Mather I H, Weber K, Keenan T W
J Dairy Sci. 1977 Mar;60(3):394-402. doi: 10.3168/jds.S0022-0302(77)83878-2.
Electrophoretic patterns of polypeptides of milk fat globule differed quantitatively depending on extent of washing during membrane preparation. This was due to selective loss of loosely associated, extrinsic membrane proteins. Major polypeptides with apparent molecular weights of 155,000 and 43,500 and membrane glycoproteins were released selectively during preparation of milk fat globule membranes. Evidence suggested that xanthine oxidase was a constituent of the selectively removed polypeptide fraction of apparent molecular weight 155,000. A major class of polypeptide with an apparent molecular weight of 62,500 was not extracted from milk fat globule membrane by treatment with dilute salts, ethylenediaminetetraacetic acid, or by nonionic and ionic detergent solutions. Milk fat globule membranes were separated into seven subfractions on isopycnic centrifugation in sucrose density gradients. Specific activities of the enzymes 5'-nucleotidase, xanthine oxidase, acid and alkaline phosphatases were similar or identical in all fractions. Electrophoretic analysis showed these seven subfractions had similar polypeptide profiles. Both phospholipid and total lipid content of subfractions were correlated inversely with fraction density. The results show that milk fat globule membrane is nearly homogenous in content of intrinsic membrane proteins and certain membrane-bound enzyme activities but is markedly heterogeneous with respect to buoyant density and lipid content.
乳脂肪球多肽的电泳图谱在数量上因膜制备过程中的洗涤程度而异。这是由于松散结合的外在膜蛋白选择性丢失所致。在乳脂肪球膜制备过程中，表观分子量为155,000和43,500的主要多肽以及膜糖蛋白被选择性释放。有证据表明，黄嘌呤氧化酶是表观分子量为155,000的选择性去除多肽部分的一个组成成分。一类表观分子量为62,500的主要多肽不能通过用稀盐、乙二胺四乙酸处理，或用非离子和离子洗涤剂溶液从乳脂肪球膜中提取出来。乳脂肪球膜在蔗糖密度梯度等密度离心中被分离成七个亚组分。5'-核苷酸酶、黄嘌呤氧化酶、酸性和碱性磷酸酶在所有组分中的比活性相似或相同。电泳分析表明这七个亚组分具有相似的多肽谱。亚组分的磷脂和总脂质含量均与组分密度呈负相关。结果表明，乳脂肪球膜在内在膜蛋白含量和某些膜结合酶活性方面几乎是均匀的，但在浮力密度和脂质含量方面明显不均一。
