An epoxy alcohol synthase pathway in higher plants: Biosynthesis of antifungal trihydroxy oxylipins in leaves of potato |
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Authors: | Mats Hamberg |
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Affiliation: | (1) Department of Medical Biochemistry and Biophysics, Division of Physiological Chemistry II, Karolinska Institutet, S-171 77 Stockholm, Sweden |
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Abstract: | 1-14C]Linoleic acid was incubated with a whole homogenate preparation of potato leaves (Solanum tuberosum 1., 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 α- and γ-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 trihydroxyoctadecenoates. 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. |
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