Plant eicosanoid

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. 

Some fatty acids are not synthesized by mammals and yet are necessary for normal growth and life. These essential fatty aeids include llnoleic and y-linolenic acids. These must be obtained by mammals in their diet (specifically from plant sources). Arachidonic acid, which is not found in plants, can only be synthesized by mammals from linoleic acid. At least one function of the essential fatty acids is to serve as a precursor for the synthesis of eicosanoids, such as... 

Eicosanoids and terpenoids are still other classes of lipids. Eicosanoids, of which prostaglandins are the most abundant kind, are derived biosynthetically from arachidonic acid, are found in all body tissues, and have a wide range of physiological activity. Terpenoids are often isolated from the essential oils of plants, have an immense diversity of structure, and are produced biosynthetically from the five-carbon precursor isopentenyl diphosphate (IPP). lsopentenyl diphosphate is itself biosynthesized from 3 equivalents of acetate in the mevalonate pathway. 

Steroids are plant and animal lipids with a characteristic tetracyclic carbon skeleton. Like the eicosanoids, steroids occur widely in body tissues and have a large variety of physiological activities. Steroids are closely related to terpenoids and arise biosynthetically from the triterpene lanosterol. Lanosterol, in turn, arises from cationic cyclization of the acyclic hydrocarbon squalene. 

Bouarab K, Adas F, Gaquerel E, Kloareg B, Salaun J-P, Potin P (2004) The innate immunity of a marine red alga involves oxylipins from both the eicosanoid and octadecanoid pathways. Plant Physiol 135 1838-1848... 

Fatty acids, such as linoleic, hnolenic, and arachidonic acids, contain two or more cis carbon-carbon double bonds and are referred to as polyunsaturated fatty acids. Several of these fatty acids, including linoleic and linolenic acids, are required nutrients for humans and must be part of a healthy diet. They are termed essential fatty acids, of which there are eight. These fatty acids cannot be synthesized by human beings but are essential to human health. Therefore, they must be consumed in adequate amounts in a healthy diet, specifically in the form of ingested plant-derived foods. A diet devoid of the essential fatty acids eventually results in a fatal condition characterized by inflammation of the skin (dermatitis), failure of wounds to heal, and poor growth. The essential fatty acids serve as precursors for complex molecules termed eicosanoids, to which we return below. 

Mammals cannot make linoleate and must obtain it from plant sources they convert exogenous linoleate to arachidonate, the parent compound of eicosanoids (prostaglandins, thromboxanes, and leukotrienes), a family of very potent signaling molecules. 

Prenylated flavonoids contain one or two isoprenyl, geranyl, dimethylallyl, and lavandulyl on the skeleton. They have limited distribution and are mostly isolated from the Moraceae family. Chi et al. isolated 19 prenylated flavonoids from six different genera of Chinese medicinal plants and evaluated inhibition activities on eicosanoid metabolisms with multiple cell line models [189]. Two 8-lavandulylated flavanones, kurarinone and sophoraflavanone G, were discovered to be dual inhibitors. They possessed the most potent COX-1 inhibitory activities with IC50 less than 1 pM comparable with that of indomethacin. Sophoraflavanone G isolated from the roots of Sophora flavescense Art (Fabaceae) also had potent 5-LOX inhibition activity with an IC50 below 0.25 pM. 

Developments in the chemistry of hormones of insects and other arthropods have shown that the simple picture of a few compounds and a few actions has become less simple. Advances in analytical methods and the increasing number of species studied have thrown up more variations on the simple rules. We are learning more about the interactions between plants and insects, and the conservation of biosynthetic routes between them, and across the phyla. The subject of eicosanoids in insects and lower animals is going to demand further investigation. 

Figure 12.16. Omega-3 fatty acids and their relevance to prostaglandin metabolism, a Omega-3 nnsatnrated fatty acids are derived from linoleic acid by a special desatnrase fonnd in plants bnt not in mammals. They are particnlarly common in cold-waterfish. b The different nnmbers and positions of donble bonds persist in the eicosanoids derived from different fatty acid precnrsors, giving rise to different homologons derivatives. This is shown here for prostaglandin E. c Biological activity of the eicosanoids derived from eicosapentanoic acid (EPA).

The fatty acid alkyl chain is susceptible to oxidation both at double bonds and adjacent allylic carbons. Eree-radical and photooxidation at aUylic carbons are responsible for deterioration of unsaturated oils and fats, resulting in rancid flavors and reduced nutritional quality, but they are also used deliberately to polymerize drying oils. Oxidation of double bonds is used in oleochemical production either to cleave the alkyl chain or to introduce additional functionality along the chain. Enzyme catalyzed oxidation is the initial step in the production of eicosanoids and jasmonates (biologically active metabolites in animals and plants respectively) but is not discussed further here. 

Fig. 13.12 Polyunsaturated fatty acids required for eicosanoid synthesis. Oleic acid is the only fatty acid synthesized by mammals de novo. Linoleic (co-3) and a-linolenic acid (9 or greater fatty acids. Ingested o>3 fatty acids are metabolized to other co-3 fatty acids with o>9 double bonds. The same applies to co-6 fatty acids. The major dietary sources of polyunsaturated fatty acids are fish and plants oils...

Linoleic acid and LNA and their long-chain derivatives arc important components of animal and plant cell membranes. When humans ingest fish or fish oil, the EPA and DHA from the diet partially replace the omega-6 fatty acids, especially AA, in the membranes of probably all cells, but especially in the membranes of platelets, erythrocytes, neutrophils, monocytes, and liver cells [reviewed in Simopoulos, 19911. A diet that has a high ratio of omega-6 omega-3 fatty acids has detrimental effects on eicosanoid metabolism and gene expression. 

---