A review and proposed nomenclature for major proteins of the milk-fat globule membrane

The characteristics and possible functions of the most abundant proteins associated with the bovine milk-fat globule membrane are reviewed. Under the auspices of the Milk Protein Nomenclature Committee of the ADSA, a revised nomenclature for the major membrane proteins is proposed and discussed in relation to earlier schemes. We recommend that proteins be assigned specific names as they are identified by molecular cloning and sequencing techniques. The practice of identifying proteins according to their Mr, electrophoretic mobility, or staining characteristics should be discontinued, except for uncharacterized proteins. The properties and amino acid sequences of the following proteins are discussed in detail: MUC1, xanthine dehydrogenase/oxidase, CD36, butyrophilin, adipophilin, periodic acid Schiff 6/7 (PAS 6/7), and fatty acid binding protein. In addition, a compilation of less abundant proteins associated with the bovine milk-fat globule membrane is presented.

Xanthine oxidase binding to glycosaminoglycans: kinetics and superoxide dismutase interactions of immobilized xanthine oxidase-heparin complexes

Xanthine oxidoreductase (XDH + XO, EC 1.2.3.2) is released into the circulation from organs rich in XO activity. Herein we report the specific high affinity binding of XO to glycosaminoglycans (GAGs) and the preferential association of XO with heparin, compared with heparan sulfate, chondroitin sulfate, and dematan sulfate. The binding of XO to Sepharose 6B-conjugated heparin (HS6B) occurs at physiological ionic strength and increased with pH, with Scatchard analysis revealing a nonlinear binding pattern at pH 7.4. The dissociation constant (Kd) for XO binding was 0.4 to 1.8 x 10(-7) M, similar to the heparin-reversible binding of lipoprotein lipase to vascular endothelium. The binding energy of 9-13 kcal/mol was concordant with noncovalent electrostatic interactions. Xanthine oxidase immobilization to HS6B rendered a catalytically active enzyme from that had kinetic characteristics distinct from XO in free solution. While the Km and Ki for xanthine in phosphate buffer at pH 7.4 were 3 microM and 1.6 mM, respectively, for free XO, they were 15 microM and 2.8 mM for immobilized XO. Inhibition constants for guanine and uric acid were also increased upon XO binding to HS6B. Changes in kinetic parameters were related to a real and not apparent decrease in binding affinity for substrate and inhibitors and were not due to diffusion-controlled processes within the gel matrix. Changes in Km and Ki for xanthine also had a significant influence on the relative quantities of O2.- and H2O2 generated by a given substrate concentration. Superoxide formed by HS6B-bound XO was partially consumed within the gel microenvironment which electrostatically excluded CuZn SOD. Immobilization of XO increased the half-life of enzyme activity in buffer and in the absence of substrate from 67 to 120 h at 4 degrees C. These data indicate that binding to cell surfaces will strongly influence the catalytic properties, oxidant producing capacity, and stability of XO.

Production and characterization of monoclonal antibodies directed against bovine milk fat globule membrane (MFGM)

Nine hybridomas secreting monoclonal antibodies (mAbs) to bovine milk fat globule membrane (MFGM) were produced from spleen cells of three immunized BALB/c mice. Several MFGM antigens recognized by some mAbs were identified as a 120 kDa protein and 67 kDa (butyrophilin), 57 kDa (PAS-6), 53 kDa (PAS-7), 33 kDa glycoproteins. The other mAbs secreted by four independent hybridoma clones recognized many broad bands ranging from 20 to 200 kDa. The 120 kDa protein and 67 kDa, 57 kDa, 53 kDa glycoproteins were detected by each mAb in the plasma membrane fraction prepared from a lactating bovine mammary gland. Moreover, mammary gland epithelium of a thin section was specifically stained with these mAbs, indicating that these mAbs directed against MFGM recognized membrane proteins and glycoproteins of lactating mammary epithelial cells. Upon heating of the MFGM in phosphate buffer, pH 7.4 at 100 degrees C for 10 min, the antigens still retained most of its reactivity to these mAbs, whereas, proteolytic cleavage by trypsin and chymotrypsin strongly reduced its reactivity to these mAbs by 60% or more except for two mAbs which recognized the 57 and 53 kDa glycoproteins, respectively.

A review of the molecular and cellular biology of butyrophilin, the major protein of bovine milk fat globule membrane

The molecular and cellular biology of the milk protein butyrophilin is reviewed. Butyrophilin constitutes more than 40% by weight of the total protein associated with the fat globule membrane of bovine milk. Closely related proteins are abundant in the fat globule membranes of many other species. Butyrophilin is synthesized as a peptide of 526 amino acids with an amino-terminal hydrophobic signal sequence of 26 amino acids, which is cleaved before secretion in association with the fat globule membrane. Hydropathy analysis and in vitro translation of butyrophilin mRNA indicate that the protein associates with membranes in a type I orientation via a single stretch of 27 hydrophobic amino acids in the approximate middle of the sequence. Evidence that butyrophilin is incorporated into fat globule membrane as a transmembrane protein and as a cytoplasmically oriented peripheral component is discussed. The carboxy-terminal sequence of butyrophilin is significantly homologous to two other proteins: ret finger protein and the 52-kDa nuclear antigen A of Sjögren's syndrome. Expression of bovine butyrophilin mRNA correlates with the onset of milk fat secretion toward the end of pregnancy and is maintained throughout lactation. The possible function of butyrophilin in the secretion of milk lipid droplets is discussed.

Characteristics of purified cows' milk xanthine oxidase and its submolecular characteristics

Xanthine oxidase (EC 1.2.3.2) was purified from fresh cows' milk by differential centrifugation and hydroxylapatite chromatography in the absence of reducing agents and proteases. The purified isolate possessed an absorbance at 280 nm:absorbance at 450 nm ratio of 4.84; an absorbance (1 cm at 280 nm 1%) of 11.9; an activity:absorbance at 450 nm of 141, a specific activity of 3.59 units/mg; and detectable dehydrogenase activity. The enzyme preparation was obtained in a reversible oxidase form that could be partially converted to xanthine dehydrogenase in the presence of 10mM dithiothreitol or 1% mercaptoethanol. Amino acid analyses revealed that the enzyme was hydrophobic in nature and that lysine constituted its N-terminal residue. The protein contained 22 disulfide and 38 sulfhydryl groups, four of which were detectable in the undenatured protein complex. Discontinuous PAGE in the presence of selected dissociation agents did not result in further resolution. Sodium dodecyl sulfate-PAGE of the purified enzyme revealed a sharp zone with a molecular weight of 151,000 +/- 4000 (i.e., monomer). The purified enzyme exhibited oxidase activity in the presence of 6 M urea and following limited proteolysis by trypsin, chymotrypsin, plasmin, pancreatin, pepsin, and papain. Proteolyzed xanthine oxidase migrated as a single zone in polyacrylamide gels in the presence and absence of dissociating agents such as 1% mercaptoethanol and 6 M urea. Restricted digestion of xanthine oxidase by proteases was indicated by the presence of three major zones with molecular weights ranging from 85,000 to 100,000, 30,000 to 35,000, and 18,000 to 20,000 commonly observed in SDS gels. Amino acid profiles of the principal peptidyl fragments of trypsin-cleaved xanthine oxidase indicated their hydrophobic nature and lysine as the N-terminal residue for all fragments.

Histochemical localization and possible antibacterial role of xanthine oxidase in the bovine mammary gland

Xanthine oxidase (XO) was demonstrated to be present in the teat canal and secretory tissue of the bovine mammary gland by histochemical techniques. Homogenates of these tissues were able to replace XO in an antibacterial assay with Streptococcus uberis. The action of XO on its substrate hypoxanthine was shown to provide an essential component for anti-streptococcal activity mediated by lactoperoxidase. A mechanism is proposed whereby the interaction of XO, lactoperoxidase and thiocyanate may provide antibacterial activity in the teat canal.

Association of sulfhydryl oxidase and xanthine oxidase in bovine mammary tissue

The presence of antigenically active xanthine oxidase was indicated in various relatively purified preparations of sulfhydryl oxidase obtained from bovine milk. Evidence for formation of a complex of the two enzymes was obtained by double immunodiffusion. Furthermore, sodium dodecylsulfate-gel electrophoresis of sulfhydryl oxidase and xanthine oxidase model mixtures indicated that high molecular weight species were present that reacted with both antisulfhydryl oxidase and antixanthine oxidase. Similar gel electrophoretic patterns visualized by protein-dye binding methods revealed a distinct band (greater than 200 kdalton) was formed upon incubation of mixtures of the two enzymes, the presence of which was unaffected by reduction of protein disulfide bonds. Immunofluorescent staining techniques showed both enzymes in the apical plasma membrane. Because sulfhydryl oxidase previously has been shown to catalyze conversion of the dehydrogenase form of xanthine oxidase to the oxidase form, this conversion may occur when xanthine oxidase contacts sulfhydryl oxidase in the apical plasma membrane. This conversion and the resulting potential for production of active oxygen species could be significant to membranotropic processes, such as fat globule secretion, and to the oxidative stability of the milk fat globule membrane.

Identification and characterization of the principal proteins of the fat-globule membrane from guinea-pig milk

The milk-fat-globule membrane (MFGM) was isolated from guinea-pig milk and the membrane-associated proteins and glycoproteins characterized by electrophoretic techniques. Major components of the membrane included PAS-I, a sialoglycoprotein of Mr greater than or equal to 200000, the redox enzyme xanthine oxidase and the glycoprotein, butyrophilin. Membrane preparations also contained two other glycoproteins, GP-80 and GP-55, of Mr 80000 and 55000, respectively. Comparison of guinea-pig xanthine oxidase and butyrophilin with proteins from bovine MFGM by peptide mapping procedures, showed that the two proteins in both species were similar, but not identical. GP-55 may also be related to glycoproteins of Mr 45000 and 48000 in the bovine membrane. The integral and peripheral components of guinea-pig MFGM were identified by treating membrane preparations with sodium carbonate solutions at high pH and by partitioning the membrane proteins in solutions of Triton X-114. By these criteria xanthine oxidase and GP-55 appeared to be peripheral components and GP-80 an integral protein of the membrane. PAS-I and butyrophilin displayed hydrophilic properties in Triton X-114 solutions, but could not be removed from membrane preparations with sodium carbonate. Possible reasons for these ambiguous data are discussed. The observed similarity between several of the proteins of guinea-pig and bovine MFGM implies that these proteins may have specific functions related to milk secretion in mammary tissue, e.g. in the budding of milk-fat globules or the exocytosis of milk protein and lactose at the apical surface.

Role of oxygen radicals in the bacteriostatic effect of whey and production of bacterial growth by free radical scavengers

The involvement of toxic oxygen intermediates in the bacteriostatic effect of milk was determined by producing bacterial growth curves using turbidimetry in the presence and absence of oxygen radical-scavenging substances. Using whey as substrate, catalase, haemoglobin combined with ascorbic acid and xanthine oxidase inhibitors all provided protection against oxygen toxicity for a strain of Staphylococcus aureus and of Streptococcus agalactiae. Superoxide dismutase and mannitol were less effective. This was evident in whey alone and in the presence of oxygen radicals produced exogenously by the t-butylhydroperoxide, H2O2 and xanthine/xanthine oxidase systems.

Homogenized milk and atherosclerotic disease: a review

The theory that consumption of homogenized milk containing active xanthine oxidase is a causative factor in development of atherosclerosis is reviewed. Biologically available xanthine oxidase in consumed milk products may be absorbed in the small intestine and enter the blood stream. However, there appears to be no unequivocal evidence that the absorbed enzyme has any pathological effects that may contribute to development of atherosclerotic heart disease.

Purification of xanthine oxidase from the fat-globule membrane of bovine milk by electrofocusing

Xanthine oxidase was purified from bovine milk-fat globule membrane by extraction with butan-1-ol, precipitation with ammonium sulphate, separation by preparative electrofocusing and chromatography on Concanavalin-A/Agarose. The enzyme had an A280/A450 ratio of 4.8 and a specific activity of 3.09. At least five to seven variants of the enzyme with isoelectric points from pH 6.9 to 7.6 were identified. Previously identified minor "variants' of the enzymes with apparently acidic isoelectric points (1) were shown to be the result of aggregation of enzyme with membrane sialoglycoproteins. Specific antibodies to xanthine oxidase were prepared by fractionating immune serum on a column of enzyme covalently bound to Sepharose 4B. A single immunoprecipitate was obtained when the purified antibodies were allowed to diffuse in agarose gels against either Triton-X-100-extracted membrane or purified xanthine oxidase. Immunoelectrophoresis of the enzyme against anti-sera to xanthine oxidase, however, revealed two precipitin lines, both of which were positive when histochemically stained for enzyme activity. The results are discussed with reference to previous purification schemes for xanthine oxidase and previous estimates for the isoelectric points of the enzyme. We also outline practical uses for the antibody prepared against the enzyme in this present study.

Characteristics of membrane-bound and soluble forms of xanthine oxidase from milk and endothelial cells of capillaries

Xanthine oxidase (xanthine:O2 oxidoreductase, EC 1.2.3.2) was purified from bovine milk lipid globules to electrophoretic homogeneity (Mr 155,000) and antibodies were raised against it in rabbits. By immunolocalization techniques, the xanthine oxidase antigen was detected in milk lipid globules and mammary gland epithelium, but also in capillary endothelium from various tissues, including liver, lung and intestine. These findings were paralleled by measurements of xanthine oxidase activities in the tissues, both in a membrane-associated and a soluble form. Addition of hypoxanthine to fractions containing native xanthine oxidase did not promote lipid peroxidation, in contrast to the widely used in vitro system for lipid peroxidation which involves addition of xanthine oxidase preparations. Extraction with buffers of high ionic strength and with nonionic detergents removed only part of the enzyme from the membranes. Immunoprecipitates from the soluble supernatant fractions, using anti-xanthine oxidase IgG, were enriched in the Mr 155,000 polypeptide. Patterns of proteolytic cleavage products of the xanthine oxidase monomer from capillaries and milk lipid globules were similar but not identical. Immunoprecipitates from soluble fractions of milk lipid globules and tissues were enriched in both xanthine oxidase and NADH-cytochrome c reductase activities. Electrophoretic separation of proteins from milk lipid globule membranes under non-denaturing conditions revealed a close correlation of xanthine oxidase and part of the NADH-cytochrome c reductase activity, but showed different activity profiles of NADH-ferricyanide reductase and xanthine oxidase.

Localization of xanthine oxidase in mammary-gland epithelium and capillary endothelium

Xanthine oxidase, an iron-sulfur molybdenum flavoprotein known to generate superoxide radical, was demonstrated in several bovine tissues. The enzyme (155 kd polypeptide) was purified from bovine milk lipid globules and antibodies were raised that allowed precipitation of the enzyme without inactivation of enzymatic activity. By immunolocalization techniques at light and electron microscope levels, the antigen was found in milk-secreting epithelial cells but not in epithelial cells of several other tissues. In a number of tissues, including mammary gland, liver, heart, lung and intestine, antibodies to xanthine oxidase stained only endothelial cells of capillaries, including sinusoids, but not endothelia of larger blood vessels and endocard. In both milk-secreting epithelial and capillary endothelial cells, xanthine oxidase was distributed throughout the cytoplasm. Results from biochemical and immunological studies suggest that xanthine oxidase is similar in the various tissues examined and may serve similar redox functions.

Separation of the proteins of bovine milk-fat-globule membrane by electrofocusing with retention of enzymatic and immunological activity

1. Proteins of fat-globule membrane from bovine milk were solubilized with the non-ionic detergent Triton X-100 in the presence of protease inhibitors. Approximately 25% of the total membrane protein was solubilized and the extracts were shown to contain a sample of most of the major membrane proteins and glycoproteins. 2. The solubilized proteins were separated in flat-beds of Ultrodex by electrofocusing and the pI values for the major proteins, glycoproteins and certain enzymes determined. Several of the proteins displayed marked heterogeneity indicating the existence of protein variants and isoenzymes. Principal pI values for the enzymes assayed were as follows: xanthine oxidase, 7.35--7.55; NADH2: iodonitrotetrazolium reductase, less than 4.5; 5'-nucleotidase, 7.15--7.4; alkaline phosphatase, 5.4--5.7; phosphodiesterase, 4.6--4.8; gamma-glutamyl transpeptidase, 4.4--4.55. 3. Fractions after electrofocusing were analyzed by 'fused rocket' immunoelectrophoresis and crossed immunoelectrophoresis after separation in polyacrylamide gels containing sodium dodecyl sulphate. Major antigens of the membrane include xanthine oxidase and glycoproteins of apparent molecular weights 67 000, 49 500 and 46 000. The latter two components share common antigenic determinants and could not be separated by gel filtration, ion-exchange chromatography, lectin-affinity chromatography or preparative electrofocusing.

Purification and identification of gamma-glutamyl transpeptidase of milk membranes

Recent evidence suggests that gamma-glutamyl transpeptidase may be involved in the transport of amino acids into the lactating mammary gland. The enzyme also is secreted in milk and is associated mainly with milk membranes. The objective of this study was to purify and characterize gamma-glutamyl transpeptidase from milk membranes. The enzyme has been purified from milk membranes by solubilization with Lubrol WX; treatment with acetone, deoxylcholate, and bromelain; and chromatography on ion exchange and molecular-sieving resins. gamma-Glutamyl transpeptidase was purified over 11,000-fold from milk. Electrophoresis on sodium dodecyl sulfate polyacrylamide gels indicates that the enzyme is composed of two subunits with molecular weights of 57,000 and 25,500. Both subunits are glycoproteins and have been identified in the sodium dodecyl sulfate polyacrylamide gel electrophoresis patterns of whole milk membrane. Kinetic characteristics of the purified enzyme are similar to those determined for intact milk membranes and lactating mammary tissue indicating that the purified enzyme has not been modified functionally by the purification procedure.

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.

Bacterial xanthine oxidase from Arthrobacter S-2

Arthrobacter S-2, originally isolated by enrichment on xanthine, produced high levels of xanthine oxidase activity, requiring as little as a 20-fold purification to approach homogeneity with some preparations. Molecular oxygen, ferricyanide, and 2,6-dichlorophenol-indophenol served as electron acceptors, but nicotinamide adenine dinucleotide did not. The enzyme was relatively specific when compared with previously studied xanthine-oxidizing enzymes, but at least one purine was observed to be oxidized at each of the three positions of the purine ring that have been subject to oxidation by this type of enzyme. The enzyme had a relatively high Km for xanthine (1.3 X 10(-4) M), and substrate inhibition was not observed with this compound, in contrast to the enzyme from cow's milk. In fact, an opposite effect was observed, and double-reciprocal plots with xanthine as the variable substrate showed a concave downward deviation at high concentrations. At 2.5 mM xanthine the enzyme had a specific activity approximately 50 times that of the most active preparations of the milk enzyme. The spectrum of the Arthrobacter enzyme resembled that of milk xanthine oxidase, suggesting a similarity of the prosthetic centers of the two enzymes. The bacterial enzyme was relatively small and may be dimeric, with approximate native and subunit molecular weights of 146,000 and 79,000, respectively.

Influence of storage, heat, and homogenization upon xanthine oxidase activity of milk

Xanthine oxidase activity in milk was determined by measuring the rate of formation of vanillic acid from vanillin. Raw milk received at a dairy plant had .208 units xanthine oxidase activity per ml and after 24-h storage at 4 C, .228 units per ml. Upon further storage activity decreased. Heating the fresh raw milk in a water bath to 55 C increased xanthine oxidase activity to .236 units per ml. Partial inactivation of the enzyme occurred when milk was heated at 60, 65, or 70 C for 5 min, and destruction was almost complete with heat at 75 C for 5 min. Raw milk heated at 48 C for 5 min and homogenized at pressures between 70.3 and 281.2 kg/cm2 had xanthine oxidase activities which were a linear function of pressure and showed that each additional kg/cm2 pressure resulted in additional xanthine oxidase activity of .16 milliunits per ml of milk.

Compositional homology of membrane-protein systems and membrane-associated proteins: comparison with milk fat globule membrane and "membrane"-derived xanthine oxidase

Amino acid compositions of the milk fat globule membrane protein and plasma membrane protein from other sources, as well as the compositions of milk fat globule membrane-derived xanthine oxidase and selected plasma membranes-associated proteins were compared by statistical difference index. Additionally, the average hydrophobicity of xanthine oxidase and selected membrane proteins were compared. These comparisons indicate high orders of apparent compositional homology between the various plasma membranes and membrane-associated proteins. Because the biological functions of membrane proteins are widely diverse, it is speculated that their "relatedness" may reflect on evolutionary convergence to similar amino acid compositions, necessitated by their in situ environment--the lipoidal bilayer. However, compositional relatedness should not imply sequential homology.