Apolipoprotein A-I localization and dipalmitoylphosphatidylcholine dynamics in reconstituted high density lipoproteins

The structure and molecular dynamics of recombinant high density lipoprotei ns (rHDL) were studied by non-radiative energy transfer (NRET), fluorescenc e anisotropy and intensity measurements. The rHDL particles contained human plasma apolipoprotein (apo) A-I and dipalmitoylphosphatidylcholine (DPPC). Fluorescent cis- and trans-parinaric acids were used both as probes of mol ecular motion in the particle lipid phase and as accepters in the Forster's energy transfer from ape A-I tryptophan residues to determine particle dim ensions, apolipoprotein localization and lipid dynamics. The probes are sen sitive to thermal wobbling (macromobility) and conformational deformations (micromobility) of phospholipid acyl chains. The experimental data fitted t o various models of the particle structure are compatible with the followin g: (a) at T<T-t the particles appeared as lens-like discs with a radius of the lipid phase of 5 nm and a mean thickness of 4 nm, the value being more by 20% in the particle centre, the alpha-helices of about 1 nm thickness we re located around the edge of the lipid core. Compared to liposomes, both m acro- and micromobility of DPPC molecules in rHDL were more rapid due to a significant disorder of the boundary lipid molecules close to the apo A-I m olecule. This disorder led to the increase of the specific surface area per one lipid molecule, S-o. The lipid phase can be divided into three regions : (i) zone I of the most tightly packed lipid (0-1.7 nm from the disc axis) with a S-o value small as 0.5 nm(2); (ii) intermediate zone II (from 1.7 t o 4.0 nm); and (iii) boundary lipid zone III (4-5 nm) of significantly diso rdered lipid with a S-o value large as 0.65 nm(2). (b) at T>T-t the S-o het erogeneity disappeared, the radius of the lipid phase did not increase sign ificantly, not exceeding 5.2-5.4 nm, but protein-induced immobilization of lipid molecules which affected about half or more of the total lipid, becam e remarkable. The overall effect was the suppression of the transition ampl itude in rHDL compared to liposomes. The structural inhomogeneity might und erlie the function of the native plasma HDL as the key component of the tra nsport and metabolism of plasma lipids. (C) 2000 Elsevier Science Ireland L td. All rights reserved.