Boesze-Battaglia K, Schimmel R
Department of Molecular Biology, University of Medicine and Dentistry of New Jersey, School of Osteopathic Medicine, Stratford, NJ 08084, USA.
J Exp Biol. 1997 Dec;200(Pt 23):2927-36. doi: 10.1242/jeb.200.23.2927.
Photoreceptor rod cells and blood platelets are remarkably different, yet both illustrate a similar phenomenon. Both are strongly affected by membrane cholesterol, and the distribution of cholesterol in the membranes of both cell types is determined by the lipid composition within the membranes. In rod cells, cholesterol strongly inhibits rhodopsin activity. The relatively higher level of cholesterol in the plasma membrane serves to inhibit, and thereby conserve, the activity of rhodopsin, which becomes fully active in the low-cholesterol environment of the disk membranes of these same cells. This physiologically important partitioning of cholesterol between disk membranes and plasma membranes occurs because the disk membranes are enriched with phosphatidylethanolamine, thus providing a thermodynamically unfavorable environment for the sterol. Cholesterol enrichment of platelets renders these cells more responsive to stimuli of aggregation. Stimuli for platelet aggregation cause a rapid transbilayer movement of cholesterol from the outer monolayer. This stimulus-dependent redistribution of cholesterol appears to result from the concomitant movement of phosphatidylethanolamine into the outer monolayer. The attractive, yet still unproven, hypothesis is that cholesterol translocation plays an important role in the overall platelet response and is intimately related to the sensitizing actions of cholesterol on these cells.
光感受器视杆细胞和血小板显著不同，但二者都说明了一种相似的现象。它们都受到膜胆固醇的强烈影响，并且这两种细胞类型膜中胆固醇的分布由膜内的脂质组成决定。在视杆细胞中，胆固醇强烈抑制视紫红质的活性。质膜中相对较高水平的胆固醇起到抑制作用，从而保存视紫红质的活性，而视紫红质在这些相同细胞的盘膜低胆固醇环境中会变得完全活跃。胆固醇在盘膜和质膜之间这种具有生理重要性的分配之所以会发生，是因为盘膜富含磷脂酰乙醇胺，从而为固醇提供了一个热力学上不利的环境。血小板的胆固醇富集使这些细胞对聚集刺激更敏感。血小板聚集的刺激会导致胆固醇从外单层快速进行跨膜运动。这种依赖刺激的胆固醇重新分布似乎是由磷脂酰乙醇胺同时向外侧单层移动所导致的。一个引人关注但尚未得到证实的假说是，胆固醇易位在血小板的整体反应中起重要作用，并且与胆固醇对这些细胞的致敏作用密切相关。
