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Summary1. Golgi complex, rough and smooth microsomes, plasma membranes, mitochondria and nuclei from rat liver were isolated and their purity assessed using specific marker enzymes.2. The various subcellular fractions were assayed for the following processes: biosynthesis of sphingomyelin, CDPdiglycerides, phosphatidylinositol, phosphatidylserine, the conversion of phosphatidylserine into phosphatidylethanolamine, the formation of lecithin via N-methylation, and the activation of palmitic and octanoic acids.3. None of these processes were found to be present in Golgi complex. 4. The endoplasmic reticulum appears to be the principal site in the cell for the synthesis of sphingomyelin, CDPdiglycerides, phosphatidylinositol, phosphatidylserine and the formation of lecithin. Interestingly, the biosynthesis of phosphatidylserine appears to be four times more active in rough than in smooth microsomes, which might suggest a ribosomal localization of this process.5. Except for CDPdiglyceride synthesis, mitochondria do not contain any of the synthesizing activities described in 4. Mitochondria are, however, the only site in the cell where phosphatidylserine is decarboxylated. This activity appears to be localized in the inner membrane.6. The activation of palmitate is localized predominantly in endoplasmic reticulum and mitochondria, though some activity was detected in plasma membranes as well. All other cell organelles, including Golgi and probably nuclei, did not contain significant palmitoyl-CoA synthetase activity. The sub-

A B S T R A C TA method is reported for the isolation of a highly purified fraction of urinary bladder membranes containing hexagonal plaques. The method uses zonal centrffugation as the final step of fractionation.The purified fraction was characterized by its electron microscopic morphology, by its enzymatic profile, by quantitative and qualitative analysis of lipids and by the protein pattern obtained by electrophoresis in polyacrylamide sodium dodecyl sulfate gels. The fraction contains 55% lipids and 35% proteins. The major protein component has a molecular weight of 27,000 daltons.Phospholipids are more than the 54% of the total lipid weight. Phosphatidylcholine, phosphatidylethanolamine, and phosphatidylinositol are the major phospholipids with 50%, 30%, and 7% of the total lipid phosphorus, respectively.The glycolipid fraction is 10% of the total lipid weight and is formed by only two components, both sulfatides. Total cholesterol makes up 35% of the total neutral lipid fraction of which cholesterol esters constitute 6 %. Glycoproteins are also found to be present in the fraction.

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