Composition and Structural and Functional Properties of Discoidal and Spherical Phospholipid--ApoE3 Complexes

M. De Pauw,¹ B. Vanloo,¹ A. D. Dergunov,^2,3 A.-M. Devreese,¹ J. Baert,⁴ R. Brasseur,⁵ and M. Rosseneu¹

¹Department of Biochemistry, University of Ghent, Belgium; fax: (3209) 225-34-89; E-mail: Maryvonne.Rosseneu@rug.ac.be

²Department of Biochemistry, Research Center for Preventive Medicine, Petroverigsky per. 10, Moscow, 101953 Russia; fax: (7-095) 928-50-63; E-mail: eqa@glas.apc.org

³To whom correspondence should be addressed.

⁴Interdisciplinary Research Center, University of Leuven Campus Kortrijk, Belgium.

⁵Laboratoire Chimie des Macromolecules aux Interfaces, Free University Brussels, Belgium.

Submitted June 25, 1996; revision submitted November 20, 1996.

To model the common structural unit in the system of reverse cholesterol transport, we studied the composition, structure, and physicochemical properties of complexes generated between dipalmitoylphosphatidylcholine (DPPC) or palmitoyllinoleoylphosphatidylcholine (PLPC) and apoE3 in the absence and in the presence of cholesterol (Chol); the data were compared with similar experiments using apoA-I, the major high-density lipoprotein. The conformation and organization of lipid-binding domains of apoE3 within the complexes were calculated by computer modeling. The transition temperatures of DPPC within discoidal complexes with mean diameters of 116 Å (GGE) or 148 Å (EM) were higher for complexes versus liposomes both in the absence and in the presence of Chol. Association of apoE3 with DPPC resulted in a more structured state of the apolipoprotein molecule versus the soluble apolipoprotein; this state was characterized by parallel orientation of alpha-helixes of apoE3 and DPPC acyl chains. Substrate efficiency of the apoE3--PLPC--Chol complexes in the lecithin-cholesterol acyltransferase (LCAT) reaction expressed as V_max/K_m was 0.5 nmole cholesteryl esters/h per 1 .M. The transformation of discoidal apoE3--DPPC--Chol complexes into spherical particles was induced by LCAT and accumulation of cholesteryl esters was ~62% of the total cholesterol. Parallel orientation of phospholipid acyl chains with helical segments disappeared in these particles. Discoidal apoE3--DPPC complexes incorporated unesterified cholesterol released from Chol-loaded J774 macrophages. The data support the concept that association of apoE3 and apoA-I with phospholipids is qualitatively similar due to similar orientation of helical repeats in the C-terminal domains of apoE3 and apoA-I.

KEY WORDS: apolipoprotein E, cholesterol efflux, lecithin-cholesterol acyltransferase, lipid--protein interaction.

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