Radhakrishnan A, Anderson T G, McConnell H M
Department of Chemistry, Stanford University, Stanford CA 94305, USA.
Proc Natl Acad Sci U S A. 2000 Nov 7;97(23):12422-7. doi: 10.1073/pnas.220418097.
Epifluorescence microscopy studies of mixtures of phospholipids and cholesterol at the air-water interface often exhibit coexisting liquid phases. The properties of these liquids point to the formation of "condensed complexes" between cholesterol and certain phospholipids, such as sphingomyelin. It is found that monolayers that form complexes can incorporate a low concentration of a ganglioside G(M1). This glycolipid is visualized by using a fluorescently labeled B subunit of cholera toxin. Three coexisting liquid phases are found by using this probe together with a fluorescent phospholipid probe. The three liquid phases are identified as a phospholipid-rich phase, a cholesterol-rich phase, and a condensed complex-rich phase. The cholera toxin B labeled ganglioside G(M1) is found exclusively in the condensed complex-rich phase. Condensed complexes are likely present in animal cell membranes, where they should facilitate the formation of specialized domains such as rafts. Condensed complexes also have a major effect in determining the chemical activity of cholesterol. It is suggested that this chemical activity plays an essential role in the regulation of cholesterol biosynthesis. Gradients in the chemical activity of cholesterol should likewise govern the rates and direction of intracellular intermembrane cholesterol transport.
对磷脂与胆固醇在气-水界面混合物的落射荧光显微镜研究常常显示出共存的液相。这些液体的性质表明胆固醇与某些磷脂（如鞘磷脂）之间形成了“凝聚复合物”。研究发现，形成复合物的单层膜能够掺入低浓度的神经节苷脂G(M1)。通过使用荧光标记的霍乱毒素B亚基可观察到这种糖脂。将该探针与荧光磷脂探针一起使用时，发现了三个共存的液相。这三个液相分别被鉴定为富含磷脂的相、富含胆固醇的相和富含凝聚复合物的相。标记有霍乱毒素B的神经节苷脂G(M1)仅存在于富含凝聚复合物的相中。凝聚复合物可能存在于动物细胞膜中，在那里它们应该有助于形成如筏状的特殊结构域。凝聚复合物在决定胆固醇的化学活性方面也有主要作用。有人提出，这种化学活性在胆固醇生物合成的调节中起着至关重要的作用。胆固醇化学活性的梯度同样应该控制细胞内膜间胆固醇转运的速率和方向。
