Fractionation and characterization of the membranes from bovine milk fat globules

Summary 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.