Lipid Composition of Rat Liver Plasma Membranes

Abstract

Rat liver plasma membranes were isolated by discontinuous sucrose gradient centrifugation. The purity of membrane preparations was checked by electron microscopy and enzyme assays. A new glycolipid, tentatively called the S-lipid, is proposed as a marker to indicate the purity of plasma membranes. It is present in most subcellular particulates (mitochondria, microsomes, lysosomes, etc.) but is absent from plasma membranes. The plasma membrane lipids represent 39.8% of the total dry weight of the membranes. Plasma membrane total lipid contains 39% neutral lipids of which free cholesterol (18.1%) and free fatty acids (7.9%) are the major components. These are present in somewhat lower amounts (4.0 and 1.3%, respectively) in lipid extracts from whole liver homogenate. The phospholipids constitute 55.4% of membrane total lipids. The phospholipid fraction is composed mainly of phosphatidylcholine (19.3% of the total lipids), phosphatidylethanolamine (10.3%), and sphingomyelin (9.8%). The minor constituents are phosphatidylserine, phosphatidylinositol, phosphatidylglycerol, phosphatidic acid, and lysophosphatidylcholine. The phospholipids of the plasma membranes, compared with those of the entire liver, have considerably higher amounts of sphingomyelin and phosphatidylserine (both about 3-fold). In the membranes, about 5.6% of the total lipids are glycolipids and lipids that are unaccounted for. Compared with the liver, the membranes, according to gas-liquid chromatographic analyses of the total lipid fatty acids and hydrocarbons, show a tendency to accumulate the longer chain fatty acids and hydrocarbons. The membranes also contain relatively higher amounts of saturated fatty acids and much less branched chain hydrocarbons.