Human Neutrophils Oxidize Low-Density Lipoprotein by a Hypochlorous Acid- Dependent Mechanism: The Role of Vitamin C

Abstract

Oxidatively modified lowdensity lipoprotein (LDL) has been strongly implicated in the pathogenesis of atherosclerosis. Peripheral blood leukocytes, such as neutrophils, can oxidize LDL by processes requiring superoxide and redoxactive transition metal ions; however, it is uncertain whether such catalytic metal ions are available in the artery wall. Stimulated leukocytes also produce the reactive oxidant hypochlorous acid (HOCl) via the heme enzyme myeloperoxidase. Since myeloperoxidasederived HOCl may be a physiologically relevant oxidant in atherogenesis, we investigated the mechanisms of neutrophilmediated LDL modification and its possible prevention by the antioxidant ascorbate (vitamin C). As a sensitive marker of LDL oxidation, we measured LDL thiol groups. Stimulated human neutrophils (5106 cells/ml) incubated with human LDL (0.25 mg protein/ml) timedependently oxidized LDL thiols (33% and 79% oxidized after 10 and 30 min, respectively). Supernatants from stimulated neutrophils also oxidized LDL thiols (33% oxidized after 30 min), implicating longlived oxidants such as Nchloramines. Experiments using specific enzyme inhibitors and oxidant scavengers showed that HOCl, but not hydrogen peroxide nor superoxide, plays a critical role in LDL thiol oxidation by neutrophils. Ascorbate (200 M) protected against neutrophilmediated LDL thiol oxidation for up to 15 min of incubation, after which LDL thiols became rapidly oxidized. Although stimulated neutrophils accumulated ascorbate during oxidation of LDL, preloading of neutrophils with ascorbate did not attenuate oxidant production by the cells. Thus, activated neutrophils oxidize LDL thiols by HOCl and Nchloramine dependent mechanisms and physiological concentrations of vitamin C delay this process, most likely due to scavenging of extracellular oxidants, rather than by attenuating neutrophil oxidant production.