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.