NITRIC-OXIDE INHIBITION OF LIPOXYGENASE-DEPENDENT LIPOSOME AND LOW-DENSITY-LIPOPROTEIN OXIDATION - TERMINATION OF RADICAL-CHAIN PROPAGATIONREACTIONS 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 atherogene sis. 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, w e assessed the influence of . NO on lipoxygenase-catalyzed oxidation o f linoleic and linolenic acid, almitoyl-2-arachidanyl-sn-glycero-3-pho sphocholine (PC) liposomes, hypercholesterolemic rabbit beta-very-low- density lipoprotein, and human low-density lipoprotein. Soybean lipoxy genase (SLO)-induced lipid oxidation was assessed by accumulation of c onjugated dienes, formation of lipid hydroperoxides, oxygen consumptio n, and liquid chromatography-mass spectrometry. Different rates of del ivery of . NO to lipid oxidation systems were accomplished either by i nfusion of . NO gas equilibrated with anaerobic buffer or via . NO rel eased from S-nitroso-glutathione. Nitric oxide alone did not induce li pid peroxidation, while exposure to SLO yielded significant oxidation of fatty acids, PC liposomes, or lipoproteins in a metal ion-independe nt mechanism. Low concentrations of . NO, which did not significantly inhibit the activity of the iron-containing lipoxygenase, induced pote nt inhibition of lipid peroxidation in a dose-dependent manner. Mass s pectral analysis of oxidation products showed formation of nitrito-, n itro-, nitrosoperoxo-, and/or nitrated lipid oxidation adducts, demons trating that . NO serves as a potent terminator of radical chain propa gation reactions. The formation of Schiffs base fluorescent conjugates between SLO-oxidized linoleic or linolenic acid and bovine serum albu min (BSA) was also inhibited by . NO via reaction with lipid hydropero xyl radicals (LOO .), thus preventing the reaction of LOO . with polyp eptide 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 pot ent oxidant-protective role in the vessel wall by inhibiting lipoxygen ase-dependent lipid and lipoprotein oxidation. This occurs via termina tion of lipid radical chain propagation reactions catalyzed by alkoxyl (LO .) and LOO . intermediates of lipid peroxidation rather than by i nhibition of lipoxygenase-catalyzed initiation reactions. (C) 1995 Aca demic Press, Inc.