PLANT PHYSIOLOGY , Vol 104, Issue 3 845-855, Copyright © 1994 by American Society of Plant Biologists
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METABOLISM AND ENZYMOLOGY |
Apparent Role of Phosphatidylcholine in the Metabolism of Petroselinic Acid in Developing Umbelliferae Endosperm
E. B. Cahoon and J. B. Ohlrogge
Department of Botany and Plant Pathology, Michigan State University, East Lansing, Michigan 48824
Studies were conducted to characterize the metabolism of the unusual fatty
acid petroselinic acid (18:1cis[delta]6) in developing endosperm of the
Umbelliferae species coriander (Coriandrum sativum L.) and carrot (Daucus
carota L.). Analyses of fatty acid compositions of glycerolipids of these
tissues revealed a dissimilar distribution of petroselinic acid in
triacylglycerols (TAG) and the major polar lipids phosphatidylcholine (PC)
and phosphatidylethanolamine (PE). Petroselinic acid comprised 70 to 75
mol% of the fatty acids of TAG but only 9 to 20 mol% of the fatty acids of
PC and PE. Although such data appeared to suggest that petroselinic acid is
at least partially excluded from polar lipids, results of [1-14C]acetate
radiolabeling experiments gave a much different picture of the metabolism
of this fatty acid. In time-course labeling of carrot endosperm,
[1-14C]acetate was rapidly incorporated into PC in high levels. Through 30
min, radiolabel was most concentrated in PC, and of this, 80 to 85% was in
the form of petroselinic acid. One explanation for the large disparity in
amounts of petroselinic acid in PC as determined by fatty acid mass
analyses and 14C radiolabeling is that turnover of these lipids or the
fatty acids of these lipids results in relatively low accumulation of
petroselinic acid mass. Consistent with this, the kinetics of
[1-14C]acetate time-course labeling of carrot endosperm and "pulse-chase"
labeling of coriander endosperm suggested a possible flux of fatty acids
from PC into TAG. In time-course experiments, radiolabel initially entered
PC at the highest rates but accumulated in TAG at later time points.
Similarly, in pulse-chase studies, losses in absolute amounts of
radioactivity from PC were accompanied by significant increases of
radiolabel in TAG. In addition, stereospecific analyses of unlabeled and
[1-14C]acetate-labeled PC of coriander endosperm indicated that
petroselinic acid can be readily incorporated into both the sn-1 and sn-2
positions of this lipid. Because petroselinic acid is neither synthesized
nor further modified on polar lipids, the apparent metabolism of this fatty
acid through PC (and possibly through other polar lipids) may define a
function of PC in TAG assembly apart from its involvement in fatty acid
modification reactions.
