Linolenic biosynthesis

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. 

Biosynthesis of triene pheromone components with a triene double bond system that is n-3 (3,6,9-) are probably produced from linolenic acid [49]. Moths in the families Geometridae, Arctiidae, and Noctuidae apparently utilize linoleic and linolenic acid as precursors for their pheromones that must be obtained in the diet,since moths can not synthesize these fatty acids [50]. Most of the Type II pheromones are produced by chain elongation and decarboxylation to form hydrocarbons [51]. Oxygen is added to one of the double bonds in the polyunsaturated hydrocarbon to produce an epoxide [49]. 

Interestingly, the first biosynthetic experiments with [3H]linolenic add and the terrestrial plant Senecio isatideus (Asteraceae) as a model system for the biosynthesis of algal pheromones were unsuccessful. If, however, labeled dodeca-3,6,9-trienoic add is administered, a rapid transformation into ectocarpene takes place (36). Nevertheless, the C12 add ultimately derives from linolenic acid via three /3-oxidations, since labeled tetradeca-5,8,11-trienoic acid, which requires only one /3-oxidation, is converted into labeled ectocarpene albeit with very low effidency (37). 

Dzhavelidze, T. A. Phenolic compounds in various parts of tea shoots. Subtrop Kult 1978 1978 154-155. Hatanaka, A., T. Kajiwara and J. Sekiya. Biosynthesis of leaf alcohol the oxygenative cleavage of linolenic acid to cis-3-hexenal and 11-formyl-cis-9-undecenoic acid from linolenic acid in tea chloroplasts. Symp Chem Nat Prod-22nd- Fukuoka, Japan 1979 657-664. 

In human being, arachidonic acid is the most important precursor for the biosynthesis of eicosanoids. Arachidonic acid is formed from linoleic acid in most mammalians by desaturation and carbon elongation to dihomog-linolenic acid and subsequent desaturation. 

Studies on the biosynthesis of lactones have shown that epoxidation of unsaturated fatty acids like, e.g., linoleic and linolenic acid may represent a common pathway to oxygenated derivatives of fatty acids. Epoxy fatty acid hydrolases were identified as key enzymes that exhibit high regioselectivity and enantiose-lectivity [25, 26]. 

A6 9-octadecadienoate rather than linoleate. However, animals need linoleate for the biosynthesis of dihomo-y-linolenate (A81114-eicosatrienoate) and arachidonate (A5 81114-eicosatetraenoate), C20 polyunsaturated fatty acid precursors of... 

A considerable amount of knowledge has accumulated about how pheromone components are produced in female moths since the first pathway was identified some 20 years ago. It appears that most female moths produce their pheromone through modifications of fatty acid biosynthesis pathways. For moths that utilize aldehydes, alcohols, or esters biosynthesis occurs in the pheromone gland. The exceptions are those that utilize linoleic or linolenic acids, which must be obtained from the diet. However, modifications of these fatty acids occur in the gland. For moths that utilize hydrocarbons or epoxides of hydrocarbons, the hydrocarbon is produced in oenocyte cells and then transported to the pheromone gland where the epoxidation step takes place. 

Rule G. S. and Roelofs W. L. (1989) Biosynthesis of sex pheromone components from linolenic acid in Arctiid moths. Arch. Insect Biochem. Physiol. 12, 89-97. 

Mammals can insert a cts-alkene into the chain, providing that it is no further away from the carbonyl group than C9. We cannot synthesize linoleic or linolenic acids (see chart a few pages back) directly as they have alkenes at Cl 2 and Cl 5. These acids must be present in our diet. And why are we so keen to have them They are needed for the synthesis of arachidonic acid, a C20 tetraenoic acid that is the precursor for some very interesting and important compounds. Here is the biosynthesis of arachidonic acid. 

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

Gamma-linolenic acid (18 3n-6) is an important unsaturated fatty acid. It is the precursor for biosynthesis of arachidonic acid that is a precursor for prostaglandin formation. Recently, y-linolenic acid has been recognized for its potential health benefits in prevention and treatment of cardiovascular disorders, premenstrual syndrome, atopic eczema, rheumatic arthritis, and alcoholism (13, 14). Seed oils of blackcurrant and other Ribes species, as well as evening primrose seed oils, are rich sources of natural y-linolenic acid. 

Figure 2. Pathways for biosynthesis of Cg aldehydes and alcohols from linolenic acid.

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