Martin F J, MacDonald R C
Biochemistry. 1976 Jan 27;15(2):321-7. doi: 10.1021/bi00647a013.
The turbidity of lipid vesicles, freshly prepared by sonicating purified dimyristoyllecithin (DML) in dilute KCl solutions, was measured as a function of time at various temperatures. A sharp maximum in the rate of increase of turbidity is found just above the crystal:liquid-crystal phase transition temperature (Tm). The initial rate of turbidity increase is first order with respect to DML concentration. Electron and light microscopy reveal large vesicles which are not present before incubation or after incubation at temperatures far from the Tm. When temperature, rather than time, is the independent variable, a sharp drop in turbidity is seen at the Tm. The magnitude of this drop and the temperature at which it occurs were used to measure the rate of lipid transfer between vesicles composed of different lipids. A mixture of DML vesicles and dipalmitoyllecithin (DPL) vesicles exhibits sharp drops in turbidity at 24 and 41 degrees, the corresponding Tm's. With time, the magnitude of the transition at 24 degrees decreases while that which was originally at 41 degrees moves to lower temperatures and increases in magnitude. At equilibrium there is a single transition at 32.5 degrees characteristic of vesicles composed of equimolar DPL and DML. The rate at which equilibrium is approached increases at around 24 degrees and again around 41 degrees. These observations indicate that vesicles are in equilibrium with monomolecular lipid, the concentration of the latter being higher the shorter the lipid acyl group or the smaller the vesicle. DML molecules are therefore lost from small vesicles to large vesicles (DML system) or lost from DML vesicles to DML-DPL vesicles (mixed system). When DML vesicles containing a few percent brain gangliosides were studied, different behavior was observed; the initial rate of increase of turbidity becomes second order in lipid concentration, and the rate constant increases with increasing concentrations of KCl. The kinetic order, coupled with the fact that electrolyte reduces intervesicle electrostatic repulsion, argues that in this situation the mechanism of vesicle growth requires vesicle collision.
通过在稀氯化钾溶液中超声处理纯化的二肉豆蔻酰卵磷脂（DML）新制备的脂质体的浊度，在不同温度下作为时间的函数进行测量。在略高于晶体：液晶相变温度（Tm）处发现浊度增加速率有一个急剧的最大值。浊度增加的初始速率相对于DML浓度呈一级反应。电子显微镜和光学显微镜显示存在大的脂质体，而在孵育前或在远离Tm的温度下孵育后不存在这种大脂质体。当温度而非时间作为自变量时，在Tm处可见浊度急剧下降。该下降的幅度及其发生的温度用于测量由不同脂质组成的脂质体之间的脂质转移速率。DML脂质体和二棕榈酰卵磷脂（DPL）脂质体的混合物在24和41摄氏度（相应的Tm）处浊度急剧下降。随着时间的推移，24摄氏度处转变的幅度减小，而最初在41摄氏度处的转变移向较低温度并幅度增加。在平衡时，存在一个32.5摄氏度的单一转变，这是由等摩尔DPL和DML组成的脂质体的特征。接近平衡的速率在24摄氏度左右以及再次在41摄氏度左右增加。这些观察结果表明脂质体与单分子脂质处于平衡状态，后者的浓度越高，脂质酰基越短或脂质体越小。因此，DML分子从小脂质体中丢失到大脂质体中（DML系统）或从DML脂质体中丢失到DML - DPL脂质体中（混合系统）。当研究含有百分之几脑苷脂的DML脂质体时，观察到不同的行为；浊度增加的初始速率在脂质浓度上变为二级反应，并且速率常数随着氯化钾浓度的增加而增加。动力学级数，再加上电解质减少囊泡间静电排斥这一事实，表明在这种情况下囊泡生长的机制需要囊泡碰撞。
