Shimakata T, Stumpf P K
J Biol Chem. 1983 Mar 25;258(6):3592-8.
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.
当在五种不同植物组织的提取物中测定脂肪酸合成酶（FAS）系统的各个酶活性时，与该系统的其他酶相比，乙酰辅酶A：酰基载体蛋白（ACP）转酰基酶以及β-酮酰-ACP合成酶I和II的比活性始终较低。然而，其中两种提取物能从[14C]丙二酰辅酶A合成相当水平的中链脂肪酸（而非C16和C18酸）；这些提取物中乙酰辅酶A：ACP转酰基酶的水平有所升高。为了更深入地探究乙酰转酰基酶的作用，从菠菜叶提取物中对该酶进行了约180倍的纯化。然后将不同浓度的转酰基酶添加到菠菜叶提取物或由高度纯化的酶组成的完全重构的FAS系统中。结果表明：（a）乙酰辅酶A：ACP转酰基酶是催化植物FAS系统限速步骤的酶；（b）该酶浓度的增加显著提高了中链脂肪酸的水平，而FAS系统中其他酶的增加则导致硬脂酸合成水平的提高；（c）β-酮酰-ACP合成酶I不参与限速步骤。有人提出，调节乙酰辅酶A：ACP转酰基酶的活性可能对合成的脂肪酸类型以及形成的脂肪酸量具有重要意义。
