title: Oxidation of 7-dehydrocholesterol and desmosterol by human cytochrome P450 46A1.
abstract: Cytochrome P450 (P450 or CYP) 46A1 is expressed in brain and has been characterized by its ability to oxidize cholesterol to 24S-hydroxycholesterol. In addition, the same enzyme is known to further oxidize 24S-hydroxycholesterol to the 24,25- and 24,27-dihydroxy products, as well as to catalyze side-chain oxidations of 7α-hydroxycholesterol and cholestanol. As precursors in the biosynthesis of cholesterol, 7-dehydrocholesterol has not been found to be a substrate of P450 46A1 and desmosterol has not been previously tested. However, 24-hydroxy-7-dehydrocholesterol was recently identified in brain tissues, which prompted us to reexamine this enzyme and its potential substrates. Here we report that P450 46A1 oxidizes 7-dehydrocholesterol to 24-hydroxy-7-dehydrocholesterol and 25-hydroxy-7-dehydrocholesterol, as confirmed by LC-MS and GC-MS. Overall, the catalytic rates of formation increased in the order of 24-hydroxy-7-dehydrocholesterol < 24-hydroxycholesterol < 25-hydroxy-7-dehydrocholesterol from their respective precursors, with a ratio of 1:2.5:5. In the case of desmosterol, epoxidation to 24S,25-epoxycholesterol and 27-hydroxylation was observed, at roughly equal rates. The formation of these oxysterols in the brain may be of relevance in Smith-Lemli-Opitz syndrome, desmosterolosis, and other relevant diseases, as well as in signal transduction by lipids.
authors.affiliation: Department of Biochemistry and Center in Molecular Toxicology, Vanderbilt University School of Medicine, Nashville, TN 37232.
authors.forename: Sandeep
authors.initials: S
authors.lastname: Goyal
authors.affiliation: Department of Biochemistry and Center in Molecular Toxicology, Vanderbilt University School of Medicine, Nashville, TN 37232.
authors.forename: Yi
authors.initials: Y
authors.lastname: Xiao
authors.affiliation: Department of Chemistry and Vanderbilt Institute of Chemical Biology, Vanderbilt University, Nashville, TN 37235.
authors.forename: Ned A
authors.initials: NA
authors.lastname: Porter
authors.affiliation: Department of Chemistry and Vanderbilt Institute of Chemical Biology, Vanderbilt University, Nashville, TN 37235.
authors.forename: Libin
authors.initials: L
authors.lastname: Xu
authors.affiliation: Department of Biochemistry and Center in Molecular Toxicology, Vanderbilt University School of Medicine, Nashville, TN 37232.
authors.forename: F Peter
authors.initials: FP
authors.lastname: Guengerich
mesh_terms: D000072556:Cholesterol 24-Hydroxylase; D003684:Dehydrocholesterols / Q000737:chemistry*; D003897:Desmosterol / Q000737:chemistry*; D006801:Humans; D007700:Kinetics; D010084:Oxidation-Reduction; D011485:Protein Binding; D013250:Steroid Hydroxylases / Q000737:chemistry*
chemical_list: D003684:Dehydrocholesterols; D003897:Desmosterol; C016705:7-dehydrocholesterol; D013250:Steroid Hydroxylases; D000072556:Cholesterol 24-Hydroxylase
keywords: 24-hydroxy-7-dehydrocholesterol; 24S,25-epoxycholesterol; 25-hydroxy-7-dehydrocholesterol; 27-hydr­oxydesmosterol