Biosynthetic origin of conjugated double bonds: Production of fatty acid components of high-value drying oils in transgenic soybean embryos

Abstract

Vegetable oils that contain fatty acids with conjugated double bonds, such as tung oil, are valuable drying agents in paints, varnishes, and inks. Although several reaction mechanisms have been proposed, little is known of the biosynthetic origin of conjugated double bonds in plant fatty acids. An expressed sequence tag (EST) approach was undertaken to characterize the enzymatic basis for the formation of the conjugated double bonds of ␣-eleostearic (18:3⌬ 9cis,11trans,13trans ) and ␣-parinaric (18:4⌬ 9cis,11trans,13trans,15cis ) acids. Approximately 3,000 ESTs were generated from cDNA libraries prepared from developing seeds of Momordica charantia and Impatiens balsamina, tissues that accumulate large amounts of ␣-eleostearic and ␣-parinaric acids, respectively. From ESTs of both species, a class of cDNAs encoding a diverged form of the ⌬ 12 -oleic acid desaturase was identified. Expression of full-length cDNAs for the Momordica (MomoFadX) and Impatiens (ImpFadX) enzymes in somatic soybean embryos resulted in the accumulation of ␣-eleostearic and ␣-parinaric acids, neither of which is present in untransformed soybean embryos. ␣-Eleostearic and ␣-parinaric acids together accounted for as much as 17% (wt͞wt) of the total fatty acids of embryos expressing MomoFadX. These results demonstrate the ability to produce fatty acid components of high-value drying oils in transgenic plants. These findings also demonstrate a previously uncharacterized activity for ⌬ 12 -oleic acid desaturase-type enzymes that we have termed ''conjugase.'' H undreds of unusual fatty acid structures are known to occur in the seed oils of various plant species (1). The biosynthetic pathways of many of these fatty acids are unknown or have not been well characterized. One such class consists of fatty acids with double bonds that are conjugated. This structural configuration is in contrast to that of linoleic (18:2⌬ 9cis,12cis ) and ␣-linolenic (18:3⌬ 9cis,12cis,15cis ) acids, the typical polyunsaturated fatty acids of plant seed oils, which contain double bonds that are separated by methylene (ϪCH 2 Ϫ) groups (Fig. 1). Among the fatty acids with conjugated double bonds that occur in the plant kingdom are ␣-eleostearic acid (18:3⌬ 9cis,11cis,13trans) and ␣-parinaric acid (18:4⌬ 9cis,11trans,13trans,15cis ) ( Fig. 1). ␣-Eleostearic acid accounts for Ͼ65% (wt͞wt) of the total fatty acids of tung oil, a high-value drying oil obtained from seeds of Aleurites fordii (2). Other sources of this fatty acid include the seed oil of Momordica charantia, which contains approximately 65% (wt͞wt) ␣-eleostearic acid (2). In addition, ␣-parinaric acid composes 30 to 65% (wt͞wt) of the seed oils of plants such as Parinarium laurinum and Impatiens species (1, 3, 4).The presence of conjugated double bonds in fatty acids markedly increases their rate of oxidation relative to polyunsaturated fatty acids with methylene-interrupted double bonds (5). This property makes seed oils, such as tung oil, that are enriched in fatty acids with conjugated double bonds...

Characteristics of seed oils and nutritional compositions of seeds from different varieties of Momordica charantiaLinn.cultivated in Bangladesh

“…The oleic acid content appeared to be fairly constant in all varieties. Regarding the polyunsaturated fatty acids, α-eleostearic acid was the major one, its average percentage being 51.80, which was lower than the value 65% detected by Cahoon et al (1999) with the same seed oil. GLC data also indicated that karela seed oils contained mainly unsaturated fatty acids 72.60-74.95%, while saturated fatty acids detected represented 25.05-27.40%.…”

“…These ratios indicate that the samples had a high content of unsaturated fatty acids, which may make them more attractive for the consumer who wishes to ingest this type of acids. Vegetable oils that contain fatty acids with conjugated double bonds, such as tung oil, are valuable drying agents in paints, varnishes, and inks (Cahoon et al 1999). Karela seed oils reported herein that are enriched in α-eleostearic acid (> 50% of the total fatty acid) possessing conjugated double bonds, are used commercially in coating materials and inks.…”

Involvement of a bifunctional fatty-acyl desaturase in the biosynthesis of the silkmoth, Bombyx mori , sex pheromone

“…In the biosynthesis of conjugated linolenic acids, enzymes termed conjugases catalyze the formation of conjugated double bonds; the Z12 double bond of linoleic acid, for instance, is converted into the two E11 and E13 double bonds found in ␣-eleostearic acid (23). Recently, several plant seed cDNAs encoding conjugases have been characterized (22)(23)(24)(25). Interestingly, one such conjugase associated with the formation of the E11,Z13-diene in punicic acid (22) has been reported to exhibit both Z12 desaturation and E11,Z13-conjugation activities.…”

Identification of bifunctional Δ12/ω3 fatty acid desaturases for improving the ratio of ω3 to ω6 fatty acids in microbes and plants

“…Jack) were transformed by using particle bombardment (45), and transgenic embryos were selected and propagated to maturity as described (46). Matured embryos from each independent event were analyzed by GC to verify expression of the F. moniliforme 3 desaturase, as described below.…”