Properties of Aqueous Dispersions of Phospholipid and Cholesterol

Abstract

Very little lipid can be extracted with diethyl ether or chloroform from aqueous dispersions of phosphatidylcholine, phosphatidylethanolamine and phosphatidic acid, or from mixed dispersions of phosphatidic acid and phosphatidylethanolamine with cholesterol. Nearly all of the cholesterol and about 60% of the phosphatidylcholine can be removed from a dispersion of these two lipids in equimolar proportions but not from a dispersion of two molecules of phosphatidylcholine to one molécule of cholesterol. NaCl and CaCl 2 greatly increase the extraction of lipids from phosphatidylcholine and phosphatidylcholine/cholesterol dispersions whereas urea and dimethylsulphoxide prevent extraction. Cholesterol molecules readily exchange between rat erythrocytes and dispersions of equimolar proportions of phosphatidylcholine and cholesterol. At lower proportions of cholesterol, the dispersions remove cholesterol from the ghosts. Up to 60% of the sterol can be removed from the ghost membrane. Phosphatidylcholine/cholesterol dispersions are readily attacked by phospholipase D in the absence of ether but dispersions of phosphatidylcholine alone need ether present to facilitate enzymic attack. These properties are compared with those of soluble and structural lipoproteins.