyapatite and hence resemble mineralized bone tissue.

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.

Localization of transglutaminase-reactive glutamine residues in bovine osteopontin

Here we report the identification of two transglutaminase-reactive glutamines (Gln-34 and Gln-36) in bovine osteopontin (OPN). Sequence alignment revealed that these glutamines are conserved in all known OPN sequences, indicating a functional importance of this region of the protein. Furthermore, immunological analysis of bovine bone demonstrated that OPN is present in high-molecular-mass complexes in vivo. These findings support the functional aspects of a transglutaminase-catalysed cross-linking of OPN in facilitating cellular attachment and tissue calcification.

Very low density lipoprotein receptor from mammary gland and mammary epithelial cell lines binds and mediates endocytosis of M(r) 40,000 receptor associated protein

We here report that the M(r) 40,000 receptor associated protein (RAP), previously found to bind to alpha 2-macroglobulin receptor/low density lipoprotein receptor related protein (alpha 2MR/LRP) and glycoprotein 330 (gp330), binds to an M(r) 105,000 membrane protein from bovine mammary gland, human mamma tumors and mammary epithelial cell lines. We have purified this protein from bovine and human sources. N-terminal amino acid sequencing and immunoblotting analyses showed that the protein was identical or closely related to very low density lipoprotein receptor (VLDL-R). Experiments with the human mamma carcinoma cell line MCF-7 showed that this receptor was able to mediate an efficient endocytosis of RAP. These novel findings strongly suggest that RAP functions as a modulator of ligand binding to VLDL-R, similarly to alpha 2MR/LRP and gp330.

Disulphide arrangement in bovine caseins: localization of intrachain disulphide bridges in monomers of kappa- and alpha s2-casein from bovine milk

Naturally occurring monomeric kappa-casein and alpha s2-casein in bovine milk were purified by ion-exchange chromatography in order to localize potential intrachain disulphide bridges. Enzymic cleavage of the proteins followed by mass spectrometry and amino acid sequence analysis of cystine-containing peptides revealed the presence of an intrachain disulphide bond in both proteins.

A unique interhelical insertion in plasminogen activator inhibitor-2 contains three glutamines, Gln83, Gln84, Gln86, essential for transglutaminase-mediated cross-linking

Plasminogen activator inhibitor type 2 (PAI-2) prevents fibrinolysis by blocking plasminogen activators. It is expressed principally by trophoblast cells and macrophages. PAI-2 in trophoblast membranes has been found cross-linked to large complexes apparently catalyzed by trophoblast transglutaminase (Jensen, P. H., Lorand, L., Ebbesen, P., and Gliemann, J. (1993) Eur. J. Biochem. 214, 141-146). Recombinant human PAI-2 was labeled with [14C]putrescine catalyzed by guinea pig liver transglutaminase. The [14C]putrescine-labeled PAI-2 was digested with cyanogen bromide and trypsin, and the peptides were purified by reverse-phase high performance chromatography. Amino acid sequencing and plasma desorption mass spectrometry of the labeled peptides revealed [14C]putrescine incorporation at Gln83, Gln84, and Gln86. These residues are present in a PAI-2-specific region of 33 amino acids that is inserted between helices C and D and which probably represents a unique solvent-exposed domain. A PAI-2 mutant lacking this insertion was determined not to be a substrate for transglutaminase by [14C]putrescine incorporation and could not form transglutaminase-catalyzed polymers. Thus, the unique PAI-2 insertion represents a functional domain that, by virtue of its transglutaminase acceptor sites, allows participation in binding reactions without affecting the inhibitory function of PAI-2.

The plasminogen activation system in bovine milk: differential localization of tissue-type plasminogen activator and urokinase in milk fractions is caused by binding to casein and urokinase receptor

We have analyzed the occurrence of components of the plasminogen activation system in bovine milk. Zymographic analyses showed that tissue-type plasminogen activator (t-PA) occurred in association with casein micelles, partially as a complex with type-1 plasminogen activator inhibitor (PAI-1), whereas urokinase-type plasminogen activator (u-PA) was confined to milk leukocytes. Whey contained a component with a plasminogen dependent proteolytic activity which was shown to be plasma prekallikrein (PPK). The u-PA in the milk leukocytes was shown to be bound to urokinase receptor (u-PAR). A purification to near-homogeneity of the bovine u-PAR was undertaken. Investigating the novel t-PA binding to casein micelles by ligand blotting and Sepharose immobilized casein, multimeric forms of kappa-casein and dimeric alpha s2-casein were identified as t-PA binding components. The kappa-casein gene and the fibrinogen gene are believed to have evolved from a common ancestor. Thus, the recent finding that casein enhances t-PA catalyzed plasminogen activation (Marcus, G., Hitt, S., Harvey, S.R. and Tritsch, G.L. (1993) Fibrinolysis 7, 229-236), and the observed t-PA/casein binding suggests that the casein micelle, which also contains plasminogen, may serve as a matrix for t-PA-catalyzed plasminogen activation in milk.

Identification of glutamine and lysine residues in Alzheimer amyloid beta A4 peptide responsible for transglutaminase-catalysed homopolymerization and cross-linking to alpha 2M receptor

The beta-amyloid peptide (beta A4), derived from a larger amyloid precursor protein, is the principal component of senile plaques in Alzheimer's disease. Here we report that the full-length (1-40) synthetic beta A4 peptide, containing one glutamine and two lysine residues, is able to form homopolymers in a transglutaminase-mediated reaction. Moreover, transglutaminase catalysed the formation of heteropolymers in reactions of beta A4 with alpha 2M receptor, a constituent of amyloid plaques, and with extracellular matrix proteins. Incorporation of site-specific probes followed by enzymatic digestion and sequencing of tracer-containing fractions demonstrated that both Lys16, Lys28 and Gln15 in beta A4 were susceptible to cross-linking by transglutaminase.

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.

Localization of two interchain disulfide bridges in dimers of bovine alpha s2-casein. Parallel and antiparallel alignments of the polypeptide chains

Carboxymethylation of bovine skimmed milk with 14C-labelled iodoacetic acid followed by purification of the alpha s2-casein dimer showed that all four cysteine residues in the protein are engaged in disulfide linkages. Mass spectrometry and sequence analysis of cystine-containing tryptic peptides revealed the presence of two interchain disulfide bridges in the protein. Sequence analysis of disulfide-linked peptides resulting from an enzymatic cleavage between the bridges demonstrated that the individual chains in the dimers are either aligned in an antiparallel or a parallel orientation. The identity of some of the disulfide-linked peptides was further verified by performic acid oxidation followed by sequence analysis of the resulting peptides.

Purification of disulphide-linked alpha s2- and kappa-casein from bovine milk

Naturally occurring disulphide-linked alpha s2- and kappa-casein in bovine milk were purified by gel chromatography on a column of Sepharose CL-6B. Four fractions (A-D) were obtained by elution with ammonium acetate-urea buffer. Fractions A and B, identified by SDS gel electrophoresis and amino acid sequence analysis, corresponded to disulphide-linked kappa-casein and alpha s2-casein respectively. Fraction C consisted of a mixture of alpha s1-, alpha s2-, and beta-casein. Separation of fraction C into its components was achieved by reversed-phase HPLC. The stability of the disulphide bridges in alpha s2- and kappa-casein was shown to differ with respect to reducing agents (dithioerythritol and 2-mercaptoethanol).