Further Evidence of Plasma Membrane Material in Skim Milk

Membrane fragments and microvillus like sacs were concentrated from bovine skim milk by differential centrifugation into a fraction resting on the casein pellet. The ratios of the free cholesterol, lipid phosphorus, and triglyceride contents of the fraction were similar to those for plasma membrane of the lactating cell. Assays for marker enzymes of Golgi apparatus (lactose synthetase), rough endoplasmic reticulum (nicotinamide adenine dinucleotide phosphate-cytochrome c reductase), and plasma membrane (glycosyl hydrolase) demonstrated the presence of only the latter. Suspension and recentrifugation of the membrane fraction produced a supernatant and a second (purified) membrane fraction resting as fluff on a second small casein pellet. Lipid analyses revealed that triglyceride with traces of free cholesterol and phospholipid were released from the original membrane fraction to the second supernantant. The relative content of these three lipid classes in conjunction with the enzyme assays provided further evidence for the co-identity of skim milk membrane material and plasma membrane of the lactating mammary cell.