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 phosphoinositid:es. Apart from the Idiacyl analogs, in some erythrocyte membranesconsiderable amounts of plasmalogens, and saturated glyeeryl 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-containing phospholipids appears to maintain the density of anionic charges at the surface of the lipid layers between certain limits. However, in a number of membranes the negatively charged phospholipids may dominate. Staphylococcus aureus harvested from a medium at pH 7 contained as a major phospholipid the anionic phosphatidyl glycerol, while an appreciable quantity of diphosphatidyl glycerol was also present. Whereas the usual, zwitterionic phosphoglycerides were lacking in this bacterium, a third phospholipid constituent appeared to be identical to an O-lysine ester of phosphatidyl glycerol. The relative quantity of this lipid was found to be increased most significantly when the pH of the medium attained a value of about 4.8, as it did following the fermentation of added glucose. Such changes in the phospholipid pattern were apparent 30 minutes after alteration of the pH of the medium. This versatility of bacteria has been utilized to prepare protoplasts of Bacillus megatherium, which have differently charged lipids in the membrane, thus giving opportunities to investigate the possible implications of the chemical make-up of the phospholipids in regard to the properties of these membranes.