Reviews of the progress of dairy science. Section C. Chemistry of milk proteins

Bovine milk acid phosphatase: II. Binding to casein substrates and heat-inactivation studies

The acid phosphatase of bovine milk was further purified to yield enzyme with an activity of about 2 units/mg. This was almost 105 times the activity present in milk and enabled a detailed study of heat inactivation to be made, together with further measurements on binding to casein substrates.

The effectiveness of caseins as inhibitors of the hydrolysis of p-nitrophenyl phosphate by acid phosphatase paralleled the phosphate content of the casein molecules, so that αs1-casein A was a more potent inhibitor with a K1 of 1·7 mM than β-casein A1A2 (K1 = 4·3 mM), which in turn was more inhibitory than κ-casein A (K1 = 5·9 mM).

The heat inactivation of acid phosphatase followed first-order kinetics at pH 4·9, 5·2 and 6·7 and values of E, the activation energy, were between 2·4×105 and 3·0×105 J mole −1 in all cases, consistent with simple protein denaturation. The presence of 1% αs1-casein A, 1% β-casein A1A2, 1% κ-casein A, 1% isoelectrically precipitated ‘whole’ casein and 1% fresh raw milk provided no substrate protection at pH 5·2 or 6·7. Acid phosphatase was somewhat less heat stable at pH 6·7 than at pH 4·9, but may be expected to survive typical milk pasteurization conditions almost completely. However, conventional milk sterilization or ultra-high-temperature (UHT) processes would be expected to give total inactivation.

Chemical changes in ultra-heat-treated milk during storage: I. Hydrolysis of casein by incubation with pronase and a peptidase mixture

Casein samples from untreated milk and stored ultra-heat-treated (UHT) milk were hydrolysed with pronase (protease ex Streptomyces griseus K-1) and subsequently with a mixture of peptidases prepared from the microsomal fraction of hog kidneys. Incubation of casein from unheated milk with pronase alone hydrolysed 70–80% of peptide bonds involving Ile, Leu, Tyr, Phe and His residues; other amino acids were released less well and proline hardly at all. The pronase/peptidase treatment resulted in 90–100% hydrolysis of peptide bonds involving all amino acids, including proline.

Caseins from stored UHT milks were more resistant to proteolysis than casein from unheated milk. Reduced release of all amino acids was observed from samples taken after storage at 37 °C for 12 months or longer and for Lys, Arg and Asn residues from samples taken after storage at 30 °C for 14 months. Resistance to proteolysis was attributed to the Maillard reaction between milk proteins and lactose during storage of UHT milk.

Heat stability of milk: influence of denaturable proteins and detergents on pH sensitivity

α-Lactalbumin and SDS in addition to β-lactoglobulin introduced pH sensitivity to the heat stability–pH curve of serum protein free casein micelles particularly by increasing stability in the pH range 6·4–6·7. Bovine serum albumin, ovalbumin and lysozyme caused marked destabilization of milk and casein micelle suspensions throughout the pH range 6·4–7·4. Tetramethyl ammonium bromide caused destabilization of milk at pH values > 7·0, but had no effect in the region of maximum stability while the non-ionic detergents Triton X-100 and Tween 80 had no effect on heat stability.

Compression and impact strength of gels, prepared from fractionated whey proteins, in relation to composition and microstructure

Gels varying in protein content and composition were prepared from whey proteins by a standard method and their compression and impact strengths were determined. No surface friction could be observed between the gels and the compression plates, and the Poisson's ratio was close to 0·5. Thus, the elastic moduli of the gels were determined. Compression and impact strengths and elastic moduli were related logarithmically to the β-lactoglobulin content of the protein powders. The strength and manner of fracture of the gels depended on the gel microstructure. Strong gels had a honeycomb structure consisting of a very tightly bound network, and failed by cracking on impact and by shear along cleavage planes in compression. Weaker gels had a loose open network with no definite means of fracture.

The effect of preheat temperature and urea addition on the seasonal variation in the heat stability of skim-milk powder

The manufacture of skim-milk powder with heat stable characteristics was investigated commercially during the course of 8 trials carried out over a 12-month period. Skim-milk was preheated to temperatures ranging from 110 to 130 °C with a 2-min holding time prior to evaporation and drying. The effect of added urea was also examined during each trial run. Heat coagulation times at 120 °C were determined upon reconstitution of the powders to 20% total solids. From February to April the heat stability of the skim-milk powders increased, with the more heat stable samples being obtained at the higher preheating temperatures. Addition of urea increased the heat stability, particularly so in those milks which had been preheated to 130 °C. The presence of added urea during preheating was not found to be necessary as an equal effect on heat stability was observed when urea was introduced at the concentrate stage before drying. Later in the season optimum heat coagulation times were obtained by maintaining preheating temperatures at 118–120 °C.

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.

The effect of the chemical structure of additives on the coagulation of casein micelle suspensions by rennet

Casein micelles in milk-salts solution adsorbed charged detergents and highly-charged polypeptides strongly, neutral detergents less strongly and low molecular-weight amines weakly. A tetra-amine was adsorbed more strongly than a tri-amine. The extent of adsorption of proteins tended to rise as the molecular weight increased. Glycerol and lactate were adsorbed to a limited extent but dextran and α-ketoglutarate were not adsorbed at all. Proline was partly adsorbed, indicating that hydrophobic binding sites were available, and caused some disruption of the casein micelles. Additives were bound to approximately the same extent by casein micelles and rennet coagula. The proportions adsorbed were constant over at least 10-fold ranges of concentration. Additives which increased the rennet clotting time (RCT) acted by binding Ca2+. Most additives decreased the RCT, the extent increasing with the amount adsorbed and the positive charge on the additive. The greatest reduction in RCT was observed with those additives which had positively-charged and hydrophobic moieties and bound most strongly to casein micelles. Of the additives tested, only sodium dodecyl sulphate affected the enzymic action of rennet. The reduction in RCT may have resulted from the neutralization of the negative charge of the micelles or enhancement of their hydrophobicity, favouring hydrophobic interactions between the particles.

Increasing cheese yields by high heat treatment of milk

Milk was heated at 97 °C/15 s to denature ∼ 30% of the whey protein and then used to make Cheshire cheese. Measurements of para-κ-casein production indicated that heating milk did not inhibit the enzymic action of rennet, but additional Ca and an initial pH of 6·4 were required for normal coagulation and curd-firming. In the experimental cheeses, about 4·5% more dry matter was recovered compared with controls made with pasteurized milk owing to a 6·7 and 0·7% increase in protein and fat recovery respectively. Experimental cheeses tended to be too moist and the curds did not fuse as well but these problems could be overcome by raising the scald temperature and cheddaring the curds. When the moisture in non-fat solids of control and experimental cheeses was similar (61%), the flavours and textures were not significantly different. The procedure requires little modification to existing commercial plant and should be suitable for varieties of cheese with higher moisture content and more crumbly texture than Cheddar.

Invited Review: β-Lactoglobulin: Binding Properties, Structure, and Function

beta-Lactoglobulin (beta-LG) is the major whey protein of ruminant species and is also present in the milks of many, but not all, other species. Its amino-acid sequence and 3-dimensional structure show that it is a lipocalin, a widely diverse family, most of which bind small hydrophobic ligands and thus may act as specific transporters, as does serum retinol binding protein. Bovine beta-LG binds a wide range of ligands, but this may not be the reason for its presence in milk. In reviewing the structure and physicochemical properties of the protein, we present the structures of the ligands cholesterol (at a resolution of 2.0A, R = 0.221; Rfree = 0.295) and vitamin D2 (at a resolution of 2.4A, R = 0.212; Rfree = 0.297) each bound to the central binding cavity of bovine beta-LG at pH 7.3. Neither ligand is fully visible in the electron density maps, and the less well-ordered regions are the polar end groups at the mouth of the binding site. In a separate experiment, a mercury ion was bound to the free Cys121 (at a resolution of 2.2A, R = 0.218; Rfree = 0.288) in a way that transmitted a small structural change through Asp137 via Arg148 to the dimer interface. It is not clear if the known dissociation that arises from the reaction of beta-LG with HgCl2 results from this perturbation. In reviewing the structural studies that reveal the ligand binding sites for long-chain fatty acids, retinoids, and steroids, only the central location, common to all lipocalins so far examined, is occupied under the conditions used. We find that there is no crystallographic evidence of another ligand binding site in our crystals grown in approximately 1.3 M citrate, although low ionic strength studies in solution indicate the possible presence of at least one other low affinity site. The apparent ability of the binding site to accommodate a wide range of ligands may point to a possible physiological function. However, by considering the lipocalin family in general, and the species distribution of beta-LG in particular, some speculation as to the physiological function can be made. beta-Lactoglobulin has been reported as being implicated, inter alia, in hydrophobic ligand transport and uptake, enzyme regulation, and the neonatal acquisition of passive immunity. However, these functions do not appear to be consistent between species. Sequence comparisons among members of the lipocalin family reveal that glycodelin, found in the human endometrium during early pregnancy, is the most closely related to beta-LG. Although the function of glycodelin is also unknown, it appears to have effects on the immune system and/or to be involved in differentiation. It is proposed that beta-LG, over-expressed in the lactating mammary gland of many, but not all, species, is primarily an important source of amino acids for the offspring of those animals that produce it, but that this function arose by gene duplication from the physiologically essential glycodelin. The other functions that have been associated with beta-LG in the neonate are, therefore, fortuitous.

The core lipocalin, bovine beta-lactoglobulin

The lipocalin family became established shortly after the structural similarity was noted between plasma retinol binding protein and the bovine milk protein, beta-lactoglobulin. During the past 60 years, beta-lactoglobulin has been studied by essentially every biochemical technique available and so there is a huge literature upon its properties. Despite all of these studies, no specific biological function has been ascribed definitively to the protein, although several possibilities have been suggested. During the processing of milk on an industrial scale, the unpredictable nature of the process has been put down to the presence of beta-lactoglobulin and certainly the whey protein has been implicated in the initiation of aggregation that leads to the fouling of heat exchangers. This short review of the properties of the protein will concentrate mainly on studies carried out under essentially physiological conditions and will review briefly some of the possible functions for the protein that have been described.

beta-lactoglobulin binds palmitate within its central cavity

Bovine beta-lactoglobulin (beta-Lg) has been studied extensively in both the isolated and the naturally occurring states. It is a commercially important whey protein of obvious nutritional value but, so far, one that has no clearly identified biological function. In common with many of the other members of the lipocalin family to which it belongs, beta-Lg binds hydrophobic ligands, and it appears possible that there are at least two distinct binding sites per monomer for a variety of ligands. By comparison with other members of the family, there is a probable binding site in the central cavity of the molecule that is formed by the eight antiparallel beta-strands that are typical of the lipocalins. We have now cocrystallized beta-Lg with palmitic acid, and the refined structure (R = 0.204, Rfree = 0.240 for 6,888 reflections to 2.5-A resolution) reveals that the ligand binds in the central cavity in a manner similar to the binding of retinol to the related lipocalin, serum retinol-binding protein. The carboxyl group binds to both Lys-60 and Lys-69 at the entrance to the cavity. The hydrophobic tail stretches in an almost fully extended conformation into the center of the protein. This is the first direct observation of a ligand binding to beta-Lg.

Isotopically labeled bovine beta-lactoglobulin for NMR studies expressed in Pichia pastoris

beta-Lactoglobulin (beta-Lg) is the major whey protein in ruminant milk and has been implicated in the irreversible denaturation of milk proteins and its associated poor processing behavior during heat treatment. In order to help understand this behavior, as well as to facilitate other studies into the relationship between the molecular structure and its behavior in solution, we have prepared and purified 15N-labeled and 13C/15N-double-labelled beta-Lg in sufficient quantities to permit a full determination of the structure and dynamics using heteronuclear NMR spectroscopy. The overexpression of the labeled protein using the Pichia pastoris system proceeds with good yield but requires the removal of significant quantities of copurifying carbohydrate which otherwise interfere with the NMR experiments. At pH 2, the resulting material gives triple resonance NMR spectra of good quality that are consistent with a monomeric, globular protein rich in beta-sheet.

Preparation and evaluation of microspheres prepared from whey protein isolate

Whey protein (WPI) microspheres were successfully produced containing hydrochlorothiazide, eosin, patent blue violet and sodium salicylate using a w/o emulsification method with glutaraldehyde cross linking. The release of these compounds from WPI microspheres occurred rapidly, with at least 70% of the incorporated material released for all systems within the first 20 min. Release of microsphere payload was essentially complete within 1 h. The degree of glutaraldehyde cross-linking was found to have no effect on the release profile for durations of cross-linking up to 24 h. Of a range of release equations examined, the experimental release data was best described by a biexponential equation and this agrees with the work of Tomlinson et al. (1984) for the release of drugs from albumin microspheres. Swelling of the microsphere systems was examined as this may contribute to the rapid release of drug from these systems.

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

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

Expression and secretion of recombinant ovine beta-lactoglobulin in Saccharomyces cerevisiae and Kluyveromyces lactis

High expression and secretion of recombinant ovine beta-lactoglobulin has been achieved in the yeast Kluyveromyces lactis. The yield of beta-lactoglobulin is 40-50 mg per litre of culture supernatant and accounts for approx. 72% of the total secreted protein. Constitutive expression is under the control of the Saccharomyces cerevisiae phosphoglycerate kinase promoter from an intronless version of the beta-lactoglobulin gene. Secretion is specified by the ovine protein's own signal sequence. this system, coupled to an efficient and novel recovery protocol, allows 30 mg of pure protein to be isolated from a typical 1 litre culture. The protein is virtually indistinguishable from beta-lactoglobulin conventionally purified from sheep milk by its behaviour in native PAGE and SDS/PAGE, reactivity to antibodies, CD, fluorescence spectroscopy and N-terminal sequencing. Attempts to achieve a similar expression and secretion system in the yeast S. cerevisiae met with only limited success, although it was found that heat-shock treatment modestly increased the yield up to approx. 3-4 mg per litre of culture supernatant. Site-directed mutagenesis showed that secretion in S. cerevisiae depended upon correct formation of the two disulphide bonds present in beta-lactoglobulin.

Transglutaminase catalyses the modification of glutamine side chains in the C-terminal region of bovine beta-lactoglobulin

The transglutaminase-catalysed incorporation of primary amines (putrescine and monodansylcadaverine) into bovine beta-lactoglobulin has been studied. In the presence of 1 mM-dithiothreitol between 1 and 2 mol of amine can be incorporated per mol of beta-lactoglobulin subunit. There is very little incorporation of amines in the absence of reducing agent. By isolating and sequencing the modified peptides, the sites of modification have been identified as Gln-159 (preferred) and Gln-155. C.d. has been used to study the structure of beta-lactoglobulin over a range of pH values and in the presence or absence of dithiothreitol. The results are discussed in terms of the X-ray-crystallographically determined structure of beta-lactoglobulin.

Large scale extraction of alpha-lactalbumin and beta-lactoglobulin from bovine whey by precipitation with polyethylene glycol and partitioning in aqueous two-phase systems

The milk proteins alpha-lactalbumin and beta-lactoglobulin have been isolated from bovine whey by fractional precipitation with polyethylene glycol (PEG) and hydrophobic partitioning in aqueous PEG-hydroxypropylstarch two-phase systems using PEG-bound palmitate as hydrophobic ligand. The possible use of this combination for large scale purification of these whey proteins is discussed.

The secondary structure of peptides derived from caseins: a circular dichroism study

Three peptides have been formed by proteolytic digestion of individual casein proteins and their secondary structures characterised by far-UV circular dichroism (CD). Peptide alpha s1(1-23), residues 1-23 of alpha s1-casein, was generated by treatment of the parent protein with chymosin. Peptides beta(1-28) and beta(1-52), residues 1-28 and 1-52 of beta-casein, were plasmin- and chymotrypsin-generated fragments, respectively. Analysis of the CD spectra revealed that in aqueous solution all three peptides have secondary structures composed exclusively of beta-sheet and random coil. A limited amount of alpha-helix was formed in two of the three peptides upon treatment with high concentrations (greater than 40% (v/v] of 2,2,2-trifluoroethanol. Partial dephosphorylation (60%) of beta(1-28) and beta(1-52) by treatment with alkaline phosphatase resulted in homogeneous preparations, as judged by polyacrylamide gel electrophoresis, which exhibited increased hydrophobicity. This reduction in the level of phosphorylation of serine residues 15, 17, 18 and 19 led to increased propensity for helix formation in the peptides in the presence of 2,2,2-trifluoroethanol, but no alpha-helical structures were detected in the dephosphorylated peptides in the absence of 2,2,2-trifluoroethanol.

The structure of beta-lactoglobulin and its similarity to plasma retinol-binding protein

Since its first isolation, bovine beta-lactoglobulin (BLG) has been an enigma: although it is abundant in the whey fraction of milk, its function is still not clear. The results of the many physicochemical studies on the protein need a structural interpretation. We report here the structure of the orthorhombic crystal form of cow BLG at pH 7.6, at a resolution of 2.8 A. It has an unusual protein fold, composed of two slabs of antiparallel beta-sheet, which shows a remarkable similarity to plasma retinol-binding protein. A possible binding site for retinol in BLG has been identified by model-building. This suggests a role for BLG in vitamin A transport and we have discovered specific receptors for the BLG-retinol complex in the intestine of neonate calves.

Ovine beta-lactoglobulin messenger RNA: nucleotide sequence and mRNA levels during functional differentiation of the mammary gland

The nucleotide sequence of ovine beta-lactoglobulin mRNA has been determined by chemical sequencing of two cDNA recombinant plasmids and primer extension products. Ovine beta-lactoglobulin mRNA consists of a 540 nucleotide coding region, flanked by 39 nucleotide 5' and 206 nucleotide 3' non-coding regions including a 20 nucleotide poly A tail. The deduced 180 amino acid sequence of pre-beta-lactoglobulin is in agreement with the previously published amino acid sequence of signal peptide and mature protein. Northern blot analysis of poly A+ RNAs from the lactating mammary glands of porcine, rabbit and rat species, allowed us to identify a homologous RNA to beta-lactoglobulin mRNA solely in the porcine species. We also detected a mRNA transcript of a size similar to that of beta-lactoglobulin mRNA in hepatic poly A+ RNA from female rat liver treated by estrogens. Furthermore, we have examined the levels of beta-lactoglobulin mRNA during the functional differentiation of the mammary gland and after hormonal stimulation. During the last third of pregnancy, the expression of beta-lactoglobulin gene is significantly more elevated than that of alpha s1- or beta-casein whose mRNA levels were found to change very slightly during this period. Both beta-lactoglobulin and casein mRNAs showed a rapid response and a wide range of change in response to cortisol treatment. However, there was a significant difference in the rate at which these processes occurred, suggesting that beta-lactoglobulin gene expression is regulated independently of the casein genes.

Effects of temperature on food proteins and its implications on functional properties

This article surveys the knowledge in the area of protein structure and chemistry of denaturation prior to an indepth review of the effects of heat on soy, milk, and egg proteins. It also reviews the methods available to assess denaturation of proteins. Protein denaturation is an ambiguous phenomenon and the consequences of denaturation on the functional properties of proteins is further confounded by this ambiguity. For each of the three food proteins, the known chemistry of individual proteins is reviewed followed by observations made on changes induced by heat in each protein group. Food proteins are not pure entities and purification and physicochemical characterization of various components of the food proteins have not been thoroughly investigated. Further, food is a complex milieu of water, fat, carbohydrate, vitamin, minerals, etc. along with proteins, and processing affects not only each individual component in the food but also the nature and intensity of intercomponent interactions in a food.

Homology of beta-lactoglobulin, serum retinol-binding protein, and protein HC

The milk protein beta-lactoglobulin has been extensively studied but its function has not been identified. A clue regarding the function of a protein can be obtained by discovering a genetic relationship with a protein of known function through comparisons of amino acid sequence. Such comparisons revealed that beta-lactoglobulin is similar to human serum retinol-binding protein and to another human protein of unknown function known as complex-forming glycoprotein heterogeneous in charge (protein HC). beta-Lactoglobulins from several species have been found to bind retinol, while the absorption and fluorescence properties reported for the unidentified heterogeneous prosthetic group of protein HC are retinoid-like. The role of serum retinol-binding protein in vitamin A transport in the circulation suggests that the other two homologous proteins may function in the binding and transport of retinoids; beta-lactoglobulin may facilitate the absorption of vitamin A from milk and protein HC may mediate the excretion of retinol-derived metabolites.

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.

Milk antigen absorption in the preterm and term neonate

The concentrations of beta-lactoglobulin was measured in the sera of 47 preterm an term neonates during the first few days of life under standardised conditions after feeding with a cows' milk'based formula. Preterm neonates, particularly those of less than 33 weeks' gestation, had higher serum concentrations of beta-lactoglobulin than term neonates given an equivalent mild feed. Prior feeding with breast milk did not diminish the amount of beta-lactoglobulin absorbed. Our results suggest tha te ability of the gastrointestinal tract to exclude antigenically intact food proteins increases with gestational age and that gut closure occurs normally before birth in man.

Measurement of thermal denaturation of beta-lactoglobulin at pH 2.5

Solutions of beta-lactoglobulin (approximately 1%) at pH 2.5, 4.5, and 6.5 were heated at 90 C for 30 min, cooled rapidly to room temperature (23 to 25 C), and examined for nature and extent of denaturation by various methods. Heating at pH 4.5 and at 6.5 caused almost complete denaturation as judged by loss of solubility at pH of heating, at pH 4.5, or in 2% trichloroacetic acid. However, after heating at pH 2.5 the protein remained in solution but partially lost solubility in 2% trichloroacetic acid and at pH 4.5. The protein fraction denatured by heating at pH 2.5, rendered insoluable at pH 4.5, was unfolded partially and irreversibly, since when resolubilized at pH 2.5 it showed increased optical levorotation and demonstrated no endothermic reaction by differential scanning calorimetry. This material was eluted at lower eluant volumes from a Sephadex G-100 column. However, it was unchanged in molecular size as judged by electrophoretic analysis and measurements of sedimentation velocity. The pH 4.5-soluble fraction from the sample heated at pH 2.5 was similar to native beta-lactoglobulin by the above methods.

The amino acid sequence of goat beta-lactoglobulin

The isolation of beta-lactoglobulin from milk of the goat is described. The purified protein was checked for purity and has been characterized by its gross composition and end groups. The native or the modified protein was then degraded by tryptic and cyanogen bromide cleavage. The cleavage products were isolated and sequenced in the sequenator using a Quadrol and propyne program. These data provide the complete sequence of beta-lactoglobulin of the goat. The results are discussed and compared particularly with bovine beta-lactoglobulin components AB. Some biological aspects are described.

The reaction of cow beta-lactoglobulin with tetracyanoaurate(III)

The reaction of cow beta-lactoglobulin with Au(CN)-4 is shown to label the free sulphydryl group on the protein by a reductive mechanism yielding a stable complex. High salt concentrations are shown to affect the pH-dependent conformational transition (N in equilibrium R yields S). It is hoped that Au(CN)-4 may be of more widespread use as a specific heavy-atom label for the isomorphous replacement method.

Comparative aspects of milk proteins

The current status of knowledge of the major proteins of milks of various species is evaluated. Most of the non-bovine milk proteins are homologous with the recognized families of those of Bos taurus, alpha S1-caseins, alpha S2-caseins, beta-caseins, kappa-caseins, beta-lactoglobulins, and alpha-lactalbumins, each family representing a separate genetic locus specific to the mammary gland. No prominent milk protein not homologous to one of these families has yet been discovered in milk of any species. Genetic polymorphism resulting from substitutions in the polypeptide chains and various degrees of post-translational phosphorylation, glycosylation, and proteolysis have been identified in milk proteins of several species. Total protein production ranges among species from about 0.5 to 10 g/d per kg0.75 maternal weight. Proportions of the several proteins vary greatly among species, but few accurate analytical data are available except for total casein and total whey protein contents.

The interfacial behaviour of three food proteins studied by the drop volume technique

The adsorption behaviour of three food proteins, a soy protein isolate, a sodium casein‐ate and a whey protein concentrate, at the air‐water interface has been studied by the drop volume method. The kinetics of surface tension decay were evaluated in terms of different rate‐determining steps at different ionic strengths and concentrations. This analysis indicates the following characteristics concerning the surface behaviour of the protein systems studied. The soy proteins diffuse slowly to the interface compared to the other proteins, probably due to the large particle size of the association complex of soy proteins. For the soy proteins, diffusion is slower in distilled water than in 0.2M‐NaCl solution and spreading of molecules at the interface is most easily performed in 0.2M‐NaCl solution. The whey proteins diffuse quickly to the interface, which is in accordance with their aqueous association; mainly small molecular complexes. Diffusion is slower and spreading easier in distilled water than in 0.2M‐NaCl solution. Although the caseinate has a complex quaternary structure, like the soy proteins, it has a very different surface behaviour. The diffusion step is rapid at concentrations above 10−3 wt % and contributes to a large extent to the interfacial tension decay, especially when the caseinate is dispersed in 0.2M‐NaCl solution. At a concentration of 10−3 wt % and below, the rate of the diffusion step is slowed down drastically, with an accompanying drop in the surface activity of the protein. This type of surface behaviour can be explained if the migration of the caseinates to the interface takes place via the casein monomers in the bulk phase.

Progressive changes in individual milk protein concentrations associated with high somatic cell counts

Progressive changes in the concentrations of milk protein components were followed after infusions ofStreptococcus agalactiaeor bacterial endotoxin into different quarters of individual cows. Both types of infusion produced similar increases in somatic cell count and resulted in similar changes in milk proteins, although the effects of the endotoxin infusion lasted for a shorter length of time.

The treatments had little effect on α-lactalbumin and β-lactoglobin concentrations, but serum albumin and immunoglobulin (Ig) concentrations increased markedly. The greatest effect on serum albumin was after the endotoxin infusion and on Ig after theStr. agalactiaeinfusion. Changes in the individual globulins indicated that passive transfer of blood proteins to milk could not account for the observed increases in IgM and IgA.

αs1-Casein and β-casein concentrations were reduced and inversely related to somatic cell count during the immediate post-infusion period, and this was accompanied by an increase in para-κ-casein. Para-κ-casein was not detected in pre-infusion or post-recovery milk samples. The decrease in β-casein was greater than that of αs1-casein. Casein concentrations returned to pre-infusion levels 2 d and 5 d after the endotoxin andStr. agalactiaeinfusions respectively.

The possibility that proteolytic enzymes are partly responsible for the changes in casein concentration is discussed.

Heterogeneity in alpha-lactalbumins. I. Human alpha-lactalbumin

alpha-Lactalbumin from human milk shows an heterogeneous behaviour when subjected to ion exchange chromatography with DEAE-Sephadex. Two components have been separated, showing identical patterns in the following studies: amino acid compositions, fluorescence and circular dichroism spectra, transition temperature of denaturation, antigenicity, lactose synthase specifying activity and hydrodynamic properties. After rechromatography of either peak, these two components appeared to be in equilibrium. This equilibrium varies with the temperature and the pH of chromatography. Moreover, an increase of n-alcohol concentration in the eluting buffer also induces an increase of the second protein peak eluting at higher ionic strength. These two peaks seem to be the result of some conformational change induced upon the binding of the protein to the solid anionic matrix.

Nomemclature of the proteins of cow's milk: fourth revision

This report reviews the nomenclature of the milk proteins of cow's milk in light of more recent advances in our knowledge. With the establishment of the primary structures of a number of these proteins, we now have a definite identification of alphas1-, kappa-, beta-, and the gamma-caseins as well as beta-lactoglobulin and alpha-lactalbumin. On the basis of new information on their primary structures and relationship to beta-casein polymorphs, changes in nomenclature have been recommended for proteins of the gamma-casein fraction. Although the primary structure serves as the unambiguous definition of proteins for which it is known, a more practical identification is necessary. We recommend that their behavior in gel electrophoresis under suitable conditions be employed for this purpose for all of the "major" milk proteins of raw skim milk except the immunoglobulins where, because of their heterogeneity and molecular genetics, physical parameters are less useful and their identification must be based upon antigenic determinants and their homology with their human counterparts. More work is needed and, with the accumulation of more information, additional changes in nomenclature can be expected for such proteins as the minor components of alphas- and kappa-caseins, alpha-lactalbumin, and the proteose-peptone fraction as well as further confirmation of the presence of immunoglobulins IgE and additional IgG subclasses. Additional components and genetic variants also can be expected.