Microsomal membranes contain phosphatidylcholine that equilibrates across the bilayer, and phosphatidylcholine that does not.

Results of experiments using phosphatidylcholine transfer protein and phospholipase C as probes indicate that there are at least two pools of phosphatidylcholine in rat liver microsomes. One of these is preferentially labelled with [14C]choline and does not equilibrate across the bilayer. The second pool is labelled with [3H]glycerol and does equilibrate across the bilayer. Our observations also confirm that phosphatidylcholine exchange protein does not modify the distribution of phospholipids or cause randomization of the inner and outer leaflet pools of phosphatidylcholine when these are differentially labelled by [14C]choline.