Intermembrane phospholipid fluxes catalyzed by bovine brain phospholipid exchange protein

Bovine brain phospholipid exchange protein catalyzes the transfer of phosphatidylinositol and phosphatidylcholine between two populations of single bilayer vesicles. The inclusion of lactosylceramide in one of the vesicle populations and the ability to precipitate those vesicles in the presence of Ricinus communis agglutinin assures the quantitative separation of donor and acceptor vesicles following incubation with exchange protein. When both vesicle populations contain phosphatidylinositol and phosphatidylcholine and transfers are monitored in both directions, the flux of phosphatidylinositol (or phosphatidylcholine) in the forward direction equals that in the reverse. When one of the vesicle populations initially lacks phosphatidylinositol, a net unidirectional transfer of that phospholipid occurs. Concurrently, a compensatory flux of phosphatidylcholine takes place in the opposite direction, such that the bidirectional fluxes of total phospholipid are equal. A net transfer of phosphatidylcholine is also demonstrated. A mechanism of true molecular exchange between vesicles, rather than net transfer, is proposed for the bovine brain phospholipid exchange protein.