Thurnhofer H, Hauser H
Laboratorium für Biochemie, Eidgenössische Technische Hochschule Zürich, Switzerland.
Biochemistry. 1990 Feb 27;29(8):2142-8. doi: 10.1021/bi00460a026.
Absorption of cholesterol by small intestinal brush border membrane from either mixed micelles or small unilamellar vesicles is protein-mediated. It is a second-order reaction. The kinetic data are consistent with a mechanism involving collision-induced transfer of cholesterol. With micelles as the donor particle, there is net transfer of cholesterol while with small unilamellar vesicles as the donor, cholesterol is evenly distributed between the two lipid pools at equilibrium. The cholesterol absorption by brush border membrane from both mixed micelles and small unilamellar vesicles reveals saturation kinetics. Proteolytic treatment of brush border membrane with papain releases about 25% of the total membrane protein. As a result, the cholesterol uptake by brush border membrane changes from a second-order reaction to a first-order one. The reaction mechanism changes from collision-induced cholesterol uptake to a mechanism involving diffusion of monomeric cholesterol through the aqueous phase. The protein(s) released into the supernatant by papain treatment of brush border membrane exhibit(s) cholesterol exchange activity between two populations of small unilamellar vesicles. The supernate-protein(s) bind(s) the spin-labeled cholesterol analogue 3-doxyl-5 alpha-cholestane.
小肠刷状缘膜从混合微团或小单层囊泡中吸收胆固醇是由蛋白质介导的。这是一个二级反应。动力学数据与涉及碰撞诱导胆固醇转移的机制一致。以微团作为供体颗粒时，存在胆固醇的净转移；而以小单层囊泡作为供体时，在平衡状态下胆固醇在两个脂质池之间均匀分布。小肠刷状缘膜从混合微团和小单层囊泡中吸收胆固醇均呈现饱和动力学。用木瓜蛋白酶对刷状缘膜进行蛋白水解处理会释放约25%的总膜蛋白。结果，刷状缘膜对胆固醇的摄取从二级反应变为一级反应。反应机制从碰撞诱导的胆固醇摄取转变为涉及单体胆固醇通过水相扩散的机制。通过木瓜蛋白酶处理刷状缘膜释放到上清液中的蛋白质在两组小单层囊泡之间表现出胆固醇交换活性。上清液蛋白与自旋标记的胆固醇类似物3 - 多昔 - 5α - 胆甾烷结合。
