The lipoxygenase pathway in sunflower roots was studied in vitro. A preliminary incubation of linoleic acid with 15 000 g supernatant of homogenate of sunflower roots (1.5-6 days after germination) revealed the predominant activity of 13-lipoxygenase. The exogenously added linoleic acid 13-hydroperoxide is further utilized through two competing pathways. One of them is directed towards formation of the ketodiene (9Z,11E)-13-oxooctadeca-9,11-dienoic acid. The second pathway, which is controlled by allene oxide synthase, leads to the formation of an alpha-ketol and a novel cyclopentenone, rac-cis-12-oxo-10-phytoenoic acid (12-oxo-PEA) via a short-lived allene oxide. Unexpectedly, the cyclopentenone 12-oxo-PEA is the predominant allene oxide synthase product. Identification of cis-12-oxo-PEA was confirmed by its UV, mass, (1)H NMR and 2D-COSY spectral data. The highest yield of 12-oxo-PEA is observed in very young roots (1.5-2 days after germination). The results of methanol-trapping experiments demonstrate that both 12-oxo-PEA and alpha-ketol are formed through the unstable allene oxide intermediate, (9Z)-12,13-epoxyoctadeca-9,11-dienoic acid, which is the primary product of allene oxide synthase. Since 12-oxo-PEA is a jasmonate congener, its biosynthesis in plants might be of physiological importance.
Source: Chembiochem. (2007) vol. 18, p. 2275-2280
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