The primary structure of donkey (Equus asinus) lysozyme contains the Ca(II) binding site of alpha-lactalbumin

Distribution of calcium, phosphorus, sulfur, magnesium, potassium, and sodium in major fractions of donkey milk

The aim of this study was to evaluate the concentrations of Ca, P, S, Mg, K, and Na, and their distribution in major fractions of donkey milk (i.e., fat, casein, whey proteins, and aqueous phase). Individual milk samples were collected by mechanical milking from 16 clinically healthy lactating donkeys. Milk yield per milking was recorded and milk gross composition, casein content, and pH were determined. Whole milk samples were centrifuged to separate fat and to obtain skim milk. Skim milk samples were ultracentrifuged to separate a sedimentable casein pellet and to obtain a supernatant whey (soluble) fraction, which was then ultrafiltered to obtain the aqueous phase of donkey milk. Whole milk and the processed samples were analyzed for the aforementioned elements by inductively coupled plasma-mass spectrometry. The concentration of elements associated with fat, casein, and whey proteins was then calculated. All the Na was present in the aqueous phase. The fat fraction in donkey milk carried very little or none of the investigated elements. The majority of Ca (62.9%) and P (53.1%) was associated with casein, and the rest of these elements was mostly present in the aqueous phase. The majority of Mg was present in the aqueous phase, but a relevant part (32.6%) was associated with the casein fraction. No K was associated with casein. On a molar basis, the ratio of colloidal Ca and P to casein (mmol/g of casein) was more than double the values reported in literature for cow milk. The correlation coefficient was negative between milk pH and P in the ultracentrifuged (r = -0.81) and ultrafiltered (aqueous) fraction (r = -0.66). Milk pH correlated positively with colloidal Ca (r = 0.59) and with the ratio of colloidal Ca to casein (mmol/g of casein; r = 0.68). Colloidal Ca and P were positively correlated (r = 0.64). These data suggest that the high ratio of colloidal Ca and P to donkey casein micelles is due to a larger amount of colloidal calcium phosphate bound to casein micelles compared with literature data on cow milk. The percentage of elements associated with whey proteins was less than 5% for Ca, P, and K, but Mg reached approximately 9% of total Mg. The majority of S (63.6%) was associated with whey proteins, and only one-fourth of this element was associated with casein, indicating a higher content of sulfur-containing amino acids in donkey whey proteins than in casein.

Proteins and bioactive peptides from donkey milk: The molecular basis for its reduced allergenic properties

The legendary therapeutics properties of donkey milk have recently been supported by many clinical trials who have clearly demonstrated that, even if with adequate lipid integration, it may represent a valid natural substitute of cow milk for feeding allergic children. During the last decade many investigations by MS-based methods have been performed in order to obtain a better knowledge of donkey milk proteins. The knowledge about the primary structure of donkey milk proteins now may provide the basis for a more accurate comprehension of its potential benefits for human nutrition. In this aspect, experimental data today available clearly demonstrate that donkey milk proteins (especially casein components) are more closely related with the human homologues rather than cow counterparts. Moreover, the low allergenic properties of donkey milk with respect to cow one seem to be related to the low total protein content, the low ratio of caseins to whey fraction, and finally to the presence in almost all bovine IgE-binding linear epitopes of multiple amino acid differences with respect to the corresponding regions of donkey milk counterparts.

A sensitive and effective proteomic approach to identify she-donkey's and goat's milk adulterations by MALDI-TOF MS fingerprinting

She-donkey’s milk (DM) and goat’s milk (GM) are commonly used in newborn and infant feeding because they are less allergenic than other milk types. It is, therefore, mandatory to avoid adulteration and contamination by other milk allergens, developing fast and efficient analytical methods to assess the authenticity of these precious nutrients. In this experimental work, a sensitive and robust matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) profiling was designed to assess the genuineness of DM and GM milks. This workflow allows the identification of DM and GM adulteration at levels of 0.5%, thus, representing a sensitive tool for milk adulteration analysis, if compared with other laborious and time-consuming analytical procedures.

MALDI-TOF mass spectrometry for the monitoring of she-donkey's milk contamination or adulteration

Donkey's milk (DM), representing a safe and alternative food in both IgE-mediated and non-IgE-mediated cow's milk protein allergy, can be categorized as precious pharma-food. Moreover, an economically relevant interest for the use of DM in cosmetology is also developing. The detection of adulterations and contaminations of DM is a matter of fundamental importance from both an economic and allergenic standpoint, and, to this aim, fast and efficient analytical approaches to assess the authenticity of this precious nutrient are desirable. Here, a rapid matrix-assisted laser desorption/ionization-time-of-flight mass spectrometry (MALDI-TOF MS)-based method aimed to the detection of bovine or caprine milk in raw DM is reported. The presence of the extraneous milks was revealed by monitoring the protein profiles of the most abundant whey proteins, α-lactalbumin (α-LA) and β-lactoglobulin, used as molecular markers. The possibility of obtaining a quantitative analysis of the level of cow or goat milk in DM based on the MALDI-TOF peak areas of α-LAs was also explored. The results showed that the experimental quantitative values were in good agreement with the real composition of each mixture. As pretreatment of the milk samples is not required, and owing to the speed and the high sensitivity of MALDI-MS, the protocol here reported could represent a reliable method for routine analyses aimed to assess the absence of contamination in raw fresh DM samples.

Characterization of the protein profile of donkey's milk whey fraction

Characterization of the protein profile of the whey fraction from a milk sample taken from an individual donkey belonging to the 'Ragusana' species of the East of Sicily is reported. Direct RP-HPLC/electrospray ionization (ESI)-MS analysis of the whey fraction allowed the detection of some unknown components, together with the identification of already known whey proteins. Matrix-assisted laser desorption/ionization (MALDI)-TOF/MS and RP-HPLC/ESI-MS/MS analysis of the enzymatic digests of the unknown components resulted the identification and characterization of (1) two beta-casein fragments; (2) the sequence of donkey's serum albumin; and (3) the oxidized methionine forms of lysozyme B and alpha-lactoalbumin. One of the two beta-casein fragments corresponds to the sequence Val(176)-Arg(189) of the horse's beta-casein. The second one corresponds the C-terminal sequence Tyr(199)-Val(226) of the horse's beta-casein, with four amino acid substitutions (Q --> R(203), L/I --> P(206), F --> L(210) and P --> A(219)). Both fragments, reasonably arising by endogenous proteases cleavage of the donkey's beta-casein, could be potential biologically active peptides. Direct mass spectrometric sequence characterization of the detected donkey's serum albumin reveals the presence of the amino acid substitution Val --> Ile at position 497 with respect to the cDNA deduced sequence. The oxidized forms of lysozyme B and alpha-lactoalbumin are selectively oxidized at methionine 79 and methionine 90, respectively.

Detection and sequence determination of a new variant beta-lactoglobulin II from donkey

The sequence determination of a new variant of beta-LG II, detected as a minor component by reversed-phase high-performance liquid chromatography/electrospray ionization mass spectrometry (RP-HPLC/ESI-MS) analysis of the whey fraction from a milk sample taken from an individual donkey belonging to the 'Ragusana' species of eastern Sicily, is reported. Direct RP-HPLC/ESI-MS analysis of the whey fraction from this milk sample allowed the identification of a new variant of beta-LG II, based on the determination of the M(r) of the intact protein. The new protein, with an experimentally determined M(r) of 18311 Da, was detected as a minor component in the whey fraction investigated. Matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF)MS and RP-HPLC/ESI-MS/MS analyses of the tryptic digest of the new protein demonstrate that it presents two amino acid substitutions with respect to the sequence of beta-LG II A, namely a substitution Pro-->Cys at position 110, and a substitution Asp-->Gly at position 162. The disulfide bonds between the four cysteines, not directly determined in donkey's and horse's beta-LG II, were shown to occur between Cys(106)-Cys(120) and Cys(66)-Cys(161), as in other mammalian beta-LGs. The new beta-LG II variant from donkey was named D.

Characterisation of proteins in the milk of fur seals

Milk protein composition was investigated throughout the lactation periods of the Australian fur seal (Arctocephalus pusillus doriferus) and Antarctic fur seal (Arctocephalus gazella). The mean protein content of the milk was found to be 10.9% and 10.6% respectively. The concentration of total protein did not change during lactation, although a decline in casein content of the milk in late lactation was apparent. Milk protein concentration during a foraging/suckling cycle of the Antarctic fur seal analysed at the time of arrival on shore, and 24 h and 72 h after arrival was 12.8%, 11.4% and 12.5% respectively. Re-feeding animals at 72 h resulted in a significant increase in milk protein content to 14.9%. Characterisation of milk protein by SDS-PAGE analysis revealed 5 casein and 10 major whey protein bands. Amino-terminal sequencing indicated that the majority of the whey fraction of the milk is beta-lactoglobulin (beta-LG). The limited amino acid sequence indicated 3 different beta-LGs were secreted in the milk. Subsequently, RT-PCR was used to extend the sequence of one of the beta-LGs and translation of the 464 bp fragment indicated that it shared 79% sequence identity with feline beta-LG II.

New genetic variants identified in donkey's milk whey proteins

Novel genetic variants for donkey milk lysozyme and beta-lactoglobulins I and II have been identified by the combined use of peptide mass mapping and sequencing by tandem mass spectrometry in association with database searching. The novel donkey lysozyme variant designated as lysozyme B (Mr 14,631 Da) differed in three amino acid exchanges, N49 --> D, Y52 --> S, and S61 --> N, from the previously published sequence. Three novel genetic variants for donkey beta-lactoglobulins were identified. One of them is a type beta-lactoglobulin I with three amino acid exchanges at E36 --> S, S97 --> T, and V150 --> I (beta-lactoglobulin I B, Mr 18,510 Da). The two others are type beta-lactoglobulins II with two amino acid exchanges at C110 --> P and M118--> T (beta-lactoglobulin II B, Mr 18,227 Da) and with three amino acid exchanges at D96 --> E, C110 --> P, and M118 -->T (beta-lactoglobulin II C, Mr 18,241 Da). All these primary structures are closely related to those of homologous proteins in horse milk (percent identity >96%).

Structural and thermodynamic responses of mutations at a Ca2+ binding site engineered into human lysozyme

Structural determinants of Ca2+ binding sites within proteins typically comprise several acidic residues in appropriate juxtaposition. Three residues (Ala-83, Gln-86, and Ala-92) in human lysozyme are characteristically mutated to Lys, Asp, and Asp, respectively, in natural Ca2+ binding lysozymes and alpha-lactalbumins. The effects of these mutations on the stability and Ca2+ binding properties of human lysozyme were investigated using calorimetry and were interpreted with crystal structures. The double mutant, in which Glu-86 and Ala-92 were replaced with Asp, clearly showed Ca2+ binding affinity, whereas neither point mutant showed Ca2+ affinity, indicating that both residues are essential. The further mutation of Ala-83 --> Lys did not affect the Ca2+ binding of the double mutant. The point mutations Ala-83 --> Lys and Glu-86 --> Asp did not affect the stability, whereas the mutation Ala-92 --> Asp was about 1.3 kcal/mol less stable. Structural analyses showed that both Asp-86 and Lys-83 were exposed to solvent. Side chains of Asp-86 and Asp-91 were rotated in opposite directions about chi1 angle, as if to reduce the electrostatic repulsion. The charged amino acids at the Ca2+ binding site did not significantly affect stability of the protein, possibly because of the local conformational change of the side chains.

Molecular divergence of lysozymes and alpha-lactalbumin

The vast number of proteins that sustain the currently living organisms have been generated from a relatively small number of ancestral genes that has involved a variety of processes. Lysozyme is an ancient protein whose origin goes back an estimated 400 to 600 million years. This protein was originally a bacteriolytic defensive agent and has been adapted to serve a digestive function on at least two occasions, separated by nearly 40 million years. The origins of the related goose type and T4 phage lysozyme that are distinct from the more common C type are obscure. They share no discernable amino acid sequence identity and yet they possess common secondary and tertiary structures. Lysozyme C gene also gave rise, after gene duplication 300 to 400 million years ago, to a gene that currently codes for alpha-lactalbumin, a protein expressed only in the lactating mammary gland of all but a few species of mammals. It is required for the synthesis of lactose, the sugar secreted in milk. alpha-Lactalbumin shares only 40% identity in amino acid sequence with lysozyme C, but it has a closer spatial structure and gene organization. Although structurally similar, functionally they are quite distinct. Specific amino acid substitutions in alpha-lactalbumin account for the loss of the enzyme activity of lysozyme and the acquisition of the features necessary for its role in lactose synthesis. Evolutionary implications are as yet unclear but are being unraveled in many laboratories.

Lysozyme and alpha-lactalbumin from the milk of a marsupial, the common brush-tailed possum (Trichosurus vulpecula)

Lysozyme and alpha-lactalbumin have been identified using N-terminal sequence analysis of whey proteins from the common brush-tailed possum, Trichosurus vulpecula after separation by two-dimensional denaturing electrophoresis. Both proteins were purified from pooled possum milk using ion exchange chromatography and gave mass values of 14,896 and 13,985 Da respectively by MALDI-TOF mass spectrometry. Clones containing the full coding sequences of the genes for both proteins were isolated from a possum mammary cDNA library and the DNA sequence of the coding region determined. The inferred protein sequences were used in phylogenetic analysis of both protein classes. These showed that the T. vulpecula alpha-lactalbumin, along with other marsupial alpha-lactalbumins, formed a family distinct from the eutherian alpha-lactalbumins and the alpha-lactalbumin of a monotreme, the platypus, consistent with the separate evolution of the marsupials. By contrast the T. vulpecula lysozyme was shown to be similar to the ruminant stomach lysozymes and primate lysozymes and quite distinct from the Ca2+-binding lysozymes found in the milk of the echidna and horse.

Thermal unfolding of beta-lactoglobulin, alpha-lactalbumin, and bovine serum albumin. A thermodynamic approach

Heat-treatment is one of the most commonly used processes in food preparation technology. An understanding of the thermodynamics of protein stability and of conformational changes of proteins, acquired through the measurement of the denaturation temperature, is therefore of particular importance. This paper attempts to shed light on the interpretation of recent calorimetric data on the thermal denaturation of bovine beta-lactoglobulin, alpha-lactalbumin, and bovine serum albumin by showing that thermodynamic parameters of heat-induced unfolding, measured by differential scanning calorimetry, are closely related to the prevailing chemical conditions such as pH, concentration of ions, protein purity, and protein concentration.

Electrophoretic polymorphism in rabbit tear lysozyme

Rabbit tears were found to contain two lysozymes which differed in their electrophoretic mobility and were designated tear lysozymes 1 and 2. Rabbit tear lysozyme 1 was purified to homogeneity by conventional purification methods. It was found to be distinct from other known mammalian c-type lysozymes, rabbit tear lysozyme 2 and the major rabbit gastrointestinal lysozyme. The activity profile is centered around the neutral region with an optimum of 7 which is slightly lower than that for chicken lysozyme. The thermal stability as well as inhibition profiles by the substrate analogues, N-acetylglucosamine (NAG) and chitotetraose (NAG)4 are comparable to those of chicken lysozyme. Based on its molecular weight and catalytic properties this isozyme is classified as a c-type lysozyme.

Models of the three-dimensional structures of echidna, horse, and pigeon lysozymes: calcium-binding lysozymes and their relationship with alpha-lactalbumins

Similarities in amino acid sequences, three-dimensional structures, and the exon-intron patterns of their genes have indicated that c-type lysozymes and alpha-lactalbumins are homologous proteins, i.e., descended by divergent evolution from a common ancestor. Like the alpha-lactalbumins, echidna milk, horse milk, and pigeon eggwhite lysozymes all bind Ca(II). Models of their three-dimensional structures, based on their amino acid sequences and the known crystal structures of domestic hen eggwhite and human lysozymes and baboon and human alpha-lactalbumins, have been built. The several structures have been compared and their relationships discussed.

cDNA and amino acid sequences of rainbow trout (Oncorhynchus mykiss) lysozymes and their implications for the evolution of lysozyme and lactalbumin

The complete 129-amino-acid sequences of two rainbow trout lysozymes (I and II) isolated from kidney were established using protein chemistry microtechniques. The two sequences differ only at position 86, I having aspartic acid and II having alanine. A cDNA clone coding for rainbow trout lysozyme was isolated from a cDNA library made from liver mRNA. Sequencing of the cloned cDNA insert, which was 1 kb in length, revealed a 432-bp open reading frame encoding an amino-terminal peptide of 15 amino acids and a mature enzyme of 129 amino acids identical in sequence to II. Forms I and II from kidney and liver were also analyzed using enzymatic amplification via PCR and direct sequencing; both organs contain mRNA encoding the two lysozymes. Evolutionary trees relating DNA sequences coding for lysozymes c and alpha-lactalbumins provide evidence that the gene duplication giving rise to conventional vertebrate lysozymes c and to lactalbumin preceded the divergence of fishes and tetrapods about 400 Myr ago. Evolutionary analysis also suggests that amino acid replacements may have accumulated more slowly on the lineage leading to fish lysozyme than on those leading to mammal and bird lysozymes.

The complete primary structure of alpha-lactalbumin isolated from pig (Sus scrofa) milk

The complete amino-acid sequence of pig alpha-lactalbumin has been determined. It was obtained by microsequencing of the native protein and the peptides derived after tryptic or cyanogen bromide cleavage. The tryptic peptides were separated by a rapid microbore HPLC method. Pig alpha-lactalbumin is 122 amino acids long and differs from the bovine homologue by 26 exchanged residues. Of the two prolines present in bovine alpha-lactalbumin, one has been deleted in the pig structure. All previously sequenced alpha-lactalbumins have shown glutamic acid at position 49, which is known to be the active site in the homologous lysozyme c structure. This residue is replaced by phenylalanine in pig alpha-lactalbumin indicating that the pig protein is the first alpha-lactalbumin with complete loss of all lysozyme functional residues.

Amino acid sequences of stomach and nonstomach lysozymes of ruminants

Complete amino acid sequences are presented for lysozymes c from camel and goat stomachs and compared to sequences of other lysozymes c. Tree analysis suggests that the rate of amino acid replacement went up as soon as lysozyme was recruited for the stomach function in early ruminants. The two lysozymes from goat stomach are the products of a gene duplication that probably took place before the divergence of cow, goat, and deer about 25 million years ago. Partial sequences of three lysozymes from goat tears indicated that (a) the goat tear family of lysozymes may have diverged from the stomach lysozyme family by an ancient duplication and (b) later duplications are probably responsible for the multiple forms of tear and milk lysozymes in ruminants.

Comparison of the binding of Ca2+ and Mn2+ to bovine alpha-lactalbumin and equine lysozyme

The enthalpy change of the binding of Ca2+ and Mn2+ to equine lysozyme was measured at 25 degrees C and pH 7.5 by batch microcalorimetry: delta H degrees Ca2+ = -76 +/- 5 kJ mol-1, delta H degrees Mn2+ = -21 +/- 10 kJ mol-1. Binding constants, log KCa2+ = 6.5 +/- 0.2 and log KMn2+ = 4.1 +/- 0.5, were calculated from the calorimetric data. Therefore, delta S degrees Ca2+ = -131 +/- 20 JK-1 mol-1 and delta S degrees Mn2+ = 8 +/- 44 JK-1 mol-1. Removal of Ca2+ induces small but significant changes in the circular dichroism spectrum, indicating the existence of a partially unfolded apo-conformation, comparable with, but different from, the apo-conformation of bovine alpha-lactalbumin.

Refined structure of baboon alpha-lactalbumin at 1.7 A resolution. Comparison with C-type lysozyme

The solution of the structure of alpha-lactalbumin from baboon milk (Papio cynocephalus) at 4.5 A resolution using the isomorphous replacement method has been reported previously. Initial refinement on the basis of these low-resolution studies was not successful because of the poor isomorphism of the best heavy-atom derivative. Because of the striking similarity between the structure of lysozyme and alpha-lactalbumin, a more cautious molecular replacement approach was tried to refine the model. Using hen egg-white lysozyme as the starting model, preliminary refinement was performed using heavily constrained least-squares minimization in reciprocal space. The model was further refined using stereochemical restraints at 1.7 A resolution to a conventional crystallographic residual of 0.22 for 1141 protein atoms. In the final model, the root-mean-square deviation from ideality for bond distances is 0.015 A, and for angle distances it is 0.027 A. The refinement was carried out using the human alpha-lactalbumin sequence and "omit maps" calculated during the course of refinement indicated eight possible sequence changes in the baboon alpha-lactalbumin X-ray sequence. During the refinement, a tightly bound calcium ion and 150 water molecules, of which four are internal, have been located. Some of the water molecules were modelled for disordered side-chains. The co-ordination around the calcium is a slightly distorted pentagonal bipyramid. The Ca-O distances vary from 2.2 A to 2.6 A, representing a tight calcium-binding loop in the structure. The calcium-binding fold only superficially resembles the "EF-hand" and presumably has no evolutionary relationship with other EF-hand structures. The overall structure of alpha-lactalbumin is very similar to that of lysozyme. All large deviations occur in the loops where all sequence deletions and insertions are found. The C terminus appears to be rather flexible in alpha-lactalbumin compared to lysozyme. The experimental evidence supports the earlier predictions for the alpha-lactalbumin structure that were based upon the assumption that alpha-lactalbumin and lysozyme have similar three-dimensional structures, with minimal deletions and insertions. A detailed comparison of the two structures shows striking features as well as throwing some light on the evolution of these two proteins from a common precursor.