Microanalysis of the amino-acid sequence of monomeric beta-lactoglobulin I from donkey (Equus asinus) milk. The primary structure and its homology with a superfamily of hydrophobic molecule transporters

A novel allelic donkey β-lactogobulin I protein isoform generated by a non-AUG translation initiation codon is associated with a nonsynonymous SNP

β-Lactoglobulin I (β-LG I) is one of the most important whey proteins in donkey milk. However, to our knowledge, there has been no study focusing on the full nucleotide sequences of this gene (BLG I). Current investigation of donkey BLG I gene is very limited with only 2 variants (A and B) characterized so far at the protein level. Recently, a new β-LG I variant, with a significantly higher mass (+1,915 Da) than known variants has been detected. In this study, we report the whole nucleotide sequence of the BLG I gene from 2 donkeys, whose milk samples are characterized by the β-LG I SDS-PAGE band with a normal electrophoretic mobility (18,514.25 Da, β-LG I B1 form) the first, and by the presence of a unique β-LG I band with a higher electrophoretic mobility (20,428.5 Da, β-LG I D form) the latter. A high genetic variability was found all over the 2 sequenced BLG I alleles. In particular, 16 polymorphic sites were found in introns, one in the 5' flanking region, 3 SNPs in the 5' untranslated region and one SNP in the coding region (g.458G > A) located at the 40th nucleotide of exon 2 and responsible for the AA substitutions p.Asp28 > Asn in the mature protein. Two SNPs (g.920-922CAC > TGT and g.1871G/A) were genotyped in 93 donkeys of 2 Italian breeds (60 Ragusana and 33 Amiatina, respectively) and the overall frequencies of g.920-922CAC and g.1871A were 0.3065 and 0.043, respectively. Only the rare allele g.1871A was observed to be associated with the slower migrating β-LG I. Considering this genetic diversity and those found in the database, it was possible to deduce at least 5 different alleles (BLG I A, B, B1, C, D) responsible for 4 potential β-LG I translations. Among these alleles, B1 and D are those characterized in the present research, with the D allele of real novel identification. Haplotype data analysis suggests an evolutionary pathway of donkey BLG I gene and a possible phylogenetic map is proposed. Analyses of mRNA secondary structure showed relevant changes in the structures, as consequence of the g.1871G > A polymorphism, that might be responsible for the recognition of an alternative initiation site providing an additional signal peptide. The extension of 19 AA sequence to the mature protein, corresponding to the canonical signal peptide with an additional alanine residue, is sufficient to provide the observed molecular weight of the slower migrating β-LG I encoded by the BLG I D allele.

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.

Major whey proteins in donkey's milk: effect of season and lactation stage. Implications for potential dietary interventions in human diseases

According to current literature, donkey's milk has been suggested as a hypoallergenic substitute in children affected by cow's milk protein allergy as well as a promising nutraceutical for aged people. However, the biologically active components of donkey's milk have not yet completely elucidated. In this framework this study is aimed at measuring α-lactalbumin (α-LA), β-lactoglobulin (β-LG), and lysozyme (LYS), the principal whey proteins in donkey's milk, in relation to lactation stage and production season. Analysis were performed by reversed-phase high-performance liquid chromatography. α-LA, β-LG, and LYS resulted to be affected by lactation stage (P < 0.01) and production season (P < 0.01). Overall, the protein content was higher (0.01 > P < 0.05) during the first four lactation's months and decreased until the month 8. The β-LG was the major protein (1.75 mg mL(-1) as mean; peak 2.24 ± 0.09 mg mL(-1)), while the α-LA had a mean concentration of 1.32 mg mL(-1) and peaked at month 1 (1.57 ± 0.09 mg mL(-1)) and LYS (0.66 mg mL(-1) as mean) showed the highest value equal to 0.76 ± 0.03 mg mL(-1). The highest (P < 0.01) concentration of all proteins was recorded at spring (α-LA: 1.69 mL(-1); β-LG: 2.07 mL(-1); LYS: 0.76 mL(-1)).

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.

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%).

Isolation and rapid sequence characterization of two novel bovine beta-lactoglobulins I and J

Two novel bovine beta-lactoglobulins I and J have been isolated from bovine milk and characterized by isoelectric focusing. Their primary structure was determined by a very rapid method consisting of a combination of Edman sequencing, mass analysis, and ladder sequencing by mass spectrometry. We found that both new beta-lactoglobulins are of the bovine beta-lactoglobulin B-variant type. beta-lactoglobulin I shows Gly instead of Glu at position 108, whereas beta-lactoglobulin J shows a Pro-to-Leu exchange at position 126.

Amino acid sequence of the beta-subunit of phycoerythrin from the cryptophyte algae Chroomonas CS 24

The full amino acid sequence of the beta-subunit of Chroomonas CS24 phycoerythrin has been determined by conventional Edman degradation and mass spectrometry. The sequence compromises 177 amino acids with a molecular mass of 18669 Da. It is 91.5% identical to the deduced amino acid sequence of Cryptomonas phi beta-phycoerythrin (Reith, M. and Douglas, S. (1990) Plant Mol. Biology 15, 585-592). The chromophores are bound by single thioether linkages. No evidence of microheterogeneity was found confirming that both beta-subunits of the holoprotein are identical.

The complete amino acid sequence of feline beta-lactoglobulin II and a partial revision of the equine beta-lactoglobulin II sequence

The amino acid sequence of feline beta-lactoglobulin (designated II) has been determined. The protein chain is 163 amino acids long with a relative molecular mass of 18,558. The primary structure was determined by sequencing of native protein (residues 1-25), BPNS-skatole cleavage fragments and the peptides obtained by proteolytic cleavage with V8 proteinase and TPCK-trypsin. Feline beta-lactoglobulin II has 53 and 57% positional identities with bovine beta-lactoglobulin A and equine beta-lactoglobulin I, respectively, and approx. 68% with a revised sequence of equine beta-lactoglobulin II. The equine beta-lactoglobulin II sequence was re-examined between positions 78 and 122 resulting in a major revision in this area with only a single insertion to give a total of 163 residues.

Covalent structure of the minor monomeric beta-lactoglobulin II component from donkey milk

The complete primary structure of the minor beta-lactoglobulin II component from donkey milk is presented. It has been established by amino-acid sequencing and mass-spectrometry analysis of intact protein and peptides obtained after enzymatic and chemical cleavages. The molecular mass and the pI of the protein are calculated to be 18,261 Da and 4.5 respectively. Despite the close structural similarity of the donkey and horse major beta-lactoglobulin I components, their minor beta-lactoglobulin II components show substantial differences in sequence. Most observed exchanges are clustered at residues 78-106 where only 6 amino-acid residues are conserved. The primary structure of donkey beta-lactoglobulin II reveals some unusual features of minor beta-lactoglobulins II and gives new light to the evolution of beta-lactoglobulins and other lipocalins involved in retinol binding or reproductive functions.

A rapid microbore HPLC method for determination of primary structure of beta-lactoglobulin genetic variants

A rapid method for determination of the primary structures for beta-lactoglobulin (beta-LG) genetic variants is described. This included rapid microbore HPLC, amino acid analyses, and wherever necessary, direct peptide sequencing. Two novel variants of beta-LG have been identified, bovine beta-LG W and ovine beta-LG C. The proteins were oxidized, digested with trypsin and separated using RP-HPLC. All peptides were recovered in a single run. Peptides with amino acid exchanges were identified by retention time and subjected to amino acid and sequence analyses. Ovine beta-LG C differs from the ovine beta-LG A variant by a single amino acid exchange at position 148 where Arg is replaced by Gln. Bovine beta-LG W differs from bovine beta-LG B by having Leu at position 56 instead of Ile. The method described here is reliable and can be used for mapping of 20-1000 pmol of material.

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

The complete primary structure of donkey lysozyme has been established by pulsed liquid-phase sequencing of tryptic and chymotryptic peptides isolated by RP-HPLC. The positions of the Cys residues were identified by labeling the Cys residues with DABIA-reagent. Donkey lysozyme is a c-type lysozyme which is 129 amino acids long. It exhibits 50% homology to the human protein. We observe the full Ca(II) binding site suggested for the homologous alpha-lactalbumines. Although horse lysozyme has been reported to contain asparagine in position 61, which was in conflict with the three-dimensional structure of lysozyme, all other known c-type lysozymes, including donkey, contain Ser 61.

Bovine beta-lactoglobulin H: isolation by preparative isoelectric focusing in immobilized pH gradients and preliminary characterization

In spite of the fact that beta-lactoglobulin (beta-lg) was first discovered in bovine milk more than fifty years ago, and that it represents the main whey protein component in all the milks in which it has been found, its biological role and genetic evolution still remain rather uncertain. From comparative studies of the primary and tertiary structures of beta-lg and of other proteins of a similar size, the existence of a new superfamily of proteins with the function of transporter of hydrophobic molecules has been conjectured. The elucidation of the structure of beta-lg either from different species or from different genetic variants of the same species should give useful information on the evolution and function of this protein family. With this aim in mind we have now undertaken the isolation and characterization of a recently discovered, new genetic variant of bovine beta-lg. A two-step purification procedure involving preparative HPLC gel filtration and preparative IEF-IPG has been successfully carried out; it affords a good recovery of the new beta-lg in highly purified form.