On the Lipid Composition of Membranes

Abstract

The lipid content of membrane preparations isolated from different origins has been demonstrated to vary from 30% to 75% on a dry weight basis. Depending on the procedure utilized for the preparation of red cell ghosts, this membrane was found to contain either 30% or 50% lipids, cholesterol and phospholipids, which together account for over 80% of the total lipids. Although some differences in the proportion between cholesterol and phospholipid are apparent among erythrocytes of different mammalian species, in general this ratio tends to be close to unity. The phospholipids consist of sphingomyelin, phosphatidyl choline (lecithin), phosphatidyl ethanolamine and anionic phosphoglycerides, e.g. phosphatidyl serine and small amounts of phosphoinositides. Apart from the diacyl analogs, in some erythrocyte membranes considerable amounts of plasmalogens, and saturated glyceryl ether phospholipids are present as well. As regards the lecithin content, (significant differences are known to exist among erythrocytes of different animals, but in general a low content of choline phosphoglyceride is compensated by an increased level of sphingomyelin having an identically charged head group. Although the lipid core of many animal membranes has a net negative charge, the abundance of choline-eontaining phospholipids appears to maintain the density of anionic charges at the surface of the lipid layers between certain limits.