Formation of γ1-A2, γ2-A2 AND γ3-A caseins by In vitro proteolysis of β-CASEIN A2 with bovine plasmin

Effect of Protein Genotypes on Physicochemical Properties and Protein Functionality of Bovine Milk: A Review

Milk protein comprises caseins (CNs) and whey proteins, each of which has different genetic variants. Several studies have reported the frequencies of these genetic variants and the effects of variants on milk physicochemical properties and functionality. For example, the C variant and the BC haplotype of αS1-casein (αS1-CN), β-casein (β-CN) B and A1 variants, and κ-casein (κ-CN) B variant, are favourable for rennet coagulation, as well as the B variant of β-lactoglobulin (β-lg). κ-CN is reported to be the only protein influencing acid gel formation, with the AA variant contributing to a firmer acid curd. For heat stability, κ-CN B variant improves the heat resistance of milk at natural pH, and the order of heat stability between phenotypes is BB > AB > AA. The A2 variant of β-CN is more efficient in emulsion formation, but the emulsion stability is lower than the A1 and B variants. Foaming properties of milk with β-lg variant B are better than A, but the differences between β-CN A1 and A2 variants are controversial. Genetic variants of milk proteins also influence milk yield, composition, quality and processability; thus, study of such relationships offers guidance for the selection of targeted genetic variants.

Plasmin activity in UHT milk: relationship between proteolysis, age gelation, and bitterness

Plasmin, the major indigenous protease in milk, is linked to quality defects in dairy products. The specificity of plasmin on caseins has previously been studied using purified caseins and in the indigenous peptide profile of milk. We investigated the specificity and proteolytic pathway of plasmin in directly heated UHT milk (>150 °C for <0.2 s) during 14 weeks of storage at 20 °C in relation to age gelation and bitter peptides. Sixty-six peptides from αS- and β-caseins could be attributed to plasmin activity during the storage period, of which 23 were potentially bitter. Plasmin exhibited the highest affinity for the hydrophilic regions in the caseins that most probably were exposed to the serum phase and the least affinity for hydrophobic or phosphorylated regions. The proteolytic pattern observed suggests that plasmin destabilizes the casein micelle by hydrolyzing casein-casein and casein-calcium phosphate interaction sites, which may subsequently cause age gelation in UHT milk.

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.

Contribution of plasmin to Cheddar cheese ripening: effect of added plasmin

Plasmin (EC 3.4.21.7) was added to cheese milk to assess its contribution to Cheddar cheese ripening; the activity of plasmin in the cheese was increased by levels ranging from 1·5 to 6 times that in the control cheeses. Even at the highest level of added plasmin, no activity was found in the whey. β-Casein was degraded faster in the experimental cheeses than in the controls, and the concentration of γ-caseins increased concomitantly. The total N in the water-soluble extract was up to ˜ 20% higher in the experimental than in the control cheeses but phosphotungstic acid-soluble N was not affected by the plasmin activity in the cheese. Several differences were apparent in the gel electropherograms of the water-soluble extracts of the experimental and control cheeses; some peptides were present at higher concentrations in the former, others in the latter, suggesting that plasmin contributes to both the formation and degradation of water-soluble peptides in cheese. The organoleptic quality of the plasmin-enriched cheeses was judged superior to that of the controls and ripening was considerably accelerated; a plasmin level 3–4 times the indigenous value appeared to be optimal. No bitterness was detected in any of the cheeses.

Milk proteins: molecular, colloidal and functional properties

The proteins of bovine milk are the best-characterized food-protein system. Lactoproteins, which have high biological value, contribute ∼25% of dietary protein in North West Europe, North America and Oceania. However, in protein-rich western diets, milk proteins are frequently more highly valued for their functional properties than for their nutritional qualities. The remarkably high heat stability of the caseinate system permits the manufacture of a range of sterilized, concentrated and dehydrated products while its gelation on very limited proteolysis is the basis of cheese manufacture. Skim-milk powders, caseinates and whey protein concentrates are the most flexible and widely used functional proteins in food processing.

This communication reviews recent studies on milk proteins with respect to molecular and colloidal properties; coagulation by Ca2+, heat and ethanol; and functional properties and their chemical and enzymic modification.

Occurrence of γ-caseins in buffalo milk

Using polyacrylamide gel electrophoresis, 2-dimensional gel electrophoresis and electrofocusing, it has been shown that 4 basic fragments were produced by action of plasmin on buffalo β-casein. These fragments were present in whole buffalo casein prepared from bulk milk. Their mol. wts, 20000, 16000 and 11000 (2 components) were determined by SDS-gradient polyacrylamide gel electrophoresis. It is suggested that 3 of them are homologous to bovine γ1-, γ2- and γ3-caseins respectively, while the last one (mol. wt 16000), which has no counterpart in plasmin digest of bovine β-casein, could arise from a cleavage at bond 68–69.

A method for determination of γ-casein and its use for investigating proteolysis in bovine milk

A convenient and sensitive method for determining γ-casein content of skim milk is described. It is based on quantitative separation in which the soluble fraction obtained from skim milk (1·5 ml) in a solvent system consisting of 50% (v/v) ethanol, 0·4 M-Na thiocyanate and 0·15 M-CaCl2 was applied to a small column of DEAE-cellulose and eluted with 0·02 M-Tris-HCl buffer (pH 8·0) containing 0·03 M-NaCl and 6 M-urea. Protein in this fraction (5 ml) was measured from its absorbance at 280 nm. The method is applicable also to heated skim milk. Its availability for investigating proteolysis in milk is expected from the fact that incubation of skim milk with porcine plasmin at 37 °C for 3 h increased γ-casein content, an increase that was lowered by addition of soyabean trypsin inhibitor. Incubation of raw skim milk with 0·02% (w/v) NaN3 resulted in increase in γ-casein with time and with increasing temperature. This was accompanied by a time lag, the length of which increased with decreasing temperature. The degree of proteolysis in skim milk, expressed as the amount of increase in γ-casein after 20 h of incubation at 37 °C, showed two pH optima, one at pH 8·0 and the other at pH 7·2–7·5. These results suggested that the present method.of γ-casein determination can be used for more precise studies of the milk plasmin system in which important factors, such as plasminogen activator and inhibitors, are involved.

Electrofocusing and two-dimensional electrophoresis of bovine caseins

The main components of bovine whole casein were characterized by electrofocusing; pi values of αs1-, β-, κ-, γ1-, γ2- and γ3-caseins were determined. A further identification of casein components was achieved by a 2-dimensional electrophoresis study. 2-Dimensional patterns of γ-caseins obtained from a hydrolysate of β-cascin by bovine plasmin are in good agreement with those of γ-caseins naturally present in whole casein.

Plasmin activity and proteose-peptone content of individual milks

The variation in plasmin activity and proteose-peptone (PP) content in milks from 81 cows of four breeds was statistically analysed. Plasmin activity increased with parity and stage of lactation and differed between breeds, but the breed effect was removed when adjustment was made for differences in milk casein content. The negative correlation found between plasmin activity and casein content seemed to be expressed only under assay conditions and was thus probably an artefact. The linear regressions of proteose-peptones in fresh (PP) and cold-stored milks (ΔPP) on plasmin activity were highly significant (P< 0·001). However, the variation in plasmin activity explained only 38 and 33%, respectively, of the variation in PP and ΔPP. At apparent zero plasmin activity, PP and ΔPP were significantly different from zero (P< 0·001) and the intercepts represented 77 and 46%, respectively, of the averages. PP and ΔPP were significantly higher in milks containing the BB genotype of ²-lactoglobulin than in milks with the AA genotype.

Natural variations in the average size of bovine casein micelles: I. Milks from individual Ayrshire cows

Milk samples from 3 groups of cows were taken at regular intervals throughout lactations following 2 spring and 1 autumn calving. The average radius of casein micelles in milk was determined either from the wavelength dependence of turbidity or from the diffusion coefficient as measured by a quasi-elastic light scattering method. A 2-fold variation in naturally occurring average radii was found both in milks from the same cow at different times and between milks from different cows at the same time.

Analysis of the peptide profile of milk and its changes during thermal treatment and storage

In this study a new method was developed for analysis of the low molecular weight protein fraction of milk, allowing a simple and fast overview of the peptide profile of various milk samples. For this purpose, immobilized metal affinity chromatography (IMAC) was coupled with matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS). By this technique, two major peptides in milk could be identified as fragments of alpha-s1-casein. During heat treatment of raw milk, five new peptides were generated, the origin of which could be assigned to the casein fraction. Storage experiments with extended shelf life milk at 4 degrees C did not show any changes in the peptide profile, whereas in ultra high temperature milk stored at room temperature, one peptide increased significantly, which was identified as the N-terminus of alpha-s1-casein. The peptide was assumed to be formed in an enzymatic reaction, which was confirmed in a storage experiment with sterilized milk. Analyses of different commercially available milk samples confirmed the results obtained with the heated and stored milk. Furthermore, differences in the peptide profiles of the samples, probably due to different cow breeds or lactation stages, were observed. These results establish IMAC prior to MALDI-TOF-MS as a valid tool for the rapid analysis of the peptide profile of milk.

The effect of conserved residue charge reversal on the folding of recombinant non-phosphorylated human β-casein

A short stretch of 13 amino acids in the central portion of human beta-casein contains four positively charged conserved residues, three Lys and one Arg. We changed these individually to Glu, reversing their charge, and compared the resulting recombinant proteins to the wild-type recombinant, monitoring thermal aggregation with turbidity as well as using the fluorescence of the intrinsic Trp, of hydrophobically bound ANS and fluorescence resonance energy transfer from Trp to ANS to detect differences in structure. The results demonstrate the need to maintain the actual or functional identity of these conserved charged amino acid residues in order to attain the protein folding and functional properties of the wild-type human beta-casein molecule. They emphasize the probability that native human beta-casein has a unique folding pattern that is important for its function of suspending minerals and delivering the protein and minerals to the neonate in a readily ingestible form.

Mechanisms involved in milk endogenous proteolysis induced by a lipopolysaccharide experimental mastitis

Experimental mastitis has been induced by the lipopolysaccharide (LPS) of Escherichia coli on six dairy cows in order to study the mechanisms involved in milk endogenous proteolysis during the inflammatory process. Variations in somatic cell count (SCC), plasmin activity, and total casein (CN) content were measured, and proteose-peptone content and the percentage of pH 4.6 insoluble peptides including gamma-CN have been considered as indicators of endogenous proteolysis. Furthermore, polymorphonuclear neutrophils (PMN) maturity has been evaluated by optical microscopy, and proteolysis by PMN proteinases has been studied at neutral and acidic pH in order to establish a link between caseinolysis, proteinase class, and PMN maturation. Two peaks of proteose-peptones content have been noticed for the six cows. First peak could be explained by both plasmin activity and SCC, while second peak was concomitant with a low plasmin activity but a SCC remaining high. The second peak of proteose-peptones content confirmed the role of cellular proteases in milk caseinolysis. Casein breakdown by cellular proteases was confirmed by SDS-PAGE electrophoresis, and a link between neutral proteinases activity and immature PMN recruitment was shown. Acidic proteinases activity was effective with mature PMN and during the recovery phase.

Impact of low concentration factor microfiltration on the composition and aging of Cheddar cheese

The effect of microfiltration (MF) on proteolysis, hardness, and flavor of Cheddar cheese during 6 mo of aging was determined. Raw skim milk was microfiltered two-fold in two cheese making trials. In trial 1, four vats of cheese were made in 1 d using unconcentrated milk (1X), 1.26X, 1.51X, and 1.82X concentration factors (CF). Casein-(CN)-to-fat ratio was constant among treatments. Proteolysis during cheese aging decreased with increasing CF due to either limitation of substrate availability for chymosin due to low moisture in the nonfat substance (MNFS), inhibition of chymosin activity by high molecular weight milk serum proteins, such as alpha2-macroglobulin, retained in the cheese or low residual chymosin in the cheese. Hardness of fresh cheese increased, and cheese flavor intensity decreased with increasing CF. In trial 2, the 1X and 1.8X CF were compared directly. Changes made in the cheese making procedure for the 1.8X CF (more chymosin and less cooking) increased the MNFS and made proteolysis during aging more comparable for the 1X and 1.8X cheeses. The significant difference in cheese hardness due to CF in trial 1 was eliminated in trial 2. In a triangle test, panelists could not differentiate between the 1X and 1.8X cheeses. Therefore, increasing chymosin and making the composition of the two cheeses more similar allowed production of aged Cheddar cheese from milk concentrated up to 1.8X by MF that was not perceived as different from aged Cheddar cheese produced without MF.

Effects of added plasmin on the formation and rheological properties of rennet-induced skim milk gels

Elevated plasmin enzyme activity has been suggested as a likely cause of impaired functional properties that occur in milk from cows either in their late-lactational period or that are experiencing mastitis. However, there are conflicting reports on the impact of plasmin on rennet coagulation properties of milk. The effects of added plasmin on the rheological properties, at small and large deformation, of rennet-induced gels were investigated. The microstructure of rennet-induced gels was studied, using confocal scanning laser microscopy. Porcine plasmin was added to reconstituted milk, and samples were incubated at 37 degrees C for between 0.5 to 8 h. The hydrolysis reaction was terminated using soybean trypsin inhibitor. The extent of degradation of caseins was determined with SDS-PAGE. The extent of breakdown of alpha(s)- and beta-caseins increased with incubation time with plasmin. Storage modulus of rennet gels decreased linearly with increasing degradation of caseins. There was an increase in the loss tangent parameter of the gels with increasing casein degradation, indicating a more liquid-like gel character. Gelation time decreased until approximately 3 h of incubation with plasmin (when the amounts of intact alpha(s)- and beta-caseins were approximately 46 and 50%, respectively); thereafter, gelation time increased considerably. Yield stress of rennet-induced gels decreased with increasing casein breakdown. When the level of casein hydrolysis was high (<40% of intact caseins), the microstructure of rennet-induced gels was drastically altered. Even when there were low levels of casein hydrolysis, the rheological properties of rennet gels were altered, which could have negative impacts on cheese yield and texture.

Comparative aspects of milk caseins

The caseins comprise the major protein component of milk of most mammals and are secreted as micelles that also carry high concentrations of calcium. They are phosphoproteins that represent the products of four genes, equivalent to those that encode the bovine alpha s1, alpha s2, beta, and kappa-caseins. There is considerable variation in the relative proportions of the particular caseins across species. The primary sequences of the alpha s1, alpha s2, and beta-caseins also show considerable species variation consistent with rapidly evolving genes that are proposed to have a common precursor. In contrast, the kappa-caseins exhibit features that demonstrate a separate origin and function where they are proposed to stabilise the micelle structure. This review focuses on comparative aspects of the caseins across a number of species for which information is now available.

Changes in casein composition of goats' milk during the course of lactation: physiological inferences and technological implications

Five British Saanen goats were milk sampled during the first 39 weeks of lactation to determine changes in casein composition. Caseins were separated by anion- and cation-exchange FPLC to determine the relative amounts of the individual caseins. Acid, alkaline and SDS-PAGE were used to determine possible genetic polymorphisms and observe any lactational changes. Total casein nitrogen was determined using a micro-Kjeldahl method and this allowed the concentrations of individual caseins to be calculated. The milk of one animal, which had the deduced genotype alpha s1-CnAB, showed higher concentrations of both total and alpha s1-casein. The remainder of the group were either heterozygous alpha s1-CnBE or, more probably, homozygous alpha s1-CnE and produced milk of a generally lower protein concentration. Both FPLC and PAGE results showed that the relative amounts and concentrations of alpha s2-casein decreased with stage of lactation, consistent with its susceptibility to proteolysis. The relative amounts of the breakdown products of plasmin attack on beta-casein, gamma-caseins, were highly negatively correlated with milk yield (r = -0.942, P < 0.001) in the declining phase of lactation, reflecting the gradual involution of the gland at this time. The relative amount of kappa-casein increased by approximately 50% after peak lactation and its concentration almost doubled near the end of lactation. These compositional changes may alter the processing qualities of goats' milk in relation to cheese production.

Review and update of casein chemistry

Of all food proteins, bovine milk proteins are probably the most well characterized chemically, physically, and genetically. The primary structures are known for most genetic variants of alpha s1-, alpha s2-, beta-, and kappa-caseins, beta-lactoglobulin, and alpha-lactalbumin. Secondary and tertiary structures of the whey proteins have been determined, and secondary structures of the caseins have been predicted from spectral studies. The caseins, although less ordered in structure and more flexible than the typical globular whey proteins, have significant amounts of secondary and, probably, tertiary structure. The amphipathic structure of the caseins is especially noteworthy; thus, these proteins most likely are divided into polar and hydrophobic domains. The presence of anionic phosphoseryl residue clusters in the calcium-sensitive casein polar domains is particularly significant because of their interaction with calcium ions, or calcium salts, or both, and the formation of micelles. Flexibility of casein structures is reflected by their susceptibilities to limited proteolysis, which dramatically changes functionality.

Proteolysis during storage of UHT milk: differences between whole and skim milk

Proteolysis during storage of UHT skim and whole milks processed by either direct or indirect systems has been studied. All the proteolysis indices determined (measurement of free amino groups, PAGE of caseins and PAGE and reversed-phase HPLC of the fraction soluble at pH 4.6) revealed greater proteolytic degradation during storage of skim milks compared with that of whole milks subjected to the same UHT treatments. Increased activities of both native milk proteinase and proteinases of bacterial origin were observed in skim UHT milks. The different behaviour of UHT skim and whole milks on storage would have to be taken into account in establishing the process conditions.

[Group separation of caseins from cow milk on hydroxyapatite]

Ion exchange-chromatography method with the use of hydroxy apatite is considered one of the methods used for the casein fractionation particularly those containing phosphate in its composition. Simplicity of hydroxyapatite-preparation and its easily use in casein fractionation in batches are advantages of this method. Sephadex G-25 or Agarose is used to easement the sample passage through the column. By using of hydroxy-apatite in batches, alpha s-casein can easily separated with the use of buffer solution 0.006 M phosphate at pH 6.6 and the elution is carried out by buffer solution containing 0.25 M KH2PO4, 0.2 M KCl, 4.5 M urea and 0.002 M di-thiotreitol. Before the elution of alpha s-casein, beta- and kappa-casein are separated using buffer solutions containing 0.005 M and 0.1 M KH2PO4 in addition to the previously mentioned components.

A 20,000-dalton casein fragment in human milk

A new peptide of 20,000 daltons was found in human milk as a constituent of the casein micelle. Enzymic digestion with plasmin or trypsin revealed that the peptide was identical with a degradation product of human beta-casein. The amino acid composition of the degradation product and the previously reported sequence in the N-terminal region of human beta-casein suggested that the peptide was a fragment of beta-casein lacking the C-terminal region. The thermal sensitivity of this peptide was higher than that of beta-casein, but the peptide lost the property of calcium-dependent precipitation, which intact beta-casein possesses.

Proteolysis in milk: the significance of proteinases originating from milk leucocytes and a comparison of the action of leucocyte, bacterial and natural milk proteinases on casein

The caseinolytic activities at pH 6.8 of polymorphonuclear (PMN) and mononuclear leucocyte homogenates (equivalent to a level of 10(6) cells/ml milk) were less than the levels of natural milk proteinase activity found in milk from healthy cows. Bulk milks contained approximately 4 times more milk proteinase activity than the composite milks from individual healthy cows. Isolated blood leucocytes, when added to raw milk of good bacteriological quality and stored at 5 degrees C, did not readily degenerate and had no detectable effect on the milk proteins even when these cells were completely disrupted by homogenization of the milk. Pasteurization of milk which contained leucocytes caused loss of cell vitality. Extracellular proteinases of psychrotrophic bacteria growing in milk were not detected until the early stationary phase of growth. The total viable count at which this occurred varied greatly. Proteinase production by a pure culture of Pseudomonas fluorescens was not detected in milk stored at 5 degrees C until a viable count of approximately 10(9) colony forming units (c.f.u.)/ml was obtained, whilst normal bulk milks stored at 5 degrees C produced detectable levels of extracellular proteinase(s) when the psychrotrophic flora reached 10(7)-10(8) c.f.u./ml. Casein proteolysis by PMN and mononuclear leucocyte homogenates resulted in similar polypeptide maps, but plasmin and bacterial proteinase isolated from a strain of Serratia marcescens resulted in polypeptide maps different from each other and from that produced by the leucocyte proteinase(s). The rate of proteolysis of caseins by the different proteinase sources appeared to be in the order alpha s1- greater than beta- greater than greater than kappa-casein for the leucocyte extracts, beta- greater than alpha s1- greater than greater than greater than kappa-casein for bovine plasmin and beta- approximately kappa- greater than alpha s1-casein for for S. marcescens proteinase.

Plasmin cleaves human beta-casein

Plasmin cleaves isolated human beta-casein to form specific fragments in a manner similar to the generation of gamma 1-, gamma 2-, and gamma 3-caseins from the bovine homologue. Identification of a protein previously isolated from human milk as a specific plasmin cleaved portion of beta-casein indicates that endogenous plasmin is active in whole milk. These findings suggest that protease activity should be considered in casein quantitation or isolation of components from human milk.

New aspects of naturally occurring proteases in bovine milk

The scientific literature on milk proteases, along with recent findings in the author's laboratory, are summarized and reviewed comprehensively. Emphasis is on detection of proteolytic enzymes and their activity, purification and kinetic characterization of the isolated enzymes, and technological problems associated with proteolytic enzymes in milk and milk products. Two serine proteinases isolated from milk are compared with plasmin of bovine blood serum. Results from these comparisons strongly suggest that milk proteinase I and plasmin are identical. Proteolysis studies with cold stored milk indicate a direct relationship between gamma-casein formation and milk proteinase association with casein micelles.

Breakdown of caseins by proteinases in bovine milks with high somatic cell counts arising from mastitis or infusion with bacterial endotoxin

Milk obtained from cows which were either infected by clinical mastitis or had been subjected to intramammary infusion of Escherichia coli endotoxin possessed high counts of somatic cells and very high levels of proteinase activity which hydrolysed the caseins almost completely in a few hours at 37 degrees C. The rate of hydrolysis of beta-casein was slightly greater than that of alpha S1-casein, but in both cases hydrolysis was enhanced by 6 cycles of freezing and thawing to disrupt somatic cell membranes. A study of the relationship between proteinase activity and cell count suggested that only some of the proteinase activity originated in the somatic cells and also that the identity of the cells making up the total cellular population was important. Maximum proteolysis occurred at 50-60 degrees C, but the temperature-activity curve was a broad peak. Likewise the pH versus activity plot was very broad and was almost flat over the pH range 6-9. Experiments with a number of inhibitors of proteinases failed to give a clear cut pattern of inhibition. All evidence obtained was consistent with the view that several different enzymes with different pH and temperature optima and different specificities contributed to the overall hydrolysis of caseins in these milks. From electrophoretic band patterns one of these enzymes was clearly plasmin, but in high cell count milks other proteinases also became significant.

14C-Methylated beta-casein as a substrate for plasmin, and its application to the study of milk protein transformations

A method was investigated for the sensitive radiochemical assay of milk protein transformations. beta-Casein was reductively methylated with NaBH4 and H14CHO giving a product of spec. act. 4.42 microCi/mg in which a maximum of 12% of the lysine residues were labelled. Methylation did not alter the electrophoretic or chromatographic properties of the protein, or its pattern of proteolysis by plasmin. The substrate susceptibility of 14C-methylated beta-casein towards plasmin was determined by measuring radioactivity transfer to the proteolytic fragments gamma 2- and gamma 3-casein. Compared with the native protein, no decrease in substrate susceptibility was detected. The presence in milk of the plasmin-like enzyme, milk proteinase, was demonstrated and its activity at 4 degrees C was quantified by examination of the fragmentation pattern of added 14C-methylated beta-casein. It was concluded that 14C-methylated protein substrates, prepared by reductive methylation, are well suited to the study of hydrolytic enzymes and that they can provide valuable information on milk protein transformations. In particular, no interference was encountered with the rate of cleavage by plasmin when the level of methylation was kept to a minimum.

The composition, structure and origin of proteose-peptone component 5 of bovine milk

Proteose-peptone component 5 has been isolated from bovine milk. Molecular weight values within the range 12000--13500 were obtained by sedimentation equilibrium, dodecylsulphate/polyacrylamide gel electrophoresis and gel filtration in urea-containing buffers. A dansylation procedure showed that the sequence Arg-Glu occupied the N-terminal position while hydrazinolysis revealed C-terminal lysine. The latter was confirmed by experiments with carboxypeptidases B and C which indicated that a mixture of molecules was present, about 80% of which had a C-terminal sequence -(Ala-Met)-Ala-Pro-Lys while about 20% had an additional -His-Lys in the terminal position. These results, together with data on the overall composition, showed that this component of the proteose-peptone fraction of milk corresponded to a mixture of molecules representing residues 1--105 and 1--107 of the beta-casein molecule, a finding that was confirmed by peptide mapping. This demonstration that proteose-peptone components correspond to the N-terminal portions of the beta-casein molecule while the gamma-caseins represent the matching C-terminal portions provides strong evidence in favour of a proteolytic mechanism for the formation of these substances in vivo and in vitro.

The composition, structure and origin of proteose-peptone component 8F of bovine milk

Proteose-peptone component 8F (or '8-fast') has been prepared from bovine milk. Sedimentation equilibrium analysis, polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulphate and gel filtration in urea-containing buffers all gave molecular weight values between 3300 and 3900. The N-terminal sequence was found to be Arg-Glu- by dansylation and Edman degradation. Hydrazinolysis released lysine from the C-terminus. A mixture of carboxypeptidases A and B showed that the C-terminal sequence was -Thr-(Arg,Ile,Asn)-Lys. The phosphate content was 3.8 mol/mol and was completely released by a short alkaline hydrolysis indicating linkage to serine. This and all other aspects of the composition were entirely consistent with the identification of this proteosepeptone as residues 1--28 of the beta-casein molecule. This identity was confirmed by a peptide mapping procedure. Thus proteose-peptone component 8F represents the N-terminal fragment when the gamma1-caseins are formed by proteolysis of beta-casein.

Effect of bovine plasmin on alpha-S1-B and kappa-A caseins

Both alpha-S1- and kappa-caseins were incubated at 37 C in the presence of bovine plasmin (.28 mg/ml) prepared from fresh blood plasma. The electrophoretic pattern of kappa-casein A was unchanged following 60-min incubation with plasmin. However, the electrophoretic band corresponding to alpha-S1-casein B gradually disappeared during the initial 30-min incubation with plasmin. Proteolysis was accompanied by the formation of one polypeptide band with electrophoretic mobility slightly slower than alpha-S1-casein B and several bands with faster electrophoretic mobilities. Two of the faster electrophoretic bands contained phosphorus. Estimates of molecular weights were 20,500, 12,300, and 10,300 daltons for three of these early degradation products of alpha-S1-casein B by plasmin.