Roda A, Minutello A, Angellotti M A, Fini A
Istituto di Chimica Analitica, Università di Messina, Italy.
J Lipid Res. 1990 Aug;31(8):1433-43.
Two independent methods have been developed and compared to determine the lipophilicity of a representative series of naturally occurring bile acids (BA) in relation to their structure. The BA included cholic acid (CA), chenodeoxycholic acid (CDCA), ursodeoxycholic acid (UDCA), deoxycholic acid (DCA), hyodeoxycholic acid (HDCA), ursocholic acid (UCA), hyocholic acid (HCA), as well as their glycine and taurine amidates. Lipophilicity was determined using a 1-octanol/water shake-flask procedure and the experiments were performed at different pH and ionic strengths and at initial BA concentrations below their critical micellar concentrations (CMC) and the water solubility of the protonated form. The experimental data show that both the protonated (HA) and ionized (A-) forms of BA can distribute in 1-octanol, and consequently a partition coefficient for HA (logP' HA) and for A- (logP' A-) must be defined. An equation to predict a weighted apparent distribution coefficient (D) value as a function of pH and pKa has been developed and fits well with the experimental data. Differences between logP for protonated and ionized species for unconjugated BA were in the order of 1 log unit, which increased to 2 for glycine-amidate BA. The partition coefficient of the A- form increased with Na+ concentration and total ionic strength, suggesting an ion-pair mechanism for its partition into 1-octanol. Lipophilicity was also assessed using reverse phase chromatography (C-18-HPLC), and a capacity factor (K') for ionized species was determined. Despite a broad correlation with the logP data, some BA behaved differently. The logP values showed that the order of lipophilicity was DCA greater than CDCA greater than UDCA greater than HDCA greater than HCA greater than CA greater than UCA for both the protonated and ionized unconjugated and glycine-amidate BA, while the K' data showed an inversion for some BA, i.e., DCA greater than CDCA greater than CA greater than HCA greater than UDCA greater than HDCA greater than UCA. The logP data fitted well with other indirect measurements of BA monomeric lipophilicity such as albumin binding or accessible total hydrophobic surface area data calculated by energy minimization and molecular computer graphics. Differences between unconjugated and amidated BA are consistent with the presence of an amide bond and a lower pKa when pH dependence was studied. Capacity factors, on the other hand, were related to properties of BA micelles such as cholesterol-solubilizing capacity and membrane disruption, reflecting the BA detergency.(ABSTRACT TRUNCATED AT 400 WORDS)
已经开发并比较了两种独立的方法来确定一系列具有代表性的天然存在的胆汁酸（BA）与其结构相关的亲脂性。这些胆汁酸包括胆酸（CA）、鹅去氧胆酸（CDCA）、熊去氧胆酸（UDCA）、脱氧胆酸（DCA）、猪去氧胆酸（HDCA）、熊胆酸（UCA）、猪胆酸（HCA），以及它们的甘氨酸和牛磺酸酰胺化物。使用1-辛醇/水摇瓶法测定亲脂性，实验在不同的pH和离子强度下进行，初始BA浓度低于其临界胶束浓度（CMC）以及质子化形式的水溶性。实验数据表明，BA的质子化（HA）和离子化（A-）形式都可以在1-辛醇中分布，因此必须定义HA（logP'HA）和A-（logP'A-）的分配系数。已经开发了一个预测加权表观分配系数（D）值作为pH和pKa函数的方程，并且该方程与实验数据拟合良好。未结合BA的质子化和离子化物种的logP之间的差异约为1个对数单位，对于甘氨酸酰胺化BA则增加到2个对数单位。A-形式的分配系数随Na+浓度和总离子强度增加，表明其分配到1-辛醇中的离子对机制。还使用反相色谱法（C-18-HPLC）评估亲脂性，并确定离子化物种的容量因子（K'）。尽管与logP数据有广泛的相关性，但一些BA的行为有所不同。logP值表明，对于质子化和离子化的未结合以及甘氨酸酰胺化BA，亲脂性顺序为DCA大于CDCA大于UDCA大于HDCA大于HCA大于CA大于UCA，而K'数据显示一些BA出现反转，即DCA大于CDCA大于CA大于HCA大于UDCA大于HDCA大于UCA。logP数据与BA单体亲脂性的其他间接测量结果拟合良好，如白蛋白结合或通过能量最小化和分子计算机图形计算的可及总疏水表面积数据。未结合和酰胺化BA之间的差异与酰胺键的存在以及研究pH依赖性时较低的pKa一致。另一方面，容量因子与BA胶束的性质有关，如胆固醇溶解能力和膜破坏，反映了BA的去污能力。（摘要截断于400字）
