Subbaiah P V, Liu M, Paltauf F
Department of Medicine, Rush Medical College, Chicago, Illinois 60612.
Biochemistry. 1994 Nov 15;33(45):13259-66. doi: 10.1021/bi00249a012.
Although human plasma lecithin-cholesterol acyltransferase (LCAT) is believed to be specific for the sn-2 position of phosphatidylcholine (PC), our recent studies showed that it derives a significant percent of acyl groups from the sn-1 position of certain PC species. To understand the physicochemical basis for this altered positional specificity, we determined the effect of sn-2 acyl group of PC on the enzyme activity and utilization of 16:0 from the sn-1 position by purified human and rat LCATs. Positional isomers of PC containing 16:0 at sn-2 were better substrates for human LCAT than the corresponding sn-1-16:0 isomers, whereas the reverse was true for rat LCAT. The positional specificity of human LCAT varied greatly depending on the nature of the acyl group at sn-2. The sn-1 contribution from various sn-1-16:0-2-acyl PCs for cholesteryl ester (CE) synthesis was 1.0% from 16:0-16:0, 1.4% from 16:0-20:5, 7.3% from 16:0-18:1, 47.0% from 16:0-20:3, 49.9% from 16:0-20:4, 54.9% from 16:0-22:6, and 72.3% from 16:0-18:0. There was a linear relationship between the percentage of 16:0 CE formed (from sn-1 position) and the acyl chain length at sn-2 position (r = 0.94). Rat LCAT also transferred some 16:0 from sn-1 position of 16:0-22:6, 16:0-20:3, and 16:0-18:0 PCs, but not from the other natural PCs tested. The phospholipase A activity of both LCATs in the presence of 16:0-20:4 PC showed the same positional specificity as CE synthesis, indicating that the specificity is determined at the formation of acyl-enzyme intermediate. These results show that the positional specificity of LCAT is influenced by the structure of PC, especially the chain length of the sn-2 acyl group.
尽管人们认为人血浆卵磷脂胆固醇酰基转移酶（LCAT）对磷脂酰胆碱（PC）的sn-2位具有特异性，但我们最近的研究表明，它有相当比例的酰基来自某些PC物种的sn-1位。为了了解这种改变的位置特异性的物理化学基础，我们通过纯化的人及大鼠LCATs，测定了PC的sn-2酰基对酶活性以及sn-1位16:0利用情况的影响。sn-2位含有16:0的PC位置异构体是人LCAT比相应的sn-1-16:0异构体更好的底物，而大鼠LCAT的情况则相反。人LCAT的位置特异性因sn-2位酰基的性质不同而有很大差异。各种sn-1-16:0-2-酰基PC对胆固醇酯（CE）合成的sn-1贡献分别为：16:0-16:0为1.0%，16:0-20:5为1.4%，16:0-18:1为7.3%，16:0-20:3为47.0%，16:0-20:4为49.9%，16:0-22:6为54.9%，16:0-18:0为72.3%。（从sn-1位）形成的16:0 CE百分比与sn-2位的酰基链长度之间存在线性关系（r = 0.94）。大鼠LCAT也从16:0-22:6、16:0-20:3和16:0-18:0 PC的sn-1位转移了一些16:0，但未从测试的其他天然PC中转移。在16:0-20:4 PC存在的情况下，两种LCAT的磷脂酶A活性显示出与CE合成相同的位置特异性，这表明该特异性是在酰基酶中间体形成时确定的。这些结果表明，LCAT的位置特异性受PC结构的影响，尤其是sn-2酰基的链长度。
