Tang D, Borchman D, Yappert M C, Cenedella R J
Department of Ophthalmology and Visual Science, University of Louisville School of Medicine, KY 40202, USA.
Exp Eye Res. 1998 May;66(5):559-67. doi: 10.1006/exer.1997.0467.
The influence of cholesterol on the binding of alpha-crystallin to pure phospholipid membranes was studied. The rationale of this investigation stems from two unique aspects of human lens cells: an unusually high level of cholesterol in the membranes and the specific binding of alpha-crystallin to membranes. In the absence of cholesterol, binding of alpha-crystallin liposomes composed of either sphingomyelin, disteroyl-phosphatidylcholine or egg-phosphatidylcholine caused a decrease in the fluorescence intensity and anisotropy of the fluorophore NBD-PE. Since this fluorescence probe resides in the polar headgroup region of the membrane, the observed changes indicated that the binding of alpha-crystallin affected the structure of these membrane regions. The ability of alpha-crystallin to modulate membrane structure suggests yet another potential role for this lens protein. Addition of cholesterol markedly decreased the binding of alpha-crystallin to liposomes composed of either sphingomyelin or disteroylphosphatidylcholine and antagonized the capacity of bound alpha-crystallin to decrease membrane surface order. This antagonism could be explained by the ability of cholesterol to directly decrease the anisotropy of the fluorophore in sphingomyelin membranes unexposed to alpha-crystallin. Thus, with cholesterol present, a further decrease in membrane order upon subsequent binding of alpha-crystallin was less likely. The results obtained with the sphingomyelin liposomes are considered most meaningful, since sphingomyelins are the principal phospholipids in the human lens nuclear membrane and cholesterol preferentially interacts with sphingomyelin. We conclude that cholesterol in lipid membranes can antagonize the binding of alpha-crystallin and thus interfere with the capacity of bound alpha-crystallin to alter membrane order. We suggest that such actions of cholesterol might serve to preserve lens membrane structure in the physiological state where the concentration of soluble alpha-crystallin is great.
研究了胆固醇对α-晶状体蛋白与纯磷脂膜结合的影响。本研究的基本原理源于人晶状体细胞的两个独特方面：膜中胆固醇水平异常高以及α-晶状体蛋白与膜的特异性结合。在没有胆固醇的情况下，由鞘磷脂、二硬脂酰磷脂酰胆碱或蛋黄磷脂酰胆碱组成的α-晶状体蛋白脂质体的结合导致荧光团NBD-PE的荧光强度和各向异性降低。由于这种荧光探针位于膜的极性头部区域，观察到的变化表明α-晶状体蛋白的结合影响了这些膜区域的结构。α-晶状体蛋白调节膜结构的能力表明这种晶状体蛋白还有另一种潜在作用。添加胆固醇显著降低了α-晶状体蛋白与由鞘磷脂或二硬脂酰磷脂酰胆碱组成的脂质体的结合，并拮抗了结合的α-晶状体蛋白降低膜表面有序性的能力。这种拮抗作用可以通过胆固醇直接降低未暴露于α-晶状体蛋白的鞘磷脂膜中荧光团的各向异性的能力来解释。因此，在存在胆固醇的情况下，随后α-晶状体蛋白结合时膜有序性进一步降低的可能性较小。用鞘磷脂脂质体获得的结果被认为最有意义，因为鞘磷脂是人类晶状体核膜中的主要磷脂，并且胆固醇优先与鞘磷脂相互作用。我们得出结论，脂质膜中的胆固醇可以拮抗α-晶状体蛋白的结合，从而干扰结合的α-晶状体蛋白改变膜有序性的能力。我们认为，在可溶性α-晶状体蛋白浓度很高的生理状态下，胆固醇的这种作用可能有助于维持晶状体膜结构。
