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Synaptosomal plasma membrane vesicles were isolated from fresh bovine cerebral cortex by Ficoll and sucrose density gradient centrifugation and their lipid compositions were determined. The cholesterol to phospholipid molar ratio is 0.59:1, and the major phospholipids are phosphatidylcholine (43%), phosphatidylethanolamine (361/6), and phosphatidylserine (141/6). Phosphatidylinositol (3%), sphingomyelin (4%) and lysophosphatidylcholine (1%) are relatively minor components, while phosphatidylglycerol is absent. The asymmetric distribution of aminophospholipids in synaptosomal plasma membrane vesicles was also determined by a chemical labeling procedure. Under the nonpenetration conditions at 4 C, bovine cerebral cortical synaptosomes were reacted with 2,4,6-trinitrobenzenesulfonic acid to label outer monolayer aminophospholipids covalently. About 13% of phosphatidylethanolamine and 20% of phosphatidylserine are present in the outer monolayer of synaptosomal plasma membrane vesicles. Each phospholipid class shows a specific fatty acid composition. Phosphatidylcholine contains high levels of hexadecanoic acid (41%). The phosphatidylethanolamine and phosphatidylserine have octadecanoic acid and 4,7,10,13,16, 19-docosahexaenoic acid as major components. Phosphatidylethanolamine and phosphatidylserine of the outer monolayer consist of more saturated fatty acids than those of the inner monolayer. In phosphatidylinositol, octadecanoic acid (35%), 5,8,11,14-eicosatetraenoic acid (25%) and 4,7,10,13,16,19-docosahexaenoic acid (24%) were the major components.
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