Oxidation of High Density Lipoproteins

Abstract

The lipids of high density lipoproteins (HDL) are initially oxidized in preference to those in low density lipoprotein when human plasma is exposed to aqueous peroxyl radicals. In this work we report on the relative susceptibility of HDL protein and lipid to oxidation and on the role HDL's ␣-tocopherol (␣-TOH) plays in modulating protein oxidation. Exposure of isolated HDL to either low fluxes of aqueous peroxyl radicals, Cu 2؉ ions, or soybean lipoxygenase resulted in the oxidation of apoAI and apoAII during the earliest stages of the reaction, i.e. after consumption of ubiquinol-10 and in the presence of ␣-TOH. Hydro(pero)xides of cholesteryl esters and phospholipids initially accumulated together with specific oxidized forms of apoAI and apoAII, separated by high pressure liquid chromatography. The specific oxidized forms of apoAI were 16 and 32 mass units heavier than those of the native apolipoproteins and contained 1 and 2 methionine sulfoxide residues per protein, respectively. The third methionine residue in apoAI, as well as Trp residues, remained unoxidized during the earliest stages of HDL oxidation examined. Exposure of isolated apoAI to peroxyl radicals, Cu 2؉ , or soybean lipoxygenase resulted in nonspecific (for peroxyl radicals) or no discernible protein oxidation (Cu 2؉ and soybean lipoxygenase). This indicated that the formation of the specific oxidized forms of apoAI observed with native HDL was not the result of direct reaction of these oxidants with the apolipoprotein. In vitro and in vivo enrichment of HDL with ␣-TOH resulted in a dosedependent increase in the extent of peroxyl radical-induced formation of HDL cholesteryl ester hydroperoxides (r ‫؍‬ 0.96) and cholesteryl ester hydroxides (r ‫؍‬ 0.92), as well as the loss of apoAI (r ‫؍‬ 0.96) and apoAII (r ‫؍‬ 0.94). ␣-TOH enrichment also enhanced HDL lipid and protein oxidation induced by Cu 2؉ or soybean lipoxygenase. These results indicate that the earliest stages of HDL oxidation are accompanied by the oxidation of specific methionine residues in apoAI and apoAII and that in the absence of co-antioxidants, ␣-TOH can promote this process.Plasma levels of high density lipoprotein (HDL) 1 cholesterol and apolipoprotein AI (apoAI) inversely correlate with the risk of developing coronary artery disease (1). An important antiatherogenic activity postulated to underlie the beneficial property of high HDL levels is the removal of cholesterol from peripheral tissues and its transport to the liver for excretion, a process known as reverse cholesterol transport (2). Other potentially anti-atherogenic properties of HDL also exist. For example, HDL preferentially transports oxidized cholesteryl esters to the liver for excretion into bile (3, 4). HDL also inhibits Cu 2ϩ -or endothelial cell-induced oxidation of low density lipoprotein (LDL) (5); there is strong evidence that LDL oxidation contributes to atherogenesis in humans (6).HDL is the major carrier of extremely low concentrations of lipid hydroperoxides in human plasma, and initially, HDL lipi...

Reagent or myeloperoxidase-generated hypochlorite affects discrete regions in lipid-free and lipid-associated human apolipoprotein A-I

“…These findings underline that methionine oxidation is important for defining the conformation in lipidfree and lipid-associated apo A-I. Recently the formation of two specific oxidized forms of apo A-I, containing one or two Met(SO) groups during the early stages of Cu# + or soybean lipoxygenase oxidation was reported [12,13].…”

“…These experiments verified the loss of peptides T1, T7, T9, T11, T16 and T22 as observed by RP-HPLC with UV detection (Figures 4 and 5). It is well documented that the treatment of apo A-I with H # O # or other oxidants generates Met(SO) groups by a two-electron oxidation step [10][11][12]. However, all investigators using oxidants weaker than OCl − reported the oxidation of only two of the three methionine residues in apo A-I (Met""# and Met"%) respectively).…”

Apolipoprotein A-I Promotes the Formation of Phosphatidylcholine Core Aldehydes That Are Hydrolyzed by Paraoxonase (PON-1) during High Density Lipoprotein Oxidation with a Peroxynitrite Donor

“…The oxidation of methionine to methionine sulfoxide by lipid hydroperoxide is a two-electron oxidation (11,12). The observation that both apoA-I and apoA-II can participate in this transformation suggests that the major requirement is for a methionine residue to be in the hydrophobic environment of the hydroperoxy fatty acyl chains.…”

“…However, HDL has been shown to protect LDL from oxidation in vitro (10). The factors protecting against formation of lipid hydroperoxides in plasma appear to be apoA-I (11)(12)(13) and the enzyme paraoxonase (PON-1) (10,14).…”

The myeloperoxidase product hypochlorous acid oxidizes HDL in the human artery wall and impairs ABCA1-dependent cholesterol transport

“…Moreover, methionine and phenylalanine residues in apoA-I are oxidized by reactive intermediates (22)(23)(24)(25), and tyrosine residues are converted to o,oЈ-dityrosine by tyrosyl radical (26). We recently showed that HOCl selectively targets tyrosine residues in apoA-I that are suitably juxtaposed to primary amino groups (27).…”