Wenk M R, Seelig J
Department of Biophysical Chemistry, Biocenter of the University of Basel, Klingelbergstr. 70, CH-4056 Basel, Switzerland.
Biochim Biophys Acta. 1998 Jul 17;1372(2):227-36. doi: 10.1016/s0005-2736(98)00059-5.
The proton-induced isothermal fusion of unilamellar lipid vesicles (Duzgunes et al., Biochemistry 24 (1985) 3091-3098) is compared with the lamellar (Lalpha)-->hexagonal (HII) phase transition of multilamellar lipid dispersions. Both lipid systems are composed of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine (POPE) and oleic acid (OA) at a 7:3 molar ratio. Using solid-state phosphorus-31 nuclear magnetic resonance (31P-NMR) it is demonstrated that the multilamellar lipid dispersions are in the bilayer state at physiological pH and undergo a Lalpha-->HII phase transition between pH 6.3 and 5.7. This phase transition can also be induced at constant pH by increasing the temperature. The midpoint of the temperature-induced Lalpha-->HII transition is Th=56 degrees C (at pH 7.4) and the corresponding transition enthalpy is DeltaH=0. 7+/-0.1 kcal/mol as determined with differential scanning calorimetry. Both the proton-induced and the temperature-induced phase transition can be completely inhibited by addition of 30 mol% of 1-palmitoyl-2-hydroxy-sn-glycero-3-phosphocholine (LPC). In a second set of experiments unilamellar vesicles are prepared either by sonication or by extrusion through polycarbonate filters at pH 7. 4 and are titrated into buffer at pH 5.7. The proton-induced fusion of the lipid vesicles is monitored with isothermal titration calorimetry, light scattering and fluorescence spectroscopy. The fusion reaction is characterized by an endothermic enthalpy of DeltaH=0.5+/-0.2 kcal/mol (at 28 degrees C). The fusion enthalpy is independent of the vesicle diameter and is only slightly reduced by an increase in temperature to 50 degrees C. Vesicle fusion is accompanied by an increase in light scattering, indicating the formation of larger lipid structures. The transition from unilamellar vesicles to fused lipid structures occurs in the same narrow pH range of 6.3-5.7 as observed for the Lalpha-->HII transition of multilamellar dispersions. Vesicle fusion can be inhibited with 30% LPC. The virtually identical set of parameters found for the Lalpha-->HII phase transition and the vesicle fusion reaction suggests that vesicle fusion also entails a Lalpha-->HII phase transition.
将质子诱导的单层脂质体等温融合（杜兹古内斯等人，《生物化学》24卷（1985年）3091 - 3098页）与多层脂质分散体的片层（Lα）→六方（HII）相转变进行了比较。两种脂质体系均由摩尔比为7:3的1 - 棕榈酰 - 2 - 油酰 - sn - 甘油 - 3 - 磷酸乙醇胺（POPE）和油酸（OA）组成。使用固态磷 - 31核磁共振（31P - NMR）证明，多层脂质分散体在生理pH值下处于双层状态，并且在pH值从6.3到5.7之间经历Lα→HII相转变。该相转变也可以通过升高温度在恒定pH值下诱导。温度诱导的Lα→HII转变的中点为Th = 56℃（在pH 7.4时），通过差示扫描量热法测定的相应转变焓为ΔH = 0.7±0.1千卡/摩尔。添加30摩尔％的1 - 棕榈酰 - 2 - 羟基 - sn - 甘油 - 3 - 磷酸胆碱（LPC）可以完全抑制质子诱导和温度诱导的相转变。在第二组实验中，单层囊泡通过超声处理或在pH 7.4下通过聚碳酸酯滤膜挤出制备，然后滴定到pH 5.7的缓冲液中。用等温滴定量热法、光散射和荧光光谱法监测脂质体的质子诱导融合。融合反应的特征是吸热焓为ΔH = 0.5±0.2千卡/摩尔（在28℃时）。融合焓与囊泡直径无关，并且温度升高到50℃时仅略有降低。囊泡融合伴随着光散射增加，表明形成了更大的脂质结构。从单层囊泡到融合脂质结构的转变发生在与多层分散体的Lα→HII转变相同的狭窄pH范围6.3 - 5.7内。囊泡融合可以用30％的LPC抑制。在Lα→HII相转变和囊泡融合反应中发现的几乎相同的参数集表明，囊泡融合也涉及Lα→HII相转变。
