Brunet S, Thibault L, Lepage G, Seidman E G, Dubé N, Levy E
Centre de Recherche, Hôpital Sainte-Justine and Département de Nutrition, Université de Montréal, Québec, Canada.
Free Radic Biol Med. 2000 Jan 1;28(1):46-54. doi: 10.1016/s0891-5849(99)00197-5.
Mammalian sterol regulatory enzymes are integral membrane proteins of the endoplasmic reticulum. They play a critical role in liver cholesterol homeostasis and the maintenance of overall cholesterol balance in different species. Because lipid peroxidation has been implicated in hepatic dysfunction and atherosclerosis, we hypothesized that its occurrence could alter the composition and properties of the bilayer lipid environment, and thereby affect the functions of these membrane proteins. Preincubation of rat liver microsomes with iron (Fe)/ascorbate (50 microM/200 microM), known to induce peroxidation, resulted in a significant inhibition of (i) the rate-limiting enzyme in cholesterol biosynthesis, HMG-CoA reductase (46%, p < .01), (ii) the crucial enzyme controlling the conversion of cholesterol in bile acids, cholesterol 7alpha-hydroxylase (48%, p < .001), and (iii) the central enzyme for cholesterol esterification: Acyl-CoA:cholesterol acyltransferase (ACAT, 80%, p < .0001). The disturbances of these key enzymes took place concomitantly with the high production of malondialdehyde (350%, p < .007) and the loss of polyunsaturated fatty acids (36.19 +/- 1.06% vs. 44.24 +/- 0.41 in controls, p < .0008). While alpha-tocopherol simultaneously neutralized lipid peroxidation, preserved microsomal fatty acid status, and restored ACAT activity, it was not effective in preventing Fe/ascorbate-induced inactivation of both HMG-CoA reductase (44%, p < .01) and cholesterol 7alpha-hydroxylase (71%, p < .0001). These results indicate that Fe/ascorbate alters the activity of the rate-determining steps in liver cholesterol metabolism, either directly or via lipid peroxidation, capable of modifying their membrane environment. The present data also suggest that the three regulatory enzymes respond differently when exposed to Fe/ascorbate or antioxidants, which may be due to dissimilar mechanisms.
哺乳动物的固醇调节酶是内质网的整合膜蛋白。它们在肝脏胆固醇稳态以及维持不同物种的整体胆固醇平衡中起着关键作用。由于脂质过氧化与肝功能障碍和动脉粥样硬化有关，我们推测其发生可能会改变双层脂质环境的组成和性质，从而影响这些膜蛋白的功能。用已知可诱导过氧化的铁（Fe）/抗坏血酸（50微摩尔/200微摩尔）对大鼠肝脏微粒体进行预孵育，导致（i）胆固醇生物合成中的限速酶HMG-CoA还原酶受到显著抑制（46%，p<.01），（ii）控制胆固醇转化为胆汁酸的关键酶胆固醇7α-羟化酶受到显著抑制（48%，p<.001），以及（iii）胆固醇酯化的核心酶：酰基辅酶A：胆固醇酰基转移酶（ACAT，8-%，p<.0001）。这些关键酶的紊乱与丙二醛的大量产生（350%，p<.007）以及多不饱和脂肪酸的损失（对照组为44.24±0.41，而实验组为36.19±1.06%，p<.0008）同时发生。虽然α-生育酚同时中和了脂质过氧化，保留了微粒体脂肪酸状态，并恢复了ACAT活性，但它在预防Fe/抗坏血酸诱导的HMG-CoA还原酶（44%，p<.01）和胆固醇7α-羟化酶（71%，p<.0001）失活方面无效。这些结果表明，Fe/抗坏血酸直接或通过脂质过氧化改变了肝脏胆固醇代谢中限速步骤的活性，脂质过氧化能够改变它们的膜环境。目前的数据还表明，这三种调节酶在暴露于Fe/抗坏血酸或抗氧化剂时反应不同，这可能是由于不同的机制。
