Characterization of a heterogeneous camel milk whey non-casein protein

Recent Advances in Camel Milk Processing

Simple Summary

The camel milk market was limited for a long time by its almost exclusive self-consumption use in nomadic camps. Significant development has been observed for the past two or three decades, including internationally, boosted by its reputation regarding its health effects for regular consumers. Such emergence has led the stakeholders in the sector to offer diversified products corresponding to the tastes of increasingly urbanized consumers, more sensitive to “modern” products. Thus, traditionally drunk in raw or naturally fermented form, camel milk has undergone unprecedented transformations such as pasteurization, directed fermentation, cheese or yoghurt processing, and manufacture of milk powder for the export market. However, the specific characteristics of this milk (composition, physical properties) mean that the technologies applied (copied from technologies used for cow milk) must be adapted. In this review, some technological innovations are presented, enabling stakeholders of the camel milk sector to satisfy the demand of manufacturers and consumers.

Abstract

Camel milk is a newcomer to domestic markets and especially to the international milk market. This recent emergence has been accompanied by a diversification of processed products, based on the technologies developed for milk from other dairy species. However, technical innovations had to be adapted to a product with specific behavior and composition. The transformation of camel milk into pasteurized milk, fermented milk, cheese, powder, or other products was supported, under the pressure of commercial development, by technological innovations made possible by a basic and applied research set. Some of these innovations regarding one of the less studied milk sources are presented here, as well as their limitations. Technical investigations for an optimal pasteurization, development of controlled fermentation at industrial scale, control of cheese technology suitable for standardized production, and improvements in processes for the supply of a high-quality milk powder are among the challenges of research regarding camel milk.

Composition, Structure, and Digestive Dynamics of Milk From Different Species-A Review

Background: The traditional dairy-cattle-based industry is becoming increasingly diversified with milk and milk products from non-cattle dairy species. The interest in non-cattle milks has increased because there have been several anecdotal reports about the nutritional benefits of these milks and reports both of individuals tolerating and digesting some non-cattle milks better than cattle milk and of certain characteristics that non-cattle milks are thought to share in common with human milk. Thus, non-cattle milks are considered to have potential applications in infant, children, and elderly nutrition for the development of specialized products with better nutritional profiles. However, there is very little scientific information and understanding about the digestion behavior of non-cattle milks. Scope and Approach: The general properties of some non-cattle milks, in comparison with human and cattle milks, particularly focusing on their protein profile, fat composition, hypoallergenic potential, and digestibility, are reviewed. The coagulation behaviors of different milks in the stomach and their impact on the rates of protein and fat digestion are reviewed in detail. Key findings and Conclusions: Milk from different species vary in composition, structure, and physicochemical properties. This may be a key factor in their different digestion behaviors. The curds formed in the stomach during the gastric digestion of some non-cattle milks are considered to be relatively softer than those formed from cattle milk, which is thought to contribute to the degree to which non-cattle milks can be easily digested or tolerated. The rates of protein and fat delivery to the small intestine are likely to be a function of the macro- and micro-structure of the curd formed in the stomach, which in turn is affected by factors such as casein composition, fat globule and casein micelle size distribution, and protein-to-fat ratio. However, as no information on the coagulation behavior of non-cattle milks in the human stomach is available, in-depth scientific studies are needed in order to understand the impact of compositional and structural differences on the digestive dynamics of milk from different species.

NMR structural studies and mechanism of action of Lactophoricin analogs as antimicrobial peptides

Antimicrobial peptides (AMPs) are effective alternatives to conventional antibiotics. They protect the host from the constant invasion of a broad range of infectious microorganisms. AMPs have been at the forefront of the response to multidrug-resistant microbial strains and appear to be ideal drug candidates. Lactophoricin (LPcin), naturally produced from bovine milk, is a typical cationic antimicrobial peptide. Three analog peptides, including LPcin-YK5, LPcin-YK8, and LPcin-YK11, with enhanced antimicrobial activity compared to the wild-type LPcin, were designed and expressed in our laboratory. We investigated the structure and antimicrobial mechanisms of action of the three novel antimicrobial peptide analogs derived from LPcin using solution NMR and solid-state NMR spectroscopy in membrane environments. Our results revealed that the three LPcin analogs exhibited helical structures with different tilt angles on the phospholipid membrane surface. We proposed three-dimensional conformations and antibacterial mechanisms of action of the three peptide analogs in the phospholipid bilayers using two-dimensional solid-state separated local field NMR experiments.

Combining different proteomic approaches to resolve complexity of the milk protein fraction of dromedary, Bactrian camels and hybrids, from different regions of Kazakhstan

Nutritional suitability of milk is not only related to gross composition, but is also strongly affected by the microheterogeniety of the protein fraction. Hence, to go further into the evaluation of the potential suitability of non-bovine milks in human/infant nutrition it is necessary to have a detailed characterization of their protein components. Combining proven proteomic approaches (SDS-PAGE, LC-MS/MS and LC-ESI-MS) and cDNA sequencing, we provide here in depth characterization of the milk protein fraction of dromedary and Bactrian camels, and their hybrids, from different regions of Kazakhstan. A total 391 functional groups of proteins were identified from 8 camel milk samples. A detailed characterization of 50 protein molecules, relating to genetic variants and isoforms arising from post-translational modifications and alternative splicing events, belonging to nine protein families (κ-, α_s1-, α_s2-, β-; and γ-CN, WAP, α-LAC, PGRP, CSA/LPO) was achieved by LC-ESI-MS. The presence of two unknown proteins UP1 (22,939 Da) and UP2 (23,046 Da) was also reported as well as the existence of a β-CN short isoform (946 Da lighter than the full-length β-CN), arising very likely in both genetic variants (A and B) from proteolysis by plasmin. In addition, we report, for the first time to our knowledge, the occurrence of a α_s2-CN phosphorylation isoform with 12P groups within two recognition motifs, suggesting thereby the existence of two kinase systems involved in the phosphorylation of caseins in the mammary gland. Finally, we demonstrate that genetic variants, which hitherto seemed to be species- specific (e.g. β-CN A for Bactrian and β-CN B for dromedary), are in fact present both in Camel dromedarius and C. bactrianus.

Proteolytic activation of proteose peptone component 3 by release of a C-terminal peptide with antibacterial properties

The milk protein proteose peptone component 3 (PP3, also known as lactophorin) is a small phosphoglycoprotein, which is exclusively expressed in the lactating mammary gland. A 23-residue synthetic peptide (lactophoricin, Lpcin S), corresponding to the C-terminal amphipathic α-helix of PP3, has previously been shown to permeabilize membranes and display antibacterial activity. Lactophorin readily undergoes proteolytic cleavage in milk and during dairy processing, and it has been suggested that PP3-derived peptides are part of milk's endogenous defense system against bacteria. Here, we report that a 26-residue C-terminal peptide (Lpcin P) can be generated by trypsin proteolysis of PP3 and that structural and functional studies of Lpcin P indicate that the peptide has antibacterial properties. The Lpcin P showed α-helical structure in both anionic and organic solvents, and the amount of α-helical structure was increased in the presence of lipid vesicles. Oriented circular dichroism showed that Lpcin P oriented parallel to the membrane surface. However, the peptide permeabilized calcein-containing vesicles efficiently. Lpcin P displayed antibacterial activity against Streptococcus thermophilus, but not against Staphylococcus aureus and Escherichia coli. The PP3 full-length protein did not display the same properties, which could indicate that PP3 functions as a precursor protein that upon proteolysis, releases a bioactive antibacterial peptide.

PP3 forms stable tetrameric structures through hydrophobic interactions via the C-terminal amphipathic helix and undergoes reversible thermal dissociation and denaturation

The milk protein proteose peptone component 3 (PP3), also called lactophorin, is a small phosphoglycoprotein that is expressed exclusively in lactating mammary tissue. The C-terminal part of the protein contains an amphipathic helix, which, upon proteolytic liberation, shows antibacterial activity. Previous studies indicate that PP3 forms multimeric structures and inhibits lipolysis in milk. PP3 is the principal component of the proteose peptone fraction of milk. This fraction is obtained by heating and acidifying skimmed milk, and in the dairy industry milk products are also typically exposed to treatments such as pasteurization, which potentially could result in irreversible denaturation and inactivation of bioactive components. We show here, by the use of CD, that PP3 undergoes reversible thermal denaturation and that the α-helical structure of PP3 remains stable even at gastric pH levels. This suggests that the secondary structure survives treatment during the purification and possibly some of the industrial processing of milk. Finally, asymmetric flow field-flow fractionation and multi-angle light scattering reveal that PP3 forms a rather stable tetrameric complex, which dissociates and unfolds in guanidinium chloride. The cooperative unfolding of PP3 was completely removed by the surfactant n-dodecyl-β-d-maltoside and by oleic acid. We interpret this to mean that the PP3 monomers associate through hydrophobic interactions via the hydrophobic surface of the amphipathic helix. These observations suggest that PP3 tetramers act as reservoirs of PP3 molecules, which in the monomeric state may stabilize the milk fat globule.

Cloning, expression, isotope labeling, purification, and characterization of bovine antimicrobial peptide, lactophoricin in Escherichia coli

Lactophoricin (LPcin-I) is a 23-amino acid peptide that corresponds to the carboxyterminal 113-135 region of component-3 of proteose peptone (PP3), a minor phosphoglycoprotein found in bovine milk. It has been reported that lactophoricin has antibacterial activity and a cationic amphipathic helical structure, but its shorter analogous peptide (LPcin-II), a 17-amino acid peptide, corresponding to the 119-135 region of PP3 does not display antibacterial activity. LPcin-I and LPcin-II have similar charge ratios and identical hydrophobic/hydrophilic sectors, according to their helical wheel projection patterns, and both peptides show cationic amphipathic helical folding and interact with membranes. However, it is known that only LPcin-I incorporates into planar lipidic bilayers to form voltage-dependent channels. In this study, the authors cloned and expressed the two recombinant peptides as ketosteroid isomerase (KSI) fusion proteins inclusion bodies in Escherichia coli. These peptides were subjected to NMR structural studies to explore their structure-activity relationships. Fusion proteins were purified by Ni-NTA affinity chromatography under denaturing conditions, and recombinant LPcin-I and LPcin-II were released from fusion by CNBr cleavage. Final purifications of LPcin-I and LPcin-II were achieved by preparative reversed-phase high performance liquid chromatography. Using these methods, we obtained several tens of milligrams of uniformly and selectively (15)N labeled peptides per liter of growth, which was sufficient for solid-state NMR spectroscopy. Peptides were identified by tris-tricine polyacrylamide gel electrophoresis and HSQC spectra. Initial structural data were obtained by solution NMR spectroscopy and compared in membrane-like environments.

Antibacterial Activity of Lactophoricin, a Synthetic 23-Residues Peptide Derived from the Sequence of Bovine Milk Component-3 of Proteose Peptone

A synthetic peptide of 23 residues corresponding to the carboxyterminal 113 to 135 region of component-3 of proteose peptone (PP3) has been investigated with regard to its antibacterial properties. This cationic amphipathic peptide that we refer to as lactophoricin, displayed a growth-inhibitory activity against both gram-positive and gram-negative bacteria. For most of the strains tested, bacterial growth was observed in the presence of lactophoricin except for Streptococcus thermophilus. In that case, lactophoricin exhibited a minimum inhibitory concentration of 10 microM and a minimum lethal concentration of 20 microM. No hemolysis of human red blood cells was detected for peptide concentrations between 2 to 200 microM, indicating that lactophoricin would be noncytotoxic when used in this concentration range.

Evidence for membrane affinity of the C-terminal domain of bovine milk PP3 component

Component PP3 is a phosphoglycoprotein isolated from bovine milk with unknown biological function, which displays in its C-terminal region a basic amphipathic alpha-helix, a feature often involved in membrane association. According to that, the behaviour of PP3 and of a synthetic peptide from the C-terminal domain (residues 113-135) was investigated in lipid environment. Conductance measurements indicated that the peptide was able to associate and form channels in planar lipid bilayers composed of neutral or charged phospholipids. Electrostatic interactions seemed to promote voltage-dependent channel formation but this was not absolutely required since the pore-forming ability of the 113-135 C-terminal peptide was also detected with the zwitterionic lipid bilayer. Additionally, a spectroscopic study using circular dichroism argues that the peptide adopts an alpha-helical conformation in interaction with neutral or charged micelles. Thus, the conducting aggregates in bilayers might be composed of a bundle of peptides in helical conformation. Besides, similar conductance measurements performed with the whole PP3 protein did not induce any channel fluctuations. However, with the latter, an early breakdown of the bilayers occurred, a finding that can be tentatively explained by a massive incorporation of PP3. In the light of the present results, it could be inferred that PP3 membrane attachment may be achieved by oligomerization of the C-terminal amphipathic helical region.

Camel (camelus dromedarius) milk PP3: evidence for an insertion in the amino-terminal sequence of the camel milk whey protein

The camel (camelus dromedarius) milk proteose peptone 3 (PP3) was purified successively by size exclusion fast protein liquid chromatography and reversed phase high performance liquid chromatography and then characterized by amino acid residue composition determination and chemical microsequencing after CNBr or trypsin cleavages. In comparison with the previously reported structure of camel milk whey protein, the camel PP3 contains an insertion in the N-terminal region which has approximately 24 residues, whereas the remaining C-terminal regions of these two homologous proteins are essentially identical. The camel PP3 seems to contain a potential O-glycosylation site localized in this insertion and 2 or 3 phosphorylated serine residues. PP3 belongs to the glycosylation-dependent cell adhesion molecule 1 (GlyCAM-1) family and could therefore play an immunological role in the camel or its suckling young.

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

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

Alternative splicing of lactophorin mRNA from lactating mammary gland of the camel (Camelus dromedarius)

The objective of this study was to determine the corrected structure of lactophorin, a major whey protein in camel milk. The protein had 60.4% amino acid sequence identity to a proteose peptone component 3 protein from bovine whey and 30.3% identity to the glycosylation-dependent cell adhesion molecule 1 in mice. The N-terminal heterogeneity of the protein was a result of alternative mRNA splicing. About 75% of the protein was expressed as a long variant A with 137 amino acid residues and a molecular mass of 15.7 kDa; about 25% was as a short variant B with 122 amino acid residues and a molecular mass of 13.8 kDa. Both proteins are probably threefold phosphorylated. In contrast to the related proteins, no glycosylation was found in camel lactophorin. Because of this difference, specific interaction with carbohydrate binding proteins, as reported for the murine protein, can be excluded, and a function of the protein other than cell recognition or rotaviral inhibition is proposed. The concentration of lactophorin in camel milk was found to be about three times higher than the concentration of the bovine homologue in bovine milk. Pronounced similarities existed between the primary and secondary structures of bovine and camel proteins. We speculated that camel lactophorin has a similar function to that of bovine protein in milk, which is supposed to be the prevention of fat globule aggregation and the inhibition of spontaneous lipolysis by lipoprotein lipase.

Conformational studies of a synthetic peptide from the putative lipid-binding domain of bovine milk component PP3

In bovine milk, a glycosylated phosphoprotein, component PP3, is known for its remarkable emulsifying properties and its capability to inhibit lipolytic activities. The determination of its primary structure is not sufficient to explain these properties. Secondary structure predictions of component PP3 and of its homologous proteins were achieved using a combination of multiple predictive methods. Based on this study, the f 119-135 region of component PP3 was proposed to be likely to adopt an amphipathic helical conformation, which is a lipid-binding motif. The conformation of the synthetic peptide corresponding to the C-terminal f 119-135 part of bovine component PP3 was analyzed by circular dichroism experiments using various media. The circular dichroism data indicated that the peptide was able to form an amphipathic alpha-helix structure in trifluoroethanol as well as in the presence of sodium dodecyl sulfate or acidic and neutral lipids, but not in water. Moreover, the conformation of this peptide is solvent dependent because it was found to adopt a beta-sheet structure for low concentrations of sodium dodecyl sulfate or a low molar ratio of acidic lipid to peptide. Tensiometric measurements showed that the amphipathic C-terminal region of component PP3 is highly tensioactive and, thus, must be responsible for the particular behavior of the protein in emulsions.

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

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

Characterization of a GlyCAM1-like gene (glycosylation-dependent cell adhesion molecule 1) which is highly and specifically expressed in the lactating bovine mammary gland

A bovine cDNA library, derived from the mammary gland of a lactating cow, was screened for identifying transcripts that specifically occur during lactation by means of differential hybridisation. Several of the clones isolated by this procedure shared 55 and 57% similarity with the mouse and rat glycosylation-dependent cell adhesion molecule 1 (GlyCAM1) cDNAs, respectively. Although the mouse and cattle proteins showed an overall similarity of only 41%, two specific regions of the proteins showed 83 and 81% similarity, respectively. The bovine protein also showed 55% similarity with a small protein isolated from the whey fraction of camel milk. Northern blot analysis showed that high-level expression of this gene was only observed in the mammary gland of lactating cows. The complete gene was isolated from a bovine genomic library and its organisation was determined. The gene was 2.5 kb in length and split into four exons. The size and organization of the gene as well as the position of the introns was identical to that of the mouse GlyCAM1 gene. In accordance with the tissue-specific expression of this gene in the mammary gland of lactating animals, potential mammary gland factor (MGF) binding sites were present in the promoter region of the gene. Based on the data presented in this study it is highly likely that this gene is the bovine homologue to the rat and mouse GlyCAM1 genes.

Characterization of a bovine mammary gland PP3 cDNA reveals homology with mouse and rat adhesion molecule GlyCAM-1

A full length PP3 (Proteose-Peptone component 3) cDNA of 679 bp was isolated from a bovine mammary gland cDNA library. The cDNA encodes a signal peptide of 18 amino acids followed by the mature PP3 sequence of 135 amino acids. This polypeptide showed homology with mouse and rat GlyCAM-1 (Glycosylation dependent Cell Adhesion Molecule 1) a protein which has been shown to act as a ligand for lymphocytes. The similarity was most profound between the signal peptides and three short regions of the mature polypeptides. Additionally structural conservation was predicted by computer analysis in the shape of a C-terminal amphipathic helix. PP3 was found to be expressed in mammary gland but not in peripheral lymph nodes, Peyer's pathes, lung, spleen, heart, and muscle.

The complete primary structure of late lactation protein from quokka (Setonix brachyurus)

The complete primary structure of the late lactation protein from the milk of quokka (Setonix brachyurus) is presented. The amino acid sequence was established by N-terminal sequence analysis of high-performance liquid chromatography purified intact protein and peptides isolated from chemical and enzymatic digests of the protein. The protein contains 158 residues including four cysteines. The sequence comparison with the tamar wallaby (Macropus eugenii) late lactation protein shows only five differences. The protein is identified as a new member of a novel late lactation protein family present in the milk of marsupials.

Phosphorylation, glycosylation and amino acid sequence of component PP3 from the proteose peptone fraction of bovine milk

Component PP3 is a phosphorylated glycoprotein with an apparent molecular mass of 28 kDa isolated from the proteose peptone fraction of bovine milk. The function of the protein is not known. The primary structure has been determined and shown to contain 135 amino acid residues (EMBL accession no. P80195). It was phosphorylated at Ser29, Ser34, Ser38, Ser40 and Ser46. Two O-linked carbohydrate groups were found at Thr16 and Thr86, while one N-linked carbohydrate group was present at Asn77. Thr16 was only approximately 50% glycosylated. The amino sugar detected by the amino acid analyser at Thr86 was mainly galactosamine but a small amount of glucosamine was also present. The amino sugars found in the carbohydrate group linked to Asn77 were both glucosamine and galactosamine. A fragment of PP3 has been isolated from milk and shown to correspond to residues 54-135. This fragment was probably generated by plasmin hydrolysing the Arg53-Ser54 bond.

Purification and characterization of three proteins isolated from the proteose peptone fraction of bovine milk

Three major proteins from the proteose peptone of bovine milk were purified by Sephadex G-75 gel chromatography, Q-Sepharose ion-exchange and additional Sephadex G-75 gel chromatography in the presence of urea. From their mobility in a gradient SDS-PAGE, the proteins were found to have molecular masses of 17, 28 and 60 kDa. The N-terminal amino acid sequence of the 17 kDa protein was found to be homologous with a camel whey protein. This protein has not previously been described in bovine milk. From the SDS-PAGE results, the 28 kDa protein was judged to be the major protein of proteose peptone, contributing approximately 25% of the total. The N-terminal amino acid sequence showed no homology to any known protein sequence, but the amino acid composition indicated that the 28 kDa protein is identical with the PP3 component from the proteose peptone fraction of bovine milk, or part of it. The 60 kDa protein was found to be bovine osteopontin, a very highly phosphorylated protein with an Arg-Gly-Asp sequence which mediates cell attachment.

Direct identification and characterization of llama (Lama glama L.) whey proteins by microsequencing after western blotting

Amino acid sequence determination is the most reliable and powerful tool to identify a protein or to classify a new one by comparison of its primary structure with already known sequences. A rapid and simple purification procedure is an essential pre-requisite for routine sequence determination. Structural characterization of llama whey proteins was undertaken for evolutionary as well as economic purposes. N-terminal sequence analyses directly on an immobilon polyvinylidene difluoride (PVDF) membrane, following Western blotting of both native and SDS-denatured llama whey proteins after polyacrylamide gel electrophoresis, revealed three different forms of glycosylated alpha-lactalbumin, and a protein with a high degree of homology with a camel whey protein of unknown function. Furthermore, by immunoblotting techniques, the electrophoretic band corresponding to serum albumin was identified.