Abstract
When individual enzyme activities of the fatty acid synthetase (FAS) system were assayed in extracts from five different plant tissues, acetyl-CoA:acyl carrier protein (ACP) transacylase and beta-ketoacyl-ACP synthetases I and II had consistently low specific activities in comparison with the other enzymes of the system. However, two of these extracts synthesized significant levels of medium chain fatty acids (rather than C16 and C18 acid) from [14C]malonyl-CoA; these extracts had elevated levels of acetyl-CoA:ACP transacylase. To explore the role of the acetyl transacylase more carefully, this enzyme was purified some 180-fold from spinach leaf extracts. Varying concentrations of the transacylase were then added either to spinach leaf extracts or to a completely reconstituted FAS system consisting of highly purified enzymes. The results suggested that: (a) acetyl-CoA:ACP transacylase was the enzyme catalyzing the rate-limiting step in the plant FAS system; (b) increasing concentration of this enzyme markedly increased the levels of the medium chain fatty acids, whereas increase of the other enzymes of the FAS system led to increased levels of stearic acid synthesis; and (c) beta-ketoacyl-ACP synthetase I was not involved in the rate-limiting step. It is suggested that modulation of the activity of acetyl-CoA:ACP transacylase may have important implications in the type of fatty acid synthesized, as well as the amount of fatty acids formed.
Highlights
When individual enzyme activities of the fatty acid (11), avocado mesocarp (12), and barley chloroplasts (13)
Materials-Acetyl-coA and malonyl-CoA were purchased from extracts or to a completely reconstituted FAS system consisting of highly purified enzymes
Fatty Acid SynthetaseSystemsforVariousPlant Tissues-Five different plant tissues were selected for the purpose of testing foarctivity levels of FAS enzymes
Summary
Fatty Acid SynthetaseSystemsforVariousPlant Tissues-Five different plant tissues were selected for the purpose of testing foarctivity levels of FAS enzymes. These were developing seeds (Cuphealutea) (19), whichcontainhigh levels of decanoic and lauric acids associatewdith triacylglycerols, safflower seeds (Carthamus tinctorius),which contain a normal pattern of palmitic, oleic, and linoleic acids associated with triacylglycerols, rapeseeds (Brassica juncea) with erucic acid as the major acyl component in triacylglycerols, leaf tissues ofyoungpea seedlings (Pisum sativum) with a normal composition of palmitic, linoleic, and linolenic acids associatedwithcomplexlipids (a 18:3 plant), andmature spinach leaves (Spinacea oleracea),which contain a normal complement offatty acids as well as hexadecatrienoic acid(a 16:3 plant) (20). When seven FAS enzymeswere assayed in the five extracts as shown in Table 11, the specific activities of acetyl transacylase and P-ketoacyl-ACPsynthetases I and I1 were markedly lower than those of the remaining FAS enzyme (8, 10)
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