Isolation and physicochemical properties of a soluble glycoprotein fraction of milk fat globule membrane.

A soluble apoprotein fraction was prepared from milk fat globule membrane lipoproteins by delipidation with a chloroform-methanol mixture and was fractionated into three fractions by gel filtration on Bio-Gel A-5m.The major fraction, Fraction II, contained about 30% of carbohydrate, i.e. 13.9% of hexoses, 8.1% of hexosamines, 8.0% of sialic acid and 0.8% of fucose, and was therefore designated a soluble glycoprotein fraction.The fraction was apparently homogeneous on sedimentation velocity analysis and DEAE-Sephadex chromatography, and had s020 ,w 6.1, D020, 3.79, v 0.719, f/f0 2.16 and molecular weight 139,000 daltons.However, the diffused pattern on disc electrophoresis and the occurrence of plural N-terminal amino acid residues suggest that the protein of this fraction is likely to be formed by intermolecular association of heterogeneous polypeptide chains.