Stymne S, Stobart A K
Biochem J. 1984 Oct 15;223(2):305-14. doi: 10.1042/bj2230305.
Acyl exchange between acyl-CoA and position 2 of sn-phosphatidylcholine occurs in the microsomal preparations of developing safflower cotyledons. Evidence is presented to show that the acyl exchange is catalysed by the combined back and forward reactions of an acyl-CoA:lysophosphatidylcholine acyltransferase (EC 2.3.1.23). The back reaction of the enzyme was demonstrated by the stimulation of the acyl exchange with free CoA and by the observation that the added CoA was acylated with acyl groups from position 2 of sn-phosphatidylcholine. Re-acylation of the, endogenously produced, lysophosphatidylcholine with added acyl-CoA occurred with the same specificity as that observed with added palmitoyl lysophosphatidylcholine. A similar acyl exchange, catalysed by an acyl-CoA:lysophosphatidylcholine acyltransferase, occurred in microsomal preparations of rat liver. The enzyme from safflower had a high specificity for oleate and linoleate, whereas arachidonate was the preferred acyl group in the rat liver microsomal preparations. The rate of the back reaction was 3-5% and 0.2-0.4% of the forward reaction in the microsomal preparations of safflower and rat liver respectively. Previous observations, that the acyl exchange in safflower microsomal preparations was stimulated by bovine serum albumin and sn-glycerol 3-phosphate, can now be explained by the lowered acyl-CoA concentrations in the incubation mixture with albumin and in the increase in free CoA in the presence of sn-glycerol 3-phosphate (by rapid acylation of sn-glycerol 3-phosphate with acyl groups from acyl-CoA to yield phosphatidic acid). Bovine serum albumin and sn-glycerol 3-phosphate, therefore, shift the equilibrium in acyl-CoA:lysophosphatidylcholine acyltransferase-catalysed reactions towards the rate-limiting step in the acyl exchange process, namely the removal of acyl groups from phosphatidylcholine. The possible role of the acyl exchange in the transfer of acyl groups between complex lipids is discussed.
酰基辅酶A与sn - 磷脂酰胆碱2位之间的酰基交换发生在发育中的红花子叶微粒体制剂中。有证据表明，酰基交换是由酰基辅酶A：溶血磷脂酰胆碱酰基转移酶（EC 2.3.1.23）的正向和逆向反应共同催化的。通过游离辅酶A对酰基交换的刺激以及添加的辅酶A被sn - 磷脂酰胆碱2位的酰基酰化的观察结果，证明了该酶的逆向反应。内源性产生的溶血磷脂酰胆碱与添加的酰基辅酶A的再酰化反应，其特异性与添加棕榈酰溶血磷脂酰胆碱时观察到的相同。大鼠肝脏微粒体制剂中也发生了由酰基辅酶A：溶血磷脂酰胆碱酰基转移酶催化的类似酰基交换。红花中的该酶对油酸和亚油酸具有高特异性，而花生四烯酸是大鼠肝脏微粒体制剂中首选的酰基。在红花和大鼠肝脏微粒体制剂中，逆向反应的速率分别为正向反应的3 - 5%和0.2 - 0.4%。先前的观察结果表明，红花微粒体制剂中的酰基交换受到牛血清白蛋白和sn - 甘油3 - 磷酸的刺激，现在可以解释为与白蛋白一起孵育的混合物中酰基辅酶A浓度降低，以及在sn - 甘油3 - 磷酸存在下游离辅酶A增加（通过sn - 甘油3 - 磷酸与酰基辅酶A的酰基快速酰化生成磷脂酸）。因此，牛血清白蛋白和sn - 甘油3 - 磷酸将酰基辅酶A：溶血磷脂酰胆碱酰基转移酶催化反应的平衡向酰基交换过程中的限速步骤，即从磷脂酰胆碱中去除酰基的方向移动。讨论了酰基交换在复合脂质之间酰基转移中的可能作用。
