A B s m A c T : A plasma membrane fraction derived from rat brain nerve endings or the synaptic plasma membrane fraction, isolated by zonal centrifugation, was analyzed for their lipid content and their composition of lipid classes and their aliphatic moieties. With the exception of glycolipids and sphingomyelin, the lipid classes of synaptic plasma membrane are very similar to whole brain. Phosphatidylcholine and phosphatidylethanolamine are the major lipids and make up 6 2 z of the total membrane lipid. Cholesterol and ceramide make up the major neutral lipids and 2 1 z of total brain lipid. The synaptic plasma membranes contain lower quantities of glyceryl ethers in phosphatidylethanolamine and phosphatidylcholine fractions than in whole brain. Certain distinctive features in fatty acid content were T he components of neuronal membranes need to be known because elucidation of the structure and dynamic function of this membrane is dependent upon such knowledge. Specific lipids of membranes undoubtedly subserve specific functions ; studies of model membranes and experiences with enzyme reconstitution illustrate the vital role and specificity of lipids in the functional properties of biomembranes. Tetrodotoxin, a molecule that specifically blocks changes in sodium conductance during nerve action potentials, interacts with and causes spreading of surface films made from nonpolar lipids extracted from squid axons, but polar lipids do not show such interactions (Villegas and Camejo, 1968;Camejo and Villegas, 1969). Phospholipids are essential for respiratory activity in mitochondria (Fleischer er a/.,

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Analysis of phospholipids in porcine, bovine and rat adipose tissue revealed a relatively high level of plasmalogens (O‐alk‐1‐enyl lipids). About 50% of the ethanolamine phospholipids in the pig and beef samples consisted of alk‐1‐enylacylglycerophosphorylethanolamine, and the corresponding value for the rat sample was near 35%. In the ethanolamine and choline phospholipid fractions, theO‐alk‐1‐enyl moieties were almost exclusively 16∶0, 18∶0 and 18∶1, whereas the acyl moieties had chain lengths ranging from 16 to 22 carbon atoms with a high degree of unsaturation.

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