Nitric oxide inhibition of lipoxygenase-dependent liposome and low-density lipoprotein oxidation: Termination of radical chain propagation reactions and formation of nitrogen-containing oxidized lipid derivatives

Lipoxygenase-induced lipid oxidation contributes to plasma lipoprotein oxidation and may be an underlying pathogenic mechanism of atherogenesis. Since inactivation of the vasorelaxant actions of nitric oxide (. NO) plays a critical role in the impaired function of atherosclerotic vessels and because . NO reacts rapidly with other radical species, we assessed the influence of . NO on lipoxygenase-catalyzed oxidation of linoleic and linolenic acid, 1-palmitoyl-2-arachidanyl-sn-glycero-3-phosphocholine (PC) liposomes, hypercholesterolemic rabbit beta-very-low-density lipoprotein, and human low-density lipoprotein. Soybean lipoxygenase (SLO)-induced lipid oxidation was assessed by accumulation of conjugated dienes, formation of lipid hydroperoxides, oxygen consumption, and liquid chromatography-mass spectrometry. Different rates of delivery of . NO to lipid oxidation systems were accomplished either by infusion of . NO gas equilibrated with anaerobic buffer or via . NO released from S-nitroso-glutathione. Nitric oxide alone did not induce lipid peroxidation, while exposure to SLO yielded significant oxidation of fatty acids, PC liposomes, or lipoproteins in a metal ion-independent mechanism. Low concentrations of . NO, which did not significantly inhibit the activity of the iron-containing lipoxygenase, induced potent inhibition of lipid peroxidation in a dose-dependent manner. Mass spectral analysis of oxidation products showed formation of nitrito-, nitro-, nitrosoperoxo-, and/or nitrated lipid oxidation adducts, demonstrating that . NO serves as a potent terminator of radical chain propagation reactions. The formation of Schiffs base fluorescent conjugates between SLO-oxidized linoleic or linolenic acid and bovine serum albumin (BSA) was also inhibited by . NO via reaction with lipid hydroperoxyl radicals (LOO .), thus preventing the reaction of LOO . with polypeptide amino groups. Mass spectrometry analysis showed that both lipid peroxidation products and nitrogen-containing oxidized lipid species decreased in the presence of BSA. We conclude that . NO can play a potent oxidant-protective role in the vessel wall by inhibiting lipoxygenase-dependent lipid and lipoprotein oxidation. This occurs via termination of lipid radical chain propagation reactions catalyzed by alkoxyl (LO .) and LOO . intermediates of lipid peroxidation rather than by inhibition of lipoxygenase-catalyzed initiation reactions. (C) 1995 Academic Press, Inc.