Don C. Zimmerman

The biosynthesis of jasmonic acid: A physiological role for plant lipoxygenase☆

Linolenic acid was converted to a cyclic product, 12-oxo-phytodienoic acid, by lipoxygenase and hydroperoxide cyclase enzymes present in Vicia faba pericarp. Isotope labeling studies in which [U-14C] 12-[180] oxo-phytodienoic acid was incubated with thin sections of pericarp tissue showed that 12-oxo-phytodienoic acid is a biosynthetic precursor to jasmonic acid, a plant growth regulator which promotes senescence. Key enzymes proposed for this pathway are a reductase enzyme which reduces a double bond in the cyclopentenone ring, and beta-oxidation enzymes which remove six carbons from the carboxyl end of the molecule.

Oxidative Systems for Modification of Fatty Acids: The Lipoxygenase Pathway

Publisher Summary This chapter discusses the advances in plant lipoxygenase A special effort has been made to focus on physiological roles in which LOX might participate. A clear theme emerging from this effort is that a unique physiological role for LOX probably does not exist in plants. It likely has a more general function of responding to plant requirements by synthesizing fatty acid hydroperoxide, the central intermediate for a variety of diverging metabolic pathways. In this regard, the LOX pathway in plants can be similar to the mammalian LOX or cyclo-oxygenase pathways. In animals, the hydroperoxide or endoperoxide products of these two enzymes are transformed into a variety of leukotrienes and prostaglandins, depending on the nature of the cellular stimulus. Substrates for LOX are fatty acids that possess a cis , cis -1,4-pentadiene structure. The most common plant fatty acids that have this structure are linoleic and linolenic acids, but other fatty acids having this structure are also effective substrates to varying degrees.

Characterization of a prostaglandin-like metabolite of linolenic acid produced by a flaxseed extract

One of the products formed upon incubation of linolenic acid (cis9,12,15-octadecatrienoic acid) with an extract of flaxseed acetone powder has been characterized as 8-[2-(cis-pent-2′-enyl)-3-oxo-cis-cyclopent-4-enyl]octanoic acid. The cyclopentenone ring structure of this acid is analogous to that of the A-type prostaglandins produced in mammalian systems.

Thermal alteration of a cyclic fatty acid produced by a flaxseed extract

Methyl 8-[2-(cis-pent-2′-enyl)-3-oxo-cis-cyclopent-4-enyl] octanoate (I) is the methyl ester of a cyclic fatty acid synthesized enzymically from an incubation of linolenic acid with an extract of flaxseed (Linum usitatissimum L.). A proposed trivial name for I is methyl 12-oxo-cis-10, 15-phytodienoate (12-oxo-PDA). The evidence presented indicated that compound I has thecis configuration of the carbon chains with respect to the cyclopentenone ring. Treatment with acid, base, or heat isomerized I to a second product (II) that has thetrans configuration of the carbon chains. Prolonged heat treatment of II yielded a third cyclic product, methyl 12-oxo-9(13),cis-15-phytodienoate (III).

Formation of 12-[18O]Oxo-cis-10,cis-15-phytodienoic acid from 13-[18O]hydroperoxylinolenic acid by hydroperoxide cyclase

Abstract13-[18O] Hydroperoxylinolenic acid was permitted to react with an extract of flaxseed acetone powder containing hydroperoxide cyclase activity. The resulting product, 12-oxo-cis-10,cis-15-phytodienoic acid (12-oxo-PDA), contained18O in the carbonyl oxygen at carbon 12, suggesting that an epoxide was an intermediate in the hyderoperoxide cyclase reaction. A substrate specificity study showed that acis double bond β,γ to the conjugated hydroperoxide group was essential for the substrate to be converted to a cyclic product by hydroperoxide cyclase.

The Lipoxygenase Pathway

After more than fifty years since its discovery, the study of the lipoxygenase pathway remains one of the most engaging, yet elusive areas of plant lipid research. Since 1932 when Andre and Hou (1) first reported on the existence of oxidizing enzymes which alter soybean oil, Chemical Abstracts has cited more than 1000 research articles under the subject of plant lipoxygenase. Despite the abundance of research effort on this topic, reviews (2–6) over the decades have been forced to report that the physiological role of lipoxygenase in plants is unknown.

Formation of γ-ketols from 13- and 9-hydroperoxides of linolenic acid by flaxseed hydroperoxide isomerase

A reexamination of the flaxseed hydroperoxide isomerase reaction showed that a minor enzymic product (ca. 5%), identified as a γ-ketol, was present. The substrates were the 13- or 9-hydroperoxides of linolenic acid, which were converted to 9-hydroxy-12-oxo-cis-15-trans-11-octadecadienoic acid, respectively. These compounds were formed in addition to the major products reported earlier: a 12,13-α-ketol and 12-oxo-cis-10,15-phytodienoic acid from the 13-isomer, and a 9,10-α-ketol from the 9-isomer.

Substrate specificity of flax hydroperoxide isomerase

Incubations of the 13- and 9-hydroperoxides of linolenic acid with a flax acetone powder extract containing hydroperoxide isomerase resulted in the formation of 13-hydroxy-12-oxo-cis-9,15-octadecadienoic acid and 9-hydroxy-10-oxo-cis-12,15-octadecadienoic acid, respectively. The rate of formation of product from 13-hydroperoxy linolenic acid was 36 times that from 9-hydroperoxy linolenic acid. Analogous results were obtained with the 13- and 9-hydroperoxides of linoleic acid. The results demonstrated the substrate specificity of flax hydroperoxide isomerase.