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

Effects of mechanical agitation of raw milk on the milk-fat globule in relation to the level of induced lipolysis

The amount of damage caused to the milk-fat globule by mechanical agitation of raw milk was assessed by several methods and related to the level of induced lipolysis resulting from the agitation. The results obtained by measuring the amount of ‘free’ fat formed, the percentage of fat in the skim-milk phase after separation, and the increase in the proportion of alkaline phosphatase associated with the skim-milk phase, showed little correlation with the level of lipolysis induced by agitation over a range of conditions. However, the level of induced lipolysis showed good correlation with the amount of lipolysis caused by addition of an exogenous lipase (from Candida cylindraceae) to the agitated milk. It is concluded that classical methods of assessing milk-fat globule damage are unsuitable for predicting the amount of fat available for lipase action and the lipolytic potential of raw milk subjected to mechanical agitation.

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.

Fractionation and characterization of the membranes from bovine milk fat globules

Bovine milk-fat globule membrane (MFGM) has been fractionated in to 3 membrane components by isopycnic sucrose density gradient centrifugation. The 3 components, designated as heavy, medium and light material, were characterized by particular polypeptide, lipid, carbohydrate and enzyme compositions.

The heavy material (density > 1·145 g/ml) was red-brown in appearance and consisted of 63% protein, 32% lipid and 5% carbohydrate. The lipid moiety contained 40% triglyceride and 60% phospholipid, while the protein consisted of 3 major polypeptides (apparent mol. wt 135000, 70000 and 53000) in the ratio of 3:6:1. This membrane class was also characterized by the presence of both xanthine oxidase and acid phosphatase. The medium density material (density 1·055–1·145 g/ml) was light red to pink in colour and consisted of 35% protein, 58% lipid and 6% carbohydrate. The lipid contained 45% triglyceride and 55% phospholipid, with nearly 70% of the original MFGM phospholipid being found in this material. The ratio of the 3 major polypeptide bands was 2:4:4 and high levels of both alkaline phosphatase and 5'-nucleotidase were present. The light material (density < 1·055 g/ml) was white to pale pink in colour and was composed of 88% lipid of which 94% was triglyceride, together with 8% protein and 4% carbohydrate. There were virtually no enzymes present and 70% of the protein consisted of a polypeptide with apparent mol. wt of 53000. The 3 membrane classes also differed in carbohydrate content and in the fatty acid composition of their triglycerides.

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.

Hydrolysis of bovine milk fat globules by lipoprotein lipase: inhibition by proteins extracted from milk fat globule membrane

Extraction of membrane proteins from milk fat globules by GuHCl or by MgCl2 made the lipids more accessible to lipolysis by added lipoprotein lipase. The increase in lipolysis paralleled the loss of membrane proteins and was continuous up to 2.5 M GuHCl, which was the highest concentration used. About twice as much protein was extracted with 2.5 M GuHCl as with buffer only. The amount of protein lost was about 50% of total milk fat globule protein. Lipolysis of milk fat globules was inhibited by addition of the extracted protein. The extracted proteins also reduced lipolysis when added to whole milk. More protein was needed to inhibit lipolysis of milk fat globules treated with GuHCl compared with globules treated with buffer only. The inhibition by a given amount of protein decreased if more milk fat globules were used. Protein extracted with MgCl2 had similar effects as those extracted with GuHCl. The major components extracted with MgCl2 migrated in the 40 to 50-kdalton region on sodium dodecyl sulfate-polyacrylamide gel electrophoresis. By gel filtration chromatography, two protein fractions were obtained, which inhibited lipolysis more efficiently than the total extract. As has previously been found for inhibition of lipolysis by skim milk, the amount of extracted protein needed to inhibit lipolysis varied between preparations of milk fat globules. Milk with propensity to cold-induced ("spontaneous") lipolysis was normalized by addition of extracted proteins.

Immunohistological study of malignant diffuse mesotheliomas of the pleura

Paraffin sections from fifteen cases of malignant diffuse mesothelioma of the pleura and five cases of bronchial adenocarcinoma infiltrating the pleura were examined with an antiserum specific for factor VIII related antigen and with antisera against various epithelial markers: keratin, carcinoembryonic antigen (CEA), fat globule membrane antigen and secretory component. In all adenocarcinomas all the epithelial markers were present whereas the factor VIII related antigen was absent. The distribution of the fat globule membrane antigens, keratin, secretory component and factor VIII related antigen varied from one mesothelioma to another. The mesotheliomas were generally negative for CEA. The three mesotheliomas which were positive for CEA were also positive for alcian blue after hyaluronidase treatment. Amongst the markers used, CEA seems the most useful for the differential diagnosis between carcinoma and mesothelioma. However, the simultaneous detection of several markers allows the characterization of various phenotypes. Some of them are close to the phenotypes of true adenocarcinoma. A relation between a given phenotype and the biological behaviour of the tumour has still to be demonstrated.

Adenocarcinoma in situ and invasive adenocarcinoma of the uterine cervix. An immunohistologic study with antibodies specific for several epithelial markers

The distribution of carcinoembryonic antigen (CEA), secretory component (SC), fat globule membrane antigens ( FGMA ), and keratin was determined immunohistochemically in 22 invasive adenocarcinomas of various types and in 9 adenocarcinomas in situ of the uterine cervix. In the invasive adenocarcinomas 77% were positive for CEA, 47% for SC, 89% for keratin, and 77% for FGMA . In adenocarcinomas in situ 67% were positive for CEA, 11% for SC, and 44% for keratin. The location of the markers was variable in the cells, and the cells in a tumor were irregularly positive. For a given histologic type there were several phenotypes. No correlation was found between histologic types of invasive adenocarcinomas and the various phenotypes. It remains to be shown whether a particular phenotype has a particular biological behavior. The detection in the serum of the markers shown in histologic preparations could be useful in the postsurgical monitoring.

Free and membrane-bound xanthine oxidase in bovine milk during cooling and heating

The effect of cold storage (5 C, 24 h) and heat treatment (60 C, 5 min) of milk on activities of free and membrane-bound xanthine oxidase has been studied. Both treatments enhanced total xanthine oxidase activity in milk. Activity of membrane-bound xanthine oxidase increased and free xanthine oxidase decreased in buttermilk while it increased in skim milk on cold storage. Heat of milk increased free and membrane-bound xanthine oxidase activities in both buttermilk and skim milk. The state of xanthine oxidase activity in skim milk from reconstituted milk, which was prepared by mixing xanthine oxidase inactivated skim milk and fresh cream, showed that only the free enzyme migrated from the cream phase to skim milk on cold storage. Very little xanthine oxidase activity was detectable in skim milk on heat treatment of the reconstituted milk sample. The overall increased activity of xanthine oxidase in milk during cold storage or heat treatment may not be due to the release of fat globule membrane enzyme to skim milk.

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.

Molecular weight profile of fat globule membrane proteins

Fat globule membrane material, isolated by churning cream that had been washed four times, was extracted sequentially with .6 M potassium chloride and centrifuged to yield pellet and supernatant fractions. Compositional data indicated that lipid components were removed preferentially into the supernatant fractions. Electropherograms of the pellet and supernatant fraction in 10% and 5% sodium dodecyl sulfate-containing polyacrylamide gels revealed that the protein species originally present in the membrane associated into higher molecular weight species concomitant with the removal of lipid moieties. Thus, a contribution of the lipid moiety to membrane integrity is suggested. Approximately 17 protein-stained zones, ranging in molecular weight from 13,500 to 208,000 daltons, were observed in gels of the membrane material.