Identification and characterization of the principal proteins of the fat‐globule membrane from guinea‐pig milk

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Abstract
The milk-fat-globule membrane (MFGM) was isolated from guinea-pig milk and the membrane-associated proteins and glycoproteins characterized by electrophoretic techniques. Major components of the membrane included PAS-I, a sialoglycoprotein of Mr≥ 200000, the redox enzyme xanthine oxidase and the glycoprotein, butyrophilin. Membrane preparations also contained two other glycoproteins, GP-80 and GP-55, of Mr 80000 and 55000, respectively. Comparison of guinea-pig xanthine oxidase and butyrophilin with proteins from bovine MFGM by peptide mapping procedures, showed that the two proteins in both species were similar, but not identical. GP-55 may also be related to glycoproteins of Mr 45000 and 48000 in the bovine membrane. The integral and peripheral components of guinea-pig MFGM were identified by treating membrane preparations with sodium carbonate solutions at high pH and by partitioning the membrane proteins in solutions of Triton X-114. By these criteria xanthine oxidase and GP-55 appeared to be peripheral components and GP-80 an integral protein of the membrane. PAS-I and butyrophilin displayed hydrophilic properties in Triton X-114 solutions, but could not be removed from membrane preparations with sodium carbonate. Possible reasons for these ambiguous data are discussed. The observed similarity between several of the proteins of guinea-pig and bovine MFGM implies that these proteins may have specific functions related to milk secretion in mammary tissue, e.g. in the budding of milk-fat globules or the exocytosis of milk protein and lactose at the apical surface.