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PLANT PHYSIOLOGY , Vol 110, Issue 4 1239-1247, Copyright © 1996 by American Society of Plant Biologists
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BIOCHEMISTRY AND ENZYMOLOGY |
Evidence That Isolated Chloroplasts Contain an Integrated Lipid-Synthesizing Assembly That Channels Acetate into Long-Chain Fatty Acids
P. G. Roughan and J. B. Ohlrogge
Horticulture and Food Research Institute of New Zealand, Private Bag 92169, Auckland, New Zealand (P.G.R.)
High rates of light-dependent fatty acid synthesis from acetate were
measured in isolated chloroplasts that were permeabilized to varying
extents by resuspension in hypotonic reaction medium. The reactions in
hypotonic medium unsupplemented with cofactors were linear with time and
were directly proportional to chlorophyll concentration, suggesting that
the enzymes and cofactors of fatty acid synthesis remained tightly
integrated and thylakoid associated within disrupted chloroplasts.
Permeabilized chloroplasts expanded to at least twice the volume of intact
chloroplasts, lost about 50% of their stromal proteins in the medium, and
metabolized exogenous nucleotides. However, neither acetyl-coenzyme A (CoA)
nor malonyl-CoA inhibited fatty acid synthesis from acetate; nor were
[1-14C]acetyl-CoA and [14C]malonyl-CoA significantly incorporated into
fatty acids. Fatty acid synthesis from acetate was independent of added
cofactors but was totally light dependent. Changes in the products of fatty
acid synthesis were consistent with the loss of endogenous
glycerol-3-phosphate from permeabilized chloroplasts. However, in
appropriately supplemented medium, the products of acetate incorporation by
spinach (Spinacia oleracea) chloroplasts were similar when reactions were
carried out in either isotonic or hypotonic medium. Taken together, the
results of this study suggest that the enzymes of fatty acid synthesis with
chloroplasts are organized into a multienzyme assembly that channels
acetate into long-chain fatty acids, glycerides, and CoA esters.
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