Fatty acids having three or four conjugated double bonds occur in various seed oils. Conjugated trienoic fatty acids include a-eleostearic acid (18:3Δ9Z,11E,13E), catalpic acid (18:3Δ9E,11E,13Z), punicic acid (18:3Δ9Z,11E,13Z), calendic acid (18:3Δ8E,10E,12Z), and jacaric acid (18:3Δ8Z,10E,12Z), whereas a-parinaric acid 18:4Δ9Z,11E,13E,15Z) is the only well-known conjugated tetraenoic acid of seed origin¹. Moreover, marine algae contain conjugated isomeric tetraene fatty acids such as bosseopentaenoic acid (20:5Δ5Z,8Z,10E,12E,14Z)^2,3.
    Conjugated triene and tetraene fatty acids are more susceptible to oxidation and polymerization reactions compared to non-conjugated analogues⁴. Thus, the use of oils containing conjugated trienoic fatty acids in paint and varnish formulations speeds up the drying process and produces more resistant coatings⁵. In biological systems, the rapid conversion of a-parinaric acid into reactive oxidized products makes it cytotoxic, especially to malignant cells⁶, and therefore its use as a chemotherapeutic agent for gliomas has been suggested⁷. In experimental work, a-parinaric acid finds use as a fluorescent probe to monitor e.g. lipid peroxidation⁸. Although conjugated triene and tetraene fatty acids may be obtained by total synthesis, a biotechnical approach involving modification of the responsible biosynthetic enzymes in transgenic plants is emerging as a promising alternative for large scale production. Three classes of enzymes operating by distinct mechanisms can produce conjugated triene and tetraene fatty acids as is briefly discussed below.

Isotopic evidence for conversion of linoleic acid into calendic acid in developing seeds of Calendula officinalis was provided in 1985⁹. Subsequently, the biosynthesis of a-eleostearic acid in developing seeds of Momordica charantia was demonstrated¹⁰. Linoleate esterified to phosphatidylcholine serves as precursor. Using molecular biology techniques, cDNAs obtained from developing seeds of M. charantia and Impatiens balsamina were expressed in soybean resulting in accumulation of a-eleostearic and a-parinaric acids, respectively¹¹. Furthermore, linoleoyl desaturases catalyzing formation of calendic acid have been cloned and expressed in yeast ^12,13 and soybean¹³. The term "conjugase" was proposed by Cahoon and coworkers for enzymes catalyzing formation of acyl moieties containing conjugated double bonds¹¹. These enzymes seem all to be structurally related to oleoyl Δ12 desaturase. Other variants of this enzyme catalyze 12-hydroxylation of oleate¹⁴, Δ12,13-epoxidation of linoleate¹⁵, and introduction of a Δ12,13-triple bond in oleate¹⁵. Biochemically, the hydrogen abstraction or oxygen insertion catalyzed by Δ12 desaturase or its variants require molecular oxygen and a reduced cofactor (NADH or NADPH).

In 1991, the presence of endogenous conjugated tetraene fatty acids was observed in the marine red alga Bossiella orbigniana². Furthermore, extracts of the alga catalyzed oxidation of arachidonic acid into 5Z,8Z,10E,12E,14Z-eicosapentaenoic acid (bosseopentaenoic acid). The reaction was O₂-dependent but its mechanism was not clarified. As shown later, also preparations of the red alga Lithothamnion corallioides were capable of the biosynthesis of bosseopentaenoic acid from arachidonic acid and in addition catalyzed the formation of a unique 13-hydroxylated derivative of arachidonic acid¹⁶. The mechanism of the biosynthesis of conjugated tetraene fatty acids in marine algae was studied with an enzyme preparation obtained from L. corallioides using γ-linolenic acid as substrate. Conversion of γ-linolenic acid into the conjugated tetraene 6Z,8E,10E,12Z-octadeca-tetraenoic acid was dependent on molecular oxygen but did not require a reducing cofactor. Interestingly, the conjugated tetraene was formed together with a stoichiometrical amount of hydrogen peroxide indicating an oxidase mechanism¹⁷. In line with this proposal, the desaturation proceeded smoothly under anaerobic conditions provided that an artificial electron acceptor such as p-benzoquinone was present. Examination of the stereochemistry of the desaturation reaction revealed a stereospecific elimination of the pro-S hydrogen from C-8 and of the pro-R hydrogen from C-11. The oxidase/desaturase of L. corallioides also acted on a-linolenic acid which was converted into a-parinaric acid¹⁸.

In 1994, a non-oxidative mechanism of formation of conjugated triene fatty acids was described ^19,20. Thus, extracts of the marine red alga Ptilota filicina catalyzed conversion of e.g. arachidonic acid into 5Z,7E,9E,14Z-eicosatetraenoic acid and of γ-linolenic acid into 6Z,8E,10E-octadecatrienoic acid. Neither dioxygen nor a cofactor were needed for these conversions. The enzyme responsible, polyenoic fatty acid isomerase, was partially purified and characterized as an oligomer having a molecular mass of 174 kDa.
    The enzymatic isomerization of γ-linolenic acid was studied by the use of stereospecifically deuterated substrates and found to occur by intramolecular transfer of the pro-S hydrogen from C-11 to C-13 (isomerization of the Δ12 double bond into the Δ11 position) followed by elimination of the pro-R hydrogen from C-8 (concerted isomerization of the Δ9,11 double bonds into the Δ8,10 positions). Possibly related to the algal polyenoic fatty acid isomerase is an enzyme isolated from the anaerobic bacterium Butyrivibrio fibrisolvins²¹.This enzyme catalyzes the stereospecific isomerization of the Δ12 double bond of linoleic acid into the Δ11 position to provide 9Z,11E-octadecadienoic acid. This conjugated linoleic acid isomer appears to serve as an intermediate in the biohydrogenation of linoleic acid which takes place in certain animal species.