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Effect of Preparation on Properties of a Soluble Glycoprotein Fraction of Milk Fat Globule Membrane

Department of Agricultural Chemistry, The University of Tokyo, Bunkyo-ku, Tokyo 113, Japan

ABSTRACT

The effect on the physicochemical properties and polypeptide composition of the major soluble glycoprotein fraction of milk fat globule membrane of the kind of solvent and the state of milk fat globule membrane at delipidation was investigated. The fraction obtained by methanol-chloroform and ethanol-ether delipidation contained .02% and .25 to .36% lipid-phosphorus, respectively. The fraction obtained by direct delipidation of solid, freeze-dried membrane or of salted-out membrane had a more heterogeneous sedimentation pattern and a lower carbohydrate content (14.7 to 17.7%) than that obtained by delipidation of an aqueous suspension of membrane (24.6 to 25.6%). Fractions by the salted-out and freeze dried procedures contained polypeptides of molecular weight 136,000 to 55,000 in addition to eight polypeptides, Periodic Acid-Schiff-I to VII and Coomassie Blue-Ill, with relatively low molecular weight (less than 40,000 except that of PAS-I) which were in the aqueous suspension preparations. The polypeptide composition of the major glycoprotein fractions obtained by gel filtration was influenced by the ionic strength of the salt solution used to extract soluble protein from delipidized membrane. Differences in preparation procedure account for the discrepancies in physicochemical properties of the major soluble apoprotein fraction that have been reported.