Chong P L, Choate D
Department of Biochemistry, Meharry Medical College, Nashville, Tennessee 37208.
Biophys J. 1989 Mar;55(3):551-6. doi: 10.1016/S0006-3495(89)82848-6.
Differential scanning calorimetry (DSC) has been employed to study the effects of cholesterol on the phase transition of C(18):C(10) phosphatidylcholine (C(18):C(10)PC). C(18):C(10)PC is an asymmetric mixed-chain phosphatidylcholine known to form mixed-interdigitated structures below the transition temperature and form partially interdigitated lipid bilayers above the transition. Three types of samples were used. The treated sample is the lipid dispersion that had undergone three freeze-thaw cycles and stored at 4 degrees C for more than 48 h. The untreated sample was made by vortexing the dry lipid in 50 mM KCl, without the above-mentioned pretreatment. The cold-treated sample was prepared by incubating the treated sample at -20 degrees C for 15 d. There is no apparent difference in the DSC curves between the treated and cold-treated samples. The data derived from the treated samples seem to be more reproducible. The DSC curves between the cholesterol/C(18):C(10)PC and cholesterol/symmetric diacylphosphatidylcholine mixtures are different in three aspects: overall appearance, the cholesterol dependence of delta H, and the effect of cholesterol on the maximal transition temperature Tm, the onset temperature To, and the completion temperature Tc. for both the treated and untreated samples, the total enthalpy change delta H of the phase transition of C(18):C(10)PC decreases with increasing cholesterol content, approaching zero at approximately 25 mol%. This level is lower than the total enthalpy changes reported previously for the cholesterol/symmetric diacylphosphatidylcholine mixtures. Both the heating and cooling thermograms show that Tm, To, and Tc decrease with increasing cholesterol content. The decreasing rates of these temperatures with cholesterol are in the neighborhood of -0.24 degree per mol% of cholesterol. This value is greater than those reported previously for cholesterol/symmetric diacylphosphatidylcholine mixtures. The phase transition between interdigitated lipid bilayers appears to be more sensitive to cholesterol than that between noninterdigitated lipid structures. The formation of highly ordered interdigitated lipid bilayers requires stringent structural conditions such as specific chain length differences and high molecular order. Apparently, in the presence of cholesterol, these stringent structural conditions are no longer satisfied. It is likely that cholesterol causes a local disordering effect on the gel phase of C(18):C(10)PC and that as a consequence the physical state of the gel phase changes continuously with the cholesterol content. The implication of the present study is that cholesterol may have a function in preventing lipids from forming highly ordered interdigitated structures in natural membranes.
差示扫描量热法（DSC）已被用于研究胆固醇对C(18):C(10)磷脂酰胆碱（C(18):C(10)PC）相变的影响。C(18):C(10)PC是一种不对称混合链磷脂酰胆碱，已知在转变温度以下形成混合叉指结构，在转变温度以上形成部分叉指脂质双层。使用了三种类型的样品。处理后的样品是经过三个冻融循环并在4℃下储存超过48小时的脂质分散体。未处理的样品是通过将干燥的脂质在50 mM KCl中涡旋制备的，没有上述预处理。冷处理的样品是通过将处理后的样品在-20℃下孵育15天制备的。处理后的样品和冷处理后的样品的DSC曲线没有明显差异。来自处理后样品的数据似乎更具可重复性。胆固醇/C(18):C(10)PC和胆固醇/对称二酰基磷脂酰胆碱混合物之间的DSC曲线在三个方面不同：整体外观、ΔH对胆固醇的依赖性以及胆固醇对最大转变温度Tm、起始温度To和完成温度Tc的影响。对于处理后的样品和未处理的样品，C(18):C(10)PC相变的总焓变ΔH随着胆固醇含量的增加而降低，在约25 mol%时接近零。该水平低于先前报道的胆固醇/对称二酰基磷脂酰胆碱混合物的总焓变。加热和冷却热谱图均显示，Tm、To和Tc随着胆固醇含量的增加而降低。这些温度随胆固醇的降低速率约为每mol%胆固醇-0.24℃。该值大于先前报道的胆固醇/对称二酰基磷脂酰胆碱混合物的值。叉指脂质双层之间的相变似乎比非叉指脂质结构之间的相变对胆固醇更敏感。高度有序的叉指脂质双层的形成需要严格的结构条件，如特定的链长差异和高分子有序性。显然，在胆固醇存在的情况下，这些严格的结构条件不再满足。胆固醇可能对C(18):C(10)PC的凝胶相产生局部无序效应，因此凝胶相的物理状态随胆固醇含量而不断变化。本研究的意义在于，胆固醇可能具有防止脂质在天然膜中形成高度有序的叉指结构的功能。
