Effects of charged lipids on the interaction of cholesteryl ester transfer protein with lipid microemulsions

This study reports the effects of charged lipids on the transfer of cholesterlyl-1 pyrene decanoate (Py-CE) between apolipoprotein-free microemulsion particles mediated by cholesteryl ester transfer protein (CETP). The surface charge characteristics of microemulsion particles composed of cholesteryl oleate and egg yolk phosphatidylcholine were altered by incorporating phosphatidyl-serine, oleate or stearylamine into the phosphatidylcholine that forms the surface monolayer of the particle. The transfer of Py-CE was measured continuously by following the decrease in excimer fluorescence that accompanies the transfer of the probe from donor to acceptor particles [Rajaram, Chan and Sawyer (1994) Biochem. J. 304, 423-430]. The inclusion of 20 mol% phosphatidylserine relative to the phospholipid in the surface monolayer of the emulsion caused a 64% decrease in the first-order rate constant describing the transfer. An increase in ionic strength caused a partial reversal of this effects, indicating that electrostatic factors are only partially responsible for the interaction with lipid. Complete inhibition of transfer was observed when 10 mol% sodium oleate was incorporated into the surface monolayer. The incorporation of stearylamine into the emulsion caused a 32% increase in the transfer rate. The binding of CETP to the different emulsion surfaces was also examined using a surface plasmon resonance biosensor. The presence of negatively charged lipid (phosphatidylserine or oleic acid) decreased the rate of association of CETP with the emulsion without a significant change in the dissociation rate constant. The presence of the positively charged lipid stearylamine increased the rate of association of CETP with the lipid surface. It is concluded that a negative surface charge on the monolayer decreases the rate of transfer by decreasing the affinity of CETP for these particles.