Use of mass spectrometry To rapidly characterize the heterogeneity of bovine alpha-lactalbumin

From a bovine whey protein fraction the nonglycosylated and glycosylated alpha-lactalbumin fractions were isolated by gel-permeation chromatography followed by reversed-phase high-performance liquid chromatography. Both fractions were studied by electrospray-ionization mass spectrometry (ESI-MS). For the nonglycosylated fraction, a mass of 14 178 Da was found, which was in accordance with the known amino acid sequence of the protein. The glycosylated fraction appeared to be a mixture of components with mass values ranging from ca. 15 840 to 16 690 Da. Given the published carbohydrate composition of the whole glyco-alpha-lactalbumin fraction, these masses could be matched to those of 14 differently glycosylated forms of alpha-lactalbumin. Further support for these forms was obtained from the results of a separate mass spectrometric analysis of the mixture of oligosaccharides released from the protein by treatment with peptide-N(4)-(N-acetyl-beta-glucosaminyl)asparagine amidase F. ESI-MS was found to be a powerful tool to gain insight into the composition of the complex mixture of glycoforms of alpha-lactalbumin without the need of further purification of these forms or of the oligosaccharides released thereof.

Functionality of beta-casein peptides: importance of amphipathicity for emulsion-stabilizing properties

To investigate structure-function relationships with regard to emulsion-stabilizing properties, peptides from bovine beta-casein (betaCN), obtained by plasmin hydrolysis and fractionation of the hydrolysate, were isolated and identified on the basis of their masses determined by electrospray ionization mass spectrometry, the primary structure of the intact protein, and the known specificity of the enzyme. An amphipathic peptide fraction was fractionated further by ion-exchange chromatography and subsequent hydrophobic interaction chromatography resulting in the components betaCN[f 1-105/107] and betaCN[f 29-105/107]. The latter peptides had poor emulsion-stabilizing properties compared to the former ones, and the stability of an emulsion formed with betaCN[f 29-105/107] was also more sensitive to hydrophobic impurities than that of an emulsion formed with betaCN[f 1-105/107]. The highly charged N-terminal part appeared to be important for the emulsion-stabilizing properties of these peptides. A hypothesis for the structure-function relationship is given.

Reversed-phase high-performance liquid chromatographic separation of bovine kappa-casein macropeptide and characterization of isolated fractions

From complex mixtures of non-glycosylated and differently glycosylated caseinomacropeptides (CMP; kappa-casein fragment 106-169; M(r) approximately 7000) various fractions were isolated and further purified by reversed-phase HPLC. The fractions were characterized by mass determination and composition analysis and also used in gel-permeation chromatography and NMR studies to investigate their molecular size behaviour as a function of pH, ionic strength, peptide concentration and degree of glycosylation. No evidence was found for association of any CMP fraction as a function of the experimental conditions applied, which is in contrast with suggestions made in the literature. The increased molecular size (apparent molecular mass approx. 30-45 kDa) is rather explained by a large voluminosity of the molecular species due to internal electrostatic and steric repulsion. Furthermore, the susceptibility of some non-glycosylated and glycosylated CMP fractions to enzymic attack by the Glu-specific endopeptidase from Staphylococcus aureus V8 was studied. Initial rates of proteolysis by this enzyme were independent of the degree of glycosylation. Only in the case of highly glycosylated CMP was further hydrolysis to smaller fragments inhibited. Hydrolytic products were identified by electrospray ionization and fast-atom bombardment mass spectrometry.

Raising the pH of the pepsin-catalysed hydrolysis of bovine whey proteins increases the antigenicity of the hydrolysates

BACKGROUND

Hypersensitivity to cow milk protein is frequently observed in infancy. Since the pH in the infant's stomach is relatively high (pH 3-4) compared with adults (pH 2), an incomplete digestion of the milk proteins is expected to occur.

OBJECTIVE

The determination of the degree of hydrolysis by pepsin of the four main proteins of bovine whey, i.e. alpha-lactalbumin (alpha La), beta-lactoglobulin (beta Lg), bovine serum albumin (BSA) and bovine immunoglobulin G (B-IgG), in the pH range 2.0-4.0 and of the antigenic properties of the resulting hydrolysates.

METHODS

Whey proteins were successively hydrolysed with pepsin at pH values ranging from 2.0 to 4.0 and with pancreatic enzymes at pH 7.5 using a pH-stat. The resulting hydrolysates were characterized by their degree of hydrolysis, and analysed by sodium dodecyl sulfate polyacrylamide gel electrophoresis, gel permeation chromatography and immunologically by competitive enzyme-linked immunosorbent assay.

RESULTS

In general, the degree of hydrolysis, the gel electrophoretic patterns, the contents of peptides of molecular mass > 5 kDa and the residual human-immunoglobulin E and G antigenicities of the hydrolysates did not differ much whether the pepsin incubation was done at pH 2.0 or 3.0. Pepsin incubation at pH 4.0, however, resulted in a decreased hydrolysis and enhanced residual antigenicity of alpha La, BSA and B-IgG, but not of beta Lg.

CONCLUSION

The poor and slow degradation of the antigenic epitopes of whey proteins when pepsin digestion occurs under conditions that prevail in the stomach of infants could be of much importance for the development of cow milk hypersensitivity. The immature gastrointestinal mucosal barrier of infants allows large antigenic fragments of these proteins to pass into the systemic circulation.

Identification of a new genetic variant of bovine beta-casein using reversed-phase high-performance liquid chromatography and mass spectrometric analysis

Various components of the beta-casein fraction from bovine milk were separated by preparative reversed-phase high-performance liquid chromatography (RP-HPLC). They included the genetic variants beta A1, beta A2, beta A3, and an unknown component previously denoted beta X [S. Visser et al., J. Chromatogr. 548 (1991) 361-370]. Tryptic digests of these components were compared by RP-HPLC and most peaks were analysed by mass spectrometry (MS). The tryptic map of beta X was closest to that of beta A1, but with a few mutually different peak components. Electrospray ionisation MS revealed that in the beta X map these components had relative molecular masses of 16 higher than the corresponding ones in the beta A1 map. The main differential peaks represented the 114-169 fragments of beta A1 and beta X, respectively, which were both purified and then cleaved with cyanogen bromide. In the resulting mixtures, each of which contained three fragments, the corresponding peptides representing the 145-156 sequence showed the 16 relative molecular mass difference. In beta X this sequence contained a Leu residue at position 152 instead of the Pro-152 in beta A1, as established by fast-atom bombardment MS-MS. The Leu could be discriminated from an Ile residue by the presence of a side-chain-specific, D-type fragment ion in the MS-MS spectrum of the beta X CNBr peptide. The sequence of the two homologous 145-156 fragments was confirmed by regular amino acid sequence analysis. In accordance with internationally accepted guidelines for the nomenclature of milk proteins, the new genetic variant has been named beta-casein F-5P.

Action of a cell-envelope proteinase (CEPIII-type) from Lactococcus lactis subsp. cremoris AM1 on bovine kappa-casein

The specificity of the cell-envelope proteinase (CEPIII-type) from Lactococcus lactis subsp. cremoris AM1 in its action on bovine kappa-casein was studied. A 4-h digest (pH 6.2, 15 degrees C) of kappa-casein was made with the purified proteinase. The pH-4.6 soluble fraction, representing more than 95% of the whole hydrolysate, was ultrafiltered to obtain a high-molecular-mass (HMM) and a low-molecular-mass (LMM) fraction, which were separately further purified by electrophoretic and chromatographic techniques. Isolated HMM and LMM products were identified by amino acid analysis, end-group determination and mass spectrometry. On-line HPLC/mass spectrometry was also used for the separation of an LMM peptide mixture and the identification of its components. The HMM products formed were the fragments 1-160, 1-151, 1-95 and 1-79 of kappa-casein, whereas the main LMM products found were the 161-169 and 152-160 fragments. The enzyme specificity was concluded to be primarily directed towards the C-terminal region of the substrate molecule by cleavage of the 160-161 and 151-152 peptide bonds. Two minor LMM products were identified as the fragments 96-104 and 103-106, indicating additional cleavage at positions 102-103, 104-105 and 106-107 of the sequence. Also several peptide bonds within the 161-169 sequence were found to be subject to secondary cleavage by the proteinase. From electrophoretic and identification data it is concluded that the lactococcal CEPI, CEPIII and several mixed-type proteinases all act on the peptide bonds at positions 79-80 and 95-96.(ABSTRACT TRUNCATED AT 250 WORDS)

Overproduction of bovine beta-casein in Escherichia coli and engineering of its main chymosin cleavage site

A cDNA clone containing the entire coding region for bovine beta-casein A3 flanked by 53 base pairs of 5' non-coding and 358 base pairs of 3' non-coding sequences was isolated from a bovine mammary cDNA phagemid library. The coding segment for mature beta-casein was subcloned into the T7 expression system, in which the expression of recombinant beta-casein was controlled by the T7 gene 10 promoter and ribosome binding site. High level expression of Met-beta-casein to approximately 20% of the total soluble proteins was obtained in Escherichia coli within 2 h after induction of T7 RNA-polymerase synthesis. In an attempt to induce secretion the coding segment for mature beta-casein was coupled to the ompA translational initiation signal and signal peptide coding sequence but no secretion of the fusion protein and no processing of the signal peptide from the fusion protein was observed. Instead, the Met-beta-casein could be isolated in a soluble form from E.coli cells after an osmotic shock, indicative of a periplasmic location. This procedure did not lyse the cells. The protein was purified to homogeneity after a pH 4.8 isoelectric precipitation followed by reversed-phase high-performance liquid chromatography. The beta-casein cDNA was altered to change the main chymosin cleavage site in beta-casein at position 192-193 in two ways, namely from Leu-Tyr to Pro-Pro and to Leu-stop.(ABSTRACT TRUNCATED AT 250 WORDS)

Phenotyping of bovine milk proteins by reversed-phase high-performance liquid chromatography

A reversed-phase high-performance liquid chromatographic method for the separation of the most common and some less common genetic variants of the bovine caseins is described. When the method is used for analysing clarified skim milk, simultaneous identification of casein variants and major they protein variants can be effected in a single run. The potential of the method for quantitative application is discussed.

Specificity of a cell-envelope-located proteinase (PIII-type) from Lactococcus lactis subsp. cremoris AM1 in its action on bovine ?-casein

The action of the cell-envelope proteinase (PIII-type) from Lactococcus lactis ssp. cremoris AM1 on bovine beta-casein was studied. The results were compared with those obtained earlier with (PI-type) proteinases from the cell envelope of other L. lactis strains. From a 4-h digest (pH 6.2; 15 degrees C) of beta-casein made with the PIII-type proteinase, 24 peptides were isolated and purified by selective precipitation followed by semi-preparative reversed-phase HPLC. Altogether, these peptides accounted for the preferential splitting of 16 peptide bonds in beta-casein by the PIII-type proteinase. In nine cases the primary cleavage site (P1-P'1) was a Glx-X or X-Glx peptide bond. In ten cases at least one large hydrophobic residue (Met, Leu, Tyr, Phe) formed part of the cleavable bond. The P2-P3 and/or P'2-P'3 regions of the substrate consisted of hydrophobic and/or negatively charged side chains or of side chains potentially involved in hydrogen bonds. Nine of the peptide bonds split were reported previously to be also susceptible to cleavage by PI-type proteinases, although the kinetics may be different. The PIII-type proteinase shows a broader specificity in its initial cleavage of beta-casein than does the PI-type.

Specificity of two genetically related cell-envelope proteinases of Lactococcus lactis subsp. cremoris towards alpha s1-casein-(1-23)-fragment

The specificity of two genetically related cell-envelope serine proteinases (PI-type and PIII-type) of Lactococcus lactis subsp. cremoris towards the alpha s1-casein-(1-23)-fragment, an important intermediate product of primary chymosin-directed proteolysis in cheese, has been established. Both enzymes showed, at pH 6.5 and under relatively low-ionic-strength conditions, a characteristic, mutually different, cleavage pattern that seems, in the first instance, to be determined by the charge N-terminal to the cleaved bond. With Pi, three cleavage sites were found in the N-terminal positively charged part of the peptide and, with PIII, three sites were found in the C-terminal negatively charged part. Comparison of the specific cleavage sites in this peptide and those in beta-casein revealed similarities with respect to the different residues which can occur N-terminally to the cleaved bond. The properties of these substrate residues match with the structural and various interactive features of the respective binding regions of the enzymes predicted on the basis of a close sequence similarity of the lactococcal proteinases with the subtilisin family. A hydrophobic interaction and/or hydrogen-bridge formation seems to govern the binding of the first amino acid residue N-terminal to the scissile bond. The more distantly N-terminally positioned sequence of residues apparently is attracted electrostatically by a negative charge in the binding region of PI and by a positive charge in that of PIII, provided that the opposite charge is is present at the appropriate position in this sequence. Hence a specific electrostatic binding may occur; additionally, hydrophobic interaction and/or hydrogen-bond formation is important.

Peptide substrates for chymosin (rennin). Interaction sites in kappa-casein-related sequences located outside the (103-108)-hexapeptide region that fits into the enzyme's active-site cleft

The role of individual amino acid residues in the 98-102 and 111-112 regions of bovine kappa-casein in its interaction with the milk-clotting enzyme chymosin (rennin) was investigated. to this end the tryptic 98-112 fragment of kappa-casein was modified in its N- and/or C-terminal part by chemical (guanidation, ethoxyformylation, repeated Edman degradation) and enzymic (carboxypeptidase) treatments. Further, use was made of short synthetic kappa-casein analogues in which His-102 had been replaced by Pro or Lys. All peptides and their derivatives were tested comparatively at various pH values for their ability to act as chymosin substrates via specific cleavage of the peptide bond at position 105-106. The results indicate that in the alternating 98-102 sequence (His-Pro-His-Pro-His) the His as well as the Pro residues contribute to the substrate activity with no predominant role of any one of these groups. Another interaction site is formed by the Lys residue at position 111 of the substrate. A model of the enzyme-substrate complex is proposed. Herein the 103-108 fragment of the substrate, to be accommodated within the enzyme's active-site cleft, is brought into position by electrostatic binding (via His-98, His-100, His-102 and Lys-111) near the entrance of the cleft. These interactions are strongly supported by Pro residues at positions 99, 101, 109 and 110 of the substrate, which act as stabilizers of the proper conformation of the substrate in the enzyme-substrate complex.

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.

Comparative Study of Action of Cell Wall Proteinases from Various Strains of Streptococcus cremoris on Bovine α s1 -, β-, and κ-Casein

Partially purified cell wall proteinases of eight strains of Streptococcus cremoris were compared in their action on bovine α s1 -, β-, and κ-casein, as visualized by starch gel electrophoresis, sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and thin-layer chromatography. Characteristic degradation profiles could be distinguished, from which the occurrence of two proteinases, represented by strain HP and strain AM 1 , was concluded. The action of the HP-type proteinase P 1 (also detectable in strains Wg 2 , C 13 , and TR) was established by electrophoretic methods to be directed preferentially towards β-casein. The AM 1 -type proteinase P III (also detectable in strain SK 11 ) was also able to degrade β-casein, but at the same time split α s1 - and κ-casein more extensively than did P I . Strain FD 27 exhibited mainly P I activity but also detectable P III degradation characteristics. The cell wall proteinase preparation of strain E 8 showed low P I as well as low P III activity. All proteinase preparations produced from κ-casein positively charged degradation products with electrophoretic mobilities similar to those of degradation products released by the action of the milk-clotting enzyme chymosin. The differences between P I and P III in mode of action, as detected by gel electrophoresis and thin-layer chromatography, were reflected by the courses of the initial degradation of methyl - 14 C-labeled β-casein and by the effect of α s1 - plus κ-casein on these degradations. The results are discussed in the light of previous comparative studies of cell wall proteinases in strains of S. cremoris and with respect to the growth of this organism in milk.