The biosynthesis of calendic acid, octadeca-(8E,10E, 12Z)-trienoic, acid, by developing marigold seeds: origins of (E,E,Z) and (Z,E,Z) conjugated triene acids in higher plants
Using an homogenate of marigold seeds, gathered 15 days after flower-drop, [1-14C] linoleic acid and [1-14C] oleic acid were incorporated into calendic acid with little randomisation of the label. [1-14C] Linolenic acid was not incorporated. Despite requiring 12,13-dehydrogenation to form linoleic acid, the putative precursor, oleic acid was better incorporated than administered linoleic acid. Stearic acid, requiring both 12,13- and 9,10-dehydrogenation, was a poor precursor. The results of a series of double-labelling experiments support and supplement these conclusions.
[8,8,11,11,16,16,17,17-2H8]Octadec-9-enoic acid was synthesised and employed in a mass-spectral experiment to show that conversion into calendic acid involves loss of two deuterium and two hydrogen atoms (deuterium at C-16 and C-17 was introduced for loading purposes only, in order to increase the sensitivity of the experiment). Taken with [3H]-labelling work, the experiment indicates that during conversion of linoleic acid into calendic acid, there is no loss of the labelled hydrogens at C-9, -10, -12, or -13, but loss of hydrogen from each of C-8 and C-11.
(9S)-Hydroxyoctadeca-(10E,l2Z)-dienoic acid (α-dimorphecolic acid) was isolated and converted into (R/S)-hydroxy- and -hydroperoxy-[9-3H]octadeca-(10E,l2Z)-dienoic acids. Neither labelled specimen was converted into calendic acid by marigold seed homogenate. Abstraction of a hydrogen atom from C-l 1 of linoleic acid is viewed as giving an (E)-allylic radical which, as in lipoxygenase reactions, can be trapped by oxygen at C-9, thus providing a source of α-dimorphecolic acid, a minor component of marigold seed oil. However, this hydroxyacid is apparently a terminus rather than an intermediate for calendic acid. Formation of the latter seems best accounted for by formal loss of a hydrogen atom from C-8 of the (E)-allylic radical. The general position relating to the formation of (E,E,Z) and (Z,E,Z)-trienes is summarised.