Physical Chemistry of Bile

Fabrication of Molecular Blocks with High Responsiveness to the Cancer Microenvironment by Ursodeoxycholic Acid

In cancer therapy, a drug delivery system (DDS) has been widely studied to achieve selective drug accumulation at the tumor site. However, DDS still has a major drawback in that it requires multistep processes for intracellular delivery, resulting in low efficiency of drug delivery. To overcome this problem, we recently reported a molecular block (MB) that disrupts cancer cell membranes in the cancer microenvironment using deoxycholic acid (DCA). However, the MB showed considerable cytotoxicity even at neutral pH, possibly due to the structural hydrophobic property of DCA. Herein, we focused on selecting the most suitable bile acid for an MB that possessed high responsiveness to the cancer microenvironment without cytotoxicity at neutral pH. Cell viabilities of the free bile acids such as DCA, chenodeoxycholic acid (CDCA), cholic acid (CA), and ursodeoxycholic acid (UDCA) were evaluated at neutral pH (pH = 7.4) and a cancer acidic environment (pH = 6.3-6.5). The half-maximal inhibition concentration (IC₅₀) value of UDCA at pH = 7.4 showed an approximately 7.5-fold higher IC₅₀ value than that at pH = 6.3, whereas the other bile acids yielded less than a 4-fold IC₅₀ value difference between the same pHs. Biocompatible poly(vinyl alcohol) (PVA) was functionalized with UDCA (PVA-UDCA) for the synthesis of higher responsiveness to the cancer microenvironment without cytotoxicity at neutral pH. Importantly, 56% pancreatic cancer cell death was observed at pH = 6.5, whereas only 10% was detected at neutral pH by the PVA-UDCA treatment. However, PVA-DCA indicated almost the same cancer cell death property, independent of pH conditions. These results suggest PVA-UDCA shows great potential for a new class of MB.

Adsorption and Micellization Behavior of Mixtures of Amphiphilic Drugs with Small Amounts of Bile Salts

Bile acid salts are the most important biological surfactant-like molecules. These compounds play an active role in the cholesterol and lipid solubilization. They are also used as drug carriers. The salts give anionic amphiphilic moiety in aqueous solution and will interact strongly when mixed with cationic amphiphiles. With this aim we have investigated mixed systems of three bile acid salts (sodium cholate (NaC), sodium deoxycholate (NaDC) and sodium taurocholate (NaTC)) with two cationic drugs (adiphenine hydrochloride (ADP) and clomipramine hydrochloride (CLP)) by surface tension measurements. It was found that the two components mix nonideally, i.e., the cmc values of the mixtures decrease with the stoichiometric mole fraction (α1) of bile salts and are lower than cmc* values (cmc at ideal mixing condition). The interaction parameters, βm and βσ, come out to be negative and large in magnitude which is obvious from the fact that the two components are of opposite charge and would experience attractive interactions. The results suggest that the contribution of bile salts in the mixed micelles (X1m) is greater than both the α1 and X1id (the contribution in ideal mixing condition). These components also form mixed interface where the contribution of bile salts is more than α1 (i.e., X1σ > α1). The minimum area occupied by a monomer decreases with increase in the content of bile salts in the solution. This confirms that the opposite charges on the two components decrease the repulsion among the head groups. All the evaluated thermodynamic parameters (i.e., ΔG0m, ΔGex, ΔG°ads and Gmin) support the above explanations.

Counterion binding in the aqueous solutions of bile acid salts, as studied by computer simulation methods

We investigate the structural and dynamical properties of counterion binding in sodium cholate and sodium deoxycholate micelles at three different concentration, namely, 30, 90, and 300 mM, by means of molecular dynamics simulations at the atomistic level. The obtained results can resolve a long-standing, apparent contradiction between different experiments that reported discordant values for the degree of counterion binding. Namely, our results suggest that certain experimental techniques, such as freezing point depression, are only sensitive to the contact counterions, and hence, the degree of contact binding of the counterions is measured. On the other hand, in experiments employing, e.g., electrode potential or nuclear magnetic resonance measurements, the solvent-separated counterions also contribute to the signal detected, and hence, the counterions that are measured as bound ones do include the solvent-separated counterions as well.

Morphology of bile salt micelles as studied by computer simulation methods

The relative arrangement of the neighboring bile ions and the shape of the hydrophobic and hydrogen-bonded primary micelles as well of the large secondary micelles formed by these ions are analyzed in detail on the basis of molecular dynamics computer simulations of 30 and 300 mM sodium cholate and sodium deoxycholate solutions. In the lower concentration considered, the systems only contain primary micelles, whereas in both of the 300 mM systems secondary micelles are also present. The simulations performed were long enough that the systems reached thermodynamic equilibrium. It is found that the neighboring cholate ions prefer alignments in which their quasi-planar tetracyclic ring systems are parallel with each other, whereas for deoxycholate an opening of the angle between these planes is observed. The shape of the micelles is characterized by the ratio of their three principal moments of inertia. The primary deoxycholate micelles are found to be rather spherical, whereas in the case of cholate somewhat flattened, disklike or oblate shaped ellipsoidal primary micelles are found, irrespective of whether these micelles are kept together by hydrogen bonds or are of hydrophobic origin. Finally, the secondary micelles are found to exhibit a large variety of shapes, ranging from flattened oblates to rodlike objects through various different irregular shapes, characterized by markedly different values of the three principal moments of inertia. The observed preferences of the relative arrangement of the neighboring ions and of the aggregate shapes as well as the differences observed in the behavior of the two bile ions studied in these respects are traced back to the molecular structure of these ions.

Molecular Aggregates in Aqueous Solutions of Bile Acid Salts. Molecular Dynamics Simulation Study

The aggregation behavior of two bile acid salts (i.e., sodium cholate and sodium deoxycholate) has been studied in their aqueous solutions of three different concentrations (i.e., 30, 90,and 300 mM) by means of molecular dynamics computer simulations. To let the systems reach thermodynamic equilibrium, rather long simulations have been performed: the equilibration period, lasting for 20-50 ns, has been followed by a 20 ns long production phase, during which the average size of the bile aggregates (regarded to be the slowest varying observable) has already fluctuated around a constant value. The production phase of the runs has been about an order of magnitude longer than the average lifetime of both the monomeric bile ions and the bonds that link two neighboring bile ions together to be part of the same aggregate. This has allowed the bile ions belonging to various aggregates to be in a dynamic equilibrium with the isolated monomers. The observed aggregation behavior of the studied bile ions has been found to be in good qualitative agreement with experimental findings. The analysis of the results has revealed that, due to their molecular structure, which is markedly different from that of the ordinary aliphatic surfactants, the bile ions form rather different aggregates than the usual spherical micelles. In the lowest concentration solution studied, the bile ions only form small oligomers. In the case of deoxycholate, these oligomers, such as the ordinary micelles, are kept together by hydrophobic interactions, whereas in the sodium cholate system, small hydrogen-bonded aggregates (mostly dimers) are also present. In the highest concentration systems, the bile ions form large secondary micelles, which are kept together both by hydrophobic interactions and by hydrogen bonds. Namely, in these secondary micelles, small hydrophobic primary micelles are linked together via the formation of hydrogen bonds between their hydrophilic outer surfaces.

Is there a link between oestrogen therapy and gallbladder disease?

Cholelithiasis is the most common form of benign gallbladder disease that results in major heath expenditure. Female sex hormones are causally related to cholesterol gallstone disease, which are more common in women than in men. The risk of development of cholelithiasis is further enhanced by the use of exogenous female sex hormones and by pregnancy. Oestrogens are used in oral contraceptives and in hormone replacement therapy (HRT). Oral contraceptives do not pose a greater risk for gallbladder disease. The findings from two randomised, controlled trials, the Heart and Oestrogen/Progestin Replacement Study and the Women's Health Initiative postmenopausal hormone trial, unequivocally confirm that oral oestrogen use in postmenopausal women is causally associated with gallbladder disease, and the magnitude of the effect is not influenced greatly by the presence or absence of progestins. A cautious approach should be observed when prescribing HRT. Women must be informed about the effect of oestrogen use on increased risk of benign gallbladder disease. HRT should be used in the lowest possible dose for the shortest possible time. Women harbouring asymptomatic gallstones should not receive oestrogens because of the possibility of developing cholecystitis.

Study of capsular polysaccharide from Vibrio parahaemolyticus

The leading cause of food poisoning in both Taiwan and Japan is Vibrio parahaemolyticus infection, whose mechanism of enteropathogenesis is still unclear. To evaluate whether surface components are responsible for the intestinal adhesion of V. parahaemolyticus, we have developed a novel method for isolating the capsular polysaccharide (CPS) from V. parahaemolyticus (serotype O4:K8). We found that culturing of V. parahaemolyticus in broth for 1 week or more changed the colony form of the bacteria on an agar plate from opaque to translucent. The translucent colonies of V. parahaemolyticus contained little CPS and exhibited a much lower level of adherence to epithelial cells (Int-407) than the opaque colonies of the bacteria. Incubation of V. parahaemolyticus in medium supplemented with bile increased the levels of CPS and adherence. Treatment of V. parahaemolyticus with anti-CPS but not anti-LPS serum decreased the level of bacterial adherence. In addition, purified CPS bound to epithelial cells in a dose-dependent manner. Intranasal administration of CPS to mice in the presence of adjuvants such as immunostimulatory sequence oligodeoxynucleotides or cholera toxin elicited CPS-specific mucosal and systemic immune responses. These results indicate that CPS plays an important role in the adherence of V. parahaemolyticus to its target cells and may be considered a potential target for the development of a vaccine against this pathogen.

Micelle formation of sodium deoxycholate and sodium ursodeoxycholate (part 1)

Micellization of sodium deoxycholate (NaDC) and sodium ursodeoxycholate (NaUDC) was studied for the critical micelle concentration (CMC), the micelle aggregation number, and the degree of counterion binding to micelle, where sodium cholate (NaC) was used as a reference. The fluorescence probe technique of pyrene was employed to determine accurately the CMC values for the bile salts, which indicated that a certain concentration range of CMC and a stepwise aggregation for micellization were reasonable. The temperature dependences of micellization for NaDC and NaUDC were studied at 288.2, 298.2, 308.2, and 318.2 K by aqueous solubility change with solution pH. Aggregations of the bile salt anions were analyzed using the stepwise association model and found to grow in size with increasing concentration, which confirmed that the mass action model worked quite well. The average aggregation number was found to be 2.5 (NaUDC) and 10.5 (NaDC) at the concentration of 20 mM and at 308.2 K. The aggregation number determined by static light scattering also agreed well with those by the solubility method in the order of size: NaUDC<NaC<NaDC at 308.2 K. The results indicated that the location of the OH group at C-7 and its orientation were the most important factors from the viewpoint of chemical structure for the growth of micelles. The activity measurement for sodium ions was made by a sodium ion selective electrode in order to confirm the low counterion binding to micelles and the validity of the present association model of bile salts, but the model did not hold good for NaDC at higher concentrations.

Solubilization of cholesterol and polycyclic aromatic compounds into sodium bile salt micelles (part 2)

The aqueous solubility of cholesterol was determined over the temperature range from 288.2 to 318.2 K with intervals of 5 K by the enzymatic method. The solubility was (3.7+/-0.3)x10(-8) mol dm(-3) (average +/- S.D.) at 308.2 K. The maximum additive concentrations of cholesterol into the aqueous micellar solutions of sodium deoxycholate (NaDC), sodium ursodeoxycholate (NaUDC), and sodium cholate (NaC) were spectrophotometrically determined at different temperatures. The cholesterol solubility increased in the order of NaUDC<NaC<NaDC; for example, 0.10 for NaUDC, 0.61 for NaC, and 2.99 mmol dm(-3) for NaDC at the concentration of 60 mmol dm(-3) and at 308.2 K. The same solubilization experiments were made at 308.2 K using polycyclic aromatic compounds (benzene, naphthalene, anthracene, pyrene) as a reference. Their solubility increase for the bile salts was in the same order as above. Thermodynamic analysis was made for the solubilization, where a micelle was regarded as a chemical species. The average number of solubilizate per micelle was less than unity throughout the experiments. From the Gibbs energy change for solubilization at the different mean aggregation numbers, the function of bile salt micelles was discussed from the viewpoint of molecular structure of solubilizates. The DeltaG(0) value for cholesterol was most negative among the solubilizates studied, which reflected that solubilization of cholesterol into bile salt micelles brought about largest thermodynamic stabilization.