Effects of oxidation, particle size, and protein composition on structure and stability of high-density lipoprotein

High-density lipoproteins (HDL) are heterogeneous complexes of proteins (mainly apoA-I and apoA-II) and lipids, which protect from atherosclerosis by removing cholesterol from arterial walls via reverse cholesterol transport (RCT) and by exerting anti-oxidant and anti-inflammatory effects. During RCT, HDL are remodeled by plasma factors and undergo protein dissociation and lipoprotein fusion and rupture, which are necessary for function. To determine the effects of lipoprotein size, composition and oxidation on the rates of HDL remodeling, we use thermal denaturation as a simple experimental model of HDL fusion and rupture.;HDL oxidation importantly affects their function and remodeling. Mild oxidation reportedly enhances cellular cholesterol uptake by HDL whereas extensive oxidation impairs it. To elucidate the underlying energetic and structural basis, we analyzed effects of copper and hypochlorite (that preferentially oxidize lipids and proteins, respectively) on thermal stability of human plasma HDL. Circular dichroism, light scattering, differential scanning calorimetry, gel electrophoresis and electron microscopy showed that mild oxidation destabilizes HDL and accelerates protein dissociation and HDL fusion, while extensive oxidation inhibits these reactions. We propose that mild oxidation lowers kinetic barriers for HDL remodeling due to diminished apolipoprotein affinity for lipids resulting from Met oxidation in apoA-I and apoA-II followed by their cross-linking into dimers or trimers. In contrast, advanced oxidation inhibits protein dissociation and HDL fusion due to massive protein cross-linking on the HDL surface. This finding helps postulate a new hypothesis: HDL stability correlates inversely with HDL function. We propose that local structural disorder in the HDL surface facilitates insertion of cholesterol and other plasma factors that remodel HDL during RCT. Thus, less stable HDL undergo faster metabolic remodeling, which accelerates cholesterol removal and benefits cardioprotection.;Our kinetic analysis using circular dichroism, light scattering and DSC showed that the stability of plasma HDL decreases with increasing particle diameter. This may facilitate preferential cholesterol ester uptake from large lipid-loaded HDL during RCT. We also showed that, contrary to the existing notion, size-matched HDL(A-I/A-II) containing apoA-I and apoA-II have comparable stability to HDL(A-I) containing only apoA-I. This helps reconcile controversial reports on the role of apoA-II in RCT.