Δ5-olefinic acids in the seed lipids from four Ephedra species and their distribution between the α and β positions of triacylglycerols. Characteristics common to coniferophytes and cycadophytes

Abstract

The fatty acid compositions of the seed lipids from four Ephedra species, E. nevadensis, E. viridis, E. przewalskii, and E. gerardiana (four gymnosperm species belonging to the Cycadophytes), have been established with an emphasis on Δ5-unsaturated polymethylene-interrupted fatty acids (Δ5- UPIFA). Mass spectrometry of the picolinyl ester derivatives allowed characterization of 5, 9- and 5, 11-18:2; 5, 9, 12-18:3; 5, 9, 12, 15-18:4; 5, 11-20:2; 5, 11, 14-20:3; and 5, 11, 14, 17-20:4 acids. Δ5-UPIFA with a Δ11 -ethylenic bond (mostly C20 acids) were in higher proportions than Δ5-UPIFA with a Δ9 double bond (exclusively C18 acids) in all species. The total A5-UPIFA content was 17-31% of the total fatty acids, with 5, 11, 14-20:3 and 5, 11, 14, 17-20:4 acids being the principal Δ5-UPIFA isomers. The relatively high level of cis-vaccenic (11-18:1) acid found in Ephedra spp. seeds, the presence of its Δ5:desaturation product, 5, 11-18:2 acid (proposed trivial name: ephedrenic acid), and of its elongation product, 13- 20:1 acid, were previously shown to occur in a single other species, Ginkgo biloba, among the approximately 170 gymnosperm species analyzed so far. Consequently, Ephedraceae and Coniferophytes (including Ginkgoatae), which have evolved separately since the Devonian period (~300 million yr ago), have kept in common the ability to synthesize C18 and C20 Δ5-UPIFA. We postulate the existence of two A5-desaturases in gymnosperm seeds, one possibly specific for unsaturated acids with a Δ9-ethylenic bond, and the other possibly specific for unsaturated acids with a Δ11-ethylenic bond. Alternatively, the Δ5-desaturases might be specific for the chain length with C18 unsaturated acids on the one hand and C20 unsaturated acids on the other hand. The resulting hypothetical pathways for the biosynthesis of Δ5-UPIFA in gymnosperm seeds are only distinguished by the position of 1118:1 acid. Moreover, 13C nuclear magnetic resonance spectroscopy of the seed oil from two Ephedra species has shown that Δ5-UPIFA are essentially excluded from the internal position of triacylglycerols, a characteristic common to all of the Coniferophytes analyzed so far (more than 30 species), with the possibility of an exclusive esterification at the sn-3 position. This structural feature would also date back to the Devonian period, but might have been lost in those rare angiosperm species containing Δ5-UPIFA.