Abstract
Myeloperoxidase (MPO)-derived hypochlorous acid induces changes in HDL function via redox modifications at the level of apolipoprotein A-I (apoA-I). As 4F and apoA-I share structural and functional properties, we tested the hypothesis that 4F acts as a reactive substrate for hypochlorous acid (HOCl). 4F reduced the HOCl-mediated oxidation of the fluorescent substrate APF in a concentration-dependent manner (ED(50) ∼ 56 ± 3 μM). This reaction induced changes in the physical properties of 4F. Addition of HOCl to 4F at molar ratios ranging from 1:1 to 3:1 reduced 4F band intensity on SDS-PAGE gels and was accompanied by the formation of a higher molecular weight species. Chromatographic studies showed a reduction in 4F peak area with increasing HOCl and the formation of new products. Mass spectral analyses of collected fractions revealed oxidation of the sole tryptophan (Trp) residue in 4F. 4F was equally susceptible to oxidation in the lipid-free and lipid-bound states. To determine whether Trp oxidation influenced its apoA-I mimetic properties, we monitored effects of HOCl on 4F-mediated lipid binding and ABCA1-dependent cholesterol efflux. Neither property was altered by HOCl. These results suggest that 4F serves as a reactive substrate for HOCl, an antioxidant response that does not influence the lipid binding and cholesterol effluxing capacities of the peptide.
Highlights
Myeloperoxidase (MPO)-derived hypochlorous acid induces changes in HDL function via redox modifications at the level of apolipoprotein A-I
In light of observations that HDL isolated from patients with coronary artery disease contains elevated levels of nitrotyrosine (N-Tyr) and chlorotyrosine (Cl-Tyr), several studies have focused on the role of MPO and hypochlorous acid (HOCl) in the modification of Tyr residues in apolipoprotein A-I (apoA-I) [16, 19]
N-Tyr and Cl-Tyr are both markers of redox injury, data suggest that tyrosine chlorination, but not nitration, is a critical determinant of impaired ABCA1dependent cholesterol efflux [16, 22]
Summary
Myeloperoxidase (MPO)-derived hypochlorous acid induces changes in HDL function via redox modifications at the level of apolipoprotein A-I (apoA-I). It was proposed that chlorination of up to two of the seven Tyr residues in apoA-I plays a critical role in the inhibition of ABCA1mediated cholesterol efflux [16, 23] Tyr chlorination under these conditions was dependent on the reaction of HOCl with the KXXY amino acid motif [24]. Other studies suggest that Cl-Tyr formation in apoA-I may represent a marker for dysfunctional HDL, Tyr chlorination per se is not a mechanism underlying the inhibition of ABCA1-mediated cholesterol efflux [25] Rather, these studies indicated that the fundamental mechanism underlying the loss of HDL function was the oxidation of Trp residues in the protein [25]
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