Abstract 113: Phospholipid Physical State Determines the Efficiency of Cholesteryl Ester Transfer by Cholesteryl Ester Transfer Protein between Discoidal High Density Lipoproteins

Abstract

The transfer of cholesteryl ester by recombinant cholesteryl ester transfer protein (CETP) between reconstituted discoidal HDL was studied. Particles contained apolipoprotein A-I, unsaturated palmitoyllinoleoylphosphatidylcholine (PLPC), palmitoyloleoylphosphatidylcholine (POPC) or saturated dipalmitoylphosphatidylcholine (DPPC) and cholesteryl ester up to 3 mol% as cholesteryl 1-pyrenedecanoate (CPD) or cholesteryl laurate (CL) in donor and acceptor rHDL, respectively. The concentration dependencies of excimerization upward deviated from a linear dependence, being more curvilinear for DPPC. Separately for DPPC complexes, monomer fluorescence compared to excimer was more efficiently quenched by acrylamide. The nonradiative energy transfer between apoA-I tryptophan residues and CPD molecules was more efficient for PLPC compared to DPPC complexes. Data fulfilled the quenching sphere-of-action model, the sphere with adjacent probe molecules being larger and more distant from apoA-I tryptophan residues in DPPC complexes. CETP-catalyzed cholesteryl ester exchange between donor and acceptor HDL followed by probe excimerization was characterized by a heterogeneous kinetics; the fast exchanging CPD pool was much higher in a case of POPC compared to DPPC complexes. Probe fraction accessible to CETP increased with temperature, suggesting a more homogeneous probe distribution. The noncompetitive inhibition of probe transfer by acceptor particles seems to satisfy the zero off-rate compatible with probe tunneling. The values of V max (0.063 μM·min -1 ) and k cat (0.42 s -1 ) together with a similarity of K m (0.9 μM CPD) and K I (2.8 μM CL) values for POPC-containing rHDL suggest the efficient cholesteryl ester transfer between nascent HDL with unsaturated phosphatidylcholine in vivo opposite to much less efficiency with HDL with saturated phosphatidylcholine. The physical state of phospholipid matrix may underlie CETP activity with discoidal HDL through self-association and location of cholesteryl ester in the bilayer, the availability of cholesteryl ester to cholesterol-binding site in apoA-I structure and the binding of apoA-I-interacting cholesteryl ester to CETP. We are grateful to Dr. A. Tall for providing the recombinant CETP.