Human neutrophils employ the myeloperoxidase/hydrogen peroxide/chloride system to oxidatively damage apolipoprotein A‐I

Abstract

The structural integrity of apolipoprotein A‐I (apo A‐I) is critical to the physiological function of high‐density lipoprotein (HDL). Oxidized lipoproteins are thought to be of central importance in atherogenesis, and oxidation products characteristic of myeloperoxidase, a heme protein secreted by activated phagocytes, have been detected in human atherosclerotic tissue. At plasma concentrations of halide ion, hypochlorous acid is a major product of the myeloperoxidase‐hydrogen peroxide‐chloride system. We therefore investigated the effects of activated human neutrophils, a potent source of myeloperoxidase and hydrogen peroxide, on the protein and lipid components of HDL. Both free and HDL‐associated apo A‐I exposed to activated human neutrophils underwent extensive degradation as monitored by RP‐HPLC and Western blotting with a polyclonal antibody to apo A‐I. Replacement of the neutrophils with reagent HOCl resulted in comparable damage (at molar oxidant : HDL subclass 3 ratio = 100) as observed in the presence of activated phagocytes. Apo A‐I degradation by activated neutrophils was partially inhibited by the HOCl scavenger methionine, by the heme inhibitor azide, by chloride‐free conditions, by the peroxide scavenger catalase, and by a combination of superoxide dismutase (SOD)/catalase, implicating HOCl in the cell‐mediated reaction. The addition of a protease inhibitor (3,4‐dichloroisocoumarin) further reduced the extent of apo A‐I damage. In contrast to the protein moiety, there was little evidence for oxidation of unsaturated fatty acids or cholesterol in HDL3 exposed to activated neutrophils, suggesting that HOCl was selectively damaging apo A‐I. Our observations indicate that HOCl generated by myeloperoxidase represents one pathway for protein degradation in HDL3 exposed to activated phagocytes.

Localization of Nitration and Chlorination Sites on Apolipoprotein A-I Catalyzed by Myeloperoxidase in Human Atheroma and Associated Oxidative Impairment in ABCA1-dependent Cholesterol Efflux from Macrophages

“…Similarly oxidation of HDL with lipid hydroperoxides has been shown to oxidize methionine residues in apoA-I and increase the ability of the apoA-I to bind lipid (52). In contrast, treatment of HDL with either HOCl or enzymatic HOCl-generating systems, including MPO, has been reported to inhibit cholesterol efflux (31)(32)(33).…”

2-Chlorohexadecanal Derived From Hypochlorite-Modified High-Density Lipoprotein–Associated Plasmalogen Is a Natural Inhibitor of Endothelial Nitric Oxide Biosynthesis

“…Apolipoprotein A-I represents a preferential target for HOCl attack. 13,16,36,37 Treatment of HDL by HOCl leads also to consumption of unsaturated fatty acids. 13 Furthermore, the formation of lysophospholipids from unsaturated phosphatidylcholines after HOCl treatment has been reported.…”

Tyrosine Modification Is Not Required for Myeloperoxidase-induced Loss of Apolipoprotein A-I Functional Activities

“…Although apoAI tyrosine chlorination was associated with the MPOmediated loss of the cholesterol acceptor activity of apoAI, tyrosine chlorination might only serve as a molecular fingerprint for apoAI modification by MPO, and not the cause of apolipoprotein loss of cholesterol efflux function, as other changes in apoAI have been found after oxidative modification (25)(26)(27)(28)(29)(30). To address the specific role of tyrosine modification in MPO-dependent inactivation of apoAI, we used site-directed mutagenesis to create a tyrosine-free recombinant human apoAI.…”