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

Experimental evidence for previously unclassified calcium phosphate structures in the casein micelle

¹H-³¹P Cross-polarization magic angle spinning (CP-MAS) measurements of 40-d-old Mozzarella cheese and 20 mM EDTA-treated casein micelles revealed that each sample had immobile phosphorus with the same spectral pattern, which did not match that of native casein micelles. To identify the immobile phosphorus bodies, ¹H-³¹P CP-MAS spectra and cross-polarization kinetics measurements were undertaken on native casein micelles, EDTA-chelated casein micelles, and reference samples of β-casein and hydroxyapatite. The results showed that the immobile phosphorus bodies in the mature Mozzarella cheese had the following characteristics: they are immobile phosphoserine residues (not colloidal calcium phosphate); they are not the product of phosphoserine to colloidal calcium phosphate bridging; the phosphate is complexed to calcium; their rigidity is localized to a phosphorus site; their rigidity and bond coupling is unaffected by protein hydration; and the immobile bodies share a narrow range of bond orientations. Combining these observations, the best explanation of the immobile phosphorus bodies is that bonding structures of phosphorus-containing groups and calcium exist within the casein micelle that are not yet clearly classified in the literature. The best candidate is a calcium-bridged phosphoserine-to-phosphoserine linkage, either intra- or inter-protein.

Detection of two minor phosphorylation sites for bovine κ-casein macropeptide by reversed-phase liquid chromatography-tandem mass spectrometry

This work addresses the characterization of phosphopeptides in bovine κ-casein macropeptide by reversed-phase liquid chromatography-electrospray ionization-tandem mass spectrometry (RPLC-ESI-MS(2)). Two different mass spectrometers, equipped with an ion trap (IT) or a quadrupole time-of-flight (Q-TOF) analyzer, were used to perform an accurate phosphorylation site assignment. A total of 8 phosphopeptides from 26 identified peptides were characterized. MS(2) spectra of phosphopeptides were dominated by the neutral loss of a phosphoric acid molecule (H(3)PO(4)) and sufficient informative fragment ions resulting from peptide backbone cleavages enabling the elucidation of the phosphopeptide sequence. A higher number of sequence informative b and y ions were detected using a Q-TOF instead of an IT analyzer. In addition to the well-established phosphorylation sites at Ser(149) and Ser(127), this study also revealed the presence of two minor phosphorylation sites at Thr(145) and Ser(166). These findings indicate that RPLC-ESI-MS(2) on a Q-TOF analyzer is a useful technique for identifying low-abundance phosphorylation sites in caseins.

Distribution and mobility of phosphates and sodium ions in cheese by solid-state 31P and double-quantum filtered 23Na NMR spectroscopy

The feasibility of solid-state magic angle spinning (MAS) (31)P nuclear magnetic resonance (NMR) spectroscopy and (23)Na NMR spectroscopy to investigate both phosphates and Na(+) ions distribution in semi-hard cheeses in a non-destructive way was studied. Two semi-hard cheeses of known composition were made with two different salt contents. (31)P Single-pulse excitation and cross-polarization MAS experiments allowed, for the first time, the identification and quantification of soluble and insoluble phosphates in the cheeses. The presence of a relatively 'mobile' fraction of colloidal phosphates was evidenced. The detection by (23)Na single-quantum NMR experiments of all the sodium ions in the cheeses was validated. The presence of a fraction of 'bound' sodium ions was evidenced by (23)Na double-quantum filtered NMR experiments. We demonstrated that NMR is a suitable tool to investigate both phosphates and Na(+) ions distributions in cheeses. The impact of the sodium content on the various phosphorus forms distribution was discussed and results demonstrated that NMR would be an important tool for the cheese industry for the processes controls.

Detection of sialylated phosphorylated kappa-casein glycomacropeptide electrophoresed on polyacrylamide gels and cellulose acetate strips by the thiobarbituric acid and malachite green dye reactions

A 64 amino acid residue sialylated phosphorylated glycomacropeptide (GMP) from bovine sweet whey can be detected as a Coomassie blue-staining peptide by electrophoresis on sodium dodecyl sulfate (SDS)-polyacrylamide gels. There is, however, limited information available concerning detection of GMP as a sialylated phosphorylated compound. Samples of GMP were electrophoresed on SDS-polyacrylamide gels or cellulose acetate strips (CAS). Immediately following electrophoresis, fractions obtained by cutting gels or strips were subjected to sialic acid determination by the thiobarbituric acid reaction and phosphorus determination by the malachite green dye reaction. Both determinations were found to be sensitive enough to detect approximately 20 and 40 microg of GMP in CAS and SDS gels, respectively. Further studies demonstrated that sialylated phosphorylated GMP can be detected on either SDS gels or CAS loaded with whey products or whey-added margarine residues.

The phosphorylation pattern of human alphas1-casein is markedly different from the ruminant species

Caseins are highly phosphorylated milk proteins assembled in large colloidal structures termed micelles. In the milk of ruminants, alphas1-casein has been shown to be extensively phosphorylated. In this report we have determined the phosphorylation pattern of human alphas1-casein by a combination of matrix-assisted laser desorption mass spectrometry and amino acid sequence analysis. Three phosphorylation variants were identified. A nonphosphorylated form, a variant phosphorylated at Ser18 and a variant phosphorylated at Ser18 and Ser26. Both phosphorylation sites are located in the amino acid recognition sequence of the mammary gland casein kinase. Notably, no phosphorylations were observed in the conserved region covering residues Ser70-Glu78, which is extensively phosphorylated in the ruminant alphas1-caseins.

Mass mapping analysis as a tool for the identification of genetic variants of bovine beta-casein

Matrix-assisted laser desorption ionisation time-of-flight mass spectrometric analysis of the tryptic digest of beta-casein A2 and beta-casein B was performed before and after the separation of the peptides by LC. The overlapping of the chromatograms showed that all peaks were present in both samples, except for one only found in the tryptic digest of the A2 variant and two in the B variant. Experimental masses could be assigned to those peptides produced by tryptic digest of beta-casein variant. This peptide mapping strategy and current methodological improvements represent a promising tool for the identification of milk genetic variants with the difference of an amino acid substitution.

Isolation and analysis of kappa-casein glycomacropeptide from goat sweet whey

Glycomacropeptide (GMP) was purified from goat sweet whey by anion-exchange and hydrophobic interaction chromatography. Approximately 0.06% (w/v) of sweet whey was recovered as GMP. Amino acid analysis of the GMP preparation showed that the content of phenylalanine (an amino acid that does not occur in goat GMP) was negligible, indicating that the GMP was of high purity. The goat GMP contained 25 microg sialic acid per mg of dry weight. This was approximately 3-fold lower than the sialic acid concentration in bovine GMP reported in the literature. Gel electrophoretic results demonstrated that most of the goat GMP occurs as a dimer. The GMP was intensely stained with Coomassie blue in 50% methanol containing 12.5% (w/v) trichloroacetic acid, but showed very weak metachromasia with the same dye in 45% methanol containing 10% acetic acid, a preparation commonly used to stain protein.

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.

Investigation of bacterial spore structure by high resolution solid-state nuclear magnetic resonance spectroscopy and transmission electron microscopy

High resolution solid-state nuclear magnetic resonance spectroscopy (NMR) in combination with transmission electron microscopy (TEM) of spores of Bacillus cereus, an outer coatless mutant B. subtilis 322, an inner coatless mutant B. subtilis 325 and of germinated spores of B. subtilis CMCC 604 were carried out. Structural differences in the coats, mainly protein of spores were reflected by NMR spectra which indicated also differences in molecular mobility of carbohydrates which was partially attributed to the cortex. Dipicolinic acid (DPA) of spores of B. cereus displayed a high degree of solid state order and may be crystalline. Heat activation was studied on spores of B. subtilis 357 lux + and revealed a structural change when analysed by TEM but this was not associated with increases in molecular mobility since no effects were measured by NMR.

Effects of hydration on molecular mobility in phase-bright Bacillus subtilis spores

The molecular mobility of 31P and 13C in dormant Bacillus subtilis spore samples with different water concentrations was investigated by high-resolution solid-state NMR. Lowest molecular mobility was observed in freeze-dried preparations. Rehydration to a 10% weight increase resulted in increases in molecular motions and addition of excess water furthered this effect. A spore slurry which had been freeze-dried displayed after addition of excess water similar NMR spectra to native wet preparations. Dipicolinic acid (DPA), which is mainly located in the core, was detected at all hydration levels in 13C cross-polarization magic angle spinning (CPMAS) but not in single-pulse magic angle spinning (SPMAS) spectra, indicating that hydration had no effect on its mobility. The molecular mobility of 31P, present mainly in core-specific components, was strongly dependent on hydration. This result suggests reversible water migration between inner spore compartments and the environment, whereas 13C spectra of DPA indicate that it is immobilized in a water-insoluble network in the core. Scanning transmission electron microscopy revealed that freeze-dried spores were significantly longer and narrower than fully hydrated spores and had a 3% smaller volume.