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Arachidonic, eicosapentaenoic, and biosynthetically related fatty acids in the seed lipids from a primitive gymnosperm, Agathis robusta
Author(s) -
Wolff Robert L.,
Christie William W.,
Pédrono Frédérique,
Marpeau Anne M.
Publication year - 1999
Publication title -
lipids
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.601
H-Index - 120
eISSN - 1558-9307
pISSN - 0024-4201
DOI - 10.1007/s11745-999-0460-y
Subject(s) - gymnosperm , polyunsaturated fatty acid , biology , eicosapentaenoic acid , arachidonic acid , biochemistry , fatty acid , botany , enzyme
The fatty acid composition of the seeds from Agathis robusta , an Australian gymnosperm (Araucariaceae), was determined by a combination of chromatographic and spectrometric techniques. These enabled the identification of small amounts of arachidonic (5,8,11,14–20∶4) and eicosapentaenoic (5,8,11,14,17–20∶5) acid for the first time in the seed oil of a higher plant. They were apparently derived from γ‐linolenic (6,9,12–18∶3) and stearidonic (6,9,12,15–18∶4) acids, which were also present, via chain elongation and desaturation, together with other expected biosynthetic intermediates [bis‐homo‐γ‐linolenic (8,11,14–20∶3) and bishomo‐stearidonic (8,11,14,17–20∶4) acids]. Also present were a number of C 20 fatty acids, known to occur in most gymnosperm families, i.e., 5,11–20∶2, 11,14–20∶2 (bishomo‐linoleic), 5,11,14–20∶3 (sciadonic), 11,14,17–20∶3 (bishomo‐α‐linolenic), and 5,11,14,17–20∶4 (juniperonic) acids. In contrast to most other gymnosperm seed lipids analyzed so far, A. robusta seed lipids did not contain C 18 Δ5‐desaturated acids [i.e., 5,9–18∶2 (taxoleic), 5,9,12–18∶3 (pinolenic), or 5,9,12,15–18∶4 (coniferonic)]. These structures support the simultaneous existence of Δ6‐ and Δ5‐desaturase activities in A. robusta seeds. The Δ6‐ethylenic bond is apparently introduced into C 18 polyunsaturated acids, whereas the Δ5‐ethylenic bond is introduced into C 20 polyunsaturated acids. A general metabolic pathway for the biosynthesis of unsaturated fatty acids in gymnosperm seeds is proposed. When compared to Bryophytes, Pteridophytes (known to contain arachidonic and eicosapentaenoic acids), and species from other gymnosperm families (without such acids), A. robusta appears as an “intermediate,” with the C 18 Δ6‐desaturase/C 18 →C 20 elongase/C 20 Δ5‐desaturase system in common with the former subphyla, and the unsaturated C 18 →C 20 elongase/C 20 Δ5‐desaturase system specific to gymnosperms. The following hypothetical evolutionary sequence for the C 18 Δ6/Δ5‐desaturase class in gymnosperm seeds is suggested: Δ6 (initial)→Δ6/Δ5 (intermediate)→Δ5 (final).