Characterization of 4-oxo-2-nonenal as a novel product of lipid peroxidation

Fe-II-mediated decomposition of 13-[S-(Z,E)]-9,11-hydroperoxyoctadecadienoic (hydroperoxylinoleic) acid resulted in the formation of three alpha,beta-unsaturated aldehydes. At low Fe-II concentrations or at early time points after the addition of Fe-II, two major products were observed. The least polar product had chromatographic properties that were identical with those of 4-oxo-2-nonenal. Conversion of this product to its bis-oxime derivative with hydroxylamine hydrochloride resulted in two syn- and two anti-oxime isomers that had chromatographic and mass spectral properties identical with the properties of products derived from an authentic standard of 4-oxo-2-nonenal, This confirmed for the first time that 4-oxo-2-nonenal is a major product of the Fe-II-mediated breakdown of lipid hydroperoxides, The more polar product had chromatographic properties that were similar to those of 4-hydroperoxy-2-nonenal. LC/MS analysis of its syn- and anti-oxime isomers confirmed this structural assignment, Thus, 4-hydroperoxy-2-nonenal is a previously unrecognized major product of lipid hydroperoxide decomposition. At high Fe-II concentrations and at longer incubation times, a third more polar product was observed with chromatographic properties that were identical to those of 4-hydroxy-2-nonenal. The syn- and anti-oxime isomers had chromatographic and mass spectral properties identical with the properties of products derived from an authentic standard of 4-hydroxy-2-nonenal. It appears that 4-hydroperoxy-2-nonenal is formed initially and that it is then converted to 4-hydroxy-2-nonenal in the presence of high Fe-II concentrations or by extended incubations in the presence of low Fe-II concentrations. It is conceivable that some of the 4-hydroperoxy-2-nonenal is also converted to 4-oxo-2-nonenal. However, we cannot rule out the possibility that it is also formed by a concerted mechanism from a rearrangement product of 13-[S-(Z,E)]-9,11-hydroperoxyoctadecadienoic acid.