Hydroperoxy fatty acids, synthesis

FIG. 3 Hydroperoxy-fatty acid synthesis from triacylglycerol with successive lipase-catalyzed hydrolysis and lipoxygenation. 

Since AA is only a minor fatty acid in higher plants, eicosanoids are not of major importance for plant physiology. However, the oxygenation metabolites of linoleic acid and a-linolenic acid, called oxylipins [5,6], do play a role in plant defence reactions, in the formation of phytohormones and in the synthesis of cutin monomers [6,40-43]. Oxylipins constitute a family of lipids that are formed from free fatty acids by a cascade of reactions involving at least one step of dioxygen-dependent oxidation. The biosynthesis of oxylipins proceeds via a large number of metabolic pathways, most of which involve an unsaturated hydroperoxy fatty acid as intermediate (Scheme 10). Conversion of the hydroperoxide via the peroxide lyase pathway, the allene oxide pathway and the recently discovered peroxygenase pathway, leads to a complex pattern of oxidized lipid mediators. 

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]. 

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