Bile acid structure-activity relationship: evaluation of bile acid lipophilicity using 1-octanol/water partition coefficient and reverse phase HPLC.

A Roda,A Minutello,Ma Angellotti,A Fini

doi:10.1016/s0022-2275(20)42614-8

Abstract

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)

Highlights

Two independent methods have been developed and compared to determine the lipophilicityof a representative series of naturally occurring bile acids (BA) in relation to their structure
On the other hand, were related to properties of BA micelles such as cholesterol-solubilizing capacity and membrane disruption, reflecting the BA detergency. The extrapolation of these data to biological phenomena must carefully consider the experimental conditions in which the interaction occurs, Le., total BA concentration, ionic strength, Na+ concentration, and pH, which in turn determine the BA species existing in solution that could interact with biological membranes or the lipid environment. -Roda, A., A
The methodological approach used for the measurements of BA lipophilicity using either 1-octanol/water partition coefficients or capacity factors is fundamental for accurate definition of this physicochemical property

Summary

Introduction

Two independent methods have been developed and compared to determine the lipophilicityof a representative series of naturally occurring bile acids (BA) in relation to their structure. On the other hand, were related to properties of BA micelles such as cholesterol-solubilizing capacity and membrane disruption, reflecting the BA detergency The extrapolation of these data to biological phenomena must carefully consider the experimental conditions in which the interaction occurs, Le., total BA concentration, ionic strength, Na+ concentration, and pH, which in turn determine the BA species existing in solution that could interact with biological membranes or the lipid environment. Supplementarykey words bile acid structure ionization interaction with membranes Since the introduction of naturally occurring bile acids (BA) such as ursodeoxycholic (UDCA) and chenodeoxycholic acid (CDCA) as drugs to dissolve cholesterol gallstones, interest in these acids has increased in the last decade [1,2,3]. The lipophilicity of a given BA may determine its effects on cholesterol transport by altering the detergency of the BA pool and the physicochemical state of bile (micelles and vesicles)

Methods

Results

Conclusion