A comparison of the properties of membranes isolated from bovine skim milk and cream

Abstract

Membrane material was isolated from skim milk and cream using the same samples of whole milk and similar purification techniques. The membranes from these two sources were characterised and compared by lipid, carbohydrate, enzymatic and electrophoretic analyses. The skim milk membranes contained higher levels of cholesterol, phospholipid and carbohydrate per mg of protein than the cream membranes. In general, the specific activities of the enzymes tested were also higher in the skim milk membranes, nucleotide pyrophosphatase, γ-glutamyltranspeptidase and sulphydryl oxidase being particularly active in these membranes. The major protein of the skim milk material had a molecular weight of approx. 85 000 and constituted 32% of the total protein. This particular protein band was almost absent in the cream membranes (only 3% of the total protein) where the major protein had a molecular weight of approx. 70 000 and constituted 34% of the total protein. Glycoprotein bands were also located in both membrane preparations but the position of these bands did not correspond with the areas which were stained with the protein staining reagent Coomassie blue. The major glycoprotein in both skim milk and cream membranes had an apparent molecular weight of 115 000. The biochemical and compositional differences between these membranes in milk provide further evidence for the skim milk membranes being more closely related to secretory cell plasma membrane than to the cream membranes. The data also lend support to the hypothesis of Keenan et al. ((1970) J. Cell Biol. 44, 80) that the cream membrane undergoes morphological and structural changes while evolving from the plasma membrane of the secretory cell.