Ellens H, Bentz J, Szoka F C
Biochemistry. 1986 Jan 28;25(2):285-94. doi: 10.1021/bi00350a001.
We have examined whether there is a relationship between the lamellar-hexagonal phase transition temperature, TH, and the initial kinetics of H+- and Ca2+-induced destabilization of phosphatidylethanolamine (PE) liposomes. The liposomes were composed of dioleoylphosphatidylethanolamine, egg phosphatidylethanolamine (EPE), or phosphatidylethanolamine prepared from egg phosphatidylcholine by transesterification (TPE). These lipids have well-spaced lamellar-hexagonal phase transition temperatures (approximately 12, approximately 45, and approximately 57 degrees C) in a temperature range that allows us to measure the initial kinetics of bilayer destabilization, both below and above TH. The liposomes were prepared at pH 9.5. The TH of EPE and TPE was measured by using differential scanning calorimetry, and it was found that the TH was essentially the same at low pH or at high pH in the presence of 20 mM Ca2+. At temperatures well below TH, either at pH 4.5 or at pH 9.5 in the presence of Ca2+, the liposomes aggregate, leak, and undergo lipid mixing and mixing of contents. We show that liposome/liposome contact is involved in the destabilization of the PE liposomes. The temperature dependence of leakage, lipid mixing, and mixing of contents shows that there is a massive enhancement in the rate of leakage when the temperature approaches the TH of the particular PE and that lipid mixing appears to be enhanced. However, the fusion (mixing of aqueous contents) is diminished or even abolished at temperatures above TH. At and above the TH, a new mechanism of liposome destabilization arises, evidently dependent upon the ability of the PE molecules to adapt new morphological structures at these temperatures. We propose that this destabilization demarks the first step in the pathway to the eventual formation of the HII phase. Thus, the polymorphism accessible to PE is a powerful agent for membrane destabilization, but additional factors are required for fusion.
我们研究了层状-六方相转变温度（TH）与H⁺和Ca²⁺诱导的磷脂酰乙醇胺（PE）脂质体去稳定化的初始动力学之间是否存在关系。脂质体由二油酰磷脂酰乙醇胺、鸡蛋磷脂酰乙醇胺（EPE）或通过酯交换从鸡蛋磷脂酰胆碱制备的磷脂酰乙醇胺（TPE）组成。这些脂质在一个温度范围内具有间隔良好的层状-六方相转变温度（分别约为12℃、约45℃和约57℃），这使我们能够在低于和高于TH的温度下测量双层去稳定化的初始动力学。脂质体在pH 9.5条件下制备。通过差示扫描量热法测量EPE和TPE的TH，发现在低pH或在20 mM Ca²⁺存在下的高pH时，TH基本相同。在远低于TH的温度下，无论是在pH 4.5还是在存在Ca²⁺的pH 9.5条件下，脂质体都会聚集、泄漏，并发生脂质混合和内容物混合。我们表明脂质体/脂质体接触参与了PE脂质体的去稳定化。泄漏、脂质混合和内容物混合的温度依赖性表明，当温度接近特定PE的TH时，泄漏速率会大幅提高，并且脂质混合似乎也会增强。然而，在高于TH的温度下，融合（水相内容物混合）会减弱甚至消失。在TH及以上，出现了一种新的脂质体去稳定化机制，显然取决于PE分子在这些温度下适应新形态结构的能力。我们提出这种去稳定化标志着最终形成HII相途径的第一步。因此，PE可实现的多态性是膜去稳定化的有力因素，但融合还需要其他因素。
