Organic nitrites and NO: Inhibition of lipid peroxidation and radical reactions

Organic nitrites, such as i-amyl nitrite (IAN), are nitrovasodilator drugs used both clinically and recreationally. Nitrites are also chemically reasonable biological products of NO metabolism, in particular in both inhibition of lipid peroxidation by NO and induction of lipid peroxidation by peroxynitrite and NO2. Nitrites are also potential products of biomolecule nitrosation and intermediates in biotransformation of nitrate vasodilators. Although mechanisms can be drawn for both prooxidant and antioxidant activity, IAN has been observed to inhibit lipid peroxidation in a variety of systems. To test if the antioxidant activity of nitrites results from NO release alone, inhibition of lipid peroxidation was studied for four organic nitrites and four NO donor NONOates. Iron-induced lipid peroxidation in synaptosomal tissue homogenates and azo compound-initiated lipid peroxidation in liposomes and linoleic acid SDS comicelles were examined. Lipid peroxidation was quantified by TBARS and oxygen uptake analysis. A good correlation of rate of NO release with IC50 for inhibition of lipid peroxidation was observed for the NONOates, compatible with lipid radical chain termination by NO, for which a chain termination stoichiometry of 0.4-0.5 mol of lipid peroxyl radicals per mole of NO was determined. In neutral aqueous solution, nitrites also spontaneously released NO as measured by chemiluminescence; however, no correlation was observed between the rate constants of NO release for the nitrites and their inhibitor potency toward lipid peroxidation. Long chain nitrites were seen to be relatively good inhibitors of lipid peroxidation by mechanisms that must involve factors in addition to simple homolysis to release NO. Evidence for direct cc-hydrogen atom abstraction from the nitrite by peroxyl radicals was obtained by analysis of aldehyde products and supported by MO calculations. The data suggest that lipid nitrites formed as NO chain termination products have the capacity to further inhibit lipid peroxidation and to release NO.