The lipoxygenase pathway in tulip (Tulipa gesneriana): detection of the ketol route.
The in vitro metabolism of [1-(14)C]linoleate, [1-(14)C]linolenate and their 9(S)-hydroperoxides was studied in cell-free preparations from tulip (Tulipa gesneriana) bulbs, leaves and flowers. Linoleate and its 9-hydroperoxide were converted by bulb and leaf preparations into three ketols: (12Z)-9-hydroxy-10-oxo-12-octadecadienoic acid (alpha-ketol), (11E)-10-oxo-13-hydroxy-11-octadecadienoic acid (gamma-ketol) and a novel compound, (12Z)-10-oxo-11-hydroxy-12-octadecadienoic acid (10,11-ketol), in the approximate molar proportions of 10:3:1. The corresponding 15, 16-dehydro alpha- and gamma-ketols were the main metabolites of [1-(14)C]linolenate and its 9-hydroperoxide. Thus bulbs and leaves possessed 9-lipoxygenase and allene oxide synthase activities. Incubations with flower preparations gave alpha-ketol hydro(pero)xides as predominant metabolites. Bulb and leaf preparations possessed a novel enzyme activity, gamma-ketol reductase, which reduces gamma-ketol to 10-oxo-13-hydroxyoctadecanoic acid (dihydro-gamma-ketol) in the presence of NADH. Exogenous linolenate 13(S)-hydroperoxide was converted mostly into chiral (9S,13S)-12-oxo-10-phytodienoate (99.5% optical purity) by bulb preparations, while [1-(14)C]linolenate was a precursor for ketols only. Thus tulip bulbs possess abundant allene oxide cyclase activity, the substrate for which is linolenate 13(S)-hydroperoxide, even though 13(S)-lipoxygenase products were not detectable in the bulbs. The majority of the cyclase activity was found in the microsomes (10(5) g pellet). Cyclase activity was not found in the other tissues examined, but only in the bulbs. The ketol route of the lipoxygenase pathway, mediated by 9-lipoxygenase and allene oxide synthase activities, has not been detected previously in the vegetative organs of any plant species.
Grechkin
A N
AN
Kazan Institute of Biochemistry and Biophysics, Russian Academy of Sciences, P.O. Box 30, Kazan, 420503, Russia. grechkin@sci.kcn.ru
Mukhtarova
L S
LS
Hamberg
M
M
D002250:Carbon Radioisotopes; D002851:Chromatography, High Pressure Liquid; D005227:Fatty Acids / Q000378:metabolism; D019787:Linoleic Acid / Q000378:metabolism; D008042:Linolenic Acids / Q000378:metabolism; D008054:Lipid Peroxides / Q000378:metabolism; D008084:Lipoxygenase / Q000378:metabolism*; D019684:Magnoliopsida / Q000201:enzymology*; D013058:Mass Spectrometry; D013056:Spectrophotometry, Ultraviolet
D002250:Carbon Radioisotopes; D005227:Fatty Acids; D008042:Linolenic Acids; D008054:Lipid Peroxides; C057375:13-hydroperoxy-9,11,15-octadecatrienoic acid; D019787:Linoleic Acid; D008084:Lipoxygenase