Mavromoustakos T, Yang D P, Makriyannis A
Institute of Organic and Pharmaceutical Chemistry, National Hellenic Research Foundation, Athens, Greece.
Biochim Biophys Acta. 1994 Aug 24;1194(1):69-74. doi: 10.1016/0005-2736(94)90204-6.
We have used small-angle X-ray diffraction and differential scanning calorimetry (DSC) to study the topographies of alphaxalone and its biologically inactive analog delta 16-alphaxalone in dimyristoylphosphatidylcholine (DMPC) and DMPC/cholesterol model membranes. Diffraction patterns were obtained and analyzed for preparations of bilayers without and with the steroids. Temperature dependence of the total period repeat distance (d-spacing) allowed us to identify equivalent temperatures at which the preparations had similar d-spacing and were in the same mesomorphic state. The combination of X-ray and DSC data showed that the anesthetic steroid alphaxalone broadens the membrane phase transition and increases the ratio of gauche: trans conformers in the membranes in contrast to the inactive steroid delta 16-alphaxalone which affects the membranes only marginally. In model DMPC membranes alphaxalone and delta 16-alphaxalone are located near the bilayer interface. This location is maintained by alphaxalone when cholesterol is incorporated in the bilayer as evidenced by the X-ray measurements. However, when delta 16-alphaxalone is incorporated in cholesterol containing bilayers, a decrease in the electron density profile of the preparation is observed. This can be explained by invoking the formation of a delta 16-alphaxalone-cholesterol complex. The delta 16-alphaxalone complex shows no periodicity and is therefore, not detected in the X-ray diffraction experiment. Presumably, this complex forms aggregates either on the surface or inside the bilayer. This explanation corroborates DSC results which show that delta 16-alphaxalone sharpens the phase transition of DMPC/cholesterol preparations, an indication that some cholesterol is excluded from the bilayer preparation after the addition of the biologically inactive steroid.
我们利用小角X射线衍射和差示扫描量热法（DSC）研究了α-香螺酮及其生物活性无的类似物δ16-α-香螺酮在二肉豆蔻酰磷脂酰胆碱（DMPC）和DMPC/胆固醇模型膜中的拓扑结构。获得了双层膜在无类固醇和有类固醇情况下的衍射图谱并进行了分析。总周期重复距离（d间距）的温度依赖性使我们能够确定制剂具有相似d间距且处于相同介晶态的等效温度。X射线和DSC数据的结合表明，与仅对膜有轻微影响的无活性类固醇δ16-α-香螺酮相比，麻醉类固醇α-香螺酮拓宽了膜相变并增加了膜中gauche：trans构象异构体的比例。在模型DMPC膜中，α-香螺酮和δ16-α-香螺酮位于双层界面附近。X射线测量证明，当胆固醇掺入双层膜中时，α-香螺酮可维持此位置。然而，当δ16-α-香螺酮掺入含胆固醇的双层膜中时，观察到制剂的电子密度分布降低。这可以通过形成δ16-α-香螺酮-胆固醇复合物来解释。δ16-α-香螺酮复合物没有周期性，因此在X射线衍射实验中未被检测到。据推测，这种复合物在双层膜的表面或内部形成聚集体。这一解释证实了DSC结果，即δ16-α-香螺酮使DMPC/胆固醇制剂的相变变尖锐，这表明在添加生物活性无的类固醇后，一些胆固醇从双层制剂中被排除。
