The oxidation of cholesterol in rat liver sub-cellular particles. The cholesterol-7-alpha-Hydroxylase enzyme system.
The oxidation of cholesterol in rat liver sub-cellular particles. The cholesterol-7-alpha-Hydroxylase enzyme system.
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大鼠肝亚细胞颗粒中胆固醇的氧化。
DOI：
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发表时间：
1971
期刊：
影响因子：
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通讯作者：
George S. Boyd
中科院分区：
文献类型：
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作者：
J. Mitton;N. A. Scholan;George S. Boyd
大鼠肝脏的各种亚细胞部分对[4-14C]-胆固醇氧化成7α-羟基-[4-14C]胆固醇和其他[4-14C]胆固醇氧化产物发挥催化作用。这些体外研究中产生的一些氧化产物似乎是异常氧化产物，因为没有证据表明它们是胆固醇生理氧化的中间体。这些异常氧化产物已被归类为自动氧化产物，尽管事实上这些细胞部分中的自动氧化反应似乎需要 NADPH、天然蛋白质以及可能某些核苷酸的参与来产生这些化合物。
催化[4-14C]胆固醇氧化为7α-羟基-[4-14C]胆固醇的酶系统可以在大鼠肝脏的18000×g上清液部分中进行测定。这种酶反应受到肝组织广泛均质化和延长潜伏期的不利影响，这两者都会增加自氧化反应。胆固醇-7α-羟化酶在 NADPH 和氧气存在下表现出最佳活性。
在洗涤的肝微粒体级分中，[4-14C]胆固醇在NADPH、ADP和亚铁离子存在下被广泛氧化为7β-羟基胆固醇、胆甾烷-3β、5α、6β-三醇和7-酮胆固醇。进一步的研究表明，还形成了另一种化合物，其在所用的薄层溶剂系统中具有与 7-酮胆固醇相同的迁移率。与参考标准相比，该化合物的还原和进一步代谢表明它可能是一种胆固醇氧化物，是在微粒体脂质可能发生过氧化的条件下形成的。 Tris 和磷酸盐缓冲液以及各种核苷酸的作用的比较表明，微粒体部分中的胆固醇自氧化可能与一般脂质过氧化系统有关。
煮沸的100000×g肝上清液制剂或2-巯基乙胺均能有效猝灭胆固醇过氧化，同时刺激胆固醇-7α-羟化酶系统。已提出一种方案来解释肝微粒体胆固醇自氧化或过氧化，并将该过程与参与酶促胆固醇-7α-羟化酶反应的混合功能氧化酶进行对比。
Various sub-cellular fractions of rat liver exert catalytic effects on the oxidation of [4-14C]-cholesterol to 7α-hydroxy-[4-14C]cholesterol and to other [4-14C]cholesterol oxidation products. Some of these oxidation products created in these studies in vitro appear to be aberrant oxidation products in that there is no evidence that they are intermediates in the physiological oxidation of cholesterol. These aberrant oxidation products have been classified as autoxidation products despite the fact that the autoxidation reactions in these cell fractions appear to require the participation of NADPH, native proteins and perhaps certain nucleotides for the production of these compounds.
The enzyme system catalysing the oxidation of [4-14C]cholesterol to 7α-hydroxy-[4-14C]cholesterol could be assayed in a 18000 ×g supernatant fraction of rat liver. This enzymic reaction was adversely affected by extensive homogenisation of liver tissue and by prolonged incubation periods, both of which tended to increase the autoxidation reactions. The cholesterol-7α-hydroxylase enzyme showed optimal activity in the presence of NADPH and oxygen.
In the washed liver microsomal fraction, [4-14C]cholesterol was extensively oxidised in the presence of NADPH, ADP and ferrous ions to 7β-hydroxycholesterol, cholestan-3β, 5α, 6β-triol and 7-ketocholesterol. Further investigation showed that another compound was also formed, having the same mobility as 7-ketocholesterol in the thin layer solvent systems used. Reduction and further metabolism of this compound compared with reference standards suggested that it may be a cholesterol oxide, formed under conditions where peroxidation of microsomal lipids could occur. A comparison of the effects of Tris and phosphate buffers, and various nucleotides suggests that cholesterol autoxidation in the microsomal fraction may be related to the general lipid peroxidation systems.
The boiled 100000 ×g liver supernatant preparation or 2-mercaptoethylamine both effectively quenched cholesterol peroxidation, at the same time stimulating the cholesterol-7α-hydroxylase system. A scheme has been suggested to explain the liver microsomal cholesterol autoxidation or peroxidation and to contrast this process with the mixed function oxidase involved in the enzymic cholesterol-7α-hydroxylase reaction.