Some aspects of the chemical composition of the milk fat globule membrane during lactation

Membrane material in bovine skim-milk from udder quarters infused with endotoxin and pathogenic organisms

The effects of infusing endotoxin and a pathogen into different quarters of the udder of the same cow on the appearance and composition of milk membrane material were studied. Milk membrane was prepared by high-speed centrifugation of skim-milk. In samples from the control quarters only, a very thin layer covering the casein pellet was observed, whereas after the infusions it appeared as an opaque diffuse fluffy layer. The fluffy layer persisted for a maximum of 3 d after endotoxin infusion. A similar layer appeared 14 d after infusion of the pathogen and persisted, in spite of antibiotic treatment, throughout the experiment. From comparisons of (a) milk acid phosphatase activities between infused and control quarters, (b) the protein composition of milk membrane, and (c) morphological observations on the distribution of membrane material in milk, it was concluded that milk membrane originates from leucocytes, cell debris from the mammary gland, and the surface of the milk-fat globule membrane. The relative contribution made by these sources to milk membrane from healthy and infected udders is discussed.

The effect of ageing cooled milk on the composition of the fat globule membrane

The effect of ageing fresh milk for 24 h at 4°C on the composition of 4 fractions of milk fat globule membrane (FGM) – microsomal pellet membrane (MPM), deoxycholate soluble membrane (DOCM), high density membrane (HDM) and low density membrane (LDM) – prepared from washed cream treated with sodium deoxycholate (DOC), was studied for milk of individual cows. Total FGM and its fractions were solubilized by treatment with sodium dodecyl sulphate (SDS), EDTA and β-mercaptoethanol, and the dissociated FGM proteins were separated by polyacrylamide gel electrophoresis in the presence of SDS.

Ageing resulted in a greater loss of phospholipid during cream preparation than was found with fresh milk, and also in a reduction in the total amount of FGM isolated. Loss of FGM material was confined to MPM, DOCM and LDM and was accompanied by compositional changes in DOCM and LDM, quantitatively the most significant fractions. Ageing also produced changes in the gel electrophoresis patterns of DOCM, where the band of greatest mobility (mol. wt about 16000) was partially lost, and of LDM, where there was an increase observed in the major protein band.

The implication of the results on the proposed models of FGM structure is discussed.

Location of proteins within the milk fat globule membrane

Radioactive labelling of intact fat globules with125I gave at least one labelled protein in each of the 4 fractions of the fat globule membrane. Similar treatment of isolated membrane gave extensive labelling of all membrane proteins in each fraction.

It was concluded that all fractions are partially exposed at the membrane surface and the existence of well-defined layers is unlikely.

Milk-fat globule membranes: chemical composition and phosphoesterase activities during lactation

The morphology, protein and glycoprotein composition, amino-acid analysis, phospholipid and carbohydrate contents and 3 phosphoesterase activities of the milk-fat globule membranes were studied in 4 cows during their whole period of lactation. No differences were found in the morphology and the overall chemical composition. However, glycoprotein composition varied with the cow and, for each cow, with the stage of lactation. The differences were more marked for membranes obtained from colostrum. Acid phosphomonoesterase was practically unchanged during the whole period of lactation, while alkaline phosphomonoesterase and phosphodiesterase increased as lactation proceeded.

Examination of the structure of the bovine milk-fat-globule membrane and of cheese by X-ray crystallography

Low-angle X-ray diffraction techniques have been applied to the study of the structure of the fat-globule membrane of milk and of Cheddar cheese. Membrane was prepared by freeze-thawing, churning and ether extraction, the endproduct of each method being a pellet produced by centrifuging. High-angle and low-angle reflexions from membrane could be related to triglyceride spacings, and low-angle reflexions due to spacings at about 5·5 nm to phospholipid bilayers. Spacings due to triglycerides were observed in mature Cheddar together with spacings that agreed well with values for some of the amino acids.

Identification of the milk fat globule membrane proteins. II. Isolation of major proteins from electrophoretic gels and comparison of their amino acid compositions

The fat globule membranes of milk are derived from the apical plasma membrane of the mammary secretory cells. The nature of the membrane proteins, as isolated from cows' milk, has been studied by the use of discontinuous and continuous SDS-gel electrophoresis. Six methods of preparation of milk fat globule membrane suggested by various authors were tested; gel electrophoresis showed that five major bands were present, independent of the method of preparation. The apparent molecular masses of these proteins as determined on SDS-gels (15% T) were 167, 142, 64, 49 and 46 kDa, respectively. The 167 kDa band stained only with periodic acid-Schiff reagent, while the 142 kDa band stained only with Coomassie blue; the last three bands stained with both. Delipidated membranes were extracted stepwise with water, 0.02 M NaCl and 0.6 M NaCl. The 64 kDa band appears to be nearly insoluble, while the bands of 142, 49 and 46 kDa are fractionated by this procedure. The resolution of all of these proteins by electrophoresis was superior to that achieved by molecular sieve chromatography, and so electrophoretic extraction was used to isolate the major proteins. Dansyl chloride derived proteins were used as markers. Amino acid compositions of the recovered proteins were obtained and are compared.

Membranes of mammary gland. XII. Loosely associated proteins and compositional heterogeneity of bovine milk fat globule membrane

Electrophoretic patterns of polypeptides of milk fat globule differed quantitatively depending on extent of washing during membrane preparation. This was due to selective loss of loosely associated, extrinsic membrane proteins. Major polypeptides with apparent molecular weights of 155,000 and 43,500 and membrane glycoproteins were released selectively during preparation of milk fat globule membranes. Evidence suggested that xanthine oxidase was a constituent of the selectively removed polypeptide fraction of apparent molecular weight 155,000. A major class of polypeptide with an apparent molecular weight of 62,500 was not extracted from milk fat globule membrane by treatment with dilute salts, ethylenediaminetetraacetic acid, or by nonionic and ionic detergent solutions. Milk fat globule membranes were separated into seven subfractions on isopycnic centrifugation in sucrose density gradients. Specific activities of the enzymes 5'-nucleotidase, xanthine oxidase, acid and alkaline phosphatases were similar or identical in all fractions. Electrophoretic analysis showed these seven subfractions had similar polypeptide profiles. Both phospholipid and total lipid content of subfractions were correlated inversely with fraction density. The results show that milk fat globule membrane is nearly homogenous in content of intrinsic membrane proteins and certain membrane-bound enzyme activities but is markedly heterogeneous with respect to buoyant density and lipid content.

Compositional and electrophoretic changes in buffalo milk-fat globule membrane proteins during lactation

Chemical analyses, polyacrylamide-gel electrophoresis and isoelectric focusing of milk-fat globule membrane proteins (FGMP) obtained from the milk of 2 Murrah buffaloes were done to determine if any change in composition occurred during lactation. Changes in the levels of sialic acid, hexose, hexosamine, N and P were found in the FGMP obtained at different stages of lactation. On the day of parturition, 8 major proteins in FGMP were determined by sodium dodecyl sulphate polyacrylamide-gel electrophoresis whereas 6 major proteins were obtained in FGMP of middle and late lactation milks. Isoelectric focusing of FGMP showed 8-9, 9-13 and 13-16 proteins from colostrum, middle and late lactation milks, respectively and the isoelectric pH of the proteins varied from 5-25 to 7-80, 5-85 to 8-30 and 5-75 to 8-61 respectively.

Isolation, preparation and the amino acid composition of 4 milk-fat globule membrane proteins solubilized by treatment with sodium dodecyl sulphate

Milk-fat globule membrane (MFGM) proteins were solubilized by treatment with SDS. Four of the major proteins were isolated as SDS complexes using column chromatography. The purity of each isolate was determined by SDS polyacrylamide gel electrophoresis and sufficient of each protein was obtained for amino acid analysis. The amino acid compositions of the isolated MFGM proteins and a total MFGM protein extract were determined. Differences in amino acid composition were found in particular between the major MFGM glycoprotein and the other 3 membrane proteins. The relationships of the amino acid composition to protein properties and structure are discussed.

A study of cow's milk containing high levels of linoleic acid: isolation and properties of the fat-globule membrane

Properties of whole milk and milk fractions from cows fed a diet that gave a greatly increased proportion of unsaturated fatty acid residues (especially of linoleic acid) in the milk lipids were studied, and this milk (high-linoleic milk) was compared with milk from cows on a control diet (control milk). The milk fractions were isolated by high-speed centrifugation of whole milk or cream and were examined by chemical analysis and electron microscopy. During centrifugation the globules of milk fat were disrupted and the membranes (fat-globule 'ghosts') floated as a layer beneath the free lipid. Membrane proteins from the 2 sorts of milk gave the same electrophoretic pattern and the amino acid compositions were the same. Lipid analysis of the membrane fraction from high-linoleic milk showed the expected increase in the proportion of unsaturated fatty acid residues in the neutral lipids, but there was an unexpected decrease in the proportion of unsaturated residues in the membrane phospholipids. No differences were found between high-linoleic and control milk in the ultrastructure of the milk-fat globules or the isolated membranes.

Studies on the structure of milk fat globule membrane

Milk fat globule membrane was solubilized with sodium dodecyl sulfate and mercaptoethanol and the membrane proteins were separated by SDS-polyacrylamide gel electrophoresis. The membrane preparations contained three major size classes of polypeptide of 155,000, 62,500 and 43,500 daltons. At least five glycopeptides were separated of which two stained intensely with periodic acid-Schiff reagent, but poorly with coomassie blue. Trypsin hydrolysis of whole cream and isolated milk fat globule membrane revealed major differences in the rates of protein hydrolysis. Many of the membrane proteins of whole cream resisted proteolysis compared with the same proteins in the isolated membrane. Two glycopeptides were resistant to trypsin digestion in either preparation. Treatment of whole cream with neuraminidase led to the release of at least 70% of the protein-bound sialic acid. Whole cream and isolated membrane samples were iodinated with 125I in the presence of lactoperoxidase and hydrogen peroxide. The membrane proteins were significantly more accessible to lactoperoxidase-125I i in isolated membrane compared with the proteins of whole cream. Polypeptides of molecular weight 43,500 and approximately 48,000 daltons were predominantly labelled in whole cream and could be eluted from the fat globules with magnesium chloride (1.5m). The results strongly suggest that the proteins of milk fat globule membrane are asymmetrically arranged in the membrane and that most of the protein-bound sialic acid is present on the external surface of milk fat globules.