title: Oxysterols from free radical chain oxidation of 7-dehydrocholesterol: product and mechanistic studies.
abstract: Free radical chain oxidation of highly oxidizable 7-dehydrocholesterol (7-DHC), initiated by 2,2'-azobis(4-methoxy-2,4-dimethylvaleronitrile), was carried out at 37 degrees C in benzene for 24 h. Fifteen oxysterols derived from 7-DHC were isolated and characterized with 1D and 2D NMR spectroscopy and mass spectrometry. A mechanism that involves abstraction of hydrogen atoms at C-9 and/or C-14 is proposed to account for the formation of all of the oxysterols and the reaction progress profile. In either the H-9 or H-14 mechanism, a pentadienyl radical intermediate is formed after abstraction of H-9 or H-14 by a peroxyl radical. This step is followed by the well-precedented transformations observed in peroxidation reactions of polyunsaturated fatty acids such as oxygen addition, peroxyl radical 5-exo cyclization, and S(H)i carbon radical attack on the peroxide bond. The mechanism for peroxidation of 7-DHC also accounts for the formation of numerous oxysterol natural products isolated from fungal species, marine sponges, and cactaceous species. In a cell viability test, the oxysterol mixture from 7-DHC peroxidation was found to be cytotoxic to Neuro2a neuroblastoma cells in the micromolar concentration range. We propose that the high reactivity of 7-DHC and the oxysterols generated from its peroxidation may play important roles in the pathogenesis of Smith-Lemli-Opitz syndrome, X-linked dominant chondrodysplasia punctata, and cerebrotendinous xanthomatosis, all of these being metabolic disorders characterized by an elevated level of 7-DHC.
authors.affiliation: Department of Chemistry and Vanderbilt Institute of Chemical Biology, Vanderbilt University, Nashville, Tennessee 37235, USA.
authors.forename: Libin
authors.initials: L
authors.lastname: Xu
authors.forename: Zeljka
authors.initials: Z
authors.lastname: Korade
authors.forename: Ned A
authors.initials: NA
authors.lastname: Porter
mesh_terms: D000818:Animals; D001391:Azo Compounds / Q000737:chemistry; D045744:Cell Line, Tumor; D002470:Cell Survival / Q000187:drug effects; D003684:Dehydrocholesterols / Q000737:chemistry* / Q000378:metabolism; D005609:Free Radicals / Q000737:chemistry / Q000378:metabolism; D051379:Mice; D009447:Neuroblastoma / Q000188:drug therapy / Q000473:pathology; D009570:Nitriles / Q000737:chemistry; D019906:Nuclear Magnetic Resonance, Biomolecular; D010084:Oxidation-Reduction; D051381:Rats; D013261:Sterols / Q000737:chemistry* / Q000378:metabolism / Q000494:pharmacology
chemical_list: D001391:Azo Compounds; D003684:Dehydrocholesterols; D005609:Free Radicals; D009570:Nitriles; D013261:Sterols; C109522:V 70; C016705:7-dehydrocholesterol