Y-Ketols

It is pertinent to mention the formation of y-ketols by photochemical addition of alcohols to enones follows the same polarity pattern [193],... 

Scheme Generation of jasmonic acid as well as a- and y-ketols according to Hamberg [160].

Water addition to the allene oxide results in generation of a- and Y-ketols (Schenae 7) [161-165]. 

Keto esters, 230-231 d-Keto esters, 127, 311 7-Keto esters, 229 Ketolactones, 420 a-Ketols, 426,530 d-Ketols, 141 y-Ketols, 141 7-Ketonitriles, 447 Ketonitrones, 281 5-Keto-l-nonene, 572... 

More than 30 years ago it was reported that flaxseed homogenates convert 13-HpOTrE into a- and -y-ketols, and the enzyme responsible for this reaction was called hydroperoxide isomerase [54]. Later studies indicated that this reaction was actually a two-step process, consisting of an enzyme-catalysed dehydration of the hydroperoxy fatty acid to a rather unstable allene oxide followed by a non-enzymic hydrolysis [55,56]. The enzyme responsible for allene oxide synthesis has been purified and shown to be a cyt E-450 isoform [57]. As an alternative to hydrolysis, the allene oxide may undergo enzymic cyclization forming 12-oxo-phytodienoic acid. This compound is subsequently converted into jasmonic acid, a phytohormone implicated in the reaction of higher plants to a number of stimuli, such as wounding, fungal elicitation, mechanical forces and osmotic stress [54]. 

The last route is important for the formation of jasmonic acid. The action of allene oxide synthase on 13-hydroperoxy linolenic acid initially results in the generation of 12,13-epoxy-octadecatrienoic acid. This unstable epoxide is either chemically hydrolyzed to a- and y-ketols and racemic 12-oxo-phytodienoic acid or, in the presence of allene oxide cyclase (AOC), is further converted to enantiomeric pure 12-oxo-PDA [7]. The ring double bond of PDA is then reduced in a NADPH dependent reaction by 12-oxo-PDA reductase and after shortening of the side chain containing the carboxy group by 3 rounds of fi>-oxidation, the biosynthesis of jasmonic acid is completed. 

Hydroperoxide isomerase was the first enzyme discovered that used the hydroperoxide product of lipoxygenase as its substrate, the products being a-and Y-ketols (14,15). The 13-hydroperoxide isomer of llnolelc or linolenic acid is converted to both 13-hydroxy-12-oxo and 9-hydroxy-12-oxo products (Fig. 2), in approximately a 4 1 ratio, respectively. From the 9-hydroper-oxlde isomer, 9-hydroxy-lO-oxo and 13-hydroxy-10-oxo products are formed. 

The most recent findings relating to lipoxygenase pathway in plants concern allene oxides, the primary products of enzymatic transformations of fatty acid hydroperoxides. Spontaneous hydrolysis of allene oxides leads to the formation of a- and y-ketols [1]. It has been also found that hydroperoxide dehydrase from flax seeds converts 8-hydroxy-155-hydroperoxy derivatives of arachidonic and eicosapentaenoic acids into a-ketols and prostaglandin A3 analogues [2]. These data allow us to hypothesize formation of ketols of 9-hydroxy derivative of a-linolenic acid. 

Recent work has revealed that lipoxygenases from some plant tissues oxidize a-ketols of a-linolenic acid to hydroperoxides of a-ketols [3]. This paper deals with the formation of similar products by the other way by hydroperoxide dehydration of double dioxygenation products. Incubation of 9-hydroxy-16-hydroperoxy-10( ),12(Z),14( )-[l-l C]octadecatrienoic acid with enzyme preparation from com seeds led to the formation of three polar metabolites. After RP- and SP-HPLC purifications two of these polar metabolites were identified by UV spectroscopy and electron-impact mass spectrometry as a- and y-ketols of 9-hydroxy derivative of a-linolenic acid 9,16-dihydroxy-15-oxo-I0(, 12(Z)-octadecadienoic acid and 9,12-dihydroxy-15-oxo-I0(jE, 13( )-octadecadienoic acid, respectively. Stmcture of the most polar metabolite is discussed. 

The amount of this metabolite, retention time 31 min (compound 3, fig.lA) accounted for approximately 40% of total recovered radioactivity at pH 7.5. When incubation medium pH was 6.3 the amount of compound 2, retention time 27 min. (fig. IB) was the same as the yield of the main product (about 10%). The change of direction of the substrate metabolism corresponds to different formation of a- and y-ketols under weak acidic and neutral pH of incubation medium. The similar stimulation of y-ketol formation under weak acidic pH values have been observed in the studying of metabolism of a-linolenic acid hydroperoxides by hydroperoxide dehydrases from different plant species [3]. The UV spectrum of metabolite 2 exhibits a typical oxoene chromophore, at 227 nm. The data obtained allow us to suggest that compound 2 is y-ketol of 9-hydroxy derivative, 9,12-dihydroxy-15-oxo-10( ),13( )-octadecadienoic acid. 

Scheme 1. The pathway of 15,16-a-ketol (4) and 12,15-y-ketol (2) formation from 9-hydroxy-16-hydroperoxy-10( ),12(Z),14( )-octadecatrienoic acid (1). 3, supposed structure of metabolite 1. HPD, hydroperoxide dehydrase.

Recent work has demonstrated that hydroperoxides of a-ketols are formed from the corresponding hydroperioxide precursors by conversion of hydroperoxides into a-ketols and following lipoxygenase oxidation of a-ketols [3]. The present work has revealed that the similar metabolites (a- and y-ketols of hydroxy derivatives) may be formed from double dioxygenation products by dehydration of hydroperoxide groups and further hydrolysis of hydroxy allene oxides. 

The obtained results allow us to conclude that com seed hydroperoxide dehydrase catalyzes dehydration of hydroperoxide groups at C-16 position with formation of a- and y-ketols of 9-hydroxylinolenate. We hypothesize the formation of cyclopentenone 9-hydroxy-ll-[2-ethyl-3-oxo-cyclopent-4-enyl]-10( )-undecenoic acid. 

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