Aminophospholipid Asymmetry in Murine Synaptosomal Plasma Membrane

The asymmetric distribution of aminophospholipids in isolated murine synaptosomal plasma membranes was determined by a chemical labeling procedure. Under nonpenetrating conditions, mouse brain synaptosomes were reacted with trinitrobenzenesulfonic acid (TNBS) to label outermonolayer aminophospholipids covalently. About 10-15% of the phosphatidylethanolamine and 20% of the phosphatidylserine were found to be in the outer monolayer of the synaptosomal plasma membrane. Furthermore, the fatty acyl group composition of the labeled phosphatidylethanolamine (outer monolayer) consisted of more saturated fatty acid than did the unlabeled phosphatidylethanolamine (inner monolayer). These results demonstrated an aminophospholipid asymmetry in synaptosomal plasma membranes which was independent of serum-lipoprotein exchange processes and also of phosphatidylethanolamine-methylatingenzymes.