Nitric Oxide Inhibition of Lipid Peroxidation:  Kinetics of Reaction with Lipid Peroxyl Radicals and Comparison with α-Tocopherol

Abstract

The reaction between nitric oxide (*NO) and lipid peroxyl radicals (LOO*) has been proposed to account for the potent inhibitory properties of *NO toward lipid peroxidation processes; however, the mechanisms of this reaction, including kinetic parameters and nature of termination products, have not been defined. Here, the reaction between linoleate peroxyl radicals and *NO was examined using 2, 2'-azobis(2-amidinopropane) hydrochloride-dependent oxidation of linoleate. Addition of *NO (0.5-20 microM) to peroxidizing lipid led to cessation of oxygen uptake, which resumed at original rates when all *NO had been consumed. At high *NO concentrations (>3 microM), the time of inhibition (Tinh) of chain propagation became increasingly dependent on oxygen concentration, due to the competing reaction of oxygen with *NO. Kinetic analysis revealed that a simple radical-radical termination reaction (*NO:ROO* = 1:1) does not account for the inhibition of lipid oxidation by *NO, and at least two molecules of *NO are consumed per termination reaction. A mechanism is proposed whereby *NO first reacts with LOO* (k = 2 x 10(9) M-1 s-1) to form LOONO. Following decomposition of LOONO to LO* and *NO2, a second *NO is consumed via reaction with LO*, with the composite rate constant for this reaction being k = 7 x 10(4) M-1 s-1. At equal concentrations, greater inhibition of oxidation was observed with *NO than with alpha-tocopherol. Since *NO reacts with LOO* at an almost diffusion-limited rate, steady state concentrations of 30 nM *NO would effectively compete with endogenous alpha-tocopherol concentrations (about 20 microM) as a scavenger of LOO* in the lipid phase. This indicates that biological *NO concentrations (up to 2 microM) will significantly influence peroxidation reactions in vivo.

Nitric Oxide Regulation of Free Radical– and Enzyme-Mediated Lipid and Lipoprotein Oxidation

“…25 With oxidizing linoleic acid, ⅐ NO first reacts with LOO ⅐ , where kϭ2ϫ10 25 to form an unstable organic peroxynitrite (LOONO) intermediate that quickly decomposes (kϭ0.1 to 0.3 s Ϫ1 20 ) to generate an alkoxyl radical (RO ⅐ ) and ⅐ NO 2 as a caged radical pair (RO ⅐ -⅐ NO 2 ). Then, a second molecule of ⅐ NO can react with RO ⅐ to form an alkyl nitrite, RNO 2 , where kϭ2ϫ10…”

“…26 Other reactions and rearrangements are also possible, yielding derivatives such as epoxyallylic radicals and aldehydes. 23,25 ⅐…”

Effects of Different Phases of Cigarette Smoke on Lipid Peroxidation and Membrane Structure in Liposomes

“…Examples par excellence are furnished by the smoke-borne NO and nitroxyl radicals. [16,30] Indeed, it is well known that both nitric oxide [31,32] and nitroxyl free radicals [33,34] are able to inhibit oxidation processes including lipid peroxidation. Clearly, this adds mechanistic intricacy to the development of the smoking-associated oxidative stress.…”

Nitric oxide as a cellular antioxidant: A little goes a long way

“…Nitric oxide has been proposed to act as a pro-oxidant at high concentrations [10] or when it reacts with superoxide (O 2 •− ) forming the highly reactive peroxynitrite (ONOO − ) [11,12,13]. On the other hand, NO • can also inhibit oxidation; NO • can terminate chain reactions during lipid peroxidation, as observed in model lipid systems [14,15], low density lipoprotein oxidation [16,17,18,19] and in cells [20,21].…”