Cholesterylester hydroperoxide reducing activity associated with isolated high- and low-density lipoproteins

Exposure of isolated high-(HDL) and low-density lipoproteins (LDL) to aqueous peroxyl radicals generated from a thermo-labile azo-compound resulted in immediate formation of cholesteryllinoleate hydroxide (Ch18:2-OH) in addition to hydroperoxides of cholesteryllinoleate (Ch18:2-OOH) and phospholipids. Ch18:2-OH was also formed in peroxyl radical-oxidizing human plasma devoid of ascorbate or low molecular weight compounds or isolated lipoproteins in the presence of desferrioxamine. In contrast, peroxyl radical-mediated oxidation of HDL or LDL lipid extracts or detergent-solubilized lipoproteins resulted in the formation of Ch18:2-OOH without concomitant formation of Ch18:2-OH. Heat treatment of the isolated lipoproteins prior to oxidation greatly reduced Ch18:2-OH formation. Compared to the concentrations of Ch18:2-OOH accumulating, formation of Ch18:2-OH was more pronounced in oxidizing HDL than LDL isolated from the same blood donor. The levels of Ch18:2-OH detected after prolonged oxidation periods were independent of the radical flux to which the lipoproteins were exposed. In the absence of peroxyl radical generator, [3H]Ch18:2-OOH associated with HDL was converted readily and in a biphasic manner into [3H]Ch18:2-OH upon incubation at 37 but not 4 degrees C. LDL-associated [3H]Ch18:2-OOH were also reduced, albeit with an initial reaction rate approximately 10 times slower than that observed with labelled HDL. Together, the results show that cholesterylester hydroxides are formed during (peroxyl) radical-mediated oxidation of isolated intact HDL and LDL under transition metal-free conditions. The findings suggest the presence of a hydroperoxide reducing activity in isolated human lipoproteins, particularly HDL.