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

Bovine antigens and the formation of circulating immune complexes in selective immunoglobulin A deficiency

We have shown that levels of circulating immune complexes are closely associated with the presence of precipitating antibodies to bovine milk proteins in individuals with selective immunoglobin (Ig)A deficiency. To test whether milk proteins are involved in immune complex formation, sera of seven IgA-deficient individuals were studied for the appearance of complexes after milk ingestion. In three of the seven, an initial fall in the level of complexes was followed by an increasing value, which peaked at 120-150 min. In another three, there was a tendency toward the formation of two peaks of complexes, the first at 30-60 min and the second at 120-150 min after drinking milk. One subject, who had had recent treatment for two separate neoplasms, had a steady level of complexes that did not change during the course of this test. After drinking milk, the molecular weight of the complexes found in the sera of one individual at the start of the milk test fell from >19S to 7-11S, and in vitro additions of progressively increasing amounts of a mixture of milk proteins or bovine gamma globulin, to sera that contained complexes produced a progressive reduction in the level of complexes detectable. We conclude that the circulating immune complexes found in some patients who lack IgA contain bovine milk proteins and that periodic fluctuation of the molecular weight of such complexes, depending upon antigen ingestion, appears likely. It remains uncertain what effect the chronic circulation of complexes has upon the clinical state of this group of patients.

The formation and structure of some proteose-peptone components

Two constituents of the proteose-peptone fraction of bovine milk have been isolated and characterized. Component 5 (PP5) has been shown to represent residues 1-105 and 1-107 of the beta-casein amino acid sequence, while component 8-fast (PP8F) corresponds to residues 1-28 of beta-casein. Thus, these proteose-peptones represent the N-terminal portions of the beta-casein molecule, produced by proteolytic cleavages which form the gamma 1, gamma 2 and gamma 3-caseins from the C-terminal part. The continuing formation of the total proteose-peptone fraction, PP5, PP8F and the gamma-caseins during storage of raw milk at 18 or 37 degrees C has been also been demonstrated.

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.

Biosynthesis and secretion of milk proteins: a review

Recent years have seen a great increase in the knowledge and understanding of milk proteins. Arising from several origins including the blood stream and various cellular sources, many of the proteins found in milk are products of the secretory cells directly involved in the synthesis and secretion processes of various milk components. The lactation-specific proteins present in major amounts are synthesized in the rough endoplasmic reticulum (RER) under genetic control and undergo further post-translational modifications in their secretory route from the RER through the Golgi apparatus and secretory vesicles before ejection into the lumen with other milk components. Various molecular aspects of these mechanisms and their control are now understood, but many remain to be described.

Stability of buffalo casein micelles

Buffalo skim-milk is less heat stable than cow skim-milk. Interchanging ultracentrifugal whey (UCW) and milk diffusate with micellar casein caused significant changes in the heat stability of buffalo casein micelles (BCM) and cow casein micelles (CCM). Buffalo UCW dramatically destabilized CCM, whereas buffalo diffusate with CCM exhibited the highest heat stability. Cow kappa-casein stabilizes alphas-casein against precipitation by Ca better than buffalo kappa-casein. About 90% of alphas-casein could be stabilized by kappa:alphas ratios of 0.20 and 0.231 for cow and buffalo, respectively. Sialic acid release from micellar kappa-casein by rennet was higher than from acid kappa-casein in both buffalo and cow caseins, the release being slower in buffalo. The released macropeptide from buffalo kappa-casein was smaller than that from cow kappa-casein as revealed by Sephadex gel filtration. Sub-units of BCM have less sialic acid (1.57 mg/g) than whole micelles (2.70 mg/g). On rennet action, 47% of bound sialic acid was released from sub-units as against 85% from whole micelles. The sub-micelles are less heat stable than whole micelles. Among ions tested, added Ca reduced heat stability more dramatically in whole micelles, whereas added phosphate improved the stability of micelles and, more strikingly, of sub-micelles. Citrate also improved the heat stability of sub-micelles but not of whole micelles.

Role of mammary casein kinase in the phosphorylation of milk proteins

Casein kinase from lactating bovine mammary gland catalyses the transfer of the terminal phosphoryl group of ATP to specific serine residues in dephosphorylated caseins. Best substrates for casein kinase are the dephosphorylated proteins (bovine alpha S1- and beta-caseins and pepsin), unphosphorylated human beta-casein and the dephosphorylated peptide (residues 1-25) from bovine beta-casein. Results obtained with bovine and human beta-caseins indicate that the two serines underlined in the cluster Ser-Leu-Ser-Ser-Ser are particularly susceptible to the action of casein kinase. Since a similar sequence is found in dephosphorylated alpha S1-casein, it is probable that serines in this region of alpha S1-casein are also phosphorylated. The results support the concept that certain serines in casein are particularly susceptible to phosphorylation by casein kinase.

Human beta-casein

Human beta-casein occurs in multiphosphorylated forms having the same amino acid composition but with 0-5 phosphate groups/molecule. Sequence analysis was used to determine whether each of the phosphorylated forms is a mixture of species having a certain number of phosphate groups randomly distributed or whether each form contains phosphate groups on specific seryl or threonyl residues. It was found that forms containing 2, 4 and 5 phosphate groups/molecule are homogeneous with respect to their phosphorylation sites. The monophosphorylated form, however, is a mixture of equal amounts of species phosphorylated at residues 9 or 10.

Separation of the proteins of bovine milk-fat globule membrane by electrofocusing

The proteins of milk-fat globule membrane have been separated by electrofocusing on both the analytical and preparative scale. Over forty separated proteins of the membrane can be identified after electrofocusing in the presence of urea, Triton X-100 and mercaptoethanol with apparent isoelectric points between pH 5.0 and 9.0. At least eight of these proteins appear to contain carbohydrate. After separation by electrofocusing the samples have been further analyzed by electrophoresis in polyacrylamide gels containing sodium dodecyl sulphate. Some of the proteins previously identified as single bands by electrophoresis in SDS are resolved into several components by electrofocusing. The major components of milk-fat globule membrane are a glycoprotein of 67 000 daltons, with an apparent isoelectric point of 5.55, and a protein of 155 000 daltons with an isoelectric point of 7.6. Partially purified fractions of the major proteins and glycoproteins can be obtained after preparative electrofocusing in flat-beds of Sephadex G-75.

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.

The differential enzyme susceptibility of bovine immunoglobulin G1 and immunoglobulin G2 to pepsin and papain

Purified bovine immunoglobulins IgG1 and IgG2 were subjected to enzymatic degradation with pepsin and papain. Results were monitored using density gradient ultracentrifugation, acrylamide electrophosesis and immunodiffusion employing subclass- and light chain-specific antisera. The results indicated a marked enzymatic susceptibility of IgG1 to digestion with pepsin. This differential susceptibility can also be demonstrated in unfractionated bovine gamma-globulin. No striking differences between the two subclasses were observed during treatment with papain in the presence of cysteine and after 24 h, most IgG1 and IgG2 was degraded to Fc and Fab fragments. The pepsin Fc fragment generated from IgG2 was larger than that generated from IgG1 although the F(ab')2 fragments were simialr in size. These results are consistent with the hypothesis that the Fc region of IgG1 contains multiple cleavage sites for pepsin whereas IgG2 has few. Rabbits immunized with the first elution peak from a 30 h pepsin digest of bovine gamma-globulin fractionated on Sephadex G-150, responded primarily to common gamma-chain and IgG2-specific determinants. Thus, the differential susceptibility of bovine subclasses to pepsin provides a method for stimulating IgG2-specific antibodies in rabbits.

Preparation and properties of proteases immobilized on anion exchange resin with glutaraldehyde

High activity alkaline protease was obtained when the enzyme was immobilized on Dowex MWA-1 (mesh 20-50) with 10% glutaraldehyde in chilled phosphate buffer (M/15, PH 6.5). Activity yields of the protease and rennet were 27 and 29, respectively. The highest activities appeared at 60 degrees C, pH 10 for alkaline protease and 50 degrees C, pH 4.0 for rennet. The properties of both proteases were not essentially changed by the immobilization except that the Km values of both enzymes were increased about tenfold as a result of immobilization. Both proteases in the immobilized state were more stable than those in the free state at 60 degrees C. Other peptide hydrolases, beta-galactosidase, invertase, and glucoamylase, were successfully immobilized with high activities, but lipase, hexokinase, glucose-6-phosphate dehydrogenase, and xanthine oxidase became inactive.

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.

Phosphorylation of casein by the lactating mammary gland: a review

The lactating mammary gland synthesizes and secretes large amounts of phosphoproteins that mainly are associated with the casein fraction of milk. The free amino acids and inorganic phosphate of blood serve as building materials for casein, and the final product appears in milk as a colloidal-sized particle, the casein micelle. According to our present concept, the biosynthesis of casein occurs in two steps: synthesis of the polypeptide chain, followed by phosphate addition. Phosphate groups are transferred to the nascent casein by a protein kinase localized in the Golgi apparatus. The enzyme uses adenosine 5'-triphosphate as the phosphate donor and requires divalent cations. Neighboring amino acids may be important in determining which serine residues in casein are phosphorylated. This review discusses historical and current research on the phosphorylation of casein.

Immunoglobulin IgG1 metabolism in new born calves

The half-life of IgG1 immunoglobulin was measured in six neonatal calves following a meal of iodine-125 labeled IgG1 in colostrum derived from their dams. The half-life as measured by the decrease in plasma concentration of IgG1 was 19.9 +/- 1.9 days. However, the half-life as measured by the disappearance of [iodine-125]IgG1 from the plasma was 11.5 +/- .6 days. The latter value is closer to the true half-life because it is not affected by endogenous production of IgG1 by the tissues of the young calf. A decrease in the specific activity of plasma [iodine-125]IgG1 with time representing the body "pool" of IgG1 (half-life 25.8 +/- 6.1 days) suggests that the calf from birth to about 20 days of age is capable of synthesizing a significant amount of IgG1 immunoglobulin.

Kinetic analysis of the binding of immunoglobulins IgG1 and IgG2 to bovine mammary cells

Previous reports were confirmed that specific binding sites exist on bovine mammary cells near parturition presumably involved in the transfer of immunoglobulins IgG1 and IgG2 across the mammary gland at the time of colostrum formation. Determination of the kinetic parameters of these binding sites using 125I-labeled IgG1 and IgG2 immunoglobulins indicated the presence of sites with association constants (Ka) of about 5 - 10(8)--10 - 10(8)M-1 for both subclasses during normal lactation with about 9000 and 3000 sites per cell for each, respectively. The number of IgG1 sites tended to increase as the time of parturition approached. In addition, a new group of sites numbering about 5000 per cell with very strong binding for IgG1 (Ka about 45 - 10(8)M-1) appeared on the cells about a week before parturition. The numbers and affinity of the IgG1 and IgG2 binding sites bear a relationship to the approximate 7 : 1 ratio of these immunglobulin subclasses found in colostrum and normal milk and to the time of maximum colostrum formation. The results support the premise that a highly selective transport mechanism exists in the bovine mammary epithelial cell for the transfer of IgG1 and IgG2 immunoglobulins from blood to the lacteal secretions.

Production and turnover of IgG1 and IgG2 immunoglobulins in the bovine around parturition

Production rates (entry rate into blood plasma) and other metabolic parameters for the IgG1 and IgG2 subclasses of immunoglobulins in mammary secretions (ratio of about 7 to 1) were determined in cows around the time of parturition by both single-injection and continuous-infusion isotope-dilution techniques. Four cows were given a single dose of 150 to 200 muCi of iodine-125 labeled IgG1 and 100 to 250 muCi of iodine-131 labeled IgG2 at 2 to 1 wk before parturition. Four cows, including two of the above, were infused continuously with constant amounts of the labeled immunoglobulins starting at 11 to 4 days before parturition. All cows were maintained until 4 to 6 days after parturition for monitoring the specific activities of iodine-125 labeled IgG1 and iodine-131 labeled IgG2 in the plasma and mammary secretions. Maximum entry rates of IgG1 and IgG2 were between 3 and 1 day prepartum with means of 125 and 60 g/500 kg body weight per day. The exchangeable pool means for IgG1 and IgG2 were 619 and 643 g/500 kg body weight, and both immunoglobulins were divided almost equally between the intra- and extravascular pools. A greatly increased production and a shortened half-life or greater turnover for plasma IgG1 occurs around the time of parturition which can account for the large accumulation of IgG1 in colostrum.