Myeloperoxidase impairs ABCA1-dependent cholesterol efflux through methionine oxidation and site-specific tyrosine chlorination of apolipoprotein A-I.

High density lipoprotein (HDL) isolated from human atherosclerotic lesions and the blood of patients with established coronary artery disease contains elevated levels of 3-chlorotyrosine. Myeloperoxidase (MPO) is the only known source of 3-chlorotyrosine in vivo, indicating that MPO oxidizes HDL in humans. We previously reported that Tyr-192 is the major site that is chlorinated in apolipoprotein A-I (apoA-I), the chief protein in HDL, and that chlorinated apoA-I loses its ability to promote cholesterol efflux from cells by the ATP-binding cassette transporter A1 (ABCA1) pathway. However, the pathways that promote the chlorination of specific Tyr residues in apoA-I are controversial, and the mechanism for MPO-mediated loss of ABCA1-dependent cholesterol efflux of apoA-I is unclear. Using site-directed mutagenesis, we now demonstrate that lysine residues direct tyrosine chlorination in apoA-I. Importantly, methionine residues inhibit chlorination, indicating that they can act as local, protein-bound antioxidants. Moreover, we observed near normal cholesterol efflux activity when Tyr-192 of apoA-I was mutated to Phe and the oxidized protein was incubated with methionine sulfoxide reductase. Thus, a combination of Tyr-192 chlorination and methionine oxidation is necessary for depriving apoA-I of its ABCA1-dependent cholesterol transport activity. Our observations suggest that biologically significant oxidative damage of apoA-I involves modification of a limited number of specific amino acids, raising the feasibility of producing oxidation-resistant forms of apoA-I that have enhanced anti-atherogenic activity in vivo.