Synthesis of LJP 993, a multivalent conjugate of the N-terminal domain of beta2GPI and suppression of an anti-beta2GPI immune response

LJP 993, a tetravalent conjugate of the amino-terminal domain (domain 1) of beta2GPI, was synthesized, and studies were carried out to explore the ability of LJP 993 to bind anti-beta2GPI antibodies and to function as a B cell toleragen. Domain 1 was expressed in Pichia pastoris, and the N-terminus was site-specifically modified by a transamination reaction converting the N-terminal glycine to a glyoxyl group. A tetravalent platform was synthesized with linkers that terminate in aminooxy groups. This was accomplished by preparing an ethylene glycol-based heterobifunctional linker that contains both a Boc-protected aminooxy group and a free primary amine. The linker was used to modify a tetravalent platform molecule by reacting the amino groups on the linker with 4-nitrophenyl carbonate esters on the platform to provide a linker-modified platform, and the Boc protecting groups were removed to provide a tetravalent aminooxy platform. Glyoxylated domain 1 was attached to the platform to provide LJP 993 by formation of oxime bonds. The protein domains of LJP 993 retain activity as evidenced by the ability of LJP 993 to bind to anti-beta2GPI antibodies. Dissociation constants (Kd) for domain 1 and LJP 993 bound to immobilized affinity-purified anti-beta2GPI antibodies from autoimmune thrombosis patients were determined using surface plasmon resonance. An immunized mouse model was developed to test the ability of LJP 993 to act as a toleragen. A thiol containing domain 1 analogue was expressed in insect cells using the baculovirus expression system, and it was used to prepare an immunogenic conjugate of domain 1 and maleimide-derivatized keyhole limpet hemocyanin (KLH). Mice were immunized with the KLH conjugate, and spleen cells were harvested from the immunized mice. The cells were incubated with various concentrations of LJP 993 and transferred to mice whose immune systems had been compromised by irradiation. The hosts were then boosted with the KLH-domain 1 conjugate, and after 7 days their antibody levels were measured. Host mice receiving cells that were treated with LJP 993 produced significantly lower amounts of anti-domain 1 antibodies than controls which received untreated cells, indicative of B cell tolerance.

Use of a fluorescent bile acid to enhance visualization of the biliary tract and bile leaks during laparoscopic surgery in rabbits

BACKGROUND

We set out to determine whether intravenously administered cholylglycylaminofluorescein (CGF), a fluorescent bile acid, would enhance the visualization of the biliary tract and bile leaks in rabbits undergoing laparoscopic cholecystectomy (LC).

METHODS

CGF was infused at doses of 1, 5, and 10 mg/kg b.w. Biliary recovery was determined spectrophotometrically (six rabbits). For LC (seven rabbits), a blue (fluorescein) filter was attached to the light source, and a fluorescein-emission filter was attached to the charge coupled device (CCD) camera. The biliary tract and bile leak (made by incising the gallbladder) was observed under standard and fluorescent illumination.

RESULTS

Apple-green fluorescence appeared in 2 min and persisted for 30-60 min, enhancing visualization of bile duct anatomy as well as the bile leak. Biliary recovery of CGF at 90 min was high (86-96% of the infused dose).

CONCLUSION

In rabbits, CGF is secreted quantitatively in bile, induces biliary fluorescence, and enhances visualization of the bile ducts and bile leaks when viewed with appropriate filters.

Sulindac is excreted into bile by a canalicular bile salt pump and undergoes a cholehepatic circulation in rats

BACKGROUND & AIMS

Dihydroxy bile acids induce a bicarbonate-rich hypercholeresis when secreted into canalicular bile in unconjugated form; the mechanism is cholehepatic shunting. The aim of this study was to identify a xenobiotic that induces hypercholeresis by a similar mechanism.

METHODS

Five organic acids (sulindac, ibuprofen, ketoprofen, diclofenac, and norfloxacin) were infused into rats with biliary fistulas. Biliary recovery, bile flow, and biliary bicarbonate were analyzed. Sulindac transport was further characterized using Tr(-) rats (deficient in mrp2, a canalicular transporter for organic anions), the isolated perfused rat liver, and hepatocyte membrane fractions.

RESULTS

In biliary fistula rats, sulindac was recovered in bile in unconjugated form and induced hypercholeresis of canalicular origin. Other compounds underwent glucuronidation and were not hypercholeretic. In the isolated liver, sulindac had delayed biliary recovery and induced prolonged choleresis, consistent with a cholehepatic circulation. Sulindac was secreted normally in Tr(-) rats, indicating that its canalicular transport did not require mrp2. In the perfused liver, sulindac inhibited cholyltaurine uptake, and when coinfused with cholyltaurine, induced acute cholestasis. With both basolateral and canalicular membrane fractions, sulindac inhibited cholyltaurine transport competitively.

CONCLUSIONS

Sulindac is secreted into bile in unconjugated form by a canalicular bile acid transporter and is absorbed by cholangiocytes, inducing hypercholeresis. At high flux rates, sulindac competitively inhibits canalicular bile salt transport; such inhibition may contribute to the propensity of sulindac to induce cholestasis in patients.

Multivalent thioether-peptide conjugates: B cell tolerance of an anti-peptide immune response

Antibodies which bind beta2-glycoprotein I (beta2GPI) are associated with antiphospholipid syndrome. Synthetic peptide mimotopes have been discovered which compete with beta2GPI for binding to selected anti-beta2GPI. A thiol-containing linker was attached to the N-terminus of two cyclic thioether peptide mimotopes, peptides 1a and 1b. The resulting peptides, with linker attached, were reacted with two different haloacetylated platforms to prepare four tetravalent peptide-platform conjugates to be tested as B cell toleragens. The linker-containing peptides were reacted with maleimide-derivatized keyhole limpet hemocyanin (KLH) to provide peptide-KLH conjugates. Peptides 1a and 1b were also modified by acylation with 3-(4'-hydroxyphenyl)propionic acid N-hydroxysuccinimidyl ester. The resulting hydroxyphenyl peptides were radioiodinated and used to measure anti-peptide antibody levels. The KLH conjugates were used to immunize mice to generate an anti-peptide immune response. The immunized mice were treated with the conjugates or saline solution and boosted with the appropriate peptide-KLH conjugate. Three of the four conjugates suppressed the formation of anti-peptide antibody. The stabilities of the conjugates in mouse serum were measured, and the relative stabilities did not correlate with ability to suppress antibody formation.

An N-acyl glycyltaurine conjugate of deoxycholic acid in the biliary bile acids of the rabbit

jj biliary bile acid composition of the adult and neonatal domestic rabbit, as well as that of the adult brush rabbit, was characterized. In adult domestic rabbits, the dominant bile acid present was deoxycholic acid (88% of total bile acids), a secondary bile acid formed by the bacterial 7-dehydroxylation of cholic acid. Although most of the bile acids present were conjugated with glycine, two exceptions were observed. About 3% of deoxycholic acid was conjugated, in N-acyl linkage, with glycyl-taurine. Chenodeoxycholic acid, which composed <1% of wile acids, was conjugated solely with taurine. The bile of neonatal rabbits contained a greater percentage of primary bile acids, and bile acids were conjugated to a much greater extent with taurine. The adult brush rabbit had a bile acid composition similar to that of the domestic rabbit, but about one-third of all bile acids were conjugated with taurine. In addition, lithocholic acid was present as its sulfated amidate, whereas in the domestic rabbit, lithocholic acid was conjugated solely with glycine. The biliary bile acid composition of rabbits appears to be unique both in terms of the predominant steroid moiety, as well as in the modes of conjugation.

Transport of fluorescent bile acids by the isolated perfused rat liver: kinetics, sequestration, and mobilization

Hepatocyte transport of six fluorescent bile acids containing nitrobenzoxadiazolyl (NBD) or a fluorescein derivative on the side chain was compared with that of natural bile acids using the single-pass perfused rat liver. Compounds were infused at 40 nmol/g liver min for 15 minutes; hepatic uptake and biliary recovery were measured; fractional extraction, intrinsic basolateral clearance, and sequestration (nonrecovery after 45 minutes of additional perfusion) were calculated. Fluorescent bile acids were efficiently extracted during the first 3 minutes (70%-97%), but net extraction decreased with time mostly because of regurgitation into the perfusate. For cholylglycine and ursodeoxycholylglycine (UDC-glycine), extraction was 94% to 99%, and regurgitation did not occur. Intrinsic hepatic clearance of fluorescent bile acids (2-7 mL/g liver x min) was lower than that of cholylglycine (9.0 +/- 0.6; mean +/- SD) and UDC-glycine (21.4 +/- 0.4). Sequestration at 60 minutes was 8% to 26% for fluorescent bile acids with a cholyl moiety (cholylglycylaminofluorescein [CGamF], cholyllysylfluorescein [C-L-F], cholyl-[N epsilon-NBD]-lysine [C-L-NBD], and cholylaminofluorescein [CamF]), 32% for ursodeoxycholylaminofluorescein (UDCamF), and 88% for ursodeoxycholyl-(N epsilon-NBD)lysine (UDC-L-NBD). Cholylglycine and UDC-glycine had <3% retention. Biliary secretion of sequestered UDCamF, but not of UDC-L-NBD, was induced by adding dibutyryl cyclic adenosine monophosphate (DBcAMP) to the perfusate, possibly by translocation to the canaliculus of pericanalicular vesicles containing fluorescent bile acids. Biliary secretion of UDC-L-NBD, but not of UDCamF, was induced by adding cholyltaurine or UDC-taurine, possibly by inhibition of binding to intracellular constituents or of transport into organelles. It is concluded that fluorescent bile acids are efficiently transported across the basolateral membrane, but in contrast to natural conjugated bile acids, are sequestered in the hepatocyte (UDC derivatives > cholyl derivatives). Two modes of hepatic sequestration of fluorescent bile acids were identified. Fluorescent bile acids may be useful to characterize sequestration processes during bile acid transport through the hepatocyte.

Fluorescent bile acid derivatives: relationship between chemical structure and hepatic and intestinal transport in the rat

Studies were performed to characterize hepatic and intestinal transport, as well as biotransformation during transport, of a spectrum of fluorescent bile acids containing a fluorophore attached to the side chain. The following two classes of compounds were studied: 1) aminofluorescein (amF) coupled directly to the carboxylic group of a bile acid which was cholic, ursodeoxycholic, or cholylglycine; and 2) nitrobenzoxadiazolyl (NBD) coupled to the epsilon-amino group of a lysine conjugated bile acid, which was cholic or ursodeoxycholic. Fluorescein, a cholephilic organic anion, was studied as a control. Fluorescent bile acids were synthesized and their structures confirmed by nuclear magnetic resonance and mass spectrometry. Using the biliary fistula rat, hepatic transport, biotransformation, and choleretic activity were defined; intestinal absorption was assessed by jejunal or ileal perfusion studies. All fluorescent bile acids had hepatic transport maxima about one-sixth that reported for cholyltaurine, but derivatives of cholylglycine were transported best. Bile acids underwent little (<5%) biotransformation during hepatocyte transport. Only the amF conjugate of cholylglycine had normal choleretic activity; other compounds were hypocholeretic or cholestatic. In contrast, fluorescein was well transported, was partly glucuronidated, and had normal choleretic activity. NBD-tagged, but not amF-tagged, bile acids were actively transported by the intestine (ileum > jejunum), and no fluorescent bile acid had passive intestinal permeability; NBD-tagged bile acids were biotransformed during intestinal transport (jejunum > ileum). We conclude that the structure of the fluorophore as well as that of the bile acid influences transport by the hepatocyte and enterocyte. These fluorescent bile acids differ from fluorescein in being impermeable to cell membranes and undergoing little biotransformation during hepatocyte transport. Of these fluorescent bile acids, cholylglycylamF has hepatocyte transport and choleretic properties most closely resembling those of a natural bile acid.

Effect of side chain length on biotransformation, hepatic transport, and choleretic properties of chenodeoxycholyl homologues in the rodent: studies with dinorchenodeoxycholic acid, norchenodeoxycholic acid, and chenodeoxycholic acid

To assess the effect of side chain length on the metabolism and physiological effects of homologues of chenodeoxycholic acid (CDCA), dinorCDCA, the C22 homologue, was synthesized and its hepatic biotransformation, transport kinetics, and choleretic properties were defined in rat and hamster biliary fistula and in isolated perfused rat liver. Results were compared with those of norCDCA, the C23 homologue, and of CDCA, the natural C24 homologue. In the rat, dinorCDCA was secreted mostly in unconjugated form (the majority as dinor-alpha-muricholic acid); the remainder was glucuronidated. In the hamster, glucuronidation was greater, and the unconjugated fraction contained equal parts of dinorCDCA and 5beta-hydroxy-dinorCDCA. NorCDCA was glucuronidated extensively (70%, rat; 40%, hamster). CDCA, in contrast, was efficiently amidated with taurine or glycine. In the perfused liver, the initial uptake rate of all three homologues was identical; later, regurgitation and/or cholehepatic shunting of dinorCDCA and norCDCA, but not of CDCA, occurred. In rats and hamsters with biliary fistulas, dinorCDCA and norCDCA, but not CDCA, induced a bicarbonate-rich hypercholeresis of canalicular origin. Hypercholeresis was not induced by the taurine conjugate of dinorCDCA. Hepatobiliary retention of both dinorCDCA and norCDCA occurred, consistent with efficient ductular absorption (calculated to be 94%) and cholehepatic cycling of the unmetabolized bile acids. It is concluded that dinorCDCA, as norCDCA, is inefficiently amidated, is metabolized as a xenobiotic, and induces hypercholeresis. DinorCDCA is the first dihydroxy bile acid to be identified that is secreted largely in unconjugated form in bile.

Carrier-mediated transport of conjugated bile acids across the basolateral membrane of biliary epithelial cells

When secreted into bile, unconjugated dihydroxy bile acids are absorbed passively by cholangiocytes according to the cholehepatic circulation hypothesis. A fraction of these are likely to be conjugated during transcellular transport. Experiments were performed using fluorescent conjugated bile acids to test whether carrier-mediated transport of conjugated bile acids is present in the basolateral domains of polarized cholangiocytes of intrahepatic bile ductules isolated from rat liver. The time course of the cellular localization of cholyl-NBDAB-Gly and chenodeoxycholyl-NBDAB-Gly, which are anionic fluorescent derivatives of the corresponding glycine-conjugated bile acids, was characterized using an image-analysis system. With 0.3-3 microM solutions, fluorescence was present at 1 and 3 min in the basolateral area of cholangiocytes. Staining in the apical region occurred later, with a peak after 15 min of incubation. The basolateral uptake of the two fluorescent bile acids was temperature dependent and Na+ independent, and was not influenced by the addition of amiloride, by lowering of the medium pH to 6.0, or by preincubation with valinomycin. Uptake was partially inhibited by the absence of Cl- or HCO3- in the perfusate, by preincubation with 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS), and by the presence of different organic anions or unconjugated and conjugated bile acids in the medium. When cells were preloaded with an ethyl ester of chenodeoxycholyl-NBDAB-Gly, which is hydrolyzed by intracellular esterases, the decrease of cell fluorescence was partly inhibited by H2DIDS, whereas it was stimulated by the presence of 20 microM cholyltaurine in the medium. It is concluded that transport of conjugated bile acid anions across the basolateral membrane of the polarized rat cholangiocyte is carrier mediated. The conjugated bile acid transporter is likely to be an anion exchanger and is likely to be involved in bile secretion whenever conjugated bile acids or other organic anions are transported from the base of the biliary ductular epithelial cells into the plasma of the periductular capillary plexus.

Hepatocyte transport of bile acids and organic anions in endotoxemic rats: impaired uptake and secretion

BACKGROUND & AIMS

In sepsis, intrahepatic cholestasis occurs frequently, suggesting impaired hepatocyte transport of bile acids and organic anions. The aim of the study was to define the magnitude, time course, and the site of impaired biliary secretion in a rat sepsis model.

METHODS

Maximal transport for two bile acids (cholyltaurine and chenodeoxycholyltaurine) and two organic anions (sulfobromophthalein and sulfolithocholyltaurine) was measured in isolated perfused livers at various times after lipopolysaccharide injection. Basolateral and canalicular liver plasma membrane vesicles were used to characterize the impairment in hepatocyte transport.

RESULTS

Maximal hepatocyte transport was reduced for all compounds by 60%-81% compared with controls. Bile acid-independent bile flow was reduced by 51%. Impairment was maximal 12 hours after endotoxin injection and recovered thereafter. In basolateral plasma membrane vesicles, sodium-dependent transport for bile acids was reduced by 36%-47%. Sodium-independent transport of organic anions was reduced by 40%-55%. Adenosine triphosphate-stimulated transport was greatly decreased in canalicular vesicles prepared from endotoxemic animals for all four compounds probably because of a reduced number of transport molecules, based on kinetic studies.

CONCLUSIONS

Basolateral and canalicular bile acid and organic anion transport are markedly impaired in endotoxemia. These mechanisms may contribute to the cholestasis of sepsis.

Carrier-mediated jejunal absorption of conjugated bile acids in the guinea pig

BACKGROUND & AIMS

Conjugated bile acid absorption is known to occur in the jejunum in mammals, but the mechanism has not been well defined. The aim of this study was to define the mechanisms by which conjugated bile acids are absorbed from the jejunum.

METHODS

The steady-state absorption of eight conjugated bile acids from a perfused jejunal segment was measured in the anesthetized biliary fistula guinea pig. Experiments defined the effect of bile acid structure, tested for competitive inhibition and saturation of transport, and compared the absorption rate of taurine conjugates with that of glycine conjugates at pH 7.6 or 5.0.

RESULTS

Dihydroxy conjugates were absorbed twice as rapidly as cholyl conjugates from the perfused jejunum; glycine and taurine conjugates of a given bile acid were absorbed at a similar rate. Absorption of ursodeoxycholate taurine showed saturability and competitive inhibition by other conjugated bile acids. When intraluminal pH was decreased to pH 5.0, the absorption rate of glycine (but not taurine) conjugates increased, indicating passive absorption of the protonated species of glycine-conjugated bile acids.

CONCLUSIONS

Uptake of glycine- or taurine-conjugated bile acids by the guinea pig jejunum occurs by at least two mechanisms: carrier-mediated transport (dihydroxy conjugates greater than trihydroxy conjugates) and passive absorption in protonated (uncharged) form of glycine conjugates.

Validation of [22,23-3H]cholic acid as a stable tracer through conversion to deoxycholic acid in human subjects

Bile acids labeled with 3H on the sterol nucleus lose a substantial fraction of label during enterohepatic cycling and conversion to secondary bile acids. We tested the isotopic stability of a side-chain 3H label, [22,23-3H]cholic acid in humans. The 3H-labeled compound was administered simultaneously with [24-14C]cholic acid to four healthy volunteers. Duodenal bile was collected daily for 5 days after isotope administration to determine the ratio of 3H/14C in bile acids. Urine was collected to determine loss of radioactivity by this route. Cholic acid and deoxycholic acid were isolated from biliary bile acids by thin-layer chromatography after deconjugation with cholylglycine hydrolase. The ratio of 3H/14C in cholic acid and deoxycholic acid remained constant and identical to that of the administered mixture in all subjects, indicating stability of the 3H label during enterohepatic cycling. Cumulative loss of 3H in urine averaged only 1.2% of administered dose and was identical to loss of 14C (average 1.3%) indicating little if any transfer of 3H from bile acid to body water. Deconjugation of biliary bile acids by alkaline hydrolysis resulted in 15-20% loss of 3H label, consistent with known base-catalyzed exchange of alpha-carbon protons on carboxylic acids. We conclude that [22,23-3H]cholic acid is a biologically stable, and therefore reliable, isotopic tracer of cholic acid in humans during enterohepatic cycling including conversion to deoxycholic acid, provided deconjugation is performed enzymatically. Because the 22,23-3H label can be inserted into most C24 bile acids, it appears the best way to tag 3H-labeled bile acids for metabolic studies.

Hepatic biotransformation in rodents and physicochemical properties of 23(R)-hydroxychenodeoxycholic acid, a natural alpha-hydroxy bile acid

The hepatic biotransformation in the rat and hamster of 23(R)-hydroxychenodeoxycholic acid (23(R)OH-CDCA), the alpha-hydroxy derivative of CDCA, was defined; some physiological and physicochemical properties were also assessed. 23(R)OH-CDCA was isolated from duck bile; [24-14C]23(R)OH-CDCA was synthesized. The compound was administered intravenously to anesthetized biliary fistula rats at doses of 1, 3, or 5 mu mol/kg-min and to hamsters at 3 mu mol/min-kg. Biliary bile acids and radioactivity were analyzed by thin-layer chromatography (TLC), high pressure liquid chromatography (HPLC), and gas chromatography-mass spectrometry (GC-MS). Recovery of radioactivity in bile was incomplete (50-70% of infused dose); some was also recovered as breath 14CO2. Radioactivity in bile was present as unchanged compound (25-50%, dose-dependent) and its conjugates (with taurine, with glycine, or with glucuronate). Nor-CDCA (C23) was present in bile (in both unconjugated and conjugated form), indicating that 23(R)OH-CDCA had undergone oxidative decarboxylation (alpha-oxidation) with loss of the C-24 carboxyl group. The alpha-oxidation was 20 +/- 5% (mean +/- SD) of administered dose in the rat and was not dose-dependent; in hamsters, alpha-oxidation was 35 +/- 8%. In rats, the S isomer of 23OH-CDCA also underwent alpha-oxidation (10 +/- 2%). Nor-CDCA also underwent 6beta-hydroxylation to form nor-alpha-muricholic acid, as well as reduction of its C-23 carboxyl group to form the C23 alcohol. The taurine conjugate of 23(R)OH-CDCA [23(R)OH-CDC-tau] was prepared synthetically and characterized by 1H-NMR. By surface tension measurements, it had a critical micellization concentration (CMC) of 3.5 mM (in 0.15 M Na+), as compared to 1.8 mM for CDC-taurine. Aqueous solubility of 23(R)OH-CDCA increased markedly above pH 5, compared to pH 7 for CDCA. When incubated with cholylglycine hydrolase, 23(R)OH-CDC-tau was deconjugated at a rate one-fourth that of CDC-tau. It is concluded that the presence of a 23(R)-hydroxyl group in a 3alpha, 7alpha-dihydroxy bile acid alters its metabolism in the rodent hepatocyte, as evidenced by inefficient conjugation with taurine or glycine, alpha-oxidation to nor (C23) bile acid, and reduction of the nor bile acid to the primary alcohol. The taurine conjugate of 23(R)OH-CDCA, a major biliary bile acid of marine mammals and wading birds, is a biological detergent with properties superior to those of the taurine conjugate of CDCA. Natural C23 nor-bile acids may be formed by alpha-oxidation of alpha-hydroxy C24 bile acids.

Method for removal of surface-active impurities and calcium from conjugated bile salt preparations: comparison with silicic acid chromatography

Some commercial preparations of common natural conjugated bile salts contain impurities (e.g., amines, lipids, and calcium) that are likely to affect their physicochemical properties. A method was developed for purifying commercial preparations of sodium salts of glycine- and taurine-conjugated bile acids. The method consists of passage of a dilute aqueous solution of the sodium bile salt through three columns in sequence: graphitized carbon, a hydrophobic bonded octadecylsilane (C18) cartridge, and a calcium-chelating resin. The final solution was extracted with chloroform, and the purified bile salt was then isolated by freeze-drying, with a yield of 65-75%. Each bile salt purified by this method was compared with the corresponding bile salt purified by conventional adsorption chromatography on a silicic acid column, using a mixture of methanol and chloroform as eluant. Purity was assessed by visible spectra, by surface tension measurements (using the maximum bubble-pressure method and a Wilhelmy wire method), by chloroform extractability of impurities in the conjugated bile acid, by liposome solubilization, and by chemical analysis of the calcium content. Both purification methods removed colored and surface-active impurities, but the new method was always as or more effective than silicic acid column chromatography. Calcium ion, present in commercial bile salts in concentrations up to 16 mmol/mol bile salt, was removed completely by the three-column method, but not by silicic acid chromatography. The new method is thus a simple, rapid, and efficient procedure for purification of the sodium salts of glycine- and taurine-conjugated bile acids for physicochemical measurements, in which elimination of surface-active impurities and polyvalent cations is desired.

Transport characteristics of three fluorescent conjugated bile acid analogs in isolated rat hepatocytes and couplets

The transport properties of three different synthetically prepared fluorescent conjugated bile acid analogs (FBA), all with the fluorophore on the side chain, were determined using isolated rat hepatocytes and hepatocyte couplets. The compounds studied were cholylglycylamidofluorescein (CGamF), cholyl(N epsilon-nitrobenzoxadiazolyl [NBD])-lysine (C-NBD-L), and chenodeoxycholyl-(N epsilon-NBD)-lysine (CDC-NBD-L). When hepatocytes were incubated at 37 degrees C with 0.3 mumol/L of FBA and 0.15 mol/L of Na+, cell fluorescence increased linearly with time at a rate (U/min) of 7.8 +/- 0.5 for CGamF, 7.2 +/- 0.3 for C-NBD-L, and 13.7 +/- 1.0 for CDC-NBD-L (mean, +/- SE; n = 40 to 90). Uptake was concentration dependent for concentrations less than 20 mumol/L and was saturable. The Michaelis constant (Km) value (mumol/L) for CGamF was 10.8, for C-NBD-L was 3.8, and for CDC-NBD-L was 3.0. In the absence of Na+, the uptake rate was decreased by 50% for CGamF and by 38% for C-NBD-L; but uptake of CDC-NBD-L was unchanged and thus Na+ independent. Cellular uptake of all three derivatives was specific to hepatocytes and was absent in several nonhepatocyte cell lines. For CGamF and C-NBD-L, both Na(+)-dependent and Na(+)-independent uptake was inhibited by 200-fold excess concentrations of cholyltaurine, dehydrocholyltaurine, and cholate, but for CDC-NBD-L, these nonfluorescent bile acids did not inhibit initial uptake. The intracellular fluorescence of CGamF was strongly pH dependent at an excitation wavelength of 495 nm, but pH independent at 440 nm excitation. In contrast, intracellular fluorescence of C-NBD-L and CDC-NBD-L was pH independent. All three FBA were secreted into the canalicular space of approximately 50% to 60% of couplets. Cellular adenosine triphosphate (ATP) depletion with either CN- or atractyloside inhibited secretion of all three FBA. The multispecific organic anion transporter (MOAT) inhibitor, chlorodinitrobenzene, blocked secretion of fluorescent MOAT substrates at a concentration of 1 mumol/L. At this concentration it did not affect secretion of the three FBA. At higher concentrations, chlorodinitrobenzene partially inhibited the canalicular secretion of CGamF but not the other two FBA. In conclusion, all three FBA are secreted by canalicular membrane bile acid transporters, but the sinusoidal uptake characteristics of CGamF and C-NBD-L are more similar than those of CDC-NBD-L to the transport properties of cholyltaurine. Therefore, C-NBD-L appears to be the best of the three for studies of conjugated trihydroxy-bile acid transport in hepatocytes.

Biliary bile acids of fruit pigeons and doves (Columbiformes): presence of 1-beta-hydroxychenodeoxycholic acid and conjugation with glycine as well as taurine

The biliary bile acid composition of 30 species of pigeons and doves belonging to seven genera in the avian order Columbiformes was determined using TLC, HPLC, GLC/MS, LSIMS, and NMR. In 23 of 25 species of fruit pigeons and doves, chenodeoxycholic acid was the major bile acid (> 50%). In only 1 species (Ptilinopus ornatus) was cholic the major bile acid. A number of species (7 of 15 species in the genus Ptilinopus, and 6 of 9 species in the genus Ducula) contained 1 beta,3 alpha,7 alpha-trihydroxy-5 beta-cholan-24-oic acid in proportions ranging from 2 to 43%. This 1 beta-hydroxy derivative of chenodeoxycholic acid has not been previously identified as a major biliary bile acid in vertebrates. Five of 15 species of the genus Ptilinopus, 5 of 9 species of the genus Ducula, and the only species examined for the genus Gymnophaps contained 23R-hydroxy chenodeoxycholic acid in detectable proportions, ranging from 1 to 4%. Bile acids were conjugated (in N-acyl linkage) with glycine and taurine in 28 species and with only taurine in 2 species. The fruit pigeons are the first non-mammalian genera identified to date in whom bile acids are conjugated with glycine, as well as with taurine. An incidental finding was that a gallbladder was present in 3 genera (Ptilinopus, Ducula, and Gymnophaps) and absent in 4 genera (Gallicolumba, Chalcophaps, Otidiphaps, and Treron).

Effects of side chain length on ionization behavior and transbilayer transport of unconjugated dihydroxy bile acids: a comparison of nor-chenodeoxycholic acid and chenodeoxycholic acid

13C-NMR spectroscopy was used to examine the effect of side chain length on the ionization properties and transmembrane transport rate of 3 alpha,7 alpha-dihydroxy bile acids. When solubilized in taurocholate micelles, [23-13C]nor-chenodeoxycholic acid (nor-CDCA) had a pKa of 6.1, similar to that of CDCA (pKa 6.2), its C24 homologue. In unilamellar phosphatidylcholine vesicles, the pKa of nor-CDCA was 7.0, whereas that of CDCA was 6.6. Lineshape analysis indicated that the rate of ionization of nor-CDCA as a micellar solute or as a vesicle component was very slow (0.4 x 10(5) sec-1) compared to that of acetic acid in water (8.7 x 10(5) sec-1). Lineshape analysis of spectra of the protonated form of nor-CDCA at acidic bulk pH indicated that the transbilayer transport rate of nor-CDCA (580 sec-1) was six times faster than that of CDCA (100 sec-1). It is proposed that the shorter side chain of the nor-CDCA molecule causes it to reside more deeply inside the vesicle bilayer than CDCA, explaining its weaker ionization and more rapid flip-flop rate. These in vitro experiments imply that, in vivo, a given C23 nor-dihydroxy bile acid will ionize less readily when present in membranes, and it will also flip-flop faster than its C24 homologue.

Transcriptional regulation of cholesterol 7 alpha-hydroxylase mRNA by conjugated bile acids in primary cultures of rat hepatocytes

The role of bile acids in the regulation of cholesterol 7 alpha-monooxygenase (EC 1.14.13.17) was characterized using primary cultures of rat hepatocytes supplemented with dexamethasone and thyroxine. Taurocholate and taurodeoxycholate (50 microM) repressed cholesterol 7 alpha-hydroxylase mRNA to 44 +/- 9 and 52 +/- 4%, respectively, of control values. Repression by these natural, relatively hydrophobic bile acids was concentration dependent, with an IC50 of about 50 microM, and time dependent with a t1/2 for repression of 22 h. In contrast, two natural hydrophilic bile acids, tauroursodeoxycholate and taurohyodeoxycholate, had no effect on cholesterol 7 alpha-hydroxylase mRNA levels. Taurochenodeoxycholate and taurolithocholate also had no effect, but these hydrophobic bile acids were rapidly hydroxylated to more hydrophilic bile acids. Hydrophilic bile acid analogues (nor (C23) bile acids and beta-hydroxy epimers) repressed cholesterol 7 alpha-hydroxylase mRNA less potently than their corresponding and more hydrophobic C24 or alpha-hydroxy derivatives. Cholesterol 7 alpha-hydroxylase specific activity was decreased by taurocholate or taurodeoxycholate (50 microM) to 26 +/- 9 and 56 +/- 3% of control, respectively; its transcriptional activity was repressed to 52 +/- 5% of control by taurocholate (50 microM). The addition of cholesterol or the induction of cholesterol biosynthesis did not influence repression of cholesterol 7 alpha-hydroxylase mRNA levels by taurocholate. Based on several lines of evidence, cAMP was not involved in bile acid-induced repression. In rat hepatocytes cultured under conditions in which cholesterol 7 alpha-hydroxylase gene expression is maintained at in vivo levels, hydrophobic bile acids repress this enzyme at the level of gene transcription independently of cholesterol availability.

Solubility of calcium salts of unconjugated and conjugated natural bile acids

The approximate solubility products of the calcium salts of ten unconjugated bile acids and several taurine conjugated bile acids were determined. The formation of micelles, gels, and/or precipitates in relation to Ca2+,Na+, and bile salt concentration was summarized by "phase maps." Because the ratio of Ca2+ to bile salt in the precipitates was ca. 1:2, and the activity of Ca2+ but not that of bile salt (BA-) could be measured, the ion product of aCa2+ [BA-]2 was calculated. The ion product (= Ksp) ranged over nine orders of magnitude and the solubility thus ranged over three orders of magnitude; its value depended on the number and orientation of the hydroxyl groups in the bile acid. Ion products (in units of 10(-9) mol/l)3 were as follows: cholic (3 alpha OH,7 alpha OH,12 alpha OH) 640; ursocholic (3 alpha OH,7 beta OH,12 alpha OH) 2300; hyocholic (3 alpha OH,6 alpha OH,7 alpha OH) 11; ursodeoxycholic (3 alpha OH,7 beta OH) 91; chenodeoxycholic (3 alpha OH,7 alpha OH) 10; deoxycholic (3 alpha OH,12 alpha OH) 1.5; 12-epideoxycholic (lagodeoxycholic, 3 alpha OH,12 beta OH) 2.2; hyodeoxycholic (3 alpha OH,6 alpha OH) 0.7; and lithocholic (3 alpha OH) 0.00005. The critical micellization temperature of the sodium salt of murideoxycholic acid (3 alpha OH,6 beta OH) was greater than 100 degrees C, and its Ca2+ salt was likely to be very insoluble. Taurine conjugates were much more soluble than their corresponding unconjugated derivatives: chenodeoxycholyltaurine, 384; deoxycholyltaurine, 117; and cholyltaurine, greater than 10,000. Calcium salts of unconjugated bile acids precipitated rapidly in contrast to those of glycine conjugates which were metastable for months. Thus, hepatic conjugation of bile acids with taurine or glycine not only enhances solubility at acidic pH, but also at Ca2+ ion concentrations present in bile and intestinal content.

Activation of mast cells by bile acids

To test whether bile acids interact with mast cells, dilute, aqueous solutions of five pure unconjugated natural bile acids and their corresponding glycine or taurine conjugates were incubated with murine PT-18 cells (a mast cell line functionally and cytochemically similar to mucosal mast cells) or with freshly isolated rat peritoneal mast cells. Bile acid solutions ranged in concentration from 0.3 to 10 mmol/L; histamine release was assessed by a fluorimetric assay, and cell lysis by cytosolic enzyme (lactate dehydrogenase) release. Lipophilic, dihydroxy bile acids (chenodeoxycholic acid and deoxycholic acid as well as their glycine and taurine conjugates) caused histamine release in a dose-related manner; cholic acid and its conjugates caused much less or no histamine release. Two hydrophilic bile acids (ursodeoxycholic acid and ursocholic acid and their conjugates) were virtually devoid of activity. Histamine release, which was independent of extracellular Ca2+, occurred at 0.3 mmol/L, well below the critical micellization concentration. For a given concentration, unconjugated bile acids and glycine-conjugated bile acids induced more histamine release than taurine-conjugated bile acids; maximal release was observed at 3 mmol/L for lipophilic, dihydroxy bile acids. To test whether bile acids could also cause histamine release from cutaneous mast cells in vivo, rats were injected intradermally with bile acid solutions and histamine release assessed by capillary leakage of Evan's blue dye. Cutaneous blueing was greater with cytotoxic bile acids, chenodeoxycholyglycine or deoxycholylglycine, than with ursodeoxycholylglycine and was inhibited by prior antihistamine treatment. Histamine release correlated highly and positively with lipophilicity and with bile acid surface activity. It was concluded that lipophilic but not hydrophilic bile acids possess concentration-dependent cytotoxicity toward mast cells causing histamine release, that unconjugated and glycine-conjugated bile acids are more potent than taurine-conjugated bile acids, and that mast cell histamine release is highly correlated with lipophilicity of bile acids as well as their surface activity.

Replacement of cholesterol gallstones by murideoxycholyl taurine gallstones in prairie dogs fed murideoxycholic acid

The effect of two hydrophilic bile acids, murideoxycholic acid (3 alpha,6 beta-dihydroxy-5 beta-cholanoic acid) and ursodeoxycholic acid, on cholesterol and bile acid metabolism and hepatic pathology and gallstone composition was studied in the prairie dog. Cholesterol gallstones were induced by feeding a diet containing 1.2% cholesterol for 75 days. The animals were divided into six groups, and gallstone regression was studied as follows: groups 2 and 5, chow plus 0.2% cholesterol; groups 3 and 6, chow plus 0.2% cholesterol plus 0.15% ursodeoxycholic acid; groups 4 and 7, chow plus 0.2% cholesterol plus 0.15% murideoxycholic acid. Animals in groups 2 to 4 were killed after an additional 6 wk; animals in groups 5 to 7 were killed after an additional 12 wk. Gallstone dissolution did not occur in any group. The gallstones in groups 2, 3, 5 and 6 were typical cholesterol aggregates, as determined by polarized light microscopy and Fourier transform infrared spectrometry. The gallstones of the murideoxycholic acid group were large, solitary, dark stones that appeared radiopaque under 22 kVp x-ray examination. Scanning electron microscopy showed that in these stones the cholesterol crystals had been replaced by an amorphous material, both within the stone and on the stone surface. Chemical analysis indicated that at the end of 12 wk the calcium/sodium salt of the taurine conjugate of murideoxycholic acid (murideoxycholyl taurine) comprised 70% of the stones; protein, cholesterol and small amounts of other bile salts were also present. In vitro studies confirmed the insolubility of the sodium and calcium salts of murideoxycholyl taurine. These studies indicate that the hydrophilic bile acids, murideoxycholic acid and ursodeoxycholic acid, did not achieve gallstone dissolution under the conditions used. In the animals fed murideoxycholic acid, an insoluble calcium salt of murideoxycholyl taurine replaced cholesterol as the major constituent of gallbladder stones. This is the first example of an insoluble dihydroxy taurine-conjugated bile acid; administration of the unconjugated bile acid induced precipitation of a kind of gallstone not previously reported. The final result was transformation of cholesterol stones to bile salt stones.

Active absorption of conjugated bile acids in vivo. Kinetic parameters and molecular specificity of the ileal transport system in the rat

Active transport of conjugated bile acids by the distal ileum is required for efficient enterohepatic cycling of bile acids. Experiments were performed in the rat to obtain accurate values for Tmax and Michaelis constant (Km) of the absorptive area of the rat ileum and to define the structural specificity of the transport system. The distal fifth (20 cm) of the small intestine from an anesthetized animal with a biliary fistula was perfused using solutions of 10 taurine-conjugated bile acids; a flow rate was used that was sufficiently high such that unstirred water layer effects were negligible and the intraluminal concentration remained unchanged throughout the perfused segment. The absorption rate was equated with the rate of hepatic bile acid secretion. Values of Tmax (mumol/min.kg) were markedly influenced by bile acid structure: cholyltaurine, 12.9; ursocholyltaurine, 9.6; ursodeoxycholyl taurine, 5.0; and lagodeoxycholyl-(3 alpha,12 beta-dihydroxy-cholanoic acid)-taurine, 1.2. Decreasing the length of the side chain of ursodeoxycholate conjugates from 8 to 6 carbon atoms was associated with a modest increase in Tmax values from 5.0 to 9.1 mumols/min.kg. Values of Km correlated with Tmax values and ranged from 0.5 to 5 mmol/L, being highest for those bile acids that were best transported. The Tmax for cholyltaurine transport was not reached when the intraluminal concentration was as high as its critical micellization concentration, precluding the definition of its Tmax; however, for ursocholyltaurine, with a critical micellization concentration of 40 mmol/L, saturation of transport was clearly shown. Kinetic parameters could not be obtained for two common dihydroxy conjugates (chenodeoxycholyltaurine and deoxycholyltaurine) because at a transport rate of 2 mumols/min.kg systemic toxicity and death occurred. These studies define the maximal transport capacity of the rat ileum for taurine-conjugated bile acids; they indicate that the ileal transport system in the rat is of low affinity and high capacity for taurine conjugates of hydrophilic bile acids, and they show that both nuclear substituents and side chain length influence the transport rate of taurine-conjugated bile acids.

Transport, metabolism, and effect of chronic feeding of lagodeoxycholic acid. A new, natural bile acid

Ursodeoxycholic acid, the 7 beta-hydroxy epimer of chenodeoxycholic acid, is more hydrophilic and less hepatotoxic than chenodeoxycholic acid. Because "lagodeoxycholic acid," the 12 beta-hydroxy epimer of deoxycholic acid, is also more hydrophilic than deoxycholic acid, it was hypothesized that it should also be less hepatotoxic than deoxycholic acid. To test this, lagodeoxycholic acid was synthesized, and its transport and metabolism were examined in the rat, rabbit, and hamster. The taurine conjugate of lagodeoxycholic acid was moderately well transported by the perfused rat ileum (Tmax = 2 mumol/min.kg). In rats and hamsters with biliary fistulas, the taurine conjugate of lagodeoxycholic acid was well transported by the liver with a Tmax greater than 20 mumol/min.kg; for the taurine conjugate of deoxycholic acid, doses infused at a rate greater than 2.5 mumol/min.kg are known to cause cholestasis and death. Hepatic biotransformation of lagodeoxycholic acid in the rabbit was limited to conjugation with glycine; in the hamster, lagodeoxycholic acid was conjugated with glycine or taurine; in addition, 7-hydroxylation occurred to a slight extent (approximately 10%). When lagodeoxycholic acid was instilled in the rabbit colon, it was absorbed as such although within hours it was progressively epimerized by bacteria to deoxycholic acid. When injected intravenously and allowed to circulate enterohepatically, lagodeoxycholic acid was largely epimerized to deoxycholic acid in 24 hours. Surgical creation of a distal ileostomy abolished epimerization in the rabbit, indicating that exposure to colonic bacterial enzymes was required for the epimerization. Lagodeoxycholic acid was administered for 3 weeks at a dose of 180 mumol/day (0.1% by weight of a chow diet; 2-4 times the endogenous bile acid synthesis rate); other groups received identical doses of deoxycholic acid (hamster) or cholyltaurine, a known precursor of deoxycholic acid (rabbit). After 3 weeks of lagodeoxycholic acid ingestion, liver test results and liver appearance were normal. The total bile acid pool expanded by 37% in the rabbit, lagodeoxycholic acid composing 10% of biliary bile acids. In the hamster, the total bile acid pool was expanded by 95%, lagodeoxycholic acid composing 22% of biliary bile acids; biliary lipid secretion remained unchanged. Tracer studies indicated that the fractional turnover rate of lagodeoxycholic acid was high (157%/day, rabbit; 116%/day, hamster) because of its rapid epimerization to deoxycholic acid in the colon. These studies indicate that lagodeoxycholic acid, the more hydrophilic epimer of deoxycholic acid, is transported and metabolized as other dihydroxy bile acids but is much less toxic than deoxycholic acid.(ABSTRACT TRUNCATED AT 400 WORDS)

Prevention of ursodeoxycholate hepatotoxicity in the rabbit by conjugation with N-methyl amino acids

The effect of dietary administration of four different amino acid (N-acyl) conjugates of ursodeoxycholic acid on biliary bile acid composition, liver tests and hepatic morphology by light microscopy was examined in the rabbit. Each group of four to five rabbits received a chow diet supplemented with a single conjugate of ursodeoxycholic acid ursodeoxycholyl-glycine, ursodeoxycholyl-sarcosine, ursodeoxycholyl-taurine or ursodeoxycholyl-N-methyltaurine for 3 wks at a dose of 50 mg/kg/day; a control group received chow alone. After 3 wks of feeding, animals receiving ursodeoxycholyl-glycine or ursodeoxycholyl-taurine had hepatotoxicity associated with abnormal liver tests. Lithocholic acid made up 11% +/- 2.7% of biliary bile acids in the ursodeoxycholyl-glycine and 10% +/- 2.2% in the ursodeoxycholyl-taurine group. In contrast, animals receiving ursodeoxycholyl-sarcosine or ursodeoxycholyl-N-methyltaurine had neither hepatotoxicity nor abnormal liver tests and the proportion of lithocholic acid in biliary bile acids increased much less. Complementary studies showed that ursodeoxycholyl-sarcosine and ursodeoxycholyl-N-methyltaurine were not biotransformed during hepatic transport and were resistant to deconjugation and dehydroxylation in the rabbit. These experiments indicate that the N-methyl amino acid conjugates of ursodeoxycholic acid are nontoxic in the rabbit and resist deconjugation and dehydroxylation. Such resistance decreases formation of lithocholic acid in the colon, thus reducing its accumulation and consequent induction of hepatotoxicity.

Transport, metabolism, and effect of chronic feeding of cholylsarcosine, a conjugated bile acid resistant to deconjugation and dehydroxylation

To test the effect in rodents of chronic ingestion of a bile acid resistant to deconjugation, cholylsarcosine was synthesized and its transport, metabolism, and effect on biliary bile acid and biliary lipid composition were determined in rabbits, hamsters, and rats. Cholylsarcosine was shown to be well absorbed from the ileum but underwent little absorption from the jejunum or colon. When cholylsarcosine was administered in the diet at 140 mumol/kg.day, it was well absorbed and underwent little biotransformation during enterohepatic cycling; however, both bacterial deconjugation and dehydroxylation (without deconjugation) occurred to a small extent. With chronic feeding, cholylsarcosine accumulated to compose 24%-29% of circulating bile acids in all 3 rodent species. It was rapidly lost from the enterohepatic circulation, with a daily fractional turnover rate of 75%-150%, depending on the species. Cholylsarcosine caused no change in liver tests or hepatic morphology and did not influence biliary lipid secretion. When cholyltaurine was fed, it was also absorbed, but, in contrast to cholylsarcosine, was rapidly deconjugated and dehydroxylated to form deoxycholic acid. The deoxycholic acid accumulated in the enterohepatic circulation, as evidenced by a slow fractional turnover rate of 26%-40% per day, depending on the species. It is concluded that cholylsarcosine is absorbed from the ileum, has an enterohepatic circulation, does not undergo appreciable deconjugation or dehydroxylation in these rodents, and is nontoxic. In the rodent, the circulating bile acids can be somewhat enriched when a bile acid resistant to deconjugation is ingested; but the effect on the steady state biliary bile acid composition is less than that obtained when cholyltaurine is administered because cholyltaurine is biotransformed to deoxycholic acid, which in turn is absorbed and has its own efficient enterohepatic circulation.