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

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.

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.