Singlet Molecular Oxygen Generated from Lipid Hydroperoxides by the Russell Mechanism: Studies Using18O-Labeled Linoleic Acid Hydroperoxide and Monomol Light Emission Measurements

The decomposition of lipid hydroperoxides into peroxyl radicals is a potential source of singlet oxygen (1O2) in biological systems. We report herein on evidence of the generation of 1O2 from lipid hydroperoxides involving a cyclic mechanism from a linear tetraoxide intermediate proposed by Russell. Using 18O-labeled linoleic acid hydroperoxide (LA18O18OH) in the presence of Ce4+ or Fe2+, we observed the formation of 18O-labeled 1O2 (18[1O2]) by chemical trapping of 1O2 with 9,10-diphenylanthracene (DPA) and detected the corresponding 18O-labeled DPA endoperoxide (DPA18O18O) by high-performance liquid chromatography coupled to tandem mass spectrometry. Spectroscopic evidence for the generation of 1O2 was obtained by measuring (i) the dimol light emission in the red spectral region (λ > 570 nm); (ii) the monomol light emission in the near-infrared (IR) region (λ = 1270 nm); and (iii) the quenching effect of sodium azide. Moreover, the presence of 1O2 was unequivocally demonstrated by the direct spectral characterization of the near-IR light emission. For the sake of comparison, 1O2 deriving from the H2O2/OCl- and H2O2/MoO42- systems or from the thermolysis of the endoperoxide of 1,4-dimethylnaphthalene was also monitored. These chemical trapping and photoemission properties clearly demonstrate that the decomposition of LA18O18OH generates 18[1O2], consistent with the Russell mechanism and pointing to the involvement of 1O2 in lipid hydroperoxide mediated cytotoxicity.