Effect of nuclear hydroxy substituents on aqueous solubility and acidic strength of bile acids

Enhancing Dissolution and Oral Bioavailability of Ursodeoxycholic Acid with a Spray-Dried pH-Modified Extended Release Formulation

Ursodeoxycholate (UDCA) has low oral bioavailability and pH-dependent solubility and permeability. Thus, we developed a pH-modified extended-release formulation of UDCA using Na₂CO₃ as the alkalizing agent and hydroxypropyl methylcellulose (HPMC) as the release-modifying agent. The optimized pH-modified controlled-release UDCA formulation, with the UDCA:HPMC:Na₂CO₃ ratio of 200:600:150 (w/w/w), was prepared using a spray-drying method. Then, the formulation’s solubility, dissolution, and pharmacokinetic properties were characterized. In a pH-modified extended-release formulation of UDCA, the solubility of UDCA was increased to 8 mg/mL with a sustained dissolution for 12 h. Additionally, the spray-dried formulation exhibited amorphous states without molecular interaction among UDCA, Na₂CO₃, and HPMC. Moreover, the plasma UDCA concentration of the formulation maintained a higher UDCA concentration for up to 48 h than that of UDCA itself or the non-extended-release UDCA formulation. Consequently, the formulation significantly increased the AUC compared to UDCA or the non-extended-release UDCA formulation in rats. In conclusion, we have improved UDCA’s solubility and dissolution profile by preparing a pH-modified extended-release formulation with the UDCA:HPMC:Na₂CO₃ ratio of 200:600:150 (w/w/w), which effectively increased the oral bioavailability of UDCA by 251% in rats.

Guts and Gall: Bile Acids in Regulation of Intestinal Epithelial Function in Health and Disease

Epithelial cells line the entire surface of the gastrointestinal tract and its accessory organs where they primarily function in transporting digestive enzymes, nutrients, electrolytes, and fluid to and from the luminal contents. At the same time, epithelial cells are responsible for forming a physical and biochemical barrier that prevents the entry into the body of harmful agents, such as bacteria and their toxins. Dysregulation of epithelial transport and barrier function is associated with the pathogenesis of a number of conditions throughout the intestine, such as inflammatory bowel disease, chronic diarrhea, pancreatitis, reflux esophagitis, and cancer. Driven by discovery of specific receptors on intestinal epithelial cells, new insights into mechanisms that control their synthesis and enterohepatic circulation, and a growing appreciation of their roles as bioactive bacterial metabolites, bile acids are currently receiving a great deal of interest as critical regulators of epithelial function in health and disease. This review aims to summarize recent advances in this field and to highlight how bile acids are now emerging as exciting new targets for disease intervention.

Nutrients central to maintaining intestinal absorptive efficiency and barrier integrity with fowl

The small intestinal mucosa acts to recover nutrients from the lumen while providing a barrier against potential hazards. Its unstirred water layer (USWL) at the lumen interface involves membrane associated mucin linearly protruding from underlying microvilli that entangles secretory mucin released from local goblet cells. Both mucin sources are dominated by repetitive O-glycosylated areas dependant on threonine, serine, glycine, and proline. Secretory mucin differs from membrane attached mucin by further employing multiple cystines that interconnect these areas into a net-like molecular sieve. All of the glycosylated areas have ionizable acidic groups credited with reducing pH from that in the lumen to create a micro environment favoring enzymes finalizing digestion while optimizing nutrient terms for absorption. Erosion of the USWL and/or abuse of the membrane due to lumen threats require continuous repair. The aforementioned amino acids are necessary in substantial amounts while vitamin B6 collaborates with vitamin A as meaningful cofactors for mucin synthesis. Marginal inadequacies of these nutrients during inordinate demand are expected to impair mucin replacement. In turn, marginal increases in feed conversion likely occur while fostering the probability of necrotic enteritis together with gizzard erosions. Abuse of the absorptive membrane is of particular concern from fatty acid hydroperoxides because of their continual presence in feed and inability of the USWL to provide protection. These hydroperoxides threaten membrane integrity by their inclusion in micelles during digestive events with fat thereby permitting transit through the USWL. Once coalesced with membrane phospholipids, structural aberrations are visualized as interfering with nutrient recovery while enabling leakage of cell contents to potentiate wet excreta. Inclusion of dietary vitamin E along with vitamin A into micelles with fatty acid hydroperoxides provides relief by quenching further peroxidation. Assuring cystine, threonine, glycine, and serine that are directly available as such together with vitamins A, E, and B6 represents one approach toward optimizing maintenance of the intestinal mucosa.

Synthesis, physicochemical properties, and biological activity of bile acids 3-glucuronides: Novel insights into bile acid signalling and detoxification

Glucuronidation is considered an important detoxification pathway of bile acids especially in cholestatic conditions. Glucuronides are less toxic than the parent free forms and are more easily excreted in urine. However, the pathophysiological significance of bile acid glucuronidation is still controversial and debated among the scientific community. Progress in this field has been strongly limited by the lack of appropriate methods for the preparation of pure glucuronides in the amount needed for biological and pharmacological studies. In this work, we have developed a new synthesis of bile acid C3-glucuronides enabling the convenient preparation of gram-scale quantities. The synthesized compounds have been characterized in terms of physicochemical properties and abilities to modulate key nuclear receptors including the farnesoid X receptor (FXR). In particular, we found that C3-glucuronides of chenodeoxycholic acid and lithocholic acid, respectively the most abundant and potentially cytotoxic species formed in patients affected by cholestasis, behave as FXR agonists and positively regulate the gene expression of transporter proteins, the function of which is critical in human conditions related to imbalances of bile acid homeostasis.

Hydrogen bonding asymmetric star-shape derivative of bile acid leads to supramolecular fibrillar aggregates that wrap into micrometer spheres

We report that star-shaped molecules with cholic acid cores asymmetrically grafted by low molecular weight polymers with hydrogen bonding end-groups undergo aggregation to nanofibers, which subsequently wrap into micrometer spherical aggregates with low density cores. Therein the facially amphiphilic cholic acid (CA) is functionalized by four flexible allyl glycidyl ether (AGE) side chains, which are terminated with hydrogen bonding 2-ureido-4[1H]pyrimidinone (UPy) end-groups as connected by hexyl spacers, denoted as CA(AGE6-C6H12-UPy)4. This wedge-shaped molecule is expected to allow the formation of a rich variety of solvent-dependent structures due to the complex interplay of interactions, enabled by its polar/nonpolar surface-active structure, the hydrophobicity of the CA in aqueous medium, and the possibility to control hydrogen bonding between UPy molecules by solvent selection. In DMSO, the surfactant-like CA(AGE6-C6H12-UPy)4 self-assembles into nanometer scale micelles, as expected due to its nonpolar CA apexes, solubilized AGE6-C6H12-UPy chains, and suppressed mutual hydrogen bonds between the UPys. Dialysis in water leads to nanofibers with lateral dimensions of 20-50 nm. This is explained by promoted aggregation as the hydrogen bonds between UPy molecules start to become activated, the reduced solvent dispersibility of the AGE-chains, and the hydrophobicity of CA. Finally, in pure water the nanofibers wrap into micrometer spheres having low density cores. In this case, strong complementary hydrogen bonds between UPy molecules of different molecules can form, thus promoting lateral interactions between the nanofibers, as allowed by the hydrophobic hexyl spacers. The wrapping is illustrated by transmission electron microscopy tomographic 3D reconstructions. More generally, we foresee hierarchically structured matter bridging the length scales from molecular to micrometer scale by sequentially triggering supramolecular interactions.

Capture and Recycling of Sortase A through Site-Specific Labeling with Lithocholic Acid

Enzyme-mediated protein modification often requires large amounts of biocatalyst, adding significant costs to the process and limiting industrial applications. Herein, we demonstrate a scalable and straightforward strategy for the efficient capture and recycling of enzymes using a small-molecule affinity tag. A proline variant of an evolved sortase A (SrtA 7M) was N-terminally labeled with lithocholic acid (LA)-an inexpensive bile acid that exhibits strong binding to β-cyclodextrin (βCD). Capture and recycling of the LA-Pro-SrtA 7M conjugate was achieved using βCD-modified sepharose resin. The LA-Pro-SrtA 7M conjugate retained full enzymatic activity, even after multiple rounds of recycling.

Key discoveries in bile acid chemistry and biology and their clinical applications: history of the last eight decades

During the last 80 years there have been extraordinary advances in our knowledge of the chemistry and biology of bile acids. We present here a brief history of the major achievements as we perceive them. Bernal, a physicist, determined the X-ray structure of cholesterol crystals, and his data together with the vast chemical studies of Wieland and Windaus enabled the correct structure of the steroid nucleus to be deduced. Today, C24 and C27 bile acids together with C27 bile alcohols constitute most of the bile acid "family". Patterns of bile acid hydroxylation and conjugation are summarized. Bile acid measurement encompasses the techniques of GC, HPLC, and MS, as well as enzymatic, bioluminescent, and competitive binding methods. The enterohepatic circulation of bile acids results from vectorial transport of bile acids by the ileal enterocyte and hepatocyte; the key transporters have been cloned. Bile acids are amphipathic, self-associate in solution, and form mixed micelles with polar lipids, phosphatidylcholine in bile, and fatty acids in intestinal content during triglyceride digestion. The rise and decline of dissolution of cholesterol gallstones by the ingestion of 3,7-dihydroxy bile acids is chronicled. Scientists from throughout the world have contributed to these achievements.

Avicholic Acid: A Lead Compound from Birds on the Route to Potent TGR5 Modulators

Grounding on our former 3D QSAR studies, a knowledge-based screen of natural bile acids from diverse animal species has led to the identification of avicholic acid as a selective but weak TGR5 agonist. Chemical modifications of this compound resulted in the disclosure of 6α-ethyl-16-epi-avicholic acid that shows enhanced potency at TGR5 and FXR receptors. The synthesis, biological appraisals, and structure-activity relationships of this series of compounds are herein described. Moreover, a thorough physicochemical characterization of 6α-ethyl-16-epi-avicholic acid as compared to naturally occurring bile acids is reported and discussed.

Resolving self-assembly of bile acids at the molecular length scale

The self-assembly behavior of the naturally occurring steroidal bile compounds cholic, deoxycholic, ursodeoxycholic, and lithocholic acid was studied by combining atomic force microscopy (AFM), polarized optical microscopy (POM), Fourier-transform infrared spectroscopy (FTIR), absorption spectroscopy (UV-vis), circular dichroism (CD), and wide-angle X-ray scattering (WAXS). Molecular solutions of these mono-, di-, and trihydroxyl substituted bile acids spontaneously evolved into supramolecular aggregates upon the incremental addition of H(2)O as a poor solvent. Highly crystalline nanostructured multilayered assemblies were formed, which revealed a very rich polymorphism of micro- and macro-structures depending on the chemical structure of the bile acid and the properties of the cosolvent (EtOH or DMSO) used. In particular, AFM allowed resolving the crystalline structure to an unprecedented level. It was thus possible to establish that bile acids associate into H-bonded chiral dimer building blocks, which organize in 2D layers of nanostructured lamellar surface topologies with unique facial amphiphilicity. The detailed understanding of the hierarchical organization in bile acid assemblies may contribute to develop strategies to design bioinspired materials with tailor-made nanostructured surface topologies.

Cell culture models for studying the development of Barrett's esophagus: a systematic review

Background

Barrett’s esophagus (BE) is a premalignant condition caused by chronic gastroesophageal reflux. BE patients have an increased risk of developing esophageal adenocarcinoma (EAC). As many aspects of this condition are still unknown, there is a need for in vitro models to study BE development.

Aim

To review the literature on cell lines and incubation conditions for studying BE development.

Methods

A literature search was performed using PubMed, EMBASE and the Cochrane library, combining the words esophagus, cell line, culture, Barrett’s, bile, acid, exposure, reflux and adenocarcinoma.

Results

A wide range of cell lines and incubation conditions to study BE development have been reported. The most commonly used cell lines are derived from epithelium from patients with BE or EAC. A 25-minute incubation with 200 μM bile salts induced cell proliferation and Akt phosphorylation. However, increased CDX2 and MUC2 expression was only observed with longer incubations or higher bile salt concentrations. Two-hundred μM bile at pH 6 showed a higher toxicity to EAC cells than the same concentration at pH 7. Multiple 5-minute exposures with 200 μM bile at pH 4 or pH 7 increased CK8/18 and COX2 in BE epithelial cells.

Conclusions

Two-hundred μM conjugated primary or secondary bile salts at pH 4 for multiple short exposures is able to induce BE specific factors in BE cell lines. In SQ and EAC cell lines; however, higher concentrations of secondary bile salts for 8 h are needed to induce BE specific molecules. Due to the high variability in reported methods, it is difficult to determine the most effective in vitro setup for studying the development of BE.

Effects of bile salts on propranolol distribution into liposomes studied by capillary electrophoresis

The objective of this study was to study the effect of four different bile salts, cholate (C), deoxycholate (DC), taurocholate (TC), monoketocholate (MKC), on the membrane binding of a cationic model drug, propranolol, using capillary electrophoresis. The apparent distribution coefficient of propranolol in a buffer/liposome system, in the absence and presence of various concentrations of the bile salts, was measured using capillary electrophoresis frontal analysis. At bile salt concentrations which did not disrupt the liposomes, the bile salts increased the apparent distribution coefficient of propranolol in a concentration-dependent manner, to various extents (DC>C>TC>MKC). The mechanisms for these increases were inferred from studies of ion pairing between bile salts and propranolol using mobility shift affinity capillary electrophoresis and from zeta potential measurements. The bile salts ion-paired with propranolol to different extents as indicated by the estimated complexation constants (K range: 30-58 M(-1)). This was found to have a minor effect on the membrane distribution of propranolol only. The major effect is proposed to be due to the insertion of bile salt into the liposomal membranes leading to a more negatively charged membrane surface thereby providing stronger electrostatic interactions with the positively charged propranolol.

Discovery of 6alpha-ethyl-23(S)-methylcholic acid (S-EMCA, INT-777) as a potent and selective agonist for the TGR5 receptor, a novel target for diabesity

In the framework of the design and development of TGR5 agonists, we reported that the introduction of a C(23)(S)-methyl group in the side chain of bile acids such as chenodeoxycholic acid (CDCA) and 6-ethylchenodeoxycholic acid (6-ECDCA, INT-747) affords selectivity for TGR5. Herein we report further lead optimization efforts that have led to the discovery of 6alpha-ethyl-23(S)-methylcholic acid (S-EMCA, INT-777) as a novel potent and selective TGR5 agonist with remarkable in vivo activity.

Detoxification of lithocholic acid, a toxic bile acid: relevance to drug hepatotoxicity

Lithocholic acid, a monohydroxy, secondary bile acid, is formed by bacterial 7-dehydroxylation of the primary bile acid chenodeoxycholic acid (CDCA) and of the secondary bile acid ursodeoxycholic acid (UDCA). Lithocholic acid and its precursor CDCA are toxic when fed to the rabbit, rhesus monkey, and baboon, but not when CDCA, as well as UDCA, is used for therapeutic purposes in man. Older studies showed that the species specific toxicity of lithocholic acid could be explained by efficient sulfation of lithocholic acid in man and in chimpanzee, but not in the rabbit, rhesus monkey, or baboon. Rodents detoxify lithocholic acid by hydroxylation, but this does not occur in species in which it is toxic. Recent studies suggest that lithocholic acid induces its own detoxification by activating nuclear receptors to promote transcription of genes encoding sulfotransferase. In addition, work with CaCo2 cells suggest that lithocholic acid may undergo sulfation in the enterocyte and be effluxed back into the intestinal lumen. The evolution of trihydroxy bile acids in vertebrates may have occurred to decrease the formation of lithocholic acid. Lithocholic acid is a rare example of a toxic endobiotic; a variety of mechanisms have evolved to solve the problem of efficient detoxification.

Physicochemical and physiological properties of 5alpha-cyprinol sulfate, the toxic bile salt of cyprinid fish

5alpha-Cyprinol sulfate was isolated from bile of the Asiatic carp, Cyprinus carpio. 5alpha-Cyprinol sulfate was surface active and formed micelles; its critical micellization concentration (CMC) in 0.15 M Na+ using the maximum bubble pressure device was 1.5 mM; by dye solubilization, its CMC was approximately 4 mM. At concentrations >1 mM, 5alpha-cyprinol sulfate solubilized monooleylglycerol efficiently (2.1 molecules per mol micellar bile salt). When infused intravenously into the anesthetized rat, 5alpha-cyprinol sulfate was hemolytic, cholestatic, and toxic. In the isolated rat liver, it underwent little biotransformation and was poorly transported (Tmax congruent with 0.5 micromol/min/kg) as compared with taurocholate. 5alpha-Cyprinol, its bile alcohol moiety, was oxidized to its corresponding C27 bile acid and to allocholic acid (the latter was then conjugated with taurine); these metabolites were efficiently transported. 5alpha-Cyprinol sulfate inhibited taurocholate uptake in COS-7 cells transfected with rat asbt, the apical bile salt transporter of the ileal enterocyte. 5alpha-Cyprinol had limited aqueous solubility (0.3 mM) and was poorly absorbed from the perfused rat jejunum or ileum. Sampling of carp intestinal content indicated that 5alpha-cyprinol sulfate was present at micellar concentrations, and that it did not undergo hydrolysis during intestinal transit. These studies indicate that 5alpha-cyprinol sulfate is an excellent digestive detergent and suggest that a micellar phase is present during digestion in cyprinid fish.

Influence of pH on the phase distribution of nascent deoxycholic acid in fresh human cecal aspirates

Prolonged large bowel transit time and an associated increase in the proportion of deoxycholic acid (DCA) in serum and bile have been implicated in the development of cholesterol-rich gallstones and colon cancer. Prolongation of intestinal transit also increases intracolonic pH that, we hypothesized, should favor the solubilization and absorption of newly formed DCA within the colon. To test this hypothesis, we performed in vitro studies on homogenized cecal aspirates (obtained at colonoscopy) that were incubated anaerobically with [14C]cholic acid for 16 h after which the pH was adjusted to between 4.0 and 7.0 in 0.5-pH unit steps. The resultant reaction mixtures were centrifuged to separate the supernatant from the precipitate, and the specific activity of [14C]DCA was quantitated in both phases. As the pH in the aspirates was manipulated from 4.0 to 7.0, the proportion of newly formed, labeled DCA increased in the supernatant and fell in the precipitate, particularly at a hydrogen ion concentration of <100 x 10(-7) (equivalent to pH 5.0-7.0). These results show that the solubility of DCA in colonic contents increases with increasing pH. If solubility is rate limiting, this should lead to increased absorption that, in turn, would explain why the proportion of DCA in serum and bile increases with the prolongation of large bowel transit time.

Dietary fibre, physicochemical properties and their relationship to health

Dietary carbohydrates that escape digestion and absorption in the small intestine include non-digestible oligosaccharides (carbohydrates with a degree of polymerisation between three and ten), resistant starch and non-starch polysaccharides. The physiological effects of this heterogeneous mixture of substrates are partly predictable on the basis of their physicochemical properties. Monosaccharide composition and chain conformation influence the rate and extent of fermentation. Water-holding capacity affects stool weight and intestinal transit time. Viscous polysaccharides can cause delayed gastric emptying and slower transit through the small bowel, resulting in the reduced rate of nutrient absorption. Polysaccharides with large hydrophobic surface areas have potentially important roles in the binding of bile acids, carcinogens and mutagens. Ispaghula is capable of binding bile acids through a large number of weak binding sites on the polysaccharide structure, and having greatest effect on the potentially more harmful secondary bile acids deoxycholic acid and lithocholic acid.

Relationship between structure and intestinal absorption of bile acids with a steroid or side-chain modification

A structure-activity relationship for bile acid (BA) intestinal absorption is known to exist. To better understand the BA structural requirements for optimal BA intestinal absorption, rabbit ileal perfusion studies were performed. Unconjugated BA: Ursodeoxycholic (UDCA) and ursocholic acid (UCA) with methyl (6MUDCA and 6MUCA) or fluoro group (6FUDCA and 6FUCA) in the 6 position and UCA with a methyl group in 23 position (23MUCA) were compared with unconjugated UDCA, UCA, deoxycholic (DCA), chenodeoxycholic (CDCA), hyocholic (HCA), and hyodeoxycholic (HDCA) acid. BA lipophilicity was evaluated by their octanol-water partition coefficient. Conjugated BA: Taurine-conjugated UDCA and UCA with a methyl group in the 23 position (T23MUDCA and T23MUCA) were compared with the corresponding taurine-conjugated natural analogs. An analog of glycine-conjugated UDCA with the C24 amide bond replaced by a -CO-CH2- in the 24 position (24PUDCA) was studied and results were compared with the natural form (GUDCA). Unconjugated BA absorption was dose dependent (i.e., passive) and followed their lipophilicity: DCA > 6MUDCA > CDCA > HDCA > UDCA > HCA > 6FUDCA > 6MUCA > 6FUCA > UCA. Conjugated BA absorption was active, and Vmax was in the following order: TCA > TUDCA > TUCA > T23MUCA > T23MUDCA > 24PUDCA > GUDCA. 24PUDCA transport was also active and higher than GUDCA.

CONCLUSION

Passive transport is dependent on BA lipophilicity. Conjugated BAs are actively transported, and the presence of a 23-C methyl group does not improve transport when compared with the natural analogs. The substitution of the C24 amide bond with a -CO-CH2-still affords interaction of the BA with the intestinal transport carrier.

Metabolism, pharmacokinetics, and activity of a new 6-fluoro analogue of ursodeoxycholic acid in rats and hamsters

BACKGROUND/AIMS

The effectiveness of ursodeoxycholic acid in treating biliary liver diseases is limited by low bioavailability and moderate activity. A new analogue of ursodeoxycholic acid was synthesized with a fluorine atom in position 6 because this should have resulted in an analogue more hydrophilic than ursodeoxycholic acid but with similar detergency.

METHODS

After synthesis, detergency, solubility, and lipophilicity of the 6-fluoro analogue in aqueous solution were determined and compared with those of natural analogues. Stability toward 7-dehydroxylation was assessed in human stools, pharmacokinetics and metabolism were evaluated in bile fistula rats and hamsters, accumulation in bile with long-term feeding was assessed in the hamsters, and the ability to prevent the hepatotoxic effects of taurochenodeoxycholic acid was evaluated in bile fistula rats after intraduodenal coinfusion.

RESULTS

6-Fluoro-ursodeoxycholic acid was more stable than its parent molecule toward 7-dehydroxylation, it was efficiently secreted in bile, and its total recovery was very high. With long-term administration of 6-fluoro-ursodeoxycholic acid, taurine and glycine amidates accounted for more than 60% of the total biliary bile acids (15% ursodeoxycholic acid). The 6-fluoro analogue prevented the hepatotoxic effects of taurochenodeoxycholic acid.

CONCLUSIONS

The results suggest that 6-fluoro-ursodeoxycholic acid has considerable potential as a pharmaceutical agent in the treatment of cholestatic liver disease.

Pan-sulfation of bile salts markedly increases hydrophilicity and essentially abolishes self- and hetero-association with lecithin

In chronic liver disease, partially and to a lesser extent completely (pan-)sulfated common bile salts are synthesized, yet little information is available concerning their physical-chemical characteristics. We studied solution properties of pan-sulfated common free, taurine and glycine-conjugated bile salts, and the interactions of taurodeoxycholate di-sulfate (TDC-S) with lecithin. By reverse-phase HPLC, pan-sulfated glycine and taurine-conjugated bile salts were very hydrophilic, with hydrophobic indices 1.7 to 2.5 units lower than their non-sulfated congeners. In contrast to non-sulfated species, pan-sulfated free and glycine-conjugated bile salts produced simple potentiometric titration curves without precipitation of bile salt below the pK'A of the carboxylic acids. By quasi-elastic light scattering, critical micellar concentrations of TDC-S fell from 28 mM in 0.15 M NaCl to 3 mM in 4.0 M NaCl, a value slightly higher than that of TDC. TDC-S formed very small micelles (hydrodynamic radii approx. 11A) that, in contrast to TDC, did not grow with increases in bile salt (7-66 mM) or NaCl (0.15-2.0 M) concentrations. TDC-S formed mixed micelles with lecithin in 0.15 M NaCl, but with a micellar zone drastically reduced compared with that of the non-sulfated congener. However, in 4 M NaCl, the micellar zone of TDC-S expanded and approached that of the non-sulfated parent compound. Therefore, under physiological conditions, pan-sulfation of common bile salts should largely eliminate their capacity to form mixed micelles with membrane lipids.

Physical and biological properties of fluorescent dansylated bile salt derivatives: the role of steroid ring hydroxylation

The hydroxyl groups of bile salts play a major role in determining their physical properties and physiologic behavior. To date, no fluorescent bile salt derivatives have been prepared which permit evaluation of the functional role of the steroid ring. We have prepared five fluorescent cholanoyl derivatives using a dansyl-ethylene diamine precursor linked to the sulfonyl group of taurine; N-(5-dimethylamino-1-naphthalenesulfonyl)-N'-(2-aminoethanesulf onyl)- ethylenediamine. The fluorescent dansyl-taurine was conjugated to the carboxyl group of free bile acids, enabling the labeling of the series: dehydrocholate, ursodeoxycholate, cholate, chenodeoxycholate and deoxycholate. Despite a systematic hydrophobic shift compared with the native bile salts (aqueous solubility and water:octanol partitioning), the influence of steroid ring hydroxylation was retained, with the dehydrocholate and cholate derivatives more water soluble than the dihydroxy derivatives. Similarly, the sequence of HPLC mobilities, reflecting relative hydrophilicity, was identical in the dansyl-taurine derivatives and the native taurine-conjugated bile salts. Cellular uptake of all five steroid derivatives was rapid, and partial inhibition of [3H]taurocholate uptake was observed in isolated hepatocytes. Rates of biliary excretion of the dansylated derivatives by the isolated perfused rat liver correlated closely with hydrophilicity. Collectively, these findings indicate that the influence of the hydroxyl groups is retained in this series of dansylated steroids, and that hydroxylation is a key determinant of their hepatocellular transport and biliary excretion. These fluorescent bile salt derivatives may thus serve as unique probes for investigating structure-function relationships in hepatic processing of steroid-based compounds.

Quantitative structure-antimicrobial activity relationship in 5 beta-cholanyl-24-benzylamine derivatives

Some representative physicochemical properties of benzylamido and amino derivatives of common bile acids have been determined and correlated with their antimicrobial activity against gram-positive bacterial strains. Steroid hydroxyls do not affect the basicity of amino derivatives; they promote solubility in a parallel way to unconjugated bile acids and mainly control hydrophobicity of this class of compounds as measured by log P values. Activity was correlated to hydrophobicity; that is, the nature of the side chain modulated activity, affected basicity, and facilitated changes in partition ability. Benzylamino derivatives proved to be even more active than the corresponding amides when ionization is taken into account. Trihydroxy derivatives possess the lowest log P values and were practically inactive. Decreased activity was also observed in those cases where, due to the orientation of the hydroxy group in the 6 or 7 position, the back beta face of the molecule had a reduced hydrophobic surface area. Antimicrobial activity, in terms of -log MIC (minimal inhibitory concentration), was found to correlate linearly with log P values of uncharged species. This linear relationship is discussed with respect to the structure of the steroid moiety and the ability of these molecules to cross cellular membranes.

Characterizing mechanisms of hepatic bile acid transport utilizing isolated membrane vesicles

Utilizing the above-outlined approaches, mechanisms of hepatic bile acid transport have been characterized in membrane vesicles of rat liver, particularly for the conjugated trihydroxy bile acid, taurocholic acid. Uptake across the sinusoidal membrane is carrier mediated and coupled to the transmembrane sodium gradient. This carrier has an apparent Km between 30 to 50 microM and a Vmax between 4 to 6 nmol mg-1 protein min-1. Furthermore, Na+ gradient-dependent sinusoidal uptake of taurocholate can be stimulated by low concentrations of albumin. There is controversy as to whether the process is electrogenic. Although photoaffinity labeling studies indicate that an additional carrier for Na(+)-dependent bile acid uptake is also present at the sinusoidal membrane, this carrier has so far not been characterized in membrane vesicles. The proposition that pH gradient-driven furosemide-sensitive cholic acid uptake into sinusoidal membrane vesicles may represent carrier-mediated hydroxyl/cholate exchange must be revised on the basis of the recent findings that (1) true initial uptake rates are not saturable; (2) pH gradient-driven cholate uptake is also found in liposomes; and (3) furosemide also inhibits pH gradient-driven cholate uptake in liposomes. The mechanisms of transcellular transport of bile acids have been studied less extensively, but Na(+)-independent carrier-mediated taurocholic acid transport has been demonstrated in purified subcellular fractions such as rat liver microsomes and Golgi membranes. Finally, transport studies in canalicular rat liver plasma membrane vesicles indicate that canalicular excretion of bile acids is also a carrier-mediated process that may be driven, at least in part, by the physiologic electrical potential gradient, and that preferentially transports trihydroxy and conjugated dihydroxy bile acids.

Structure-activity relationship studies on natural and synthetic bile acid analogs

The objective of our research was to develop ursodiol analogs that are structurally modified to modulate hepatic side-chain amidation and prevent 7-dehydroxylation by intestinal bacteria while at the same time maintaining the critical micellar concentration (CMC) and hydrophilicity of ursodiol. More than 20 naturally occurring bile acids were screened for physicochemical properties. Then, two generations of analogs were studied, and those with physicochemical properties similar to ursodiol's were analyzed for physiologic properties. The first generation of analogs included molecules with steric and/or electronic hindrance on the side chain; the second group consisted of the same molecules conjugated with glycine or taurine and also "pseudoconjugated" analogs (23-hydroxylated, esterified, and amidated with other amino acids). Of the first-generation analogs, only cyclopropane D derivative and trans-olefin were useful to our purposes, being conjugated by the liver and almost completely recovered in bile. These two analogs were deconjugated and 7-dehydroxylated but with slower kinetics. The hydrophilicity of the molecules could be augmented by increasing the polarity of the steroid ring. Among the pseudoconjugated analogs, the CMC values were similar to those of the natural analogs, although hydrophobicity differed among the group. The analogs that were not deconjugated were not 7-dehydroxylated either. All of the pseudoconjugated bile acids were efficiently taken up by the liver, and their recovery in bile was similar to that of glycine and taurine ursodiol. From these studies we now know that side chain configuration and conformation are important in the conjugation and deconjugation processes. Mild modification of the side chain can prevent 7-dehydroxylation and thus yield a bile acid more resistant to intestinal bacteria and more bioavailable. Prevention of hepatic conjugation improves biliary secretion and recovery of many analogs.

beta-Muricholic acid; potentiometric and cholesterol-dissolving properties

Some physicochemical properties of beta-muricholic acid (3 alpha,6 beta,7 beta-trihydroxy-5 beta-cholanic acid), a major bile acid biosynthesized by rat liver, were determined and compared to those of ursodeoxycholic and chenodeoxycholic acids. From potentiometric studies, the following characteristics of beta-muricholic acid were shown: a low monomer solubility (13 microM), a high equilibrium precipitation pH (7.92 for 30 mM solution), an apparent critical micellar concentration of 4 mM, and a very low micellar capacity of the bile salt to dissolve the protonated bile acid. Sodium beta-muricholate solution (30 mM) poorly solubilized cholesterol, as indicated by a bile salt/cholesterol molar ratio of 1430, whereas saturation ratios obtained with chenodeoxycholate and ursoseoxycholate were 24 and 384, respectively. Sodium beta-muricholate (30 mM)/phosphatidylcholine/cholesterol mixtures contained non-micellar aggregates from very low cholesterol concentrations. At physiological phosphatidylcholine concentrations, sodium beta-muricholate (100 mM) dissolved cholesterol crystals via essentially lamellar liquid-crystal formation. These solubilizing properties might have important physiological relevance to the dissolution of cholesterol gallstones in man.

Effect of ursocholic acid on bile lipid secretion and composition

To further clarify the relationship between physical-chemical characteristics of bile acids and biliary lipid secretion, we investigated the effect of ursocholic acid, the 7 beta-hydroxyepimer of cholic acid, on bile lipid secretion and composition. The study included acute duodenal infusion (1 g/h for 5 h) of ursocholic acid contrasted with a less hydrophilic bile acid, ursodeoxycholic acid, in 3 T-tube patients and short-term oral administration (2 wk) of ursocholic acid (10-15 mg/kg X day) to 10 gallstone patients. Following acute infusion, ursocholic acid, similarly to ursodeoxycholic acid, accounted for greater than 80% of the biliary bile acids. However, ursocholic acid induced (per micromole of secreted bile acid) a significantly lower (p less than 0.01) secretion of cholesterol (0.013 mumol) and phospholipids (0.054 mumol) than that induced by ursodeoxycholic acid (0.034 mumol of cholesterol and 0.138 mumol of phospholipids). Biliary alkaline phosphatase activity during ursocholic acid administration was significantly lower (p less than 0.01) than during ursodeoxycholic acid administration. After short-term oral administration, ursocholic acid, undetectable before treatment, constituted 20.50% +/- 8.60% of the biliary bile acids. The percentage of deoxycholic acid increased from 32.35% +/- 18.79% to 47.53% +/- 16.19% (p less than 0.05). Mean saturation index decreased from a pretreatment value of 1.23 +/- 0.22 to 0.99 +/- 0.17 (p less than 0.05), but only in 4 of 10 subjects did bile become undersaturated. It is concluded that ursocholic acid, due to its higher hydrophilicity, stimulates a lower cholesterol and phospholipid output than ursodeoxycholic acid. Consequently, despite the low enrichment of the biliary bile acids with ursocholic acid, oral administration of ursocholic acid induces a reduction of bile cholesterol saturation.