Phospholipid-exchange proteins as membrane probes.

Phospholipid-exchange proteins, first discovered in rat liver, have now been identified in a variety of animal and plant tissues. In rat liver one protein transfers phosphatidylcholine with a high degree of specificity. Another protein fraction is capable of transferring all major types of phospholipids as well as cholesterol. Beef heart is a good source of phospholipid-exchange protein, which transfers phosphatidylinositol and phosphatidylcholine. Studies with phospholipid-exchange protein from beef heart, beef liver and rat liver show that these proteins are useful for the study of membrane structure. When unilamellar vesicles of isotopically labeled phosphatidylcholine are incubated with nonlabeled mitochondria in the presence of phospholipid-exchange protein, only the outer portion of the phosphatidylcholine bilayer is exchangeable and translocation of lipids between inner and outer parts of the bilayer is exceedingly slow. Resealed red blood cell ghosts show an equilibration of phosphatidylcholine between the inner and outer portions of the bilayer with a t1/2 of two hours. An even faster equilibration of various phospholipids in the microsomal mmebrane appears to take place in rat-liver microsomes.