Membranes of Mammary Gland. XII. Loosely Associated Proteins and Compositional Heterogeneity of Bovine Milk Fat Globule Membrane

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Abstract
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 MW of 155,000 and 43,500 and membrane glycoproteins were released selectively during preparation of milk fat globule membranes. Xanthine oxidase was apparently a constituent of the selectively removed polypeptide fraction of apparent MW 155,000. A major class of polypeptide with an apparent MW of 62,500 was not extracted from milk fat globule membrane by treatment with dilute salts, EDTA, or by nonionic and ionic detergent solutions. Milk fat globule membranes were separated into 7 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 7 subfractions had similar polypeptide profiles. Both phospholipid and total lipid content of subfractions were correlated inversely with fraction density. Milk fat globule membrane is probably nearly homogeneous in content of intrinsic membrane proteins and certain membrane-bound enzyme activities but is markedly heterogeneous with respect to buoyant density and lipid content.

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