McMullen T P, Lewis R N, McElhaney R N
Department of Biochemistry, University of Alberta, Edmonton, Alberta T6G 2H7, Canada.
Biophys J. 2000 Oct;79(4):2056-65. doi: 10.1016/S0006-3495(00)76453-8.
We have examined the effects of cholesterol on the thermotropic phase behavior and organization of aqueous dispersions of a homologous series of linear disaturated phosphatidylserines by high-sensitivity differential scanning calorimetry and Fourier transform infrared spectroscopy. We find that the incorporation of increasing quantities of cholesterol progressively reduces the temperature, enthalpy, and cooperativity of the gel-to-liquid-crystalline phase transition of the host phosphatidylserine bilayer, such that a cooperative chain-melting phase transition is completely or almost completely abolished at 50 mol % cholesterol, in contrast to the results of previous studies. We are also unable to detect the presence of a separate anhydrous cholesterol or cholesterol monohydrate phase in our binary mixtures, again in contrast to previous reports. We further show that the magnitude of the reduction in the phase transition temperature induced by cholesterol addition is independent of the hydrocarbon chain length of the phosphatidylserine studied. This result contrasts with our previous results with phosphatidylcholine bilayers, where we found that cholesterol increases or decreases the phase transition temperature in a chain length-dependent manner (1993. Biochemistry, 32:516-522), but is in agreement with our previous results for phosphatidylethanolamine bilayers, where no hydrocarbon chain length-dependent effects were observed (1999. Biochim. Biophys. Acta, 1416:119-234). However, the reduction in the phase transition temperature by cholesterol is of greater magnitude in phosphatidylethanolamine as compared to phosphatidylserine bilayers. We also show that the addition of cholesterol facilitates the formation of the lamellar crystalline phase in phosphatidylserine bilayers, as it does in phosphatidylethanolamine bilayers, whereas the formation of such phases in phosphatidylcholine bilayers is inhibited by the presence of cholesterol. We ascribe the limited miscibility of cholesterol in phosphatidylserine bilayers reported previously to a fractional crystallization of the cholesterol and phospholipid phases during the removal of organic solvent from the binary mixture before the hydration of the sample. In general, the results of our studies to date indicate that the magnitude of the effect of cholesterol on the thermotropic phase behavior of the host phospholipid bilayer, and its miscibility in phospholipid dispersions generally, depend on the strength of the attractive interactions between the polar headgroups and the hydrocarbon chains of the phospholipid molecule, and not on the charge of the polar headgroups per se.
我们通过高灵敏度差示扫描量热法和傅里叶变换红外光谱法，研究了胆固醇对一系列同系线性二饱和磷脂酰丝氨酸水分散体的热致相行为和组织结构的影响。我们发现，随着胆固醇含量的增加，主体磷脂酰丝氨酸双层从凝胶态到液晶态的相变温度、焓和协同性逐渐降低，以至于在胆固醇含量为50摩尔%时，协同链熔化相变完全或几乎完全消失，这与之前的研究结果相反。我们同样无法在二元混合物中检测到单独的无水胆固醇或胆固醇一水合物相，这也与之前的报道不同。我们进一步表明，添加胆固醇引起的相变温度降低幅度与所研究的磷脂酰丝氨酸的烃链长度无关。这一结果与我们之前对磷脂酰胆碱双层的研究结果相反，在磷脂酰胆碱双层中我们发现胆固醇以链长度依赖的方式升高或降低相变温度（1993年。《生物化学》，32:516 - 522），但与我们之前对磷脂酰乙醇胺双层的研究结果一致，在磷脂酰乙醇胺双层中未观察到烃链长度依赖效应（1999年。《生物化学与生物物理学报》，1416:119 - 234）。然而，与磷脂酰丝氨酸双层相比，胆固醇对磷脂酰乙醇胺双层相变温度的降低幅度更大。我们还表明，添加胆固醇促进了磷脂酰丝氨酸双层中片层结晶相的形成，就像在磷脂酰乙醇胺双层中一样，而在磷脂酰胆碱双层中，胆固醇的存在会抑制这种相的形成。我们将之前报道的胆固醇在磷脂酰丝氨酸双层中有限的混溶性归因于在样品水化之前从二元混合物中去除有机溶剂过程中胆固醇和磷脂相的分步结晶。总体而言，我们目前的研究结果表明，胆固醇对主体磷脂双层热致相行为的影响程度及其在磷脂分散体中的混溶性，通常取决于磷脂分子极性头部基团与烃链之间吸引力相互作用的强度，而不是极性头部基团本身的电荷。
