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Plant Physiol, December 1999, Vol. 121, pp. 1217-1226
Testing Models of Fatty Acid Transfer and Lipid Synthesis in
Spinach Leaf Using in Vivo Oxygen-18 Labeling1
Mike
Pollard* and
John
Ohlrogge
Department of Botany and Plant Pathology, Michigan State
University, East Lansing, Michigan 48824
Oxygen-18 labeling has been applied
to the study of plant lipid biosynthesis for the first time.
[13C218O2]Acetate was
incubated with spinach (Spinacia oleracea) leaves and
the 18O content in fatty acid methyl esters isolated from
different lipid classes measured by gas chromatography-mass
spectometry. Fatty acids isolated from lipids synthesized within the
plastid, such as monogalactosyldiacylglycerol, show an 18O
content consistent with the exogenous acetate undergoing a single activation step and with the direct utilization of acyl-acyl carrier protein by the acyl transferases of the chloroplast. In contrast, fatty
acids isolated from lipids assembled in the cytosol, such as
phosphatidylcholine, show a 50% reduction in the 18O
content. This is indicative of export of the fatty acyl groups from the
plastid via a free carboxylate anion, and is consistent with the
acyl-acyl carrier protein thioesterase:acyl-coenzyme A (CoA) synthetase
mediated export mechanism. If this were not the case and the acyl group
was transferred directly from acyl-acyl carrier protein to an acyl
acceptor on the cytosolic side, there would be either complete
retention of 18O or, less likely, complete loss of
18O, but not a 50% loss of 18O. Thus, existing
models for fatty acid transfer from the plastid and for spatially
separate synthesis of "prokaryotic" and "eukaryotic" lipids
have both been confirmed.
1
This work was supported by a grant from the
Department of Energy (no. DE-FG02-87ER12729).
*
Corresponding author; e-mail pollard9{at}pilot.msu.edu; fax
517-353-1926.
© 1999 American Society of Plant Physiologists
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