Speciation of Human Plasma High-Density Lipoprotein (HDL): HDL Stability and Apolipoprotein A-I Partitioning

The distribution of apolipoprotein (apo) A-I between human high-density lipoproteins (HDL) and water is an important component of reverse cholesterol transport and the atheroprotective effects of HDL. Chaotropic perturbation (CP) with guanidinium chloride (Gdm-Cl) reveals HDL instability by inducing the unfolding and transfer of apo A-I but not apo A-II into the aqueous phase while forming larger apo A-I deficient HDL-like particles and small amounts of cholesteryl ester-rich microemulsions (CERMs). Our kinetic and hydrodynamic studies of the CP of HDL species separated according to size and density show that (1) CP mediated an increase in HDL size, which involves quasi-fusion of surface and core lipids, and release of lipid-free apo A-I (these processes correlate linearly), (2) >94% of the HDL lipids remain with an apo A-I deficient particle, (3) apo A-II remains associated with a very stable HDL-like particle even at high levels of Gdm-Cl, and (4) apo A-I unfolding and transfer from HDL to water vary among HDL subfractions with the larger and more buoyant species exhibiting greater stability. Our data indicate that apo A-I's on small HDL (HDL-S) are highly dynamic and, relative to apo A-I on the larger more mature HDL, partition more readily into the aqueous phase, where they initiate the formation of new HDL species. Our data suggest that the greater instability of HDL-S generates free apo A-I and an apo A-I deficient HDL-S that readily fuses with the more stable HDL-L. Thus, the presence of HDL-L drives the CP remodeling of HDL to an equilibrium with even larger HDL-L and more lipid-free apo A-I than with either HDL-L or HDL-S alone. Moreover, according to dilution studies of HDL in 3 M Gdm-Cl, CP of HDL fits a model of apo A-I partitioning between HDL phospholipids and water that is controlled by the principal of opposing forces. These findings suggest that the size and relative amount of HDL lipid determine the HDL stability and the fraction of apo A-I that partitions into the aqueous phase where it is destined for interaction with ABCA1 transporters, thereby initiating reverse cholesterol transport or, alternatively, renal clearance.

Metal ion complexes of EDTA: a solute system for density gradient ultracentrifugation analysis of lipoproteins

In the study reported here, we apply some of the features of coordination chemistry to solve a long-standing problem in the separation and characterization of lipoprotein particles. Lipoproteins are circulating micelle-like particles responsible for lipid transport. They exist in three major classes: very-low-density lipoprotein, low-density lipoprotein, and high-density lipoprotein in well-defined density ranges using the density gradient ultracentrifugation (DGU) method. The analytical instrumentation of DGU has improved over the years in response to clinical evidence that certain lipoprotein species are linked to a high risk for developing cardiovascular disease. A long-standing problem has been a lack of appropriate gradient-forming solutes that can generate a useful gradient from a homogeneous solution. We have found that a new class of solutes based on metal ion complexes has the potential of providing a wide selection of compounds where the features can be modulated by choice of ligand, complexing metal ion, and counterion. In this study, we have chosen the cesium salt of BiEDTA (CsBiEDTA) and have investigated the dynamics of density gradient formation in the ultracentrifuge. We show that a useful density gradient can be formed within a few hours beginning with a homogeneous solution. We also present data on the migration behavior of lipoproteins under gradient-forming conditions and show that high-resolution density profiles can be obtained with good precision. The resolution of the CsBiEDTA profile is compared with those obtained using high molecular weight organic solutes.

Electronegative LDLs from familial hypercholesterolemic patients are physicochemically heterogeneous but uniformly proapoptotic

A highly electronegative fraction of human plasma LDLs, designated L5, has distinctive biological activity that includes induction of apoptosis in bovine aortic endothelial cells (BAECs). This study was performed to identify a relationship between LDL density, electronegativity, and biological activity, namely, the induction of apoptosis in BAECs. Plasma LDLs from normolipidemic subjects and homozygotic familial hypercholesterolemia subjects were separated into five subfractions, with increasing electronegativity from L1 to L5, and into seven subfractions according to increasing density, D1 to D7. L1 to L5 were also separated according to density, and D1 to D7 were separated according to charge. The density profiles of L1 to L5 were similar (maximum density = 1.030 +/- 0.002 g/ml). Induction of apoptosis by all seven density subfractions was confined to the highly electronegative fraction, L5, and within each density subfraction the magnitude of apoptosis correlated with the L5 content. Electronegative LDL is heterogeneous with respect to density and composition, and induction of apoptosis is more strongly associated with LDL electronegativity than with LDL size or density.