Nitric oxide inhibition of lipid peroxidation: kinetics of reaction with lipid peroxyl radicals and comparison with alpha-tocopherol.

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.