Oxidation of high density lipoproteins. I. Formation of methionine sulfoxide in apolipoproteins AI and AII is an early event that accompanies lipid peroxidation and can be enhanced by alpha-tocopherol

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 alpha-tocopherol (alpha-TOH) plays in modulating protein oxidation. Exposure of isolated HDL to either low fluxes of aqueous peroxyl radicals, Cu2+ 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 alpha-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, Cu2+, or soybean lipoxygenase resulted in nonspecific (for peroxyl radicals) or no discernible protein oxidation (Cu2+ 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 alpha-TOH resulted in a dose-dependent 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). alpha-TOH enrichment also enhanced HDL lipid and protein oxidation induced by Cu2+ 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, alpha-TOH can promote this process.