Freeman D A
Department of Internal Medicine, Veterans Administration Medical Center, Dallas, Texas 75216.
Endocrinology. 1989 May;124(5):2527-34. doi: 10.1210/endo-124-5-2527.
The plasma membrane cholesterol content of MA-10 Leydig tumor cells is depleted by trophic hormone stimulation and repleted by incubating the cells with low density lipoprotein. The present studies used subcellular fractionation to investigate the membranes involved in steroid hormone synthesis. The results showed that the plasma membrane was the major source of cholesterol substrate and that the cholesterol content changed independently of any mass changes in membrane protein or phospholipid. Membrane phospholipid composition also did not change as membrane cholesterol content decreased or increased, a finding inconsistent with the proposal that phospholipid composition dictates the amount of cholesterol contained in a membrane. The mitochondria of the MA-10 cells were cholesterol rich, containing more cholesterol per unit protein or phospholipid than the plasma membrane. This cholesterol was presumably in the outer mitochondrial membrane, since virtually all of the cholesterol of intact mitochondria was accessible to cholesterol oxidase. Although there was a high concentration of mitochondrial cholesterol, this cholesterol was largely inert as a substrate for steroidogenesis, and plasma membrane cholesterol was incorporated into steroid hormones without ever equilibrating with the mitochondrial cholesterol pool.
促性腺激素刺激会使MA-10睾丸间质细胞瘤细胞的质膜胆固醇含量减少，而通过用低密度脂蛋白孵育细胞可使其恢复。本研究采用亚细胞分级分离法来研究参与类固醇激素合成的膜。结果表明，质膜是胆固醇底物的主要来源，且胆固醇含量的变化独立于膜蛋白或磷脂的任何质量变化。随着膜胆固醇含量的减少或增加，膜磷脂组成也没有变化，这一发现与磷脂组成决定膜中胆固醇含量的观点不一致。MA-10细胞的线粒体富含胆固醇，每单位蛋白质或磷脂所含胆固醇比质膜更多。这些胆固醇大概存在于线粒体外膜中，因为完整线粒体的几乎所有胆固醇都可被胆固醇氧化酶作用。尽管线粒体胆固醇浓度很高，但作为类固醇生成的底物，这种胆固醇基本上是惰性的，质膜胆固醇被整合到类固醇激素中，而从未与线粒体胆固醇池达到平衡。
