Electrofocusing and two-dimensional electrophoresis of bovine caseins

Measurement of casein in milk by Kjeldahl and sodium dodecyl sulfate-polyacrylamide gel electrophoresis

Our objectives were to determine if milk casein as a percentage of true protein (CN%TP) estimated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) is equivalent to CN%TP estimated by Kjeldahl, and to determine the proportion of casein (CN), casein proteolysis products (CNPP), and serum protein (SP) from milk true protein (TP) that goes into the Kjeldahl noncasein nitrogen (NCN) filtrate and the proportion that stays in the NCN precipitate using SDS-PAGE. Raw milk samples were collected from 16 mid-lactation Holstein cows twice a week for 2 wk. These milks were analyzed for Kjeldahl total nitrogen, nonprotein nitrogen, and NCN content in duplicate, and by SDS-PAGE. The CN%TP determined by Kjeldahl was compared with the CN%TP estimated by SDS-PAGE calculated in 2 ways: as a percentage of only intact caseins divided by TP and as a percentage of both intact caseins and CNPP divided by TP. Three milks varying in fat, lactose, TP, CN, and SP content were formulated. These milks were analyzed in duplicate for Kjeldahl total nitrogen, nonprotein nitrogen, and NCN content, and each of the NCN filtrate and NCN precipitate were analyzed in duplicate by SDS-PAGE for relative quantity (%) of CN, CNPP, and SP. We found that the estimate of CN%TP by Kjeldahl was higher than the estimate of CN%TP by SDS-PAGE that was calculated as only intact CN divided by the total of all protein bands. However, no difference was detected in the estimate of CN%TP by Kjeldahl compared with CN%TP by SDS-PAGE when CNPP were included as CN in the calculation of SDS-PAGE results. Based on SDS-PAGE results, we found that a majority (89%) of the CNPP from the milk (approximately 10.13 out of 11.41% TP) were retained in the Kjeldahl NCN precipitate. Thus, CN%TP measured by Kjeldahl underestimates the amount of proteolytic damage that has been done to CN in milk. It is important for the dairy industry to correctly and rapidly measure the extent of proteolytic damage to milk protein to correctly value milk from a product quality and yield point of view. A rapid and quantitative measure of proteolytic damage to milk protein is needed.

Effect of αS1-casein genotype on phenolic compounds and antioxidant activity in goat milk yogurt fortified with Rhus coriaria leaf powder

The aim of this work was to evaluate the phenolic compounds and antioxidant activity of goat milk yogurt characterized by different α_S1-casein genotypes and fortified with Rhus coriaria leaf powder. The α_S1-casein genotype was determined by isoelectric focusing, total phenol content was determined by the Folin-Ciocalteu method, phenolic compounds were identified and quantified by HPLC-UV analysis, and antioxidant activity was measured using 2,2'-azino-bis-3-ethylbenzothiazoline-6-sulfonic acid and ferric-reducing antioxidant power. The statistical analysis showed a significant effect of the studied factors. Comparing different genotypes it emerged that yogurt from goats with weak alleles at CSN1S1 loci (FF) showed the lowest phenolic compounds and therefore a lower antioxidant activity compared with yogurt from goats with strong alleles at CSN1S1 loci (AA, BB, AB). Rhus coriaria-fortified yogurt showed a significant increase in total phenolic compounds and antioxidant activity in comparison with plain yogurt. The FF-fortified yogurt presented the lowest total phenol content and antioxidant activity. This could be due to a greater capacity of proteins and peptides in this yogurt to form stable complexes with phenols. The different total phenol content detected in R. coriaria-fortified yogurt indicates that the α_S1-casein genotype influenced the amount of added phenols that are bound to the caseins and, therefore, the part that remains free and that affects the biological capacity of the final product.

The influence of casein haplotype on quality, coagulation, and yield traits of milk from Italian Holstein cows

The aim of this work was to investigate the effect of casein haplotype (CSN1S1, CSN2, and CSN3) on quality, coagulation, and yield traits of milk from Italian Holstein cows. The casein haplotype was determined by isoelectric focusing; milk clotting properties were determined by using a mechanical lacto-dynamographic instrument; and the yields of pressed and pasta filata cheeses were expressed as kilograms of cheese per 100kg of milk processed. Statistical analysis showed a significant effect of the casein haplotype. In particular, BB-A(1)A(1)-AA milk showed the highest fat content (4.01g/100g), whereas BB-A(2)A(2)-BB milk had a higher protein content, the best coagulation characteristics, and the highest yields in pressed and pasta filata cheeses, and, consequently, better ability to retain fat and protein in the curd. The results of this study suggest that knowledge of milk protein polymorphisms not only allows the production of milk with specific qualitative and quantitative characteristics, but it could also be used as a specific marker within a breed to identify milk suitable for cheesemaking, which confers an economical advantage for dairy producers.

The influence of casein haplotype on morphometric characteristics of fat globules and fatty acid composition of milk in Italian Holstein cows

The aim of this work was to investigate the effect of casein haplotypes (αS1-, β-, and κ-caseins) on morphometric characteristics of fat globules and fatty acid composition of Italian Holstein milk. Casein haplotypes were determined by isoelectric focusing; milk fat globule size was measured by using a fluorescence microscope; and fatty acid profile was determined by gas chromatography. Casein haplotype significantly affected the fat globule size, the percentage incidence of each globule size class on total measured milk fat globules, and fatty acid composition. A higher incidence of smaller milk fat globules was associated with the BB-A(2)A(2)-BB genotype (αS1-, β-, and κ-casein haplotypes, respectively), whereas small globules were not detected in BB-A(2)A(1)-AA milk, but that milk had the highest percentage of large globules. A higher content of monounsaturated fatty acids was associated with the BB-A(2)A(2)-AB genotype, whereas higher contents of conjugated linoleic acid and docosahexaenoic acid were detected in BB-A(1)A(1)-AA milk. Our results indicate that casein haplotype could affect fat characteristics and, therefore, the nutritional and technological quality of milk.

Effect of genetic type and casein haplotype on antioxidant activity of yogurts during storage

The aim of this work was to investigate the antioxidant activity of yogurt made from the milk of 2 breeds-Italian Brown and Italian Holstein-characterized by different casein haplotypes (αS1-, β-, and κ-caseins) during storage up to 15 d. The casein haplotype was determined by isoelectric focusing; antioxidant activity of yogurt was measured using 2,2'-azino-bis-(3-ethylbenzothiazoline-6-sulfonic acid). The statistical analysis showed a significant effect of the studied factors. Antioxidant activity increased during storage of both yogurt types, but yogurt produced with Italian Brown milk showed higher antioxidant activity than those produced with Italian Holstein milk. A high scavenging activity was present in yogurts with the allelic combination of BB-A(2)A(2)-BB. The results of this study suggest that the genetic type and the haplotype make a significant contribution in the production of yogurts with high nutraceutical value.

A gel electrophoretic study of caprine casein

To compare the resolving power of starch-urea gel (SUGE) and polyacrylamide-agarose gel electrophoresis (PAAGE) in caprine casein analysis, polyacrylamide gel isoelectric focusing (PAGIF) was used as reference method. The PAAGE or SUGE patterns in the first dimension were allowed to migrate by PAGIF in an orthogonal direction giving rise to two-dimensional (2-D) separations. Using this procedure, some individual bands considered to be homogeneous by SUGE or PAAGE were found to be complex mixtures of casein components. A detailed analysis of the analytical capabilities of SUGE and PAAGE was performed. Procedures based on the densitometric reading of SUGE and PAAGE patterns seemed to give inaccurate results on the quantitative composition of caprine casein. Among the electrophoretic methods assayed, the 2-D procedure gave the best resolution of casein fractions.

Purification and characterization of porcine κ-casein

κ-casein from porcine milk was isolated and purified by affinity chromatography on thiopropyl Sepharose followed by hydroxyapatite chromatography. The amino acid composition of this protein is similar to that of bovine κ-casein. The sugar composition was established and sow κ-casein was found to be highly glycosylated. It contains 12N-acetylneuraminic acid, 10N-acetylgalactosamine, 2N-acetylglucosamine and 22 galactose residues.

Effect of aspartyl proteinases ofPenicillium caseicolumandPenicillium roquefortion caseins

The aspartyl proteinases ofPenicillium caseicolumandP. roquefortiacted identically on β-casein; both enzymes split at least 3 bonds: Lys29–Ile30, Lys97–Val98and Lys99–Glu100. From αsl-casein, these proteinases released 6 main degradation products which arose from the splitting of 4 bonds;P. roquefortiaspartyl proteinase was found to cleave 1 bond at a higher rate thanP. caseicolumaspartyl proteinase. A hypothetical sequential hydrolysis mechanism of αsl-casein by these 2 enzymes is proposed from a study of the degradation by isoelectric focusing and by 2-dimensional electrophoresis.

Discrete phosphorylation generates the electrophoretic heterogeneity of ovine β-casein

The heterogeneity of ovine β-casein, apparent after discontinuous gel electrophoresis at alkaline pH and gel isoelectric focusing either by staining with Coomassie blue or immunoblotting, was investigated in detail. Electrospray mass spectrometry was used to characterize ovine casein by direct examination of whole casein samples. β-Casein in individual milks consisted of several components which were characterized through the accurate determination of their molecular masses. It was demonstrated that protein species occurring simultaneously in individual ovine milks differed mainly in the number of phosphate groups.

A study of proteolysis during Camembert cheese ripening using isoelectric focusing and two-dimensional electrophoresis

Isoelectric focusing and 2-dimensional electrophoresis were used to study the development of the pH 4·6-insoluble fraction during Camembert cheese ripening. Modifications of this fraction were due mainly to the action of 5 proteinases: rennet (chymosin + bovine pepsin), plasmin and Penicillium caseicolum aspartyl-and metalloproteinases. Rennet was inactive on β-casein, but acted very early on αs1-casein. However, rennet and P. caseicolum aspartyl-proteinase had a very similar action on the latter substrate, which prevented clear definition of the respective actions of these proteinases on αs1-casein after 7 d of ripening. Plasmin action on β-casein was important from 21 and 35 d of ripening at the surface and in the centre of the cheese respectively, suggesting an important influence of pH changes during maturation. The respective activities of the metallo-and aspartyl-proteinases of P. caseicolum were characterized and followed using β-casein degradation products as markers. The metallo-proteinase activity was detectable immediately after the development of the Penicillium (7 d), while that of the aspartyl-proteinase was observed 3 d later. Thereafter, the amount of β-casein degradation peptides resulting from the metalloproteinase decreased while that resulting from the aspartyl-proteinase increased, suggesting a more important role of the latter enzyme.

Fast protein liquid chromatography purification of hydrophobic fraction of bovine milk proteose-peptone and characterization by bidimensional electrophoresis

Bovine milk Hydrophobic fraction of proteose-peptone was prepared by hydrophobic interaction fast protein liquid chromatography. This method has several advantages such as high rapidity, simple good reproducibility and less denaturation. The proteose-peptone was eluted from a TSK-Phenyl-5PW column with a 1 M-0 M ionic strength gradient of NaH2PO4, pH 6·8, using a 6 ml/min flow rate for 56 min. The quantity of protein injected was 62·5 mg; however, it could be increased up to 100 mg. The elution order was β-CN-4P < BSA (1·6% of total N) < β-CN-5P < β-CN-1P. The hydrophobic fraction was obtained in pure water at the end of the gradient (17·3% of total N). A proteose-peptone cartograph was achieved by bidimensional electrophoresis. This hydrophobic fraction represented three principal zones of Mr 30000–28000, 19000 and 11000, which were respectively composed of 13, 4 and 2 principal spots distributed between 4·9 and 6·1 isoelectric points (IP). These spots corresponded to glycoproteins. ·7, 5·0 and 5·1 IP which migrated to Mr 18000 while β-CN-1P was identified as Mr 9000 in two spots of 5·1 and 5·3 IP.

Procedure for simultaneous phenotyping of genetic variants in cow's milk by isoelectric focusing

A method is described which allows the simultaneous separation of all polymorphic protein fractions in cow's milk in one single run. The separation could be achieved by isoelectric focusing in ultrathin-layer polyacrylamide gels. The method is especially suitable for screening purposes because it combines low costs, high resolution and short separation time.

Natural Polysaccharide-Based Gels for Dairy Food Preservation

The innovative packaging systems described in the present work, based on natural gels, have been shown to increase the shelf life of the Mozzarella cheese, without adding any chemical substance and without thermal procedures. Physical, physicochemical, microbiological, analytical, and mechanical analyses were used to monitor the quality of the cheese as a function of storage type and storage time. In particular, microbiological analysis confirmed that the characteristics of the Mozzarella cheese stored at 4 degrees C in gel are maintained for more than 15 d, whereas samples stored in the mother solution lost important characteristics after 5 d. A penetration test (texture) confirmed that the Mozzarella cheese preserved in the gel maintained mechanical properties similar to those of the fresh product, even after storage for 30 d at 4 degrees C.

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.

Hydrolysis of caprine beta-casein by plasmin

The proteolytic activity of plasmin on soluble caprine beta-casein (CN) was studied in 50 mM Tris.HCI buffer, pH 8.0, at 37 degrees C. Electrophoretic studies showed that hydrolysis of this protein results in an electrophoretic pattern that is similar to the pattern obtained from plasmin hydrolysis of bovine beta-CN (gamma-CN and complementary N-terminal fragments), suggesting that plasmin probably attacks the same regions that are susceptible to cleavage in bovine beta-CN. As determined by SDS-PAGE, the gamma-like components of caprine milk consisted of two fragments with relative molecular mass of 9200 and two with relative molecular mass of 21,400 that could differ in the level of phosphorylation. Apparently, the high molecular mass components are homologous to bovine beta-CN (f 29-209) (gamma 1-CN), and the low molecular mass components are homologous to bovine beta-CN (f 106-209) and beta-CN (f 108-209) (gamma 2- and gamma 3-CN). Complementary N-terminal fragments had values for molecular masses in the range 13,600 to 8500 and urea-PAGE patterns that were more complex than those obtained in bovine casein because of the different phosphorylation levels in caprine beta-CN. These fragments were also present in the hydrolysate of whole caprine casein that had been treated with plasmin.

Two dimensional electrophoresis and immunoblotting for the study of ovine whey protein polymorphism

Different electrophoretic techniques have been applied to the study of ovine whey protein polymorphism. Isoelectric focusing (IEF) over the pH gradient 3.5-9.5 was a suitable method for resolving both genetic variants of ovine beta-lactoglobulin (beta-lg); using this type of IEF, ovine beta-lg A seemed to be defined by a major and a minor 'satellite' band. Approximate isoelectric point, relative molecular mass, amino acid composition and N-terminal sequence of this minor band were determined. A good separation of ovine beta-lg was achieved using ultrathin layer isoelectric focusing (UTLIEF) over the pH gradient 2.5-7.0. However, addition of urea in UTLIEF gels led to some differences in the patterns of the ovine whey proteins when compared with those obtained by IEF on gels not containing urea. Two dimensional electrophoretic techniques provided a considerable separation of ovine whey proteins. Immunoblotting applied to a two dimensional separation permitted the identification of the bands belonging to beta-lg and alpha-lactalbumin fractions.

Gel electrophoresis and immunoblotting for the detection of casein proteolysis in cheese

The whole N fraction of six samples of hard and semi-hard pressed cheeses was analysed using PAGE, polyacrylamide gel isoelectric focusing and immunoblotting with polyclonal antibodies against beta- and alpha s1-casein. The origin of some electrophoretic bands corresponding to peptides produced from the enzymic degradation of the casein fractions was established. A number of these peptides were also present in the in vitro hydrolysates of casein with plasmin and chymosin. Thus, it was also possible to determine which casein was the source of each peptide and which enzymes were active in cheese. Compared with the traditional Coomassie staining procedures, immunoblotting is more sensitive and specific, making the interpretation of each electrophoretic profile easy. Thus, it was also possible to obtain a clear picture of the state of each casein fraction in a cheese variety. Two main peptides were isolated from the pH 4.6-insoluble N fraction of Parmigiano-Reggiano using DEAE-cellulose chromatography and identified, from the amino acid sequence of the N- and C-terminal ends, as gamma 3-casein (beta-casein(f108-209)) and alpha s1-PL1 (alpha s1-casein(f80-199). In both cases, a Lys-X bond was hydrolysed, indicating the action of a trypsin-like enzyme in beta- and alpha s1-casein hydrolysis during the ripening of this variety of hard pressed cheese.

Discontinuous electrophoresis of beta-caseins for the determination of bovine caseins in milk and dairy products

Discontinuous acidic anodic polyacrylamide gel electrophoresis enables the separation of bovine beta-caseins from those of ovine and caprine. Interfering protein bands as a consequence of ripening or processing have not been detected. After evaluation of the stained gel by laser densitometry, quantification was performed with calibration standards on the same gel by the ratio of the peak areas from bovine to ovine and bovine to caprine, respectively. Thus, independence from the extractability of proteins affected by denaturation and ripening (which might in some cases raise the limit of detection) is achieved. The range of quantification extends from 5 to approximately 70% bovine casein in relation to total casein.

New alpha s2-casein variant from caprine milk

A new alpha s2-casein variant was isolated from goat milk by means of covalent chromatography. With gel electrophoresis at alkaline pH, the alpha s2-casein variant showed a double band with the highest anodic mobility amongst the casein components. The mobility of the isolated alpha s2-casein variant was similar to that occurring in three individual samples of whole casein. Two dimensional electrophoresis revealed a more pronounced heterogeneity of this protein. Developing the two dimensional plate with polyclonal antibodies obtained against the four main bovine casein fractions, we demonstrated that the variant in question belongs to the alpha s2-casein family. Since the alpha s2-casein fractions, immunospecifically developed, spread along two different zones on the gel, it was concluded that a heterozygous alpha s2-casein form was present in the sample.

Determination of ovine casein heterogeneity using gel electrophoresis and immunochemical techniques

Discontinuous PAGE at alkaline and acid pH, polyacrylamide gel isoelectric focusing, two dimensional electrophoresis and immunoblotting have been used to study the heterogeneity of sheep caseins. Three main phenotypes were selected either because of mobility at alkaline and acid pH of the individual alpha s components or because of their relative intensity. On electrophoresis at alkaline pH, one phenotype showed two distinct bands of lower electrophoretic mobility than beta 1- and beta 2-caseins. A detailed study of these components using immunospecific detection with beta-casein antiserum showed that these minor components of ovine casein may have a polypeptide chain similar to that of beta 1- and beta 2-caseins. Complete electrophoretic patterns of the casein components in some individual milks are also presented.

Molecular heterogeneity and genetics of Vicia faba seed storage proteins

Legumin and vicilin were purified from seeds of Vicia faba L. var. Scuro, characterized in different electrophoretic systems, and used to produce polyclonal antibodies in rabbits. Two-dimensional electrophoretic studies showed a wide range of heterogeneity in the subunits of both legumin and vicilin. Legumin was found to be composed of 29 disulphide-linked subunit pairs with different molecular weight and/or isoelectric point. Western blot analysis of legumin of several mutants revealed molecular polymorphism based on a corresponding gene family. Three different α-major legumin patterns were found, and inheritance studies showed that the 34.3-kD legumin polypeptide is the product of one locus, Lg-1α, which is the first legumin genetic locus described in Vicia faba. Vicilin was found to be composed of as many as 59 subunits distributed in a molecular weight range of 65.7 to 42.8 kD (major polypeptides) and 37.2 to 15.2 kD (minor polypeptides), with different isoelectric points. A model is proposed that explains the possible formation of the minor subunits and the major subunits of 48.2 and 46 kD molecular weight (MW) from proteolytic cleavages and/or glycosilation of precursor polypeptides. Ten different vicilin electrophoretic patterns were observed among the analyzed accessions, which showed large molecular polymorphism that proved to be under genetic control.

Fast isoelectric focusing of milk proteins on small ultrathin polyacrylamide gels containing urea

The preparation of 0.45 mm thin polyacrylamide gels, containing urea, for horizontal micro isoelectric focusing of milk proteins with PhastSystem is described. Isoelectric focusing in the small gels, stained either with Coomassie Brilliant Blue R-250 or with the more sensitive silver stain, affords a fast and sensitive procedure for an analysis of milk and cheese proteins. The procedure can be effectively exploited in detecting adulteration in ovine cheese with bovine milk.

The prevention of sub-surface demineralization of bovine enamel and change in plaque composition by casein in an intra-oral model

The ability of bovine milk phosphoprotein (casein) to be incorporated into plaque, prevent enamel sub-surface demineralization, and affect bacterial composition was determined using a modified intra-oral caries model. The intra-oral model consisted of a removable appliance containing a left and right pair of bovine enamel slabs placed to simulate an approximal area. Supragingival plaque was collected and impacted into the left and right inter-enamel spaces. The left side of the appliance was exposed to various sugar and salt solutions, while the right side was exposed to sugar and casein solutions. Sodium caseinate, the major fraction alpha s1-casein, and a tryptic digest of alpha s1-casein (TD-casein) were studied. Sodium caseinate at a level of 2% w/v in a 3% sucrose-3% glucose-salt solution (pH 7.0) prevented sub-surface enamel demineralization over a ten-day period as shown by microradiography and microhardness. Two exposures of a 2% w/v sodium caseinate, alpha s1-casein, or TD-casein solution (pH 7.0) per day prevented sub-surface enamel demineralization caused by six exposures of a 3% sucrose-3% glucose-salt solution per day over a ten-day period. Intact alpha s1-casein and tryptic peptides were shown immunochemically to be incorporated into the inter-enamel plaque. The incorporation of casein and its breakdown in plaque did not produce a significant change in the amount or composition of plaque bacteria. The ability of casein and tryptic peptides to prevent enamel demineralization was related to their incorporation into plaque, thereby increasing plaque calcium phosphate and acid-buffering capacity by the phosphoseryl, histidyl, glutamyl, and aspartyl residues and indirectly through catabolism by plaque bacteria.

Variability in true protein, casein, nonprotein nitrogen, and proteolysis in high and low somatic cell milks

Monthly variation in milk protein (total nitrogen X 6.38) and milk fat was determined for 115 farms over 2 yr. Yearly average milk protein and fat tests in each year were 3.16 and 3.62%, respectively. The mean regression coefficient for milk protein with respect to milk fat was .47 for the entire period. Twenty-four farms were selected and grouped high or low based on their previous 2-yr somatic cell history. Monthly milk samples for each farm were tested for direct microscopic somatic cell count, total nitrogen, noncasein nitrogen, and nonprotein nitrogen. No differences in monthly nonprotein nitrogen, true protein, and casein were found between groups. Casein as a percent of total nitrogen was significantly higher for the low somatic group for seven of the 12 mo studied but was significantly higher for 9 mo when expressed as a percent of true protein. The average increase in tyrosine value for incubated preserved milk was significantly higher for the high somatic cell milk, indicating higher proteolytic activity in high somatic cell milk. Electrophoretic analysis of high and low somatic cell milk indicated that there was substantial proteolytic breakdown of alpha S-casein and beta-casein by proteases associated with elevated somatic cell counts.

Fractionation of bovine caseins by reverse phase high performance liquid chromatography: identification of a genetic variant

Samples of reduced whole casein from genetically typed individual cows were quantitatively separated into their main components, alpha s1-, alpha s2-, beta- and kappa-caseins by reverse phase high performance liquid chromatography (RP-HPLC) using a mobile phase of phosphate-buffered aqueous propan-2-ol containing sodium dodecyl sulphate and an octadecylsilyl stationary phase. One casein sample was found to give two peaks of beta-casein of approximately equal areas. RP-HPLC of tryptic digests of the two separated peak fractions gave identical patterns with the exception of one peptide peak with different retention times. Amino acid analyses performed on both fractions showed that they corresponded to peptide 114-169 of beta-casein A1. The atypical beta-casein differed from the typical one by a Pro----Leu substitution in region 114-169.

Rapid analysis of bovine milk proteins by fast protein liquid chromatography

The separation of bovine milk proteins by fast protein liquid chromatography has been investigated by ion-exchange chromatography on Mono Q and Mono S columns and by gel filtration on a column of Superose 12. The four major casein components (alpha s1, alpha s2, beta and kappa) as well as the minor gamma-caseins were generally well separated on the Mono S column with urea-containing buffers at pH 3.8 in as short a time as 7 min, although there was considerable overlap between alpha s1- and alpha s2-casein peaks. Peak area measurements indicated that the four caseins alpha s1, alpha s2, beta and kappa were present in total casein in the approximate proportions of 3.0:0.5:3.4:0.9, in good agreement with other literature values. Whey proteins were not separated on the Mono S column, but were all well resolved by rapid analysis on the Mono Q column at pH values between 6 and 8 in buffers free of urea or 2-mercaptoethanol. Both urea and 2-mercaptoethanol were required for casein analyses on the Mono Q column, but all the casein components were then separable over a broad pH range (5.0-11.0). While urea levels of 4.5-8.0 M and pH values of 7.0 to 8.0 were most generally useful, the resolution of some components was affected by urea concentration or pH, so conditions may have to be modified for specific analysis problems. The caseins were too similar in size to be separated on the Superose 12 column but high-speed gel filtration in as little as 15 min separated all the whey proteins well, molecular weight values obtained being in good agreement with literature values.

Preparation and amino acid sequence of human kappa-casein

Human kappa-casein was prepared from whole casein by successive hydroxyapatite and thiol-Sepharose chromatographies. The primary structure of its 99-residue N-terminal fragment has been determined by sequencing peptides obtained by tryptic and chymotryptic digestions of the whole protein. This fragment overlaps the known sequence of the 65-residue C-terminal fragment. The 158-residue sequence of human kappa-casein was compared to those of goat, ewe, cow and rat kappa-caseins. Only 22% of the residues are identical in homologous positions. The rate of divergence of the 93-residue N-terminal segment (para-kappa-casein) appears to be higher than that of the rest of the molecule.

Sunflower seed protein: size and charge heterogeneity in subunits of the globulin fraction

The subunit heterogeneity of the globulin fraction of sunflower seeds was investigated by two dimensional electrophoresis, using isoelectric focusing in the first dimension and sodium dodecyl sulphate polyacrylamide gel electrophoresis in the second dimension. Under non reducing conditions, intermediary subunits B, C and D (molecular weight 54 000, 48 000 and 40 000, respectively) were focused within a pI range 5.4-6.0 but intermediary subunits A (molecular weight 60 000) focused within a pI range 6.3-6.8. Under reducing conditions the electrophoretic patterns show that intermediary subunits consist in large "acidic" and small "basic" subunits linked by disulphide bonds. The large subunits of B species are more acidic and less heterogeneous than the corresponding subunits of the A species. These results confirm that helianthinin had a "legumin-type" structure.

[Analysis of proteins in food with electrophoretic and chromatographic methods]

The efficiency of electrophoretic methods (gel electrophoresis, isoelectric focusing, twodimensional techniques) and of chromatographic methods (size exclusion and ion exchange chromatography, reversed phase HPLC) to analyze proteins in foods is reviewed. Several selected applications are discussed in detail. The large diversity of proteins in a particular food results in a unique electrophoretic or chromatographic pattern, that can be used for identification purposes, by means of the so called indicator proteins. The adaptability and resolving power of the methods assure their extended application to many protein containing foods. The uniqueness of the patterns obtained warranties differentiations of even closely related animal or plant foods as well as mixtures of them. The methods also allow quantitative determinations of mixtures of foods. Their ease of handling and good reproducibility and reliability favours their use in routine analyses. Numerous investigations on fish, meat and derived products, non-meat proteins in meat products, milk, cheese, cereals and products made of cereals, oilseed proteins, legumes, fruits and vegetables described in the literature are here presented.

Kinetic studies of in vivo digestion of bovine unheated skim-milk proteins in the rat stomach

The in vivo action of gastric proteinases on bovine milk proteins was studied in rats fed with skim-milk, by analysing gastric contents after 30, 60 and 240 min of digestion. Gastric proteolysis was already marked after 30 min of digestion and liberated a large number of peptides with different molecular weights. alpha S1-Casein and beta-casein were still the main components of the stomach contents together with alpha-lactalbumin and beta-lactoglobulin, which were little degraded, even after 240 min of digestion. A statistical analysis (multivariate method), made on several parameters (such as pH, N, and NPN) showed changes in the stomach contents during the digestion. The amino acid composition of the protein fraction was close to that of the diet, whilst that of the non-protein fraction was very different, being between the amino acid composition of the endogenous proteins and that of the diet.

Study of the heterogeneity of bovine pepsin A by pseudo isoelectric focusing

Pseudo isoelectric focusing (PIEF), i.e. focusing stopped before the equilibrium is reached, has been successfully applied to analyse bovine pepsins A obtained from abomasum mucosa extracts and from 3 individual abomasal juices. Whole bovine pepsin A from mucosa extracts, was resolved in PIEF, using an acidic pH gradient(2-4), into five major and active components. Its fractionation on hydroxyapatite only gave four components which were not homogeneous in PIEF, suggesting that interactions between the different components occur during the chromatographic procedure. After treatment with potato acid phosphatase, whole bovine pepsin A showed only one band in PIEF, displaying enzymic activity and with a mobility identical to that of the less anodic band in untreated pepsin. These results, together with organic phosphate determinations, obviously confirm that the heterogeneity of bovine pepsin A is due to the phosphate content which appears to range between 0 and 3 mol./mol. for the 5 components. Two of them, the phosphate content of which appears to be 1 mol./mol., seem to differ in the location of this phosphate group in the molecule. Whole bovine pepsin A preparations from abomasal juices shared the same pattern in PIEF, identical to that observed with bovine pepsin A from mucosa extracts, thus excluding that dephosphorylation might be involved in the secretory process, as well as disproving the theory that dephosphopepsinogen might be an intermediate in the synthesis of pepsinogen, as it was previously suggested.