Advances in separation and identification of biologically important milk proteins and peptides

Milk is a rich source of biologically important proteins and peptides. In addition, milk contains a variety of extracellular vesicles (EVs), including exosomes, that carry their own proteome cargo. EVs are essential for cell-cell communication and modulation of biological processes. They act as nature carriers of bioactive proteins/peptides in targeted delivery during various physiological and pathological conditions. Identification of the proteins and protein-derived peptides in milk and EVs and recognition of their biological activities and functions had a tremendous impact on food industry, medicine research, and clinical applications. Advanced separation methods, mass spectrometry (MS)-based proteomic approaches and innovative biostatistical procedures allowed for characterization of milk protein isoforms, genetic/splice variants, posttranslational modifications and their key roles, and contributed to novel discoveries. This review article discusses recently published developments in separation and identification of bioactive proteins/peptides from milk and milk EVs, including MS-based proteomic approaches.

Effect of Ultrafiltered Milk on the Rheological and Microstructure Properties of Cream Cheese Acid Gels

The addition of ultrafiltered (UF) milk retentate is known to impact milk properties during mozzarella and cheddar cheese production, but the effect during cream cheese production is less well understood. Little is known about the impact of UF retentate on the intermediate stages of manufacture, such as protein assembly and the formation of hydrated acid gel structures. Here, milk prepared for cream cheese manufacture using a concentration factor of 2.5 or 5 had a similar particle size distribution to unconcentrated cheese milk after homogenization but increased viscosity and a slower rate of acidification, which could be altered by increasing starter culture concentration. The acid gels formed contained more protein and fat, resulting in a higher storage modulus, firmness, and viscosity. A denser microstructure was observed in acid gels formed with UF retentate addition, and quantitative two- or three-dimensional analysis of confocal images found a greater volume percentage of protein and fat, decreased porosity, and increased coalescence of fat. The mobility of water, as assessed by proton nuclear magnetic resonance, was reduced in the dense UF gel networks. Water movement was partially obstructed, although diffusion was possible between interconnected serum pores. These insights improve our understanding of acid gel formation. They can be used by manufacturers to further optimize the early and intermediate stages of cream cheese production when using concentrated milk to reduce acid whey production and lay the foundation for larger pilot scale studies of intermediate and final cream cheese structure.

Characterization of Sample Loss Caused by Competitive Adsorption of Proteins in Vials Using Sodium Dodecyl Sulfate-Polyacrylamide Gel Electrophoresis

Sample loss caused by competitive protein adsorption on solid surfaces from complex samples remains to be a major hurdle in sensitive analyses of proteins. No label-free techniques can easily quantify individual proteins adsorbed on irregular surfaces of Eppendorf vials or Falcon tubes, which are commonly used to contain complex biological samples. Multiplexed characterization of such adsorption by different proteins is technically challenging. Herein, we developed a direct protein analysis based on sodium dodecyl sulfate-polyacrylamide gel electrophoresis for the characterization of sample loss occurred on the curved surface with limited area. Using this simple and easily accessible method, we discovered the effect of ethylenediaminetetraacetic acid on surface adsorption of different milk proteins, specifically an augmented loss of milk proteins in low-binding sample vials. In this study, we also identified severe biases of silver staining and established proteomics-based mapping of protein distribution in biological samples for absolute quantification of competitive protein adsorption on irregular surfaces.

Quantitation of human milk proteins and their glycoforms using multiple reaction monitoring (MRM)

Human milk plays a substantial role in the child growth, development and determines their nutritional and health status. Despite the importance of the proteins and glycoproteins in human milk, very little quantitative information especially on their site-specific glycosylation is known. As more functions of milk proteins and other components continue to emerge, their fine-detailed quantitative information is becoming a key factor in milk research efforts. The present work utilizes a sensitive label-free MRM method to quantify seven milk proteins (α-lactalbumin, lactoferrin, secretory immunoglobulin A, immunoglobulin G, immunoglobulin M, α1-antitrypsin, and lysozyme) using their unique peptides while at the same time, quantifying their site-specific N-glycosylation relative to the protein abundance. The method is highly reproducible, has low limit of quantitation, and accounts for differences in glycosylation due to variations in protein amounts. The method described here expands our knowledge about human milk proteins and provides vital details that could be used in monitoring the health of the infant and even the mother. Graphical Abstract The glycopeptides EICs generated from QQQ.

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.

Peptidomic profile of milk of Holstein cows at peak lactation

Bovine milk is known to contain naturally occurring peptides, but relatively few of their sequences have been determined. Human milk contains hundreds of endogenous peptides, and the ensemble has been documented for antimicrobial actions. Naturally occurring peptides from bovine milk were sequenced and compared with human milk peptides. Bovine milk samples from six cows in second-stage peak lactation at 78-121 days postpartum revealed 159 peptides. Most peptides (73%) were found in all six cows sampled, demonstrating the similarity of the intramammary peptide degradation across these cows. One peptide sequence, ALPIIQKLEPQIA from bovine perilipin 2, was identical to another found in human milk. Most peptides derived from β-casein, αs1-casein, and αs2-casein. No peptides derived from abundant bovine milk proteins such as lactoferrin, β-lactoglobulin, and secretory immunoglobulin A. The enzymatic cleavage analysis revealed that milk proteins were degraded by plasmin, cathepsins B and D, and elastase in all samples.

Multiple reaction monitoring-based determination of bovine α-lactalbumin in infant formulas and whey protein concentrates by ultra-high performance liquid chromatography-tandem mass spectrometry using tryptic signature peptides and synthetic peptide standards

The determination of α-lactalbumin in various dairy products attracts wide attention in multidiscipline fields because of its nutritional and biological functions. In the present study, we quantified the bovine α-lactalbumin in various infant formulas and whey protein concentrates using ultra-high performance liquid chromatography coupled to tandem mass spectrometer in multiple reaction monitoring mode. Bovine α-lactalbumin was quantified by employing the synthetic internal standard based on the molar equivalent relationship among the internal standard, bovine α-lactalbumin and their signature peptides. This study especially focused on the recovery rates of the sample preparation procedure and robust quantification of total bovine α-lactalbumin in its native and thermally denatured form with a synthetic internal standard KILDKVGINNYWLAHKALCSE. The observed recovery rates of bovine α-lactalbumin ranged from 95.8 to 100.6% and the reproducibility was excellent (RSD<6%) at different spiking levels. The limit of quantitation is 10 mg/100 g for infant formulas and whey protein concentrates. In order to validate the applicability of the method, 21 brands of infant formulas were analyzed. The acquired contents of bovine α-lactalbumin were 0.67-1.84 g/100g in these infant formulas in agreement with their label claimed values. The experiment of heat treatment time showed that the loss of native α-lactalbumin enhanced with an increasing intensity of heat treatment. Comparing with Ren's previous method by analysis of only native bovine α-lactalbumin, the present method at the peptide level proved to be highly suitable for measuring bovine α-lactalbumin in infant formulas and whey protein concentrates, avoiding forgoing the thermally induced denatured α-lactalbumin caused by the technological processing.

Protein composition and variation of caprine colostrum (Murciano-Granadina breed) by means of polyacrylamide-sds gel electrophoresis

The variation in the content of total protein, total casein, whey protein, α-casein, β-casein, k-casein, α-lactalbumin, β-lactoglobulin and ‘other’ whey proteins of the mammary secretion was studied in 44 goats of the Murciano-Granadina breed (Spain) from the 1st to the 4th day after parturition.

The concentration of all the protein variables showed a marked decrease from the 1st to the 4th day after parturition (P < 0·001). In whey proteins this decrease was more marked from the 1st (67·1 g/1) to the 2nd day after parturition (21·6 g/1), whereas the total caseins sharply decreased from the 2nd (62·6 g/1) to the 3rd day after parturition (39·0 g/1).

Stepwise discriminant analysis revealed that, except for the k-casein, all the proteins resulting from the electrophoretic fractionation had a discriminant power between the dates of sampling (P < 0·001). The multivariant analysis did not show statistical differences in the electrophoretic fractions corresponding to the samples on the 3rd and 4th days after parturition. According to such fractionation, the transition from colostrum to milk should take place on the 2nd day after parturition.

Variation in the N-acetyl neuraminic acid content of bovine κ-casein

A total of 555 milk samples was collected from 53 Holstein cows for 1 year. Samples were analysed for N-acetyl neuraminic acid (NANA) content, κ-casein content and somatic cell count. The extent of glycosylation of κ-casein was estimated through the NANA content of κ-casein in each milk sample. The overall mean value for the NANA content was 63·6 ± 20 μg/mg of κ-casein with a range of 23–166 μg/mg. Least squares analysis showed significant (P < 0·01) effects of parity, test day, stage of lactation, phenotype for κ-casein, somatic cell count, and cow within phenotype. The NANA content of κ-casein decreased with increasing parity number and increased significantly when somatic cell count was above 200000 cells/ml. After reaching a minimum value between 2 and 3 months after calving, the NANA content of κ-casein increased during the later part of the lactation. NANA content was higher in the milk of cows phenotyped AB for κ-casein than for cows phenotyped for κ-casein AA, thus suggesting that B variant is more efficiently glycosylated than A variant.

Validation of a new reversed-phase high-performance liquid chromatography method for separation and quantification of bovine milk protein genetic variants

A new RP-HPLC method for the separation and quantification of the most common genetic variants of bovine milk proteins is described. A reversed-phase analytical column C8 (Zorbax 300SB-C8 RP, 3.5 microm, 300A, 150 x 4.6 I.D.) was used. All the most common casein (CN) and whey protein genetic variants, including beta-CN(I) were detected and separated simultaneously in less then 40 min, with the exception of alpha(S1)-CN(B) and CN(C) variants. Purified protein genetic variants were employed in calibration and showed different absorbances at 214 nm. The procedure was developed using 40 raw individual milk samples of cows belonging to four different breeds and certified skim milk powder BCR-063R. Method validation consisted in testing linearity, repeatability, reproducibility and accuracy. A linear relationship (R(2)>0.99) between the concentrations of proteins and peak areas was observed over the concentration range, with low detection limits. Repeatability and reproducibility were satisfactory for both retention times and peak areas. The RSD of peak areas ranged from 0.92 to 4.32% within analytical day and from 0.85 to 9.52% across analytical days. The recoveries, calculated using mixtures of samples previously quantified, ranged from 98.1 to 103.7%.

The primary structure of a low-Mr multiphosphorylated variant ofβ-casein in equine milk

Highly phosphorylated casein with a low molecular mass was isolated from Haflinger mare's milk by RP-HPLC. It accounts for 4.0% of the casein content. Its mass was determined by LC-ESI-MS before and after treatment by alkaline phosphatase. The molecular mass found for the apo-form (10,591 +/- 2 Da) is in agreement with its primary structure, which was established by ESI-MS/MS from tryptic peptides. It appeared that this short protein (94 amino acid residues) is an internally truncated form of the full-length equine beta-casein (226 residues). This low-Mr variant of equine beta-casein displays a large deletion (residues 50-181), due to a cryptic splice site usage occurring within exon 7 during the course of primary transcripts processing. The phosphorylation pattern of this equine beta-casein variant was investigated by LC-ESI-MS and 2-DE. Seven phosphorylation forms were identified with one to seven phosphate groups with pIs ranging between 4.67 and 4.01. The major isoforms carry five and six phosphate groups.

Determination of the phosphorylation level and deamidation susceptibility of equine β-casein

beta-Casein was isolated from Haflinger mare's milk by RP-HPLC, and displayed microheterogeneity by urea-electrophoresis and 2-DE probably due to a variable degree of phosphorylation. To investigate the degree of phosphorylation, the primary structure of equine beta-casein was determined by tryptic hydrolysis and MS of peptides released and by MS of the protein treated by alkaline phosphatase. The molecular mass found for the apo-form of Haflinger mare's beta-casein (25 514 +/- 3 Da) was close to the theoretical mass of the reported sequence (GenBank AAG43954) modified by insertion of a region (residues 27-34) encoded by an exon sometimes out-spliced (25 511.40 Da). Hence, the beta-casein isolated from Haflinger mare's milk corresponded to a variant of 226 amino acid residues. The latter was composed by highly multi-phosphorylated isoforms with three to seven phosphate groups, and pIs, determined by 2-DE, ranging from 4.74 to 5.30. Moreover, the equine beta-casein was able to deamidate spontaneously, at the level of Asn in the potential deamidation motif (135)Asn-Gly(136). Approximately 80% of the protein was deamidated after 96 h of incubation under physiological conditions.

Capillary electrochromatographic separation of bovine milk proteins using a G-quartet DNA stationary phase

DNA oligonucleotides that form G-quartet structures were used as stationary phase reagents for separation of bovine milk proteins, including alpha-casein, beta-casein, kappa-casein, alpha-lactalbumin and beta-lactoglobulin. Both artificial protein mixtures and a skim milk sample were analyzed. The separations were performed using open-tubular capillary electrochromatography, in which the oligonucleotides were covalently attached to the inner surface of a fused-silica capillary. Better resolution was achieved using the G-quartet-coated capillaries than was achieved using either a bare capillary or a capillary coated with an oligonucleotide that does not form a G-quartet structure. A 4-plane G-quartet-forming stationary phase was able to resolve three peaks for alpha-casein and to detect thermal denaturation of the proteins in the milk sample. The results suggest that G-quartet stationary phases could be used to separate very similar protein structures, such as those arising from genetic variations or post-translational modifications.

Separation and characterization of mares' milk alpha(s1)-, beta-, kappa-caseins, gamma-casein-like, and proteose peptone component 5-like peptides

The equine alpha(s1)- and beta-caseins (CN) were purified by chromatography on DEAE-cellulose and by reversed-phase HPLC. The alpha(s1)-, beta-, and kappa-CN were characterized either by monodimensional urea-PAGE or sodium dodecylsulfate (SDS)-PAGE or by bidimensional electrophoresis. Kappa-casein was characterized after electrophoresis by glycoprotein-specific staining. To identify alpha(s1)-CN without ambiguity, internal sequences were determined after trypsin or chymosin digestion of purified alpha(s1)-CN. These sequences, that could be estimated to correspond to 62% of the full protein, presented strong identities with regions of alpha(s1)-CN primary structures of other species. In particular, 51, 48, 43, and 40% identities were obtained with corresponding regions of sow, dromedary, cow, and human alpha(s1)-CN, respectively. On the other hand, trace amounts of equine gamma-CN-like and proteose peptone component 5-like peptides were found in the whole CN. They were identified by microsequencing and corresponded to beta-CN peptides generated by plasmin action on the whole CN. The equine alpha(s1), beta-, and kappa-CN were separated by bidimensional electrophoresis in numerous isoelectric variants with apparent isoelectric points distributed between pH 4.4 to 6.3, 4.4 to 5.9, and 3.5 to 5.5, respectively. The beta- and kappa-CN displayed a more acidic character in the mare than in the cow.

Milk composition and apparent digestibilities of dietary fatty acids in lactating dairy cows abomasally infused with Cis or Trans fatty acids

Fat supplementation of diets for dairy cows produces changes in nutrient supply and milk composition. The effect of abomasal infusion of either cis-C18:1 or trans-C18:1 fatty acid isomers on the digestibility of fatty acids and milk composition was determined in lactating dairy cows. Six multiparous midlactation Holstein cows were used and fed a control diet containing 50% forage and 50% concentrate. Treatments were (per day): no infusion, infusion of a 630-g fat mixture high in cis-C18:1 isomers, and infusion of a 623-g fat mixture high in trans-C18:1 isomers using two 3 x 3 Latin squares with 4-wk experimental periods. Fat infusion did not affect total dry matter intake and increased apparent digestibilities of total fatty acids. Apparent digestibilities of C18 fatty acids were directly related to the number of double bonds within isomers, and cis-C18:1 isomers were slightly more digestible than trans-C18:1 isomers. The lower yield of C12:0, C14:0, and C16:0 fatty acids in milk fat and higher milk citrate observed when cows were infused with trans-C18:1 suggests a depressed de novo milk fatty acid synthesis. Effects of trans infusion on milk fat were independent of ruminal fermentation, fatty acid apparent absorption, and fatty acid plasma concentrations. Lower milk protein yield in cows infused with fat may have been caused by a decrease in milk protein synthesis.

Simultaneous separation and quantitation of the major bovine whey proteins including proteose peptone and caseinomacropeptide by reversed-phase high-performance liquid chromatography on polystyrene-divinylbenzene

A precise, sensitive and reliable RP-HPLC method was developed to enable not only unequivocal determination of alpha-lactalbumin and beta-lactoglobulin in bovine whey samples, but also simultaneous measurement of proteose peptone, caseinomacropeptide, bovine serum albumin and immunoglobulin G. The optimised method on the Resource RPC column allowed separation of the proteins in 30 min and could be applied to the analysis of soluble proteins in a variety of commercial and laboratory whey products. Furthermore, some qualitative information on protein heterogeneity and quality could be derived from the RP-HPLC analyses with additional data available from on-line electrospray mass spectrometry. Within- and between-day repeatability over a wide range of concentrations was excellent (RSD< or =5%) for all proteins except immunoglobulin G and bovine serum albumin where RSD was 7-10%. Analysis of grouped data from whey protein concentrate and whey protein isolate samples gave a limit of detection of < or =0.3% powder mass and a limit of quantitation of < or =1.0% powder mass for all proteins except immunoglobulin G. Limits of detection and quantitation were 0.6% and 2.0%, respectively, for this protein. Quantitative data obtained by the RP-HPLC method compared very favourably with data obtained by alternative methods of whey protein analysis.

Isolation, purification, and alteration of some functional groups of major milk proteins: a review

This review covers selected methods of isolation and purification of mainly alpha s-casein, beta-casein, kappa-casein, beta-lactoglobulin, and alpha-lactalbumin. Selected methods of alteration of some functional groups of these proteins also were reviewed. Isolation and purification of milk proteins per se are methods of modifying the individual milk proteins. Gram quantities of these proteins can now be purified in a relatively short time using ion-exchange resins. Due to the prominent use of non-food-grade reagents in the procedures for preparation of these milk proteins, individual proteins are not maximally utilized for the manufacture of food/feed and pharmaceutical products. Therefore, intensive research efforts are needed to obviate the problems associated with underutilization of milk proteins.

Role of the O-phosphoserine clusters in the interaction of the bovine milk alpha s1-, beta-, kappa-caseins and the PP3 component with immobilized iron (III) ions

alpha s1- and beta-Caseins have a sequence cluster -Ser(P)-Ser(P)-Ser(P)-Glu-Glu- which is not present in kappa-casein and the whey PP3 component. The affinity of these phosphoproteins for the iron(III)-iminodiacetic acid (IDA) complex immobilized on Sepharose was studied as a function of pH, urea concentration, calcium ion concentration, enzymatic dephosphorylation and temperature. The affinity of the three polyphosphorylated proteins (alpha s1- and beta-caseins, PP3) was similar. The sequence cluster was not a specific recognition pattern of the iron(III) ion. These three proteins presented a site of high affinity and a site of weak affinity. kappa-Casein, which had only one Ser(P) residue, presented only the site of weak affinity. Their primary site which was absent after dephosphorylation or calcium ion addition required the presence of at least two Ser(P) residues close in space. Their secondary site was sensitive to the presence of urea. It was sensitive to pH variation for PP3 and kappa-casein. The study of the affinity of a few free amino acids towards iron(III)-IDA showed that the secondary site involved tryptophan and tyrosine residues for alpha s1- and beta-caseins, histidine residues for PP3 and cysteine residues for kappa-casein.

Purification of beta-lactoglobulin from whey protein concentrate by pepsin treatment

beta-Lactoglobulin was purified from whey protein concentrate by a combination of pepsin treatment and membrane filtration. Porcine pepsin was added to whey protein (1:200, wt/wt), and the mixture was then incubated at pH 2.0 and 37 degrees C for 60 min. The protein fraction was collected by ammonium sulfate precipitation, and the precipitate was either dialyzed against water using a dialysis membrane (20-kDa pore size) or filtered using an UF membrane (30-kDa pore size). The beta-LG did not differ from standard beta-LG as measured by chromatography, SDS-PAGE, native PAGE, differential scanning calorimetry, or UV spectrum. Based on the results, a simplified procedure was developed, consisting of pepsin treatment and UF, to purify beta-LG directly from whey.

The use of caprylic acid for the extraction of the immunoglobulin fraction from egg yolk of chickens immunised with ovine alpha-lactalbumin

The extraction and purification of serum-derived immunoglobulin fraction in the egg yolk of hens by the combined treatment of the raw egg yolk with caprylic (octanoic) acid and ammonium sulphate is described. This simple two-step method proved to be both rapid, reproducible and suitable for batch processing of pooled egg yolk. The method recovered in excess of 130 mg of immunoglobulin per egg yolk. Two chickens were inoculated at two weekly intervals with 100 micrograms each of ovine alpha-lactalbumin over a ten week period. The alpha-lactalbumin antigen was purified by a hydrophobic-interaction chromatographic procedure and further purified by a gel excision-elution process. No precipitating antibodies could be demonstrated in gel diffusion techniques with this antibody. The specificity and specific activity of the antibody were monitored by western blotting and demonstrated the presence of highly specific antibodies to ovine alpha-lactalbumin in the treated egg yolk. The extraction procedure had no adverse effects on antibody titre. We concluded, and confirmed previous reports, that the use of chickens for the production of highly specific antibodies to mammalian proteins with particular reference to milk proteins presented numerous advantages over conventional procedures.

Supplemental dietary fat and ruminally protected amino acids for lactating Jersey cows

Eight Jersey cows receiving a 50:50 ratio of forage to concentrate on a DM basis were used in a replicated 4 x 4 Latin square design to determine the effects of added fat (3.4% of dietary DM) and ruminally protected AA (8 g of Met and 24 g of Lys daily) on yield and composition of milk. Treatments were 1) basal control, 2) added fat, 3) added AA, and 4) fat plus AA. Compared with no added fat, fat supplementation increased 4% FCM yield (24.7 vs. 23.0 kg/d) and milk fat yield (1.05 vs. .97 kg), depressed milk protein content (3.58 vs. 3.74%), and altered fatty acid composition of milk. Blood triglyceride and NEFA were elevated (34.4 vs. 29.5 mg/dl and 175.1 vs. 143.7 microeq/L, respectively) by added fat. Supplementation with AA elevated blood Lys, Met, and urea N without increasing milk protein yield. Increase in blood NEFA was further augmented by fat plus AA supplementation, but no changes in concentrations of Lys or Met in blood were found. Addition of AA did not alleviate the depression of milk protein content when supplemental fat was added to the diet for Jersey cows.

Effect of somatotropin administration and duodenal infusion of methionine and lysine on lactational performance and nutrient flow to the small intestine

Lack of sufficient methionine and lysine delivered post-ruminally may limit milk production response to bovine somatotropin (bST). To test this hypothesis, four Holstein cows fitted with rumen and duodenal cannulas were used in a 4 x 4 Latin square design with 14 d periods. Treatments were: (1) control, (2) continuous duodenal infusion of 8 g methionine and 24 g lysine/d, (3) injection of 25 mg bST/d and (4) infusion of methionine and lysine plus injection of bST. Infusion of amino acids led to trends for small increases in milk (3%), fat (5.5%), and protein (3.7%) yield. Larger and significant increases (8.7, 14 and 6.9% for milk, fat and protein yield respectively) were achieved with bST administration which also increased milk fat content. Plasma levels of urea-N and essential amino acids were reduced with bST. Duodenal nutrient flow was generally unaffected by treatment. The production response to bST was not enhanced in cows producing an average of 34 kg milk when provided additional methionine and lysine post-ruminally in this short-term study.

Chromatographic and electrophoretic methods used for analysis of milk proteins

Current knowledge of milk proteins and their behavior in dairy foods is based on early applications of chromatography and electrophoresis. Electrophoretic identification of the number and genetic variety of milk proteins inaugurated a research effort in which chromatographic techniques were successfully applied to the isolation of each milk protein, thus facilitating the characterization and further study of milk and dairy products. This review focuses on recent applications of chromatography for separations and analysis and on analytical applications of electrophoresis.

Enzyme-linked immunosorbent assays for bovine alpha-lactalbumin and beta-lactoglobulin in serum and tissue culture media

Enzyme-linked immunosorbent assays for bovine alpha-lactalbumin and beta-lactoglobulin have been developed for measurements of serum and tissue culture samples. Either alpha-lactalbumin or beta-lactoglobulin antiserum was coated on ELISA plates. Biotinylated proteins were used in competition with unknown amount of proteins in samples. After unbound proteins were washed off, ExtrAvidin-peroxidase and tetramethylbenzidine were then used as a detection system. Crossreactivity of caseins or bovine serum albumin was less than .0001% in either alpha-lactalbumin or beta-lactoglobulin ELISA. Parallel curves from serial dilutions were obtained in serum and media samples. The additivity of alpha-lactalbumin and beta-lactoglobulin ELISA was validated in either serum or medium samples. The intraassay and interassay coefficients of variation for alpha-lactalbumin and beta-lactoglobulin ELISA were below 10% over 51 and 47 assays. The ELISA are useful in mammary gland biology studies for measuring milk whey protein in serum or culture media.

The identification of the kappa-casein genotype in Holstein dairy cattle using the polymerase chain reaction

The polymerase chain reaction (PCR) was used to amplify a 99-bp region from the kappa-casein gene of Holstein dairy cattle which contains nucleotide substitutions that are diagnostic of the two major protein variants of kappa-casein. Identity of the amplified product was confirmed by direct sequencing. Digestion of the PCR product with MboII (A-variant specific) or TaqI (B-variant specific) allowed direct determination of the genotype of the animal (homozygous or heterozygous). A total of 58 lactating cows with known kappa-casein phenotype were tested using PCR. In all cases, the measured genotype confirmed the phenotype. We have also tested the genotype of 42 sires that were top ranked for milk yield by the CIAQ (Centre d'insemination artificielle du Quebec). The B-allele of kappa-casein which occurred at a frequency of 0.13 among the proven bulls is associated with superior milk for industrial applications. Identification of the kappa-casein genotype by PCR in bulls and calves would provide a means for rapidly changing the frequency of the B-allele in the breeding population by selection.

Application of polyacrylamide gel electrophoresis/neutron activation analysis for protein quantification

A combination of two methods, polyacrylamide gel electrophoresis (PAGE) and neutron activation analysis (NAA), has been applied to solutions containing phosphoproteins for the purpose of protein quantification. The proteins were separated by molecular weight using PAGE, and then the whole gel was activated by neutron bombardment. Densitometric measurements of the developed bands from 32P, taken from autoradiographs of the activated gels, resulted in quantification of the phosphorus, and then the related protein. This PAGE/NAA method was applied to several phosphoprotein-containing materials, including commercial milk products and reference materials, i.e., IAEA A-11, milk powder, and SRM 1845, Cholesterol in Egg Powder.