The participation of nitric oxide in cell free- and its restriction of macrophage-mediated oxidation of low-density lipoprotein.
The potential role of nitric oxide radical (NO .) in macrophage-mediated oxidation and conversion of human low density lipoprotein (LDL) to a high-uptake form was examined by exposing LDL to aerobic solutions of either NO . or 3-morpholino-sydnonimine-hydrochloride (SIN-1, a compound that spontaneously forms NO . and superoxide anion radical) or to mouse peritoneal macrophages in the presence and absence of modulators of cellular NO . synthesis. Incubation with NO . alone caused oxidation of LDL's ubiquinol-10 and accumulation of small amounts of lipid hydroperoxides, but failed to form any high-uptake ligand for endocytosis by macrophages and did not alter the LDL particle charge or the integrity of apoB. Exposure of LDL to SIN-1 resulted in complete consumption of all antioxidants and substantial formation of lipid hydroperoxides, but again had little effect on the lipoprotein particle charge or generation of high-uptake form. Preincubation of macrophages with interferon-gamma increased the cells ability to generate reactive nitrogen metabolites. The extent of cell-mediated oxidation of LDL and the generation of high-uptake LDL was substantial in resident cells in which NO . synthesis was barely detectable, depressed in cells active in NO . synthesis and restored when NO . synthesis was suppressed by the arginine analogue, NMMA. These results suggest that, while together with superoxide anion radical, NO . can oxidize LDL, its synthesis is not required for macrophage-mediated oxidation of LDL in vitro; rather it exerts a protective role in preventing oxidative LDL modification by macrophages.