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

Abstract

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/MoO₄^2- 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.