Intramolecular H-Atom Transfers in Alkoxyl Radical Intermediates Underlie the Apparent Oxidation of Lipid Hydroperoxides by Fe(II)
Dmitry D. Saraev, Zijun Wu, Hye‐Young Kim, Ned A. Porter, Derek A. Pratt
Abstract
The one-electron reduction of lipid hydroperoxides by low-valent iron species is believed to be a driver of cellular lipid peroxidation and associated ferroptotic cell death. We investigated reactions of cholesterol 7α-OOH, the primary cholesterol autoxidation product, with Fe 2+ to find that 7-ketocholesterol (7-KC, an oxidation product) is the major product under these (reducing) conditions. Mechanistic studies reveal the intervention of a 1,2-H-atom shift upon formation of the 7-alkoxyl radical to yield a ketyl radical that can be oxidized by either Fe 3+ or O 2 to give 7-KC, the most abundant oxysterol in vivo . We also investigated the corresponding reduction of the isomeric cholesterol 5α-OOH and again found that an oxidation product (5-hydroxycholesten-3-one) predominates under reducing conditions. An intramolecular H-atom shift (this time 1,4-) in the initially formed 5-alkoxyl radical is suggested to yield a ketyl radical that is oxidized to give the observed product. It would appear that a 1,2-H shift also accounts for the predominance of ketones over alcohols when unsaturated fatty acid hydroperoxides are exposed to iron-based reductants, which had previously been reported with hematin and demonstrated here with Fe 2+ . The predominance of 7-KC over the corresponding alcohol is maintained when cholesterol 7α-OOH embedded in phospholipid liposomes is treated with Fe 2+ or when ferroptosis is induced in mouse embryonic fibroblasts. Our observation that 7-KC accumulates in ferroptotic cells suggests that it may be a good biomarker for ferroptosis.