Topological Biosynthesis of Phosphatidylcholine in Brain Microsomes

: The sidedness of the biosynthesis of phosphatidylcholine and its transbilayer movement in brain microsomes were investigated. Microsomes were labelled in vitro or in vivo either through Kennedy's pathway or by the base‐exchange reaction. The vesicles were treated with phospholipase C under conditions where only the phospholipids present in the external leaflet were hydrolyzed. The incubation of microsomes with CDP‐[14C]choline or [14C]choline showed that most of the newly synthesized phosphatidylcholine molecules were localized in the external leaflet. With time a few molecules were transferred into the inner leaflet. When phosphatidylcholine was labelled in vivo by intraventricular injection of [3H]choline the specific activities of the phosphatidylcholine in the outer leaflet were higher than those in the inner leaflet after short times of labelling but became similar after long times of labelling. The results suggest that in brain microsomes the synthesis of phosphatidylcholine through Kennedy's pathway or by the base‐exchange reaction takes place on the external leaflet which corresponds to the cytoplasmic one in situ. The transfer of these molecules from the outer leaflet to the inner one is a slow process and the mechanisms that control the transbilayer movement of the phosphatidylcholine seem to be independent of those that control their biosynthesis.