Mechanism of C-5 double bond introduction in the biosynthesis of cholesterol by rat liver microsomes.
Mechanism of C-5 double bond introduction in the biosynthesis of cholesterol by rat liver microsomes.
复制标题
大鼠肝微粒体胆固醇生物合成中 C-5 双键引入的机制。
DOI：
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发表时间：
1976
影响因子：
4.8
通讯作者：
E. Caspi
中科院分区：
文献类型：
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作者：
V. Reddy;D. Kupfer;E. Caspi
在大鼠肝微粒体和微粒体丙酮粉末制剂中，使用[3 α-3H]胆甾-7-烯-3 β-醇，研究了在存在NADH的情况下胆甾-7-烯-3 β-醇（I）脱氢为胆甾-5，7-二烯-3 β-醇（II）的反应。甲萘醌、二磷酸腺苷、铁氰化钾和细胞色素c均能抑制该反应，而对甲酚则无影响。这些结果表明微粒体电子传递系统参与胆甾-7-烯-3 β-醇的脱氢。当孵育混合物中包含抗坏血酸时，在不存在NADH的情况下也观察到胆甾-7-烯-3 β-醇转化为胆甾-5，7-二烯-3 β-醇。然而，抗坏血酸催化脱氢不被铁氰化钾抑制。获得了微粒体细胞色素b5参与（I）至（II）脱氢的免疫学证据。在家兔体内诱导出大鼠肝微粒体细胞色素b5的特异性抗体。抗细胞色素b5免疫球蛋白组分抑制大鼠肝微粒体NADH-细胞色素c还原酶，但不抑制NADPH-细胞色素c还原酶。此外，细胞色素b5的还原程度不受抗体的影响。在微粒体蛋白：免疫球蛋白比例为1：5.6时，抗细胞色素b5免疫球蛋白可抑制大鼠肝微粒体将（I）转化为（II）（73%）。这些结果与微粒体细胞色素b5参与胆固醇生物合成中C-5双键的引入是一致的。脂肪酸和胆甾-7-烯-3 β-醇的微粒体脱氢的密切相似性是显而易见的，这表明两种反应的机制可能相似。
The dehydrogenation reaction of cholest-7-en-3beta-ol (I) to cholesta-5,7-dien-3beta-ol (II) in the presence of NADH was studied in rat liver microsomes and in microsomal acetone powder preparations, using [3alpha-3H]cholest-7-en-3beta-ol. It was found that the reaction was inhibited by menadione, adenosine diphosphate, potassium ferricyanide, and cytochrome c while p-cresol had no effect. These results indicated the participation of a microsomal electron transport system in the dehydrogenation of cholest-7-en-3beta-ol. The conversion of cholest-7-en-3beta-ol to cholesta-5,7-dien-3beta-ol was also observed in the absence of NADH when ascorbic acid was included in the incubation mixture. However, the ascorbic acid-catalyzed dehydrogenation was not inhibited by potassium ferricyanide. Immunological evidence that microsomal cytochrome b5 is involved in the dehydrogenation of (I) to (II) was obtained. Antibodies specific for rat liver microsomal cytochrome b5 were elicited in rabbits. The anticytochrome b5 immunoglobulin fraction inhibited rat liver microsomal NADH-cytochrome c reductase but not NADPH-cytochrome c reductase. Also, the extent of reduction of cytochrome b5 was not affected by the antibodies. The conversion of (I) to (II) by rat liver microsomes was inhibited (73%) by anticytochrome b5 immunoglobulin at a ratio of microsomal protein:immunoglobulin of 1:5.6. These results are consistent with the participation of microsomal cytochrome b5 in the introduction of the C-5 double bond in cholesterol biosynthesis. A close analogy of the microsomal dehydrogenation of fatty acids and of cholest-7-en-3beta-ol is apparent and this suggests a possible similarity in the mechanisms of the two reactions.