Steinmetz A, Hocke G, Saïle R, Puchois P, Fruchart J C
Abt. Endokrinologie und Stoffwechsel, Philipps-Universität Marburg, F.R.G.
Biochim Biophys Acta. 1989 Nov 28;1006(2):173-8. doi: 10.1016/0005-2760(89)90192-6.
Lecithin-cholesterol acyltransferase (EC 2.3.1.43, LCAT) is the enzyme responsible for the formation of the bulk of cholesteryl ester in human plasma. The LCAT-reaction takes place mainly on high-density lipoproteins and requires an apolipoprotein as activator. Besides apolipoprotein (apo) A-I several other potent activator apolipoproteins (AIV, E and CI) were identified, furthermore apo A-II was shown to be a modulator of the enzyme's reaction in the presence of apo A-I. Serum amyloid A, an apolipoprotein mainly associated with high-density lipoprotein, massively accumulates in plasma upon acute phase reactions. We therefore studied the possible influence of this acute phase reactant on cholesterol esterification in human plasma. There was a significant decrease of esterified cholesterol in plasma during acute phase reaction. We found a highly significant correlation between the unesterified part of plasma cholesterol and serum amyloid A levels (r = 0.694, P = 0.0001). Also, plasma LCAT activity was negatively correlated with serum amyloid A levels. Lipoproteins containing apo A-I and A-II (LpA-I: A-II) and lipoproteins containing apo A-I but no A-II (LpA-I) decreased significantly with the appearance in plasma of serum amyloid A. To study the influence of serum amyloid A on the LCAT reaction, artificial substrates were prepared either by a detergent dialysis procedure or by addition of apolipoprotein to a sonicated aqueous dispersion of lipid. In addition two different molar ratios of apolipoprotein/phospholipid (PC) (1:50 and 1:310) were chosen at a constant molar ratio of total cholesterol/PC of 1:20. The various substrates were incubated with purified LCAT enzyme. DMPC - or egg yolk phosphatidylcholine - cholesterol-[4-14C]cholesterol-serum amyloid A complexes per se did not stimulate LCAT activity significantly. However, apo serum amyloid A incorporated together with apo A-I by a detergent dialysis procedure lead at low concentrations of serum amyloid A to a marked increase in cholesteryl ester formation as compared to apo A-I alone but inhibited the cholesteryl ester formation at high concentrations. Thus, the low levels of esterified cholesterol in acute phase plasma are to some extent due to decreased plasma enzyme activity and in part may be due to interference of apo serum amyloid A with the natural substrate complexes of plasma HDL.
卵磷脂胆固醇酰基转移酶（EC 2.3.1.43，LCAT）是负责在人血浆中形成大部分胆固醇酯的酶。LCAT反应主要发生在高密度脂蛋白上，并且需要载脂蛋白作为激活剂。除了载脂蛋白（apo）A-I外，还鉴定出了其他几种有效的激活载脂蛋白（AIV、E和CI），此外，在apo A-I存在的情况下，apo A-II被证明是该酶反应的调节剂。血清淀粉样蛋白A是一种主要与高密度脂蛋白相关的载脂蛋白，在急性期反应时会在血浆中大量积聚。因此，我们研究了这种急性期反应物对人血浆中胆固醇酯化的可能影响。在急性期反应期间，血浆中酯化胆固醇显著减少。我们发现血浆胆固醇的未酯化部分与血清淀粉样蛋白A水平之间存在高度显著的相关性（r = 0.694，P = 0.0001）。此外，血浆LCAT活性与血清淀粉样蛋白A水平呈负相关。含有apo A-I和A-II的脂蛋白（LpA-I:A-II）以及含有apo A-I但不含A-II的脂蛋白（LpA-I）随着血清淀粉样蛋白A在血浆中的出现而显著减少。为了研究血清淀粉样蛋白A对LCAT反应的影响，通过去污剂透析程序或向脂质的超声处理水分散体中添加载脂蛋白来制备人工底物。此外，在总胆固醇/磷脂酰胆碱（PC）的恒定摩尔比为1:20的情况下，选择了两种不同摩尔比的载脂蛋白/磷脂酰胆碱（PC）（1:50和1:310）。将各种底物与纯化的LCAT酶一起孵育。二肉豆蔻酰磷脂酰胆碱 - 或蛋黄卵磷脂 - 胆固醇 - [4-14C]胆固醇 - 血清淀粉样蛋白A复合物本身并未显著刺激LCAT活性。然而，通过去污剂透析程序与apo A-I一起掺入的载脂蛋白血清淀粉样蛋白A在低浓度的血清淀粉样蛋白A下与单独使用apo A-I相比导致胆固醇酯形成显著增加，但在高浓度下抑制胆固醇酯形成。因此，急性期血浆中酯化胆固醇水平较低在一定程度上是由于血浆酶活性降低，部分可能是由于载脂蛋白血清淀粉样蛋白A对血浆高密度脂蛋白天然底物复合物的干扰。
