Steck T L, Kezdy F J, Lange Y
Department of Biochemistry and Molecular Biology, University of Chicago, Illinois 60637.
J Biol Chem. 1988 Sep 15;263(26):13023-31.
We report the results of experiments which show that cholesterol transfer between membranes cannot proceed by aqueous diffusion, as widely held, but must involve a more complex mechanism. (a) The rate of transfer of [3H]cholesterol from red blood cells was found to vary inversely with the size of the acceptor particle (ghosts, vesicles of ghosts, liposomes, and plasma lipoproteins). (b) The transfer of [3H]cholesterol from red blood cells to ghosts was accelerated by the presence of plasma, even though the plasma competed with the ghosts as an acceptor. (c) The rate of transfer of [3H]cholesterol from red blood cells to ghosts decreased to zero with increasing dilution but was not simply second-order. (d) The cholesterol in retinal rod disc membranes is not at equilibrium with plasma lipoproteins in that disc cholesterol increased when the homogenates were incubated in vitro with plasma. (e) The kinetics of cholesterol transfer cannot be limited by unstirred layer effects since the transfer of lysolecithin in the same system was faster than that of cholesterol by 3 orders of magnitude. The simplest model compatible with all the data suggests a two-step pathway involving a first-order followed by a second-order process. The first step could be a unimolecular activation event, perhaps the movement of the sterol in the donor particle to a more exposed (hydrated) position. In the second step, the activated sterol would be transferred during transient collisions between donor and acceptor particles. When collision is not rate-limiting, the overall process would appear to be simply first-order, hence kinetically indistinguishable from the aqueous diffusion mechanism. The activation-collision model thus not only rationalizes our data but is also consistent with the simpler kinetics previously reported for the transfer of both membrane phospholipids and sterols.
我们报告了实验结果，这些结果表明，膜之间的胆固醇转移并非如广泛认为的那样通过水相扩散进行，而是必定涉及一种更为复杂的机制。(a) 发现[3H]胆固醇从红细胞的转移速率与受体颗粒（血影、血影囊泡、脂质体和血浆脂蛋白）的大小成反比。(b) 血浆的存在加速了[3H]胆固醇从红细胞向血影的转移，尽管血浆作为受体与血影存在竞争。(c) [3H]胆固醇从红细胞向血影的转移速率随着稀释度增加而降至零，但并非简单的二级反应。(d) 视网膜杆状盘膜中的胆固醇与血浆脂蛋白未达到平衡，因为当匀浆在体外与血浆一起孵育时，盘状胆固醇会增加。(e) 胆固醇转移的动力学不可能受未搅拌层效应限制，因为在同一系统中溶血卵磷脂的转移比胆固醇快3个数量级。与所有数据相符的最简单模型表明存在一个两步途径，包括一个一级反应接着一个二级反应过程。第一步可能是一个单分子活化事件，也许是固醇在供体颗粒中移动到一个更暴露（水合）的位置。在第二步中，活化的固醇会在供体和受体颗粒之间的短暂碰撞过程中转移。当碰撞不是限速步骤时，整个过程在动力学上似乎只是一级反应，因此在动力学上与水相扩散机制无法区分。因此，活化 - 碰撞模型不仅使我们的数据合理化，而且也与先前报道的膜磷脂和固醇转移的更简单动力学一致。
