Boll M, Weber L W, Plana J, Stampfl A
Institut für Toxikologie, GSF-National Research Center for Environment and Health, Neuherberg, Germany.
Z Naturforsch C J Biosci. 1999 May-Jun;54(5-6):371-82. doi: 10.1515/znc-1999-5-612.
The activities of 3-hydroxy-3-methylglutaryl CoA reductase (HMGCoA reductase; EC 1.1.1.34), rate-limiting enzyme of cholesterol biosynthesis, and cholesterol 7 alpha-hydroxylase (EC 1.14.13.17), key enzyme of the neutral bile acid synthesis pathway, were measured in the microsomal fraction of rat liver and in rat liver cells to investigate the coordinate regulation of the two pathways. Both enzyme activities exhibited the same diurnal rhythm and responded in a coordinate fashion to fasting or bile acid-feeding (decrease) and to cholestyramine-feeding (increase). Cholesterol-feeding decreased the activity of HMGCoA reductase, increased that of cholesterol 7 alpha-hydroxylase, and concomitantly increased free cholesterol in microsomes. In an ex vivo setting using primary hepatocytes from animals fed a high cholesterol diet the activity of HMGCoA reductase was initially low and that of cholesterol 7 alpha-hydroxylase was elevated. Release of cholesterol into the medium with ongoing incubation caused HMGCoA reductase activity to increase, and that of cholesterol 7 alpha-hydroxylase to decline. Incubation of hepatocytes with a cholesterol-containing lipoprotein fraction stimulated the activity of cholesterol 7 alpha-hydroxylase, but left HMGCoA reductase activity unaffected. The results confirm the idea of a joint regulation of the two key enzymes of cholesterol metabolism in response to the levels of substrate and metabolites, and support the notion that with respect to bile acid and cholesterol levels, respectively, regulation of HMGCoA reductase activity may be secondary to that of cholesterol 7 alpha-hydroxylase. The in vitro studies supply evidence that the effects of cholesterol and bile acid excess or deficiency are direct and do not involve accessory changes of hormone levels or mediators.
胆固醇生物合成的限速酶3-羟基-3-甲基戊二酰辅酶A还原酶（HMGCoA还原酶；EC 1.1.1.34）以及中性胆汁酸合成途径的关键酶胆固醇7α-羟化酶（EC 1.14.13.17）的活性，在大鼠肝脏微粒体部分和大鼠肝细胞中进行了测定，以研究这两条途径的协同调节。两种酶的活性均呈现相同的昼夜节律，并以协同方式对禁食或胆汁酸喂养（降低）以及消胆胺喂养（升高）作出反应。胆固醇喂养降低了HMGCoA还原酶的活性，增加了胆固醇7α-羟化酶的活性，并同时增加了微粒体中的游离胆固醇。在使用高胆固醇饮食喂养动物的原代肝细胞的体外实验中，HMGCoA还原酶的活性最初较低，而胆固醇7α-羟化酶的活性则升高。随着孵育的进行，胆固醇释放到培养基中导致HMGCoA还原酶活性增加，而胆固醇7α-羟化酶的活性下降。用含胆固醇的脂蛋白组分孵育肝细胞刺激了胆固醇7α-羟化酶的活性，但对HMGCoA还原酶的活性没有影响。结果证实了胆固醇代谢的两种关键酶响应底物和代谢物水平而进行联合调节的观点，并支持以下观点：就胆汁酸和胆固醇水平而言，HMGCoA还原酶活性的调节可能继发于胆固醇7α-羟化酶的调节。体外研究提供了证据，表明胆固醇和胆汁酸过量或缺乏的影响是直接的，不涉及激素水平或介质的辅助性变化。
