Potentialities of magnetoliposomes in studying symmetric and asymmetric phospholipid transfer processes

Using high-gradient magnetophoresis, the non-protein-mediated transfer and exchange of phosphatidylglycerol (PG) molecules between sonicated phospholipid dispersions and magnetoliposomes is studied. The latter structures consist of nanometer-sized magnetite (Fe3O4) cores which are enwrapped by a phospholipid bilayer. Their dimensions are similar to those of small unilamellar vesicles (De Cuyper and Joniau (1988) Eur. J. Biophys. 15, 311-319). Using these particles, spontaneous lipid movements were studied in three different cases. In a first setup, symmetric exchange between dimyristoylphosphatidylglycerol (DMPG) magnetoliposomes, labelled with [3H]DMPG, and DMPG vesicles was followed. Within the time scale of the experiment (1 day) both the lipid molecules residing in the inner and outer leaflet of the magnetoliposomes participate in the exchange process, although 'flip-flop' movements have a retarding effect. In the second approach a unidirectional flux of DMPG from DMPG magnetoliposomes to distearoylphosphatidylglycerol (DSPG) acceptors is noted. In this case, the outer phospholipid leaflet of the magnetoliposomes (in contrast to the inner one) can be largely stripped off; the extent of depletion is determined by the relative amount of the DSPG receiving structures. Furthermore, it is found that with a 15-fold molar excess of receptors, the whole depletion course can be described by a single first-order rate expression. The reluctancy of the inner shell phospholipids to migrate is further illustrated by the virtual lack of transfer, observed with monolayer-coated Fe3O4 colloids. In the third case, asymmetric bidirectional PG transfer is followed between equimolar amounts of DMPG magnetoliposomes and dipentadecanoylphosphatidylglycerol vesicles. In the initial stage of the incubation period, the mmol PG/g Fe3O4 ratio decreases, but progressively restores later on. By quantitatively measuring the transfer rate of each of the individual components, this complex behavior could be unravelled.