title: Isolation and characterization of two geometric allene oxide isomers synthesized from 9S-hydroperoxylinoleic acid by cytochrome P450 CYP74C3: stereochemical assignment of natural fatty acid allene oxides.
abstract: Specialized cytochromes P450 or catalase-related hemoproteins transform fatty acid hydroperoxides to allene oxides, highly reactive epoxides leading to cyclopentenones and other products. The stereochemistry of the natural allene oxides is incompletely defined, as are the structural features required for their cyclization. We investigated the transformation of 9S-hydroperoxylinoleic acid with the allene oxide synthase CYP74C3, a reported reaction that unexpectedly produces an allene oxide-derived cyclopentenone. Using biphasic reaction conditions at 0 °C, we isolated the initial products and separated two allene oxide isomers by HPLC at -15 °C. One matched previously described allene oxides in its UV spectrum (λmax 236 nm) and NMR spectrum (defining a 9,10-epoxy-octadec-10,12Z-dienoate). The second was a novel stereoisomer (UV λmax 239 nm) with distinctive NMR chemical shifts. Comparison of NOE interactions of the epoxy proton at C9 in the two allene oxides (and the equivalent NOE experiment in 12,13-epoxy allene oxides) allowed assignment at the isomeric C10 epoxy-ene carbon as Z in the new isomer and the E configuration in all previously characterized allene oxides. The novel 10Z isomer spontaneously formed a cis-cyclopentenone at room temperature in hexane. These results explain the origin of the cyclopentenone, provide insights into the mechanisms of allene oxide cyclization, and define the double bond geometry in naturally occurring allene oxides.
authors.affiliation: From the Departments of Pharmacology and. Electronic address: alan.brash@vanderbilt.edu.
authors.forename: Alan R
authors.initials: AR
authors.lastname: Brash
authors.affiliation: From the Departments of Pharmacology and.
authors.forename: William E
authors.initials: WE
authors.lastname: Boeglin
authors.affiliation: Chemistry and the Vanderbilt Institute of Chemical Biology, Vanderbilt University, Nashville, Tennessee 37232 and.
authors.forename: Donald F
authors.initials: DF
authors.lastname: Stec
authors.affiliation: Chemistry and the Vanderbilt Institute of Chemical Biology, Vanderbilt University, Nashville, Tennessee 37232 and.
authors.forename: Markus
authors.initials: M
authors.lastname: Voehler
authors.affiliation: From the Departments of Pharmacology and.
authors.forename: Claus
authors.initials: C
authors.lastname: Schneider
authors.affiliation: the Department of Chemistry, Wayne State University, Detroit, Michigan 48202.
authors.forename: Jin K
authors.initials: JK
authors.lastname: Cha
mesh_terms: D002851:Chromatography, High Pressure Liquid; D056148:Chromatography, Reverse-Phase; D003500:Cyclization; D003517:Cyclopentanes / Q000737:chemistry / Q000378:metabolism; D003577:Cytochrome P-450 Enzyme System / Q000378:metabolism*; D004852:Epoxy Compounds / Q000737:chemistry* / Q000302:isolation & purification*; D019597:Flax / Q000201:enzymology*; D008041:Linoleic Acids / Q000737:chemistry* / Q000302:isolation & purification* / Q000378:metabolism*; D009682:Magnetic Resonance Spectroscopy; D008968:Molecular Conformation; D054883:Oxylipins / Q000737:chemistry / Q000378:metabolism; D013237:Stereoisomerism
chemical_list: D003517:Cyclopentanes; D004852:Epoxy Compounds; D008041:Linoleic Acids; D054883:Oxylipins; C011006:jasmonic acid; D003577:Cytochrome P-450 Enzyme System
keywords: Cytochrome P450; Lipid Oxidation; Lipid Structure; Lipoxygenase Pathway; NMR