Linolenic acid cascade

Chemical synthesis of the metabolites on the linolenic acid cascade Biosynthesis and Metabolism Biosynthetic pathway in plants... 

Eicosanoids and related compounds in plants Linolenic acid cascade... 

Scheme 1 The a-linolenic acid cascade. See also Scheme 1 in Chapter 8...

Detailed accounts of the biosynthesis of the prostanoids have been pubUshed (14—17). Under normal circumstances arachidonic acid (AA) is the most abundant C-20 fatty acid m vivo (18—21) which accounts for the predominance of the prostanoids containing two double bonds eg, PGE2 (see Fig. 1). Prostanoids of the one and three series are biosynthesized from dihomo-S-linolenic and eicosapentaenoic acids, respectively. Concentrations ia human tissue of the one-series precursor, dihomo-S-linolenic acid, are about one-fourth those of AA (22) and the presence of PGE has been noted ia a variety of tissues (23). The biosynthesis of the two-series prostaglandins from AA is shown ia Eigure 1. These reactions make up a portion of what is known as the arachidonic acid cascade. Other Hpid products of the cascade iaclude the leukotrienes, lipoxins, and the hydroxyeicosatetraenoic acids (HETEs). Collectively, these substances are termed eicosanoids. 

Enzymic lipid peroxidation reactions are a common reaction in mammalian cells as response to stress from outside. In contrast to plants mammalian cell membranes contain arachidonic acid instead of linolenic acid. The latter is known to generate by LPO processes physiological potent compounds prostaglandines, prostacyclines, thromboxanes and leukotiienes [221]. In addition a great number of other LPO products derived not only from arachidonic acid but also from linoleic acid have been detected in blood, especially low density lipoprotein (LDL) and tissue samples of humans [222-227]. These are increased especially in diseases combined with cell degradation [228], indicating that cell death in plant and mammalian cells is connected with a similar cascade of events. 

C20H32O2, Mr 304.47, mp. -49.5 °C, bp. 163°C (133 Pa). A biochemically important essential fatty acid, precursor of various eicosanoids. In animals, A. is formed in several steps from linoleic acid by introduction of 2 double bonds and chain extension by 2 carbon atoms linoleic acid - y- linolenic acid - di-homo-y-linolenic acid arachidonic acid. In nature A. occurs in animal lipids, especially in phospholipids of cell membranes, from which phospholipase A2 releases A. on stimulation by hormones or mediators. A. serves as starting material for the biosynthesis of numerous physiologically active oxygen derivatives (especially hydroxy-, hydroperoxy-, and epoxy compounds) known as the eicosanoids. Their separation into different groups in the so-called A. cascade is shown in the figure... 

Biosynthesis From a-linolenic acid by the so-called jasmonate cascade. For synthesis, see Lit . 

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. 

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