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

Abstract

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-arachidonyl-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-nitrosoglutathione. 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 Schiff's 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.