A procedure for the partial fractionation of the α s -casein complex

Mammals, Milk, Molecules, and Micelles

After a brief description of my family background and school days, my professional career as a dairy scientist is described under three headings: research, teaching, and writing. My research activities fall into four areas: biochemistry of cheese, fractionation and characterization of milk proteins, heat stability of milk, and dairy enzymology. Finally, I offer some advice to young scientists.

Simultaneous presence of PrtH and PrtH2 proteinases in Lactobacillus helveticus Strains improves breakdown of the pure alphas1-casein

Lactobacillus helveticus can possess one or two cell envelope proteinases (CEPs), called PrtH2 and PrtH. The aim of this work was to explore the diversity of 15 strains of L. helveticus, isolated from various origins, in terms of their proteolytic activities and specificities on pure caseins or on milk casein micelles. CEP activity differed 14-fold when the strains were assayed on a synthetic substrate, but no significant differences were detected between strains possessing one or two CEPs. No correlation was observed between the proteolytic activities of the strains and their rates of acidification in milk. The kinetics of hydrolysis of purified α(s1)- and β-casein by L. helveticus whole cells was monitored using Tris-Tricine sodium dodecyl sulfate (SDS) electrophoresis, and for four strains, the peptides released were identified using mass spectrometry. While rapid hydrolysis of pure β-casein was observed for all strains, the hydrolysis kinetics of α(s1)-casein was the only criterion capable of distinguishing between the strains based on the number of CEPs. Fifty-four to 74 peptides were identified for each strain. When only PrtH2 was present, 22 to 30% of the peptides originated from α(s1)-casein. The percentage increased to 41 to 49% for strains in which both CEPs were expressed. The peptide size ranged from 6 to 33 amino acids, revealing a broad range of cleavage specificities, involving all classes of amino acids (Leu, Val, Ala, Ile, Glu, Gln, Lys, Arg, Met, and Pro). Regions resistant to proteolysis were identified in both caseins. When strains were grown in milk, a drastic reduction in the number of peptides was observed, reflecting changes in accessibility and/or peptide assimilation during growth.

Proteolysis of αs1-casein by chymosin: influence of pH and urea

The specificity of chymosin on αs1-casein was shown to be dependent on the reaction pH and on the state of aggregation of the substrate. In aqueous solution αs1-casein was optimally hydrolysed to αs1-I at pH 5·8; if the casein was solubilized in the isoelectric region by the use of 5 M-urea, optimum proteolysis occurred at pH 2·8. Hydrolysis of αs1-I to yield αs1-II, αs1-III and αs1-IV occurred at pH values > 5·8 in the presence or absence of urea. In the isoelectric region αs1-II, αs1-III and αs1-IV were not formed in the absence of urea where the substrate was aggregated: instead a peptide αs1-V was produced; at the same pH and using urea as a solubilizing agent αs1-II, αs1-III and αs1-IV were formed together with a further peptide αs1-VI.

Binding of Mg2+ and Ca2+ to β-casein A1: a multi-nuclear magnetic resonance study

25Mg, 43Ca and 31P NMR have been used to study the binding of Mg2+ and Ca2+ ions to β-casein A1. The concentration dependence of the line width of the 25Mg NMR signal shows that β-casein contains at least two different types of binding sites for Mg2+ ions, one with strongly bound, slowly exchanging ions and one with more weakly bound ions which undergo fast exchange. The strong Mg2+ binding site has an unexpectedly high binding constant, Kbstrong 104 M–1, which has not been reported earlier. Mg2+ and Ca2+ compete for the Ca2+ binding sites of β-casein, while Na+ does not compete for these binding sites under physiological conditions. The dependence of the 43Ca NMR chemical shifts on total concentration of Mg2+ and Ca2+, in the presence of β-casein, could be equally well fitted with a model assuming up to five identical and independent sites as with a model assuming five or more sites with negative cooperativity. The proton dissociation constant, pka, for the strongest Ca2+ binding site was found to be 7·1.

A model for the assembly of bovine casein micelles from F2 and F3 subunits

Casein micelles were reassembled from mixtures of F2 and F3 subunits, the major subunits of bovine casein micelles, in varying ratios. The average radius of the reassembled micelles was inversely proportional to the F2 content. Based on this relationship, a model for casein micelle assembly is proposed in which the F3 subunit forms the core of the micelle with F2 subunit occurring on the surface. Micelle size can subsequently be defined by the F2 content.

Separation of the submicelles from micellar casein by high performance gel chromatography on a TSK-GEL G4000SW column

Bovine caseins from skim-milk (micellar casein and acid precipitated casein) were separated into 4 discrete components by high performance gel chromatography on a TSK-GEL G4000SW columm. The resolution was better than that obtained by conventional gel chromatography which resulted in separation into 2 components at best. Our results showed that the second and third components consisted of αs1- and κ-caseins and αs1- and β-caseins respectively. They are presumed to be submicellar fractions. Samples prepared by removal of Ca phosphate from large and small micelles were rich in the third and the second components respectively. It is suggested that casein micelles consist of αs1- and κ-casein complexes and complexes and polymers of αs1- and β-casein which are situated preferentially on the periphery and in the interior of the micelles respectively.

Potassium iodate-induced proteolysis in ultra heat treated milk during storage: the role of β-lactoglobulin and plasmin

The report that addition of KI03(0·1 mm) to milk before ultra high temperature (UHT) treatment induces extensive proteolysis during subsequent storage at 37 °C was confirmed. None was produced by addition of H202KMn04or K2Cr207. The pH optimum for KI03-induced proteolysis was between 7·0 and 8·0 and the temperature optimum 37—45 °C. β-Casein was particularly susceptible and the proteolysis pattern was similar to that caused by indigenous alkaline milk proteinase (MPA, plasmin). Addition of plasmin to milk before UHT treatment (140 °C/10 s) caused slight proteolysis during subsequent storage but addition of 0·1 mm-KI03and plasmin caused extensive proteolysis which was prevented by addition of soyabean trypsin inhibitor, indicating the probable involvement of plasmin in KI03-induced proteolysis in UHT-treated milk. Equally extensive proteolysis occurred in serum protein-free casein micelle systems (SPFCM), with or without KI03, during storage at 37 °C following UHT treatment, indicating a role for whey proteins in KI03-induced proteolysis. Addition of β-lactoglobulin (β-lg) to a SPFCM system inhibited proteolysis, but extensive proteolysis occurred in a SPFCM system containing both β-lg and KI03. MPA-free Na caseinate (prepared by heating at 140 °C for 7 min) underwent extensive proteolysis when treated with plasmin before UHT treatment; proteolysis was inhibited by addition of °-lg to this system and KI03reversed the inhibitory effect of β-lg. Plasmin proteolysis of isolated αs1-casein was inhibited by denatured β-lg (90 °C/30 min) at a level of 4 mg/ml but not by native β-lg. When denatured in the presence of KI03, β-lg had a lower free SH content than the control and was less inhibitory for plasmin in proteolysis of isolated αsl-casein. The results show that denatured β-lg inhibits plasmin proteolysis of caseins in UHT milk and that inhibition is prevented by KI03. This inhibition may occur via thiol–disulphide interchange, which is prevented if the SH group of ²-lg is oxidized by KI03, thus permitting the stimulatory effect of KI03on proteolysis in UHT-treated milk.

Kinetics of fibril formation of bovine kappa-casein indicate a conformational rearrangement as a critical step in the process

S-carboxymethylated (SCM) kappa-casein forms in vitro fibrils that display several characteristics of amyloid fibrils, although the protein is unrelated to amyloid diseases. In order to get insight into the processes that prevent the formation of amyloid fibrils made of kappa-caseins in milk, we have characterized in detail the reaction and the roles of its possible effectors: glycosylation and other caseins. Given that native kappa-casein occurs as a heterogeneous mixture of carbohydrate-free and carbohydrate-containing chains, kinetics of fibril formation were performed on purified glycosylated and unglycosylated SCM kappa-caseins using the fluorescent dye thioflavin T in conjunction with transmission electron microscopy and Fourier transform infrared spectroscopy for morphological and structural analyses. Both unglycosylated and glycosylated SCM kappa-caseins have the ability to fibrillate. Kinetic data indicate that the fibril formation rate increases with SCM kappa-casein concentration but reaches a plateau at high concentrations, for both the unglycosylated and glycosylated forms. Therefore, a conformational rearrangement is the rate-limiting step in fibril growth of SCM kappa-casein. Transmission electron microscopy images indicate the presence of 10- to 12-nm spherical particles prior to the appearance of amyloid structure. Fourier transform infrared spectroscopy spectra reveal a conformational change within these micellar aggregates during the fibrillation. Fibrils are helical ribbons with a pitch of about 120-130 nm and a width of 10-12 nm. Taken together, these findings suggest a model of aggregation during which the SCM kappa-casein monomer is in rapid equilibrium with a micellar aggregate that subsequently undergoes a conformational rearrangement into a more organized species. These micelles assemble and this leads to the growing of amyloid fibrils. Addition of alpha(s1)-and beta-caseins decreases the growth rate of fibrils. Their main effect was on the elongation rate, which became close to that of the limiting conformation change, leading to the appearance of a lag phase at the beginning of the kinetics.

Improved method for the simultaneous determination of whey proteins, caseins and para-κ-casein in milk and dairy products by capillary electrophoresis

A capillary electrophoresis method for the simultaneous determination of whey proteins, caseins and their degradation products, such as para-kappa-casein, was proposed. The effect of several parameters (pH, ionic strength and concentration of urea in the electrophoresis buffer and applied voltage) on the analysis time and on the separation efficiency of the major milk proteins was studied. Using a hydrophilically coated capillary, in combination with electrophoresis buffer 0.48 M citric acid-13.6 mM citrate-4.8 M urea at pH 2.3, and a separation voltage of 25 kV, a complete separation of beta-lactoglobulin and para-kappa-casein was achieved, permitting the quantification of both components.

Structural studies on casein micelles of human milk: dissociation of beta-casein of different phosphorylation levels induced by cooling and ethylenediaminetetraacetate

Information on the structure of human casein micelles has been obtained from dissociation of beta-casein (CN). Two approaches were used: cooling at 4 degrees C and addition of EDTA. An initial loss of about 80% of the protein optical density occurred upon cooling to 4 degrees C. Dissociation was time dependent, and at > or = 24 h about 10% remained. However, mean size and voluminosity of micelles increased, as indicated by laser light scattering and viscosity measurements. This process was reversible, and 95% of the protein reentered the micelles upon incubation for 3 h at 37 degrees C. Upon cooling, amounts of nonphosphorylated beta-CN increased, and singly phosphorylated beta-CN levels were almost constant relative to the total beta-CN in micelles. Upon addition of EDTA (0 to 5 mM), the forms with three to five phosphates were the major dissociating constituents; EDTA that was added by dialysis produced similar results but at lower concentrations. These data suggest that, in the absence of significant amounts of alpha s1-CN, nonphosphorylated and singly phosphorylated human beta-CN may form a framework, as proposed for alpha s1-CN for bovine milk, along with the colloidal calcium phosphate for the development of the final micelle structure by addition of the more highly phosphorylated forms. The results also indicate that human casein micelles have a less rigid structure than those of other species.

Secondary structures in beta-casein peptide 1-42: a two dimensional nuclear magnetic resonance study

Two dimensional NMR spectroscopy was used to study the structure of a peptide composed of the N-terminal 42 amino acid residues of beta-casein. The peptide was obtained by enzymic cleavage using endoproteinase Asp-N. Complete sequence-specific 1H NMR assignment was performed for the peptide at three Ca2+ concentrations (0, 22 and 37 mM). The NMR results show that the peptide was highly flexible and adopted multiple conformations. No stable secondary structures were present; however, the peptide had some regions with non-random structure. The region between residues Leu16 and Asn27 adopted conformations with an increased contribution of alpha-helical structure, a so-called nascent helix. Two regions, Glu11-SerP15 and Lys29-Phe33 showed an increased population of conformations with extended structures. Addition of Ca2+ induced chemical shift changes for the backbone amide protons, especially around the phosphoserine region and around the suggested alpha-helical structure, indicating that the addition of Ca2+ stabilized the structure already present in the apo form of the peptide.

1C5 antigen: cancer-associated protein expressed by cervical adenocarcinomas and beta-casein

The monoclonal antibody 1C5 reacts with an antigenic determinant present in 90% of the cases of adenocarcinoma of the uterine cervix. The antigen defined by the 1C5 antibody exhibits immunological characteristics similar to those of skim milk (bovine buttermilk). 1C5-defined antigen obtained from tumor extract and skim milk binds specifically to wheat germ agglutinin (WGA) lectin. The 1C5-defined antigenic activity of a WGA lectin-bound fraction was eluted at 0.7-0.8 M NaCl off a Mono Q column. Use of an inhibition assay and a dot immunobinding assay revealed that the antigenic epitope defined by the 1C5 MoAb from skim milk exists within the first 28 amino acids of the beta-casein peptide.

Differences in defining residues relevant to antibody binding by ELISA and proteolysis protection at the level of peptide antigenic determinants

The interaction of monoclonal antibody (mAb) 1C3 with bovine beta-casein was analyzed by two methods. The competitive enzyme-linked immunosorbent assay in which mAb 1C3 bound competitively to a peptide in solution or beta-casein adsorbed to wells in microtitration plates showed that beta-casein (f194-200) (peptide of residues 194 to 200 of beta-casein) was the shortest peptide among the tested peptides that had binding ability for mAb 1C3. Protection against proteinase digestion was analyzed by incubating proline-specific endopeptidase, carboxypeptidase Y or aminopeptidase T with beta-casein (f193-202) in the presence of mAb 1C3 or nonspecific mAb 31A4. Proteolysis of peptide bonds between residues 200 and 201, and 201 and 202 was depressed in the presence of mAb 1C3. However, peptide bonds between 193 and 194, and 194 and 195 were cleaved in the presence of mAb 1C3 as easily as in the presence of mAb 31A4, suggesting that the region of residues 200 to 202 was obscured by, or within the antibody binding site, but that the region of residues 193 to 195 was not. The apparent antibody binding site shown from the protection against proteolysis by mAb was clearly not identical to the shortest antigen peptide, beta-casein (f194-200), indicated from the competitive binding assay.

Binding of zinc to bovine and human milk proteins

Zn binding by whole bovine and human casein and by purified bovine caseins and whey proteins was investigated by equilibrium dialysis. Bovine alpha s1-casein had the greatest Zn-binding capacity (approximately 11 atoms Zn/mol). Protein aggregation was observed as Zn concentration was increased and the protein precipitated at a free Zn concentration of 1.7 mM. Zn binding increased with increasing pH in the range 5.4-7.0 and decreased with increasing ionic strength. Competition between Zn and Ca was observed for binding to alpha s1-casein indicating common binding sites for these two metals. Bovine beta-casein bound up to 8 atoms Zn/mol and precipitated at a free Zn concentration of approximately 2.5 mM, while kappa-casein bound 1-2 atoms Zn/mol. Whole bovine and human casein bound 5-8 atoms Zn/mol and precipitated at a free Zn concentration of approximately 2.0 mM. Scatchard plots for Zn binding to caseins showed upward convexity, possibly due to Zn-induced association of caseins. Apparent average association constants (Kapp) for all caseins were similar (log Kapp 3.0-3.2). Enzymic dephosphorylation of alpha s1- or whole bovine casein markedly reduced, but did not eliminate, Zn binding. Thus, phosphoserine residues appeared to be the primary Zn-binding sites in caseins. With the exception of bovine serum albumin, which bound over 8 atoms Zn/mol, the bovine whey proteins, beta-lactoglobulin, alpha-lactalbumin and lactotransferrin, had little capacity for Zn binding.

Purification and Characterization of a Substrate-Size-Recognizing Metalloendopeptidase from Streptococcus cremoris H61

During the ripening of Gouda-type cheese, two kinds of endopeptidases were found to participate in the degradation of αs1-CN(f1-23), a specific product from αs1-casein hydrolyzed by chymosin. One of the endopeptidases, lactic acid bacteria endopeptidase (LEP-II), which can recognize the size of its substrates, has already been purified and characterized (T. R. Yan, N. Azuma, S. Kaminogawa, and K. Yamauchi, Eur. J. Biochem. 163:259-265, 1987). The other endopeptidase, LEP-I, was purified to homogeneity by conventional chromatographic techniques from Streptococcus cremoris H61. The enzyme appeared to be monomeric, with an apparent molecular weight of 98,000, and its isoelectric point was 5.1. For the hydrolysis of αs1-CN(f1-23), the enzyme had an optimum pH and temperature of 7.0 to 7.5 and 40�C, respectively. Its activity was inhibited by such chelating agents as EDTA and 1,10-phenanthrolin, and it could be fully reactivated by Mn 2+ . Inhibitors specific for serine and thiol proteases had no effect on the protease activity. The enzyme showed a high affinity toward the Glu-Asn peptide bond of αs1-CN(f1-23) and αs1-CN(f91-100) but showed no hydrolysis activity toward αs1-CN(f1-52), αs1-CN(61-122), αs1-CN(136-196), αs1-casein, β-casein, κ-casein, α-lactalbumin, and β-lactoglobulin. The K m and V max of LEP-I for αs1-CN(f1-23) were 14.2 pM and 139 U, respectively.

Purification and characterization of a novel metalloendopeptidase from Streptococcus cremoris H61. A metalloendopeptidase that recognizes the size of its substrate

An endopeptidase (LEP-II), which has a unique substrate specificity, was purified to homogeneity by conventional chromatographic techniques from Streptococcus cremoris H61. The enzyme was a metalloendopeptidase since it was inhibited by EDTA and 1,10-phenanthroline; the metal-depleted enzyme could be fully reactivated by micromolar levels of Zn2+ and was not inhibited by specific inhibitors for serine or thiol protease. The molecular mass of the enzyme was estimated to be 80 kDa by Sephacryl S-300 gel filtration and high-performance liquid chromatography with a TSK-G3000SW column. The enzyme consisted of two identical subunits and the N-terminal sequence of LEP-II was determined up to the 19th residue. Although the enzyme had a broad substrate specificity it specifically hydrolyzed the peptide bonds involving the amino groups of hydrophobic amino acid residues. Various small polypeptides, such as alpha s1-CN(f1-23), alpha s1-CN(f91-100), oxidized insulin B chain, glucagon and some biologically active peptides were hydrolyzed. However, a variety of larger polypeptides or proteins, such as alpha s1-CN(f1-54), alpha s1-CN(f61-123), alpha s1-CN(f136-196), alpha s1-casein, beta-casein, and kappa-casein were not hydrolyzed. LEP-II recognized the size of its substrates, which were limited below a molecular mass of about 3.5 kDa.

Casein micelle structure: susceptibility of various casein systems to proteolysis

The susceptibility of the components of various caseinate systems (skim-milk, β-casein-depleted milks, colloidal phosphate-free (CPF) milk, sodium caseinate and isolated β-casein) to proteolysis was investigated. Isolated αs1- and β-caseins were quite susceptible to proteolysis, but their susceptibility decreased in heterogeneous soluble systems and even more so in heterogeneous aggregated systems. In skim-milk and β-casein-depleted milks only about 50% of both αs1- and β-casein was hydrolysable by high levels of rennin, and in CPF milk all αs1- and 70% of the β-casein was hydrolysable. It is suggested that about 50% of micellar β-casein is firmly fixed within the micelle and is unavailable for proteolysis, while the remainder can dissociate from the micelle on cooling and is then readily hydrolysable.

The compatibility of the data with the various published models of the casein micelle is discussed, and a modification of Rose's (1969) model is proposed.