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

The structural integrity of apolipoprotein A-I (apo A-I) is critical to the physiological function of high-density lipoprotein (HDL). Oxidized lipopro teins are thought to be of central importance in atherogenesis, and oxidati on products characteristic of myeloperoxidase, a heme protein secreted by a ctivated 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 inve stigated the effects of activated human neutrophils, a potent source of mye loperoxidase and hydrogen peroxide, on the protein and lipid components of HDL. Both free and HDL-associated apo A-I exposed to activated human neutro phils underwent extensive degradation as monitored by RP-HPLC and Western b lotting with a polyclonal antibody to apo A-I. Replacement of the neutrophi ls 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 t he HOCl scavenger methionine, by the heme inhibitor azide, by chloride-free conditions, by the peroxide scavenger catalase, and by a combination of su peroxide dismutase (SOD)/catalase, implicating HOCl in the cell-mediated re action. The addition of a protease inhibitor (3,4-dichloroisocoumarin) furt her 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 cho lesterol in HDL3 exposed to activated neutrophils, suggesting that HOCl was selectively damaging apo A-I. Our observations indicate that HOCl generate d by myeloperoxidase represents one pathway for protein degradation in HDL3 exposed to activated phagocytes.