title: Asymmetry of the site of choline incorporation into phosphatidylcholine of rat liver microsomes.
abstract: [14C]Choline was incorporated into microsomal membranes in vivo, and from CDP-[14C]choline in vitro, and the site of incorporation determined by hydrolysis of the outer leaflet of the membrane bilayer using phospholipase C from Clostridium welchii. Labelled phosphatidylcholine was found to be concentrated in the outer leaflet of the membrane bilayer with a specific activity approximately three times that of the inner leaflet. During incorporation of CDP-choline and treatment with phospholipase C the vesicles retained labelled-protein contents indicating that they remained intact. When the microsomes were opened with taurocholate after incorporation of [14C]choline in vivo, the labelled phosphatidylcholine behaved as a single pool. Selective hydrolysis of labelled phosphatidylcholine in intact vesicles is not, therefore, a consequence of specificity of phospholipase C. These results indicate that the phosphatidylcholine of the outer leaflet of the microsomal membrane bilayer is preferentially labelled by the choline-phosphotransferase pathway and that this pool of phospholipid does not equilibrate with that of the inner leaflet.
mesh_terms: D000818:Animals; D002794:Choline / Q000378:metabolism*; D004492:Edetic Acid / Q000494:pharmacology; D007425:Intracellular Membranes / Q000378:metabolism; D008297:Male; D008563:Membrane Lipids / Q000096:biosynthesis; D008862:Microsomes, Liver / Q000187:drug effects / Q000378:metabolism*; D010713:Phosphatidylcholines / Q000096:biosynthesis*; D051381:Rats; D010738:Type C Phospholipases
chemical_list: D008563:Membrane Lipids; D010713:Phosphatidylcholines; D004492:Edetic Acid; D010738:Type C Phospholipases; D002794:Choline