The primary objectives of this study were to determine whether analogs to native discoidal
apolipoprotein (
apo)E-containing
high-density lipoproteins (HDL) could be prepared in vitro, and if so, whether their conversion by
lecithin-cholesterol acyltransferase (LCAT; EC 2.3.1.43) produced particles with properties comparable to those of core-containing, spherical,
apoE-containing HDL in human plasma. Complexes composed of
apoE and POPC, without and with incorporated unesterified
cholesterol, were prepared by the
cholate-dialysis technique. Gradient gel electrophoresis showed that these preparations contain discrete species both within (14-40 nm) and outside (10.8-14 nm) the size range of discoidal
apoE-containing HDL reported in
LCAT deficiency. The isolated complexes were discoidal particles whose size directly correlated with their POPC:
apoE molar ratio: increasing this ratio resulted in an increase in larger complexes and a reduction in smaller ones. At all POPC:
apoE molar ratios, size profiles included a major peak corresponding to a discoidal complex 14.4 nm long. Preparations with POPC:
apoE molar ratios greater than 150:1 contained two distinct groups of complexes, also in the size range of discoidal
apoE-containing HDL from patients with
LCAT deficiency. Incorporation of unesterified
cholesterol into preparations (molar ratio of 0.5:1, unesterified
cholesterol:POPC) resulted in component profiles exhibiting a major peak corresponding to a discoidal complex 10.9 nm long. An increase of unesterified
cholesterol and POPC (at the 0.5:1 molar ratio) in the initial mixture, increased the proportion of larger complexes in the profile. Incubation of isolated POPC-
apoE discoidal complexes (mean sizes, 14.4 and 23.9 nm) with purified LCAT and a source of unesterified
cholesterol converted the complexes to spherical,
cholesteryl ester-containing products with mean diameters of 11.1 nm and 14.0 nm, corresponding to
apoE-containing HDL found in normal plasma. Conversion of smaller
cholesterol-containing discoidal complexes (mean size, 10.9 nm) under identical conditions resulted in spherical products 11.3, 13.3, and 14.7 nm across. The mean sizes of these conversion products compared favorably with those (mean diameter, 12.3 nm) of
apoE-containing HDL of human plasma. This conversion of
cholesterol-containing complexes is accompanied by a shift of some
apoE to the
LDL particle size interval. Our study indicates that
apoE-containing complexes formed by the
cholate-dialysis method include species similar to discoidal
apoE-containing HDL and that incubation with LCAT converts most of them to spherical core-containing particles in the size range of plasma
apoE-containing HDL. Plasma HDL particles containing
apoE may arise in part from direct conversion of discoidal
apoE-containing HDL by LCAT.