Interplay of Oxygen, Vitamin E, and Carotenoids in Radical Reactions following Oxidation of Trp and Tyr Residues in Native HDL₃ Apolipoproteins. Comparison with LDL. A Time-Resolved Spectroscopic Analysis^†

It has been recently shown that the inhibition of apolipoprotein A-I (apoAI) reverse cholesterol transport activity during oxidation of HDL by myeloperoxidase may involve myeloperoxidase electron transfer pathways other than those leading to tyrosine chlorination. To better understand how such mechanisms might be initiated, the role of semioxidized Tyr and Trp residues in loss of apoAI and apolipoprotein A-II (apoAII) integrity has been assessed using selective Trp and Tyr one-electron oxidation by ^•Br₂^- radical-anions in HDL₃ as well as in unbound apoAI and apoAII. Behavior of these radicals in apolipoprotein B of LDL has also been assessed. Formation of semioxidized Tyr in HDL₃ is followed by partial repair during several milliseconds via reaction with endogenous α-tocopherol to form the α-tocopheroxyl radical. Subsequently, 2% of α-tocopheroxyl radical is repaired by HDL₃ carotenoids. With LDL, a faster repair of semioxidized Tyr by α-tocopherol is observed, but carotenoid repair of α-tocopheroxyl radical is not. Only a small fraction of HDL₃ particles contains α-tocopherol and carotenoids, which explains limited repair of semioxidized Tyr by α-tocopherol. All LDL particles normally contain multiple α-tocopherol and carotenoid molecules, and the lack of repair of α-tocopheroxyl radical by carotenoids probably results from hindered mobility of carotenoids in the lipid core. Western blots of γ-irradiated HDL₃ comparable to those reported for apoAI myeloperoxidase oxidation show that the incomplete repair of semioxidized Tyr and Trp induces apoAI and apoAII permanent damage including formation of a heterodimer of one apoAI with a monomeric apoAII at about 36 kDa.