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 [O-2 ((1)Delta(g))] in biological systems. We report herein on evidence of the generation of O-2 ((1)Delta(g)) 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 O-2 ((1)Delta(g)) monomolecular decay, was observed for each ChOOH isomer or in liposomes containing ChOOHs. Moreover, the presence of O-2 ((1)Delta(g)) was unequivocally demonstrated using the direct spectral characterization of near-infrared light emission. Using O-18-labeled cholesterol hydroperoxide (Ch(18)O(18)OH), we observed the formation of O-18-labeled O-2 ((1)Delta(g)) [O-18(2) ((1)Delta(g))] by the chemical trapping of O-18(2) ((1)Delta(g)) with 9,10-diphenylanthracene (DPA) and detected the corresponding O-18-labeled DPA endoperoxide (DPA(18)O(18)O) and the O-18-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 Ch(18)O(18)OH generates O-18(2) ((1)Delta(g)), which is consistent with the Russell mechanism and points to the involvement of O-2 ((1)Delta(g)) in cholesterol hydroperoxide-mediated cytotoxicity.