Chakraborty Hirak, Sarkar Munna
Chemical Sciences Division, Saha Institute of Nuclear Physics, 1/AF, Bidhannagar, Calcutta-700 064, India.
Biophys Chem. 2007 Feb;125(2-3):306-13. doi: 10.1016/j.bpc.2006.09.002. Epub 2006 Sep 28.
Small unilamellar vesicles (SUVs) formed from a mixture of dimyristoylphosphatidylcholine (zwitterionic lipid with bulkier headgroup) and dimyristoylphosphatidylglycerol (anionic lipid with relatively smaller headgroup) allows better modulation of the physical properties of lipid bilayers compared to SUVs formed by a single type of lipid, providing us with a better model system to study the effect of membrane parameters on the partitioning of small molecules. Membrane parameter like packing of the vesicles is more pronounced in the gel phase and hence the study was carried out in the gel phase. Mixed vesicles formed from DMPG and DMPC with the mole percent ratio of 100:0, 90:10 and 80:20 were used for this study. As examples of polar solutes, piroxicam and meloxicam, two Non Steroidal Anti-inflammatory Drugs (NSAIDs) were chosen. The pH was adjusted to 2.8 in order to eliminate the presence of anionic forms of the drugs that would not approach the vesicles containing negatively charged DMPG (50% deprotonated at pH 2.8). Surface potential measured by using TNS (2,6-p-toluidinonaphthalene sulfonate, sodium salt) as surface charge sensitive probe showed no significant changes in the surface electrostatics in increasing DMPC content from 0 to 20%. Transmission electron microscopy (TEM) was used to characterize SUVs of different composition at pH 2.8. The average diameter of the mixed vesicles was found to be smaller than that formed by DMPG and DMPC alone. Partition coefficient (K(P)) of piroxicam and meloxicam was measured using intrinsic fluorescence of these molecules. K(P) value of piroxicam decreases with increase in DMPC content whereas it increases with DMPC content in case of meloxicam. This anomalous behavior of partitioning is unexpected since there was no significant change in surface pH of the vesicles and has been explained in terms of lipid packing and water penetration in the lipid bilayer.
由二肉豆蔻酰磷脂酰胆碱（具有较大头部基团的两性离子脂质）和二肉豆蔻酰磷脂酰甘油（具有相对较小头部基团的阴离子脂质）的混合物形成的小单层囊泡（SUV），与由单一类型脂质形成的SUV相比，能更好地调节脂质双层的物理性质，为我们提供了一个更好的模型系统来研究膜参数对小分子分配的影响。膜参数如囊泡的堆积在凝胶相中更为明显，因此该研究在凝胶相中进行。由DMPG和DMPC以100:0、90:10和80:20的摩尔百分比比例形成的混合囊泡用于本研究。作为极性溶质的例子，选择了两种非甾体抗炎药吡罗昔康和美洛昔康。将pH调节至2.8，以消除药物阴离子形式的存在，这些阴离子形式不会接近含有带负电荷DMPG的囊泡（在pH 2.8时50%去质子化）。使用TNS（2,6 - 对甲苯胺基萘磺酸钠）作为表面电荷敏感探针测量的表面电位显示，在DMPC含量从0增加到20%时，表面静电没有显著变化。在pH 2.8下，使用透射电子显微镜（TEM）对不同组成的SUV进行表征。发现混合囊泡的平均直径小于单独由DMPG和DMPC形成的囊泡。使用这些分子的固有荧光测量吡罗昔康和美洛昔康的分配系数（K(P)）。吡罗昔康的K(P)值随着DMPC含量的增加而降低，而美洛昔康的K(P)值则随着DMPC含量的增加而增加。这种异常的分配行为是出乎意料的，因为囊泡表面pH没有显著变化，并且已根据脂质双层中的脂质堆积和水渗透进行了解释。
