An epoxy alcohol synthase pathway in higher plants: biosynthesis of antifungal trihydroxy oxylipins in leaves of potato.
[1-14C]Linoleic acid was incubated with a whole homogenate preparation of potato leaves (Solanum tuberosum L., var. Bintje). The methyl-esterified product was subjected to straight-phase high-performance liquid chromatography and was found to contain four major radioactive oxidation products, i.e., the epoxy alcohols methyl 10(S),11(S)-epoxy-9(S)-hydroxy-12(Z)-octadecenoate (14% of the recovered radioactivity) and methyl 12(R), 13(S)-epoxy-9(S)-hydroxy-10(E)-octadecenoate (14%), and the trihydroxy derivatives methyl 9(S),10(S),11(R)-trihydroxy-12(Z)-octadecenoate (18%)and methyl 9(S), 12(S),13(S)-trihydroxy-10(E)-octadecenoate (30%). The structures and stereochemical configurations of these oxylipins were determined by chemical and spectral methods using the authentic compounds as references. Incubations performed in the presence of glutathione peroxidase revealed that lipoxygenase activity of potato leaves generated the 9- and 13-hydroperoxides of linoleic acid in a ratio of 95:5. Separate incubations of these hydroperoxides showed that linoleic acid 9(S)-hydroperoxide was metabolized into epoxy alcohols by particle-bound epoxy alcohol synthase activity, whereas the 13-hydroperoxide was metabolized into alpha- and gamma-ketols by a particle-bound allene oxide synthase. It was concluded that the main pathway of linoleic acid metabolism in potato leaves involved 9-lipoxygenase-catalyzed oxygenation into linoleic acid 9(S)-hydroperoxide followed by rapid conversion of this hydroperoxide into epoxy alcohols and a slower, epoxide hydrolase-catalyzed conversion of the epoxy alcohols into trihydroxy-octadecenoates. Trihydroxy derivatives of linoleic and linolenic acids have previously been reported to be growth-inhibitory to plant-pathogenic fungi, and a role of the new pathway of linoleic acid oxidation in defense reactions against pathogens is conceivable.
Hamberg
M
M
Department of Medical Biochemistry and Biophysics, Karolinska Institutet, Stockholm, Sweden. Mats.Hamberg@mbb.ki.se
D000935:Antifungal Agents; D002250:Carbon Radioisotopes; D002851:Chromatography, High Pressure Liquid; D004851:Epoxide Hydrolases / Q000378:metabolism; D005979:Glutathione Peroxidase / Q000378:metabolism; D019787:Linoleic Acid / Q000378:metabolism; D008041:Linoleic Acids / Q000378:metabolism; D008054:Lipid Peroxides / Q000378:metabolism; D008084:Lipoxygenase / Q000378:metabolism; D015394:Molecular Structure; D009829:Oleic Acids / Q000096:biosynthesis* / Q000737:chemistry; D010084:Oxidation-Reduction; D018515:Plant Leaves / Q000201:enzymology; D011198:Solanum tuberosum / Q000201:enzymology*; D017962:alpha-Linolenic Acid / Q000378:metabolism
C402672:9,10,11-trihydroxy-12-octadecenoate; C402673:9,12,13-trihydroxyoctadecenoate; D000935:Antifungal Agents; D002250:Carbon Radioisotopes; D008041:Linoleic Acids; D008054:Lipid Peroxides; D009829:Oleic Acids; D017962:alpha-Linolenic Acid; C038649:13-hydroperoxy-9,11-octadecadienoic acid; C049606:9-hydroperoxy-11,12-octadecadienoic acid; D019787:Linoleic Acid; D005979:Glutathione Peroxidase; D008084:Lipoxygenase; D004851:Epoxide Hydrolases