Cholesterol Hydroperoxides Generate Singlet Molecular Oxygen [O2 (1Δg)]: Near-IR Emission, 18O-Labeled Hydroperoxides, and Mass Spectrometry

In mammalian membranes, cholesterol is concentrated in lipid rafts. The generation of cholesterol hydroperoxides (ChOOHs) and their decomposition products induces various types of cell damage. The decomposition of some organic hydroperoxides into peroxyl radicals is known to be a potential source of singlet molecular oxygen [O2 (1Δg)] in biological systems. We report herein on evidence of the generation of O2 (1Δg) from ChOOH isomers in solution or in liposomes containing ChOOHs, which involves a cyclic mechanism from a linear tetraoxide intermediate originally proposed by Russell. Characteristic light emission at 1270 nm, corresponding to O2 (1Δg) monomolecular decay, was observed for each ChOOH isomer or in liposomes containing ChOOHs. Moreover, the presence of O2 (1Δg) was unequivocally demonstrated using the direct spectral characterization of near-infrared light emission. Using 18O-labeled cholesterol hydroperoxide (Ch18O18OH), we observed the formation of 18O-labeled O2 (1Δg) [18O2 (1Δg)] by the chemical trapping of 18O2 (1Δg) with 9,10-diphenylanthracene (DPA) and detected the corresponding 18O-labeled DPA endoperoxide (DPA18O18O) and the 18O-labeled products of the Russell mechanism using high-performance liquid chromatography coupled to tandem mass spectrometry. Photoemission properties and chemical trapping clearly demonstrate that the decomposition of Ch18O18OH generates 18O2 (1Δg), which is consistent with the Russell mechanism and points to the involvement of O2 (1Δg) in cholesterol hydroperoxide-mediated cytotoxicity.