Human beta-casein. Amino acid sequence and identification of phosphorylation sites

A long-term study of stable isotope ratios of fingernail keratin and amino acids in a mother-infant dyad

OBJECTIVE

To evaluate the potential of compound-specific isotope analysis of amino acids (CSIA-AA) for investigating infant feeding practices, we conducted a long-term study that compared infant and maternal amino acid (AA) nitrogen isotope ratios.

MATERIALS AND METHODS

Fingernail samples were collected from a single mother-infant dyad over 19 months postpartum. Carbon and nitrogen stable isotope ratios were measured in the bulk keratin of the fingernail samples. Selected samples were then hydrolyzed and derivatized for compound-specific nitrogen isotope analysis of keratin AAs.

RESULTS

As in previous studies, infant bulk keratin nitrogen isotope values increased during exclusive breastfeeding and fell with the introduction of complementary foods and eventual cessation of breastfeeding. Infant trophic AAs had elevated nitrogen isotope values relative to the mother, while the source AAs were similar between the mother and infant. Proline and threonine appeared to track the presence of human milk in the infant's diet as the isotopic composition of these AAs remained offset from maternal isotope values until the cessation of breastfeeding.

DISCUSSION

Although CSIA-AA is costly and labor intensive, it appears to hold potential for estimating the duration of breastfeeding, even after the introduction of complementary foods. Through the analysis of a full suite of AAs, it may also yield insights into infant physiology and AA synthesis.

The Aggregated and Micellar Forms of β-Casein Purified from Donkey and Bovine Milk Present Potential as Carriers for Bioactive Nutritional Compounds

In recent years, there has been a growing interest in the pure casein fraction of milk protein, particularly β-casein due to its physicochemical properties as well as its bio- and techno-functional properties. The utilization of self-assembled β-caseins from bovine origin as nanocarriers for the delivery of nutraceutical compounds or drugs has increased dramatically. Concerning β-caseins from other milk sources, the use of hypoallergenic donkey β-caseins as a potential delivery vehicle for nutraceutical hydrophobic compounds is beginning to generate interest. The present review deals with casein micelles models, bovine and donkey β-casein molecular structures, as well as their physical-chemical properties that account for their exploitation in nutraceutics and pharmaceutics. This review work suggests the possibility of developing delivery systems for hydrophobic bioactive compounds using β-casein purified from hypoallergenic donkey milk, highlighting the potential of this protein as an innovative and promising vehicle for enhancing the enrichment and bioavailability of various bioactive substances in food products.

Lithium Chloride Promotes Milk Protein and Fat Synthesis in Bovine Mammary Epithelial Cells via HIF-1α and β-Catenin Signaling Pathways

Lithium is one of the trace elements with many physiological properties, such as being anti-cancer, anti-viral, and anti-inflammatory. However, little is known about its effect on milk synthesis during lactation. Therefore, we selected different concentrations (5 mM, 10 mM, and 20 mM) of lithium chloride (LiCl) and assessed the effect of LiCl on bovine mammary epithelial (MAC-T) cells that underwent 4 days of differentiation induction. Moreover, we analyzed the effect of LiCl on the expression of genes related to milk fat and milk protein synthesis. Herein, LiCl (5-20 mM) significantly increased the expression of β-casein, promoted mRNA expression and phosphorylated protein expression of the signal transduction molecule and activator of transcription 5β (STAT5-β), and inhibited mRNA and protein expression of suppressor of cytokine signaling 2 (SOCS2). In contrast, 5 and 10 mM LiCl significantly inhibited expression of SOCS3. LiCl at concentration of 5-20 mM enhanced phosphorylation level of mTOR protein; at 10 mM and 20 mM, LiCl significantly promoted expression and phosphorylation of downstream ribosomal protein S6 kinase beta-1 (S6K1) protein. Considering milk fat synthesis, mRNA expression of acetyl CoA carboxylase (ACC) and lipoprotein lipase (LPL) genes was considerably increased in the presence of LiCl (5-20 mM). Additionally, increased protein expression levels of stearoyl-CoA desaturase (SCD), peroxisome proliferator-activated receptor-γ (PPARγ), and sterol regulatory element-binding protein 1 (SREBP1) were observed at all LiCl concentrations tested. Subsequently, LiCl (5-20 mM) significantly promoted protein expression and phosphorylation of β-catenin, while 10 mM and 20 mM of LiCl significantly promoted protein expression of hypoxia-inducible factor-1α (HIF-1α). Collectively, it has been shown that 10 mM LiCl can effectively activate HIF-1α, β-catenin, and β-catenin downstream signaling pathways. Conversely, at 10 mM, LiCl inhibited SOCS2 and SOCS3 protein expression through JAK2/STAT5, mTOR, and SREBP1 signaling pathways, improving synthesis of milk protein and fat. Therefore, LiCl can be used as a potential nutrient to regulate milk synthesis in dairy cows.

Investigating the Conformation of Surface-Adsorbed Proteins with Standing-Wave X-ray Fluorescence

Protein adsorption to surfaces is at the heart of numerous technological and bioanalytical applications, but sometimes, it is also associated with medical risks. To deepen our insights into processes involving layers of surface-adsorbed proteins, high-resolution structural information is essential. Here, we use standing-wave X-ray fluorescence (SWXF) in combination with an optimized liquid-cell setup to investigate the underwater conformation of the random-coiled phosphoprotein β-casein adsorbed to hydrophilic and hydrophobized solid surfaces. The orientation of the protein, as determined through the distributions of sulfur and phosphorus, is found to be sensitive to the chemical nature of the substrate. While no preferred orientations are observed on hydrophobized surfaces, on hydrophilic Al oxide, β-casein is adsorbed as a diblock copolymer with the phosphorylated domain I attached to the surface. Our results demonstrate that targeting biologically relevant chemical elements with SWXF enables a detailed investigation of biomolecular layers under near-physiological conditions.

Monitoring Human Milk β-Casein Phosphorylation and O-Glycosylation Over Lactation Reveals Distinct Differences between the Proteome and Endogenous Peptidome

Human milk is a vital biofluid containing a myriad of molecular components to ensure an infant's best start at a healthy life. One key component of human milk is β-casein, a protein which is not only a structural constituent of casein micelles but also a source of bioactive, often antimicrobial, peptides contributing to milk's endogenous peptidome. Importantly, post-translational modifications (PTMs) like phosphorylation and glycosylation typically affect the function of proteins and peptides; however, here our understanding of β-casein is critically limited. To uncover the scope of proteoforms and endogenous peptidoforms we utilized mass spectrometry (LC-MS/MS) to achieve in-depth longitudinal profiling of β-casein from human milk, studying two donors across 16 weeks of lactation. We not only observed changes in β-casein's known protein and endogenous peptide phosphorylation, but also in previously unexplored O-glycosylation. This newly discovered PTM of β-casein may be important as it resides on known β-casein-derived antimicrobial peptide sequences.

The Function and Alteration of Immunological Properties in Human Milk of Obese Mothers

Maternal obesity is associated with metabolic changes in mothers and higher risk of obesity in the offspring. Obesity in breastfeeding mothers appears to influence human milk production as well as the quality of human milk. Maternal obesity is associated with alteration of immunological factors concentrations in the human milk, such as C-reactive protein (CRP), leptin, IL-6, insulin, TNF-Alpha, ghrelin, adiponectin, and obestatin. Human milk is considered a first choice for infant nutrition due to the complete profile of macro nutrients, micro nutrients, and immunological properties. It is essential to understand how maternal obesity influences immunological properties of human milk because alterations could impact the nutrition status and health of the infant. This review summarizes the literature regarding the impact of maternal obesity on the concentration of particular immunological properties in the human milk.

Effect of protein composition of a model dairy matrix containing various levels of beta-casein on the structure and anti-inflammatory activity of in vitro digestates

An increasing body of evidence demonstrates that differences in protein composition in the food matrix can significantly affect its biological functionality. The present research hypothesized that a matrix containing the same level of dairy protein, but with different composition, even when showing similar properties during digestion, may have a different biological functionality. To test this hypothesis, three matrices, containing 2.8% protein and similar amounts of fat and solid were prepared, either with 100% whey proteins, or with a ratio of caseins to whey protein of 40 : 60, but differing in β-casein ratio. The mixtures were subjected to in vitro digestion, and the digestates were used in uptake experiments using Caco-2 cell monolayers. The basolateral fraction metabolized by the cells was used to stimulate human LPS-stimulated THP-1 macrophages and the concentration of selected cytokines were measured, as an indication of potential differences in biological functionality between the different dairy matrices. All three digestates induced a significant reduction in IL-1β cytokines, with the casein-containing treatments inducing a greater decrease compared to that containing only whey proteins. The matrix containing the highest ratio of β-casein induced the lowest secretion of proinflammatory cytokines TNF-a and IL-6. This study demonstrated that milk protein composition does not only affect the rate of gastric proteolysis and structure of the gastric digestate, but will cause differences in physiological effects. This research stressed the role of milk protein components during digestion, and of β-casein in particular, and their potential to modulate biological functions in the gastrointestinal tract.

Bioactive Proteins in Human Milk-Potential Benefits for Preterm Infants

Human milk contains many bioactive proteins that are likely to be involved in the better outcomes of breast-fed infants compared with those fed infant formula. Bovine milk proteins or protein fractions may be able to provide some of these benefits and may, therefore, be used for preterm infants. Recombinant human milk proteins are likely to exert bioactivities similar to those of the native human milk proteins, but considerable research is needed before they can be used in routine care of preterm infants.

The relationships among bovine αS-casein phosphorylation isoforms suggest different phosphorylation pathways

Casein (CN) phosphorylation is an important posttranslational modification and is one of the key factors responsible for constructing and stabilizing casein micelles. Variation in phosphorylation degree of αS-CN is of great interest because it is suggested to affect milk technological properties. This study aimed to investigate the variation in phosphorylation degree of αS-CN among milk of individual cows and to explore relationships among different phosphorylation isoforms of αS-CN. For this purpose, we analyzed morning milk samples from 529 French Montbéliarde cows using liquid chromatography coupled with electrospray ionization mass spectrometry. We detected 3 new phosphorylation isoforms: αS2-CN-9P, αS2-CN-14P, and αS2-CN-15P in bovine milk, in addition to the known isoforms αS1-CN-8P, αS1-CN-9P, αS2-CN-10P, αS2-CN-11P, αS2-CN-12P, and αS2-CN-13P. The relative concentrations of each αS-CN phosphorylation isoform varied considerably among individual cows. Furthermore, the phenotypic correlations and hierarchical clustering suggest at least 2 regulatory systems for phosphorylation of αS-CN: one responsible for isoforms with lower levels of phosphorylation (αS1-CN-8P, αS2-CN-10P, and αS2-CN-11P), and another responsible for isoforms with higher levels of phosphorylation (αS1-CN-9P, αS2-CN-12P, αS2-CN-13P, and αS2-CN-14P). Identifying all phosphorylation sites of αS2-CN and investigating the genetic background of different αS2-CN phosphorylation isoforms may provide further insight into the phosphorylation mechanism of caseins.

Simultaneous quantification of α-lactalbumin and β-casein in human milk using ultra-performance liquid chromatography with tandem mass spectrometry based on their signature peptides and winged isotope internal standards

In recent years, there is an increasing need to measure the concentration of individual proteins in human milk, instead of total human milk proteins. Due to lack of human milk protein standards, there are only few quantification methods established. The objective of the present work was to develop a simple and rapid quantification method for simultaneous determination of α-lactalbumin and β-casein in human milk using signature peptides according to a modified quantitative proteomics strategy. The internal standards containing the signature peptide sequences were synthesized with isotope-labeled amino acids. The purity of synthesized peptides as standards was determined by amino acid analysis method and area normalization method. The contents of α-lactalbumin and β-casein in human milk were measured according to the equimolar relationship between the two proteins and their corresponding signature peptides. The method validation results showed a satisfied linearity (R(2)>0.99) and recoveries (97.2-102.5% for α-lactalbumin and 99.5-100.3% for β-casein). The limit of quantification for α-lactalbumin and β-casein was 8.0mg/100g and 1.2mg/100g, respectively. CVs for α-lactalbumin and β-casein in human milk were 5.2% and 3.0%. The contents of α-lactalbumin and β-casein in 147 human milk samples were successfully determined by the established method and their contents were 205.5-578.2mg/100g and 116.4-467.4mg/100g at different lactation stages. The developed method allows simultaneously determination of α-lactalbumin and β-casein in human milk. The quantitative strategy based on signature peptide should be applicable to other endogenous proteins in breast milk and other body fluids.

Proteolytic Systems in Milk: Perspectives on the Evolutionary Function within the Mammary Gland and the Infant

Milk contains elements of numerous proteolytic systems (zymogens, active proteases, protease inhibitors and protease activators) produced in part from blood, in part by mammary epithelial cells and in part by immune cell secretion. Researchers have examined milk proteases for decades, as they can cause major defects in milk quality and cheese production. Most previous research has examined these proteases with the aim to eliminate or control their actions. However, our recent peptidomics research demonstrates that these milk proteases produce specific peptides in healthy milk and continue to function within the infant's gastrointestinal tract. These findings suggest that milk proteases have an evolutionary function in aiding the infant's digestion or releasing functional peptides. In other words, the mother provides the infant with not only dietary proteins but also the means to digest them. However, proteolysis in the milk is controlled by a balance of protease inhibitors and protease activators so that only a small portion of milk proteins are digested within the mammary gland. This regulation presents a question: If proteolysis is beneficial to the infant, what benefits are gained by preventing complete proteolysis through the presence of protease inhibitors? In addition to summarizing what is known about milk proteolytic systems, we explore possible evolutionary explanations for this proteolytic balance.

Predictors of High Serum Casein Antibody Levels among Malnourished Infants and Young Children with Congenital Heart Disease

BACKGROUND

Factors predictive of growth retardation and malnutrition in patients with congenital heart disease remain unclear.

OBJECTIVES

This study aimed to measure antibody response to bovine casein through assessing serum casein antibody levels in malnourished patients three year or younger with CHD, and to determine its relationship to gastrointestinal symptoms, anthropometric measures, and laboratory data.

SUBJECTS AND METHODS

This cross sectional case control study was conducted in sixty patients with CHD aged 4 to 72 months. They were subdivided into thirty patients with cyanotic and thirty patients with acyanotic CHD compared with thirty apparently healthy children.

RESULTS

On comparison with controls, patients showed highly significant lower anthropometric measures, calcium, iron, hemoglobin levels, and higher serum levels of casein antibody, total iron binding capacity, and alkaline phoshatase activity (P<0.000). Serum levels of casein antibody showed significantly positive correlations with serum total iron binding capacity and alkaline phosphatase activities and negatively correlated with the age at onset of symptoms, anthropometric measures, serum calcium, and iron levels.

CONCLUSION

Serum casein antibody levels play a significant role in the pathogenesis of malnutrition. Encouragement of breast feeding and avoidance of early cow's milk consumption could prevent the development of antibody response to bovine 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.

Extracellular phosphorylation and phosphorylated proteins: not just curiosities but physiologically important

Mining of the literature and high-throughput mass spectrometry data from both healthy and diseased tissues and from body fluids reveals evidence that various extracellular proteins can exist in phosphorylated states. Extracellular kinases and phosphatases (ectokinases and ectophosphatases) are active in extracellular spaces during times of sufficiently high concentrations of adenosine triphosphate. There is evidence for a role of extracellular phosphorylation in various physiological functions, including blood coagulation, immune cell activation, and the formation of neuronal networks. Ectokinase activity is increased in some diseases, including cancer, Alzheimer's disease, and some microbial infections. We summarize the literature supporting the physiological and pathological roles of extracellularly localized protein kinases, protein phosphatases, and phosphorylated proteins and provide an analysis of the available mass spectrometry data to annotate potential extracellular phosphorylated proteins.

Longitudinal analysis of protein glycosylation and β-casein phosphorylation in term and preterm human milk during the first 2 months of lactation

Human milk proteins provide term and preterm infants with both nutrition and protection. The objective of the present study was to examine longitudinal changes in the protein composition of term and preterm milk during the first 2 months of lactation, focusing on protein phosphorylation and glycosylation. Using gel electrophoresis, the relative concentration and glycosylation status of lactoferrin, secretory Ig A, β-casein, α-lactalbumin, serum albumin, bile salt-stimulated lipase, xanthine oxidoreductase, tenascin and macrophage mannose receptor 1 were measured in milk collected on days 7, 10, 14, 18, 21, 28 and 60 postpartum from preterm mothers (28-32 weeks gestation, n 17). The phosphorylation status of β-casein was also investigated. To determine if these variables differ in term and preterm milk, samples from term mothers (38-41 weeks gestation, n 8) collected on days 7, 14 and 30 of lactation were also analysed. The concentration of the abundant milk proteins decreased during lactation in term and preterm milk (P <0·05). No difference in protein glycosylation was observed, except for the glycoproteins serum albumin and tenascin. The phosphorylation of β-casein varied significantly between term and preterm milk. Further investigation is required to determine whether these modifications affect protein function and are clinically important to preterm infants.

Proteomic characterization of specific minor proteins in the human milk casein fraction

Human milk contains many bioactive proteins that are likely to support the early development of the newborn. The aim of this study was to identify whether there are specific minor proteins associated with the human milk casein micelle prepared by the acid precipitation method. Protein identification was performed by liquid chromatography tandem mass spectrometry analysis. Eighty-two proteins were identified in the casein micelle, 18 of which are not present in their whey compartment. Thirty-two of these proteins specifically associated with the casein micelle have not previously been identified in human milk or colostrum. Proteins involved in immune function comprised the major part (28%) of total proteins, and another significant part is involved in metabolism/energy production (22%). Most of the proteins were of extracellular or cytoplasmic origin (accounting for 50 and 29%, respectively). This study indicates that various soluble proteins should be considered as part of the casein compartment, prepared by the acid precipitation method. The data provide new insight not only into the proteomic profile of the human milk casein micelle and its physiological significance, but also into the proper proportion of casein and casein-associated proteins to use in infant formula.

Label-free liquid chromatography-tandem mass spectrometry analysis with automated phosphopeptide enrichment reveals dynamic human milk protein phosphorylation during lactation

Protein phosphorylation is a critical posttranslational modification that affects cell-cell signaling and protein function. However, quantifying the relative site-specific changes of phosphorylation occupancies remains a major issue. An online enrichment of phosphopeptides using titanium dioxide incorporated in a microchip liquid chromatography device was used to analyze trypsin-digested human milk proteins with mass spectrometry. The method was validated with standards and used to determine the dynamic behavior of protein phosphorylation in human milk from the first month of lactation. α-Casein, β-casein, osteopontin, and chordin-like protein 2 phosphoproteins were shown to vary during this lactation time in an independent manner. In addition, changes in specific regions of these phosphoproteins were found to vary independently. Novel phosphorylation sites were discovered for chordin-like protein 2, α-lactalbumin, β-1,4-galactosyl transferase, and poly-Ig (immunoglobulin) receptor. Coefficients of variation for the quantitation were comparable to those in other contemporary approaches using isotopically labeled peptides, with a median value of 11% for all phosphopeptide occupancies quantified.

Identification of phosphorylation sites of equine beta-casein isoforms

Equine beta-casein is phosphorylated at variable degrees and isoforms carrying 3 to 7 phosphate groups (3P-7P) have been found in milk, but the phosphorylated amino acid residues of each isoform are not yet identified. In the present work, the different phosphorylation variants were first isolated by ion-exchange chromatography and then hydrolysed by trypsin to generate caseinophosphopeptides (CPPs), each containing all the potential phosphorylation sites. The equine CPPs were prepared by metal oxide affinity chromatography, a method based on the affinity of phosphate groups towards titanium dioxide immobilized onto a micro-column. This method turned out to be an efficient tool to separate the CPPs Arg(1)-Lys(34) and Glu(4)-Lys(34) from non-phosphorylated peptides. Purification was achieved by reversed-phase high-performance liquid chromatography (RP-HPLC) and each CPP was hydrolyzed by endoproteinase Glu-C. Finally, the digests were analyzed by RP-HPLC/electrospray ionization mass spectrometry (RP-HPLC/ESI-MS) and identified by nano-electrospray ionization tandem mass spectrometry (nESI-MS/MS) to locate the phosphorylated sites of the beta-casein isoforms 4P-7P with accuracy. Thus, the isoform 4P was found to be phosphorylated on residues Ser(9), Ser(23), Ser(24), and Ser(25). Addition of phosphate groups on Ser(18), Thr(12), and Ser(10) led to the formation of the isoforms 5P-7P, respectively. The results indicated that the in vivo phosphorylation of the equine beta-casein follows a sequential way and is not randomly performed.

Chaperone-like activities of different molecular forms of beta-casein. Importance of polarity of N-terminal hydrophilic domain

As a member of intrinsically unstructured protein family, beta-casein (beta-CN) contains relatively high amount of prolyl residues, adopts noncompact and flexible structure and exhibits chaperone-like activity in vitro. Like many chaperones, native beta-CN does not contain cysteinyl residues and exhibits strong tendencies for self-association. The chaperone-like activities of three recombinant beta-CNs wild type (WT) beta-CN, C4 beta-CN (with cysteinyl residue in position 4) and C208 beta-CN (with cysteinyl residue in position 208), expressed and purified from E. coli, which, consequently, lack the phosphorylated residues, were examined and compared with that of native beta-CN using insulin and alcohol dehydrogenase as target/substrate proteins. The dimers (beta-CND) of C4-beta-CN and C208 beta-CN were also studied and their chaperone-like activities were compared with those of their monomeric forms. Lacking phosphorylation, WT beta-CN, C208 beta-CN, C4 beta-CN and C4 beta-CND exhibited significantly lower chaperone-like activities than native beta-CN. Dimerization of C208 beta-CN with two distal hydrophilic domains considerably improved its chaperone-like activity in comparison with its monomeric form. The obtained results demonstrate the significant role played by the polar contributions of phosphorylated residues and N-terminal hydrophilic domain as important functional elements in enhancing the chaperone-like activity of native beta-CN. (c) 2009 Wiley Periodicals, Inc. Biopolymers 91: 623-632, 2009.This article was originally published online as an accepted preprint. The "Published Online" date corresponds to the preprint version. You can request a copy of the preprint by emailing the Biopolymers editorial office at biopolymers@wiley.com.

Stimulation of Leishmania tropica protein kinase CK2 activities by platelet-activating factor (PAF)

Leishmania tropica is one of the causative agents of cutaneous leishmaniasis. Platelet-activating factor (PAF) is a phospholipid mediator in diverse biological and pathophysiological processes. Here we show that PAF promoted a three-fold increase on ecto-protein kinase and a three-fold increase on the secreted kinase activity of L. tropica live promastigotes. When casein was added to the reaction medium, along with PAF, there was a four-fold increase on the ecto-kinase activity. When live L. tropica promastigotes were pre-incubated for 30 min in the presence of PAF-plus casein, a six-fold increase on the secreted kinase activity was observed. Also, a protein released from L. tropica promastigotes reacted with polyclonal antibodies for the mammalian CK2 alpha catalytic subunit. Furthermore, in vitro mouse macrophage infection by L. tropica was doubled when promastigotes were pre-treated for 2 h with PAF. Similar results were obtained when the interaction was performed in the presence of purified CK2 or casein. TBB and DRB, CK2 inhibitors, reversed PAF enhancement of macrophage infection by L. tropica. WEB 2086, a competitive PAF antagonist, reversed all PAF effects here described. This study shows for the first time that PAF promotes the activation of two isoforms of CK2, secreted and membrane-bound, correlating these activities to infection of mouse macrophages.

An E. coli over-expression system for multiply-phosphorylated proteins and its use in a study of calcium phosphate sequestration by novel recombinant phosphopeptides

Phosphoproteins and phosphopeptides were expressed by E. coli to give yields of 30-200mg of purified protein per litre with an average degree of phosphorylation at multiple sites of 61-83%. The method employed two compatible cohabiting plasmids having low and high copy number, expressing a protein kinase and, more abundantly, the substrate (poly)peptide, respectively. It was used to phosphorylate recombinant beta-casein or osteopontin at multiple casein kinase-2 sites. Two constructs were designed to produce shorter peptides containing one or more clusters of phosphorylation sites resembling the phosphate centres of caseins. In the first, a 53-residue 6-His tagged phosphopeptide was expressed at a 5-fold higher molar yield. The second had multiple tandem repeats of a tryptic phosphopeptide sequence to give a similar increase in efficiency. Each recombinant phosphopeptide was purified (30-100mg) and small-angle X-ray scattering measurements showed that they, like certain casein and osteopontin phosphopeptides, sequester amorphous calcium phosphate to form calcium phosphate nanoclusters. In principle, the method can provide novel phosphopeptides for the control of biocalcification or be adapted for use with other kinases and cognate proteins or peptides to study the effect of specific phosphorylations on protein structure. Moreover, the insertion of a phosphate centre sequence, possibly with a linker peptide, may allow thermodynamically stable, biocompatible nanoparticles to be made from virtually any sequence.

Serum activity of dipeptidyl peptidase IV (DPPIV; EC 3.4.14.5) in breast-fed infants with symptoms of allergy

Beta-casomorphins, opioid peptides present in mother's milk, are a good substrate for DPPIV (EC 3.4.14.5) which is a major factor limiting the half-life of biologically active peptides. Serum DPPIV activity of two groups of infants (healthy and atopic dermatitis) and contents of beta-casomorphin-5 and -7 in their mothers' milk were determined in the study. We have found correlation between those two parameters in the group of children with atopic dermatitis syndromes, while no such a correlation was found in the control group.

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.

Cadmium-induced disturbances in lactating mammary glands of mice

Previous studies in various species have demonstrated that cadmium levels in milk are low and that cadmium is retained in the mammary glands of lactating rodents. The objective of the present study was to examine mammary glands following cadmium exposure during peak lactation. Mice were given 5 microg, 100 microg or 2000 microg cadmium/kg body weight subcutaneously on lactation days 8-10 and mammary glands were dissected on lactation day 11 for histology, quantitative gene expression and mineral analyses. Cadmium exposure induced morphological changes in the lactating mammary gland. A remodelling of the lactating mammary tissue including an increase in fat content, a less active feature of the mammary alveolar epithelial cells and more condensed appearance of the milk alveoli were observed. Although these changes were most prominent in the animals exposed to the highest cadmium dose similar morphological alterations were indicated at the two lower doses. A significant negative dose-response relationship between beta-casein gene expression and exposure of cadmium was demonstrated, while mRNA levels of alpha-lactalbumin were not affected by cadmium treatment. Furthermore, reduced levels of calcium in the mammary glands of the dams exposed to the highest cadmium dose and a positive correlation between calcium and beta-casein were demonstrated. No other differences were detected among the cadmium dose groups in mammary levels of calcium, zinc, iron or copper. In conclusion, our results indicate that cadmium disturbs the function of the lactating mammary gland, which consequently may impair the development of the suckling offspring.

Milk biologically active components as nutraceuticals: review

Milk contains components that provide critical nutritive elements, immunological protection, and biologically active substances to both neonates and adults. Milk proteins are currently the main source of a range of biologically active peptides. Concentrates of these peptides are potential health-enhancing nutraceuticals for food and pharmaceutical applications. Several bioactive peptides may be used as nutraceuticals, for example, in the treatment of diarrhea, hypertension, thrombosis, dental diseases, as well as mineral malabsorption, and immunodeficiency. Minor whey proteins, such as lactoferrin, lactoperoxidase, lysozyme, and immunoglobulins, are considered antimicrobial proteins. Milk also contains some natural bioactive substances. These include oligosaccharides, fucosylated oligosaccharides, hormones, growth factors, mucin, gangliosides, and endogenous peptides, which are present in milk at secretion. Most of the claimed physiological properties of milk bioactive components have been carried out in vitro or in animal model systems, and these hypothesized properties remain to be proven in humans. Whether these milk bioactive components will replace drugs entirely in the immediate future is still unclear, but the increasing appreciation of "drug foods" or nutraceuticals plays a complementary rather than a substitutional role to the synthetic pharmacological drugs.

Casein-hydrolyzing activity of sIgA antibodies from human milk

During pregnancy and immediately after delivery (i.e. at the beginning of lactation), the female organism is frequently characterized by an immune status similar to that of patients with autoimmune diseases. In addition, lactation is associated with an appearance of catalytically active antibodies or abzymes (Abzs) with DNAse, RNase, ATPase, amylolitic, protein kinase and lipid kinase activities in breast milk. However, until now there were no examples of human milk Abzs with a proteolytic activity. We present the first evidence that electrophoretically and immunologically homogeneous human milk sIgAs possess a beta-casein-hydrolyzing activity different from known proteases. Abzs specifically hydrolyze both human and bovine beta-caseins but not many other proteins tested. Using different methods including in situ analysis of proteolytic activity in a gel after SDS-PAGE it was shown that the observed proteolytic activity is an intrinsic property of human milk polyclonal sIgAs. Specific inhibitors of acidic and thiol proteases demonstrated a weak effect on proteolytic activity of Abzs, while a specific inhibitor of serine proteases (AEBSF) significantly inhibited the proteolytic activity of the abzymes. The K(M) value for human casein as a substrate was estimated (7.3 microM). Our findings suggest that the immune system of clinically healthy mothers can generate IgAs with a beta-casein-specific serine protease-like activity.

The effect of conserved residue charge reversal on the folding of recombinant non-phosphorylated human β-casein

A short stretch of 13 amino acids in the central portion of human beta-casein contains four positively charged conserved residues, three Lys and one Arg. We changed these individually to Glu, reversing their charge, and compared the resulting recombinant proteins to the wild-type recombinant, monitoring thermal aggregation with turbidity as well as using the fluorescence of the intrinsic Trp, of hydrophobically bound ANS and fluorescence resonance energy transfer from Trp to ANS to detect differences in structure. The results demonstrate the need to maintain the actual or functional identity of these conserved charged amino acid residues in order to attain the protein folding and functional properties of the wild-type human beta-casein molecule. They emphasize the probability that native human beta-casein has a unique folding pattern that is important for its function of suspending minerals and delivering the protein and minerals to the neonate in a readily ingestible form.

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.

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.

Suspension of the calcium-sensitive human beta-caseins by human kappa-casein

The beta-casein (CN) fraction of human milk exists as a single protein entity phosphorylated at various levels from zero to five (beta-CN-0P to beta-CN-5P). Since the beta-CN fraction is precipitated by the calcium ions in milk, a stabilizing protein is needed to form a suspension of casein micelles for ready ingestion by the infant. That stabilization is known to be carried out by kappa-CN but it is also thought possible that the 0P and 1P beta-CN moieties may play a role. To examine the effects of different phosphorylation levels, 10 mM CaCl2 was added to each of the purified proteins phosphorylated (P) at different levels. Without kappa-CN, precipitation of the different beta-CN forms varied from 78 to 99%. Human kappa-CN was then added to each to give kappa/beta molar ratios varying from 0.01 to 0.25. Some stabilization was observed even at the lowest ratio and more than 90% of the protein was suspended in all cases at the highest ratio. Interaction of low levels of kappa-CN with the different forms of beta-CN to create a suspension was somewhat dependent on the phosphorylation level and the possibility of forming intra- or intermolecular Ca2+ bridges or cross-links. Similar ratios of the beta-CN-4P to either the 0P or 1P form and of the 2P to the 1P form showed that neither the 0P nor 1P form had any stabilizing ability. In fact, coprecipitation occurred so that with either the 4P or 2P forms present, higher percentages of the 0P and 1P forms precipitated.

Kinetic and crystallographic analysis of complexes formed between elastase and peptides from β-casein

Human beta-casomorphin-7 (NH2-Tyr-Pro-Phe-Val-Glu-Pro-Ile-CO2H) is a naturally occurring peptide inhibitor of elastase that has been shown to form an acyl-enzyme complex stable enough for X-ray crystallographic analysis at pH 5. To investigate the importance of the N-terminal residues of the beta-casomorphin-7 peptide for the inhibition of elastase, kinetic and crystallographic analyses were undertaken to identify the minimum number of residues required for effective formation of a stable complex between truncated beta-casomorphin-7 peptides and porcine pancreatic elastase (PPE). The results clearly demonstrate that significant inhibition of PPE can be effected by simple tri-, tetra-and pentapeptides terminating in a carboxylic acid. These results also suggest that in vivo regulation of protease activity could be mediated via short peptides as well as by proteins. Crystallographic analysis of the complex formed between N-acetyl-Val-Glu-Pro-Ile-CO2H and PPE at pH 5 (to 1.67 A resolution) revealed an active site water molecule in an analogous position to that observed in the PPE/beta-casomorphin-7 structure supportive of its assignment as the 'hydrolytic water' in the deacylation step of serine protease catalysis.

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.