Bile acids in porcine fetal bile

A study of the biliary bile acid composition in porcine fetus compared with that of the adult pig is described. Biles, collected during gestation (weeks 4, 15 to 17 and at birth), aged six months and two years old, were analyzed by gas-liquid chromatography and capillary GC-MS. Bile acids were separated into different conjugate groups by chromatography on the lipophilic anion exchange gel, piperidinohydroxypropyl Sephadex LH-20. All and one fourth of the total bile acids in the bile of weeks 4 and 15 of gestation, respectively, were present as unconjugated form, however, only a trace of unconjugated bile acids was present in bile of late gestation, the young and the adult pigs. The ratio of glycine/taurine (G/T) conjugates in the conjugated fraction of the fetal bile at 15 weeks gestation was less than 1, which markedly contrasted with the conjugation pattern for adult bile where the ratio of G/T conjugates was approximately more than 9. The predominant acids identified in porcine fetal bile of the 4 weeks gestation were cholic acid (3alpha,7alpha,12alpha-trihydroxy-5beta-chola n-24-oic acid) and chenodeoxycholic acid (3alpha,7alpha -dihydroxy-5beta-cholan-24-oic acid). However, cholic acid in late gestation, young, and adult bile was the smallest component, whereas chenodeoxycholic acid was still the major constituent of these biles. The presence of small but valuable amounts of allocholic acid (3alpha,7alpha,12alpha-trihydroxy-5alpha-chol an-24-oic acid) and cholic acid in early gestation suggested the presence of 12alpha-hydroxylase activity of steroid nucleus in fetal liver. Considerable amounts of glycine-conjugated hyodeoxycholic acid were found in the bile of the gestation periods, suggesting the placental transfer of this bile acid from maternal circulation.     

Group separation of free and conjugated bile acids by pre-packed anion-exchange cartridges

A rapid and simple procedure for the group separation of the major free and conjugated bile acids of man is described. After initial extraction with Bond Elut C18 cartridges, the analytes are fractionated into the unconjugated, glycine- and taurine-conjugated forms using disposable Bond Elut SAX columns and methanol-acetate buffer eluents. The method is found to be accurate and reproducible and to afford complete resolution between fractions. Free and conjugated bile acids present in human bile and gastric juice are assayed by high-performance liquid chromatography, after extraction and group separation according to the described procedure.     

Hydrogen bonding in salicylsalicylic acid (salsalate) crystals

An X-ray crystallographic study of the drug salsalicylic acid (salsalate) has been performed. Crystal formation of the drug is influenced by both inter- and intra-molecular hydrogen bonding. In addition an OH group in salsalate can occupy alternate ortho positions resulting in two hydrogen bonding motifs within a single crystal.     

Endpoints as human biomarkers in exposure assessment of triazoles fungicides

Potential endpoint biomarkers were evaluated in the assessment of exposure to triazoles, in the southern region of Minas Gerais, Brazil. Volunteers were divided into three groups: occupationally exposed and rural residents (n = 21), non-occupationally exposed and rural residents (n = 35) and non-occupationally exposed and urban residents (n = 30). Of all endpoints evaluated, plasma concentration of androstenedione (p < 0.001) and glycine-conjugated bile acids presented statistical differences in the three studied groups (p < 0.05). However, our findings concerning oxidative stress and testosterone levels, plus that related to unconjugated and taurine conjugated bile acids, suggested that more studies are necessary to evaluate their potential as biomarkers for triazole exposure, as statistical significance was not attained between the groups. Our human population data contributes to the development of triazole exposure risk assessment with respect to these potential effect biomarkers, in potentially vulnerable groups and individuals.     

Hepatic uptake and biliary secretion of bile acids in the perfused rat liver.

Hepatic uptake and biliary secretion have been evaluated in the isolated perfused rat liver for cholic, chenodeoxycholic, ursodeoxycholic acid, both free and taurine-conjugated; the physicochemical properties of the bile acids have also been calculated and related to these experimental parameters. Cholic acid disappearance rate from the perfusate was the fastest, followed by that of ursodeoxycholic and chenodeoxycholic; it was also faster for taurine-conjugated bile acids than for their respective unconjugated forms. The recovery in bile was higher for conjugated than for unconjugated bile acids, and among each class, was higher for cholic than for chenodeoxycholic and ursodeoxycholic. The hepatic uptake correlated negatively (r = -0.99) with the bile acid lipophilicity, while the biliary secretion correlated with the solubility of the molecules. These results show the effect of the physicochemical properties of BA on their hepatic handling, at the physiological concentration of BA in the portal blood.     

Determination of chenodiol bioequivalence using an immobilized multi-enzyme bioluminescence technique

Measurement of the bioequivalence of formulations of chenodiol, a bile acid which is used for gallstone dissolution, is difficult because its high first-pass clearance results in low plasma levels after ingestion of usual dosages. To solve this problem, a new method was developed to determine the bioequivalence of several chenodiol formulations. The method included the following steps: isolation of all bile acids from serum by absorption to a hydrophobic resin, elution of bile acids from the resin by methanol, separation of the unconjugated bile acid fraction by an ion-exchange procedure, and bioluminescence measurement of the unconjugated 7 alpha-hydroxy bile acids using Sepharose beads containing co-immobilized 7 alpha-hydroxysteroid dehydrogenase, diaphorase, and luciferase. The isolation method gave complete recovery, and the bioluminescence procedure was simple, rapid, and sensitive. The peak level of systemic chenodiol occurred 1 to 2 h following oral ingestion and ranged from 4 to 8 microM. This method appears superior to previously reported methods for determining the bioequivalence of chenodiol preparations. In principle, the method is suitable for measurement of the bioequivalence of other bile acids provided the appropriate hydroxysteroid dehydrogenase is available.     

INFLUENCE OF BILE ACIDS ON THE BILIARY TRANSPORT MAXIMUM OF PHENOLSULPHONPHTHALEIN IN THE RAT

1. The effect of changes in bile acid secretion induced by cholestyramine treatment or taurocholate infusion on the biliary transport maximum (Tm) of phenolsulfonphthalein (PSP) was studied in Wistar rats. 2. Five hours after oral administration of cholestyramine (1.5 g/kg bodyweight) the biliary output of bile acids decreased to 51% and bile flow to 76% of control values. The percentage of conjugated and unconjugated PSP excreted into bile and the Tm of the dye were not significantly modified by cholestyramine pretreatment. 3. Administration of sodium taurocholate at increasing rates (60-480 nmol/100 g bodyweight per min) enhanced bile flow and the biliary output of bile acids in a linear dose-related fashion. The Tm of PSP increased progressively until a maximum of 29% above the control values was reached at a taurocholate dose of 240 nmol/100 g bodyweight per min). The enhancement corresponded mainly to the unconjugated dye, the excretion of conjugated PSP not being significantly modified by the infusion of the bile acid. 4. The results indicate that bile acids can influence to some extent biliary excretion of PSP in the rat, although this component is of minor importance at low bile acid secretory rates.     

Chromatographic determination of bile acids in biological fluids with sensitive and selective detection

Separation and determination of bile acids in biological fluids by high-performance liquid chromatography are reviewed. The capacity ratios of bile acids on an ODS column were affected by the number, position and configuration of the hydroxyl group on the steroid nucleus, and the chromatographic behavior was markedly influenced by the pH of a mobile phase according to the conjugated form at C-24. A new pre-column derivatization reagent, 1-anthroyl nitrile, was developed and applied to the analysis of bile acids in biological fluids. Bile acids were derivatized through the 3 alpha-hydroxyl group into the corresponding esters, separated on an ODS column, and monitored by a fluorescence detector with detection limit of 20 fmol. The sensitive method for the determination of bile acids in biological materials by gas chromatography (GC) in combination with negative ion chemical ionization (NICI) mass spectrometry is also described. Of various derivatives for the carboxyl group, the pentafluorobenzyl (PFB) ester provided the highest value of the ratio of the negative to positive ion current. A characteristic carboxylate anion [M-181]- was produced as the most abundant ion by the loss of the PFB group in NICI. PFB esters of bile acids were further derivatized into the dimethylethylsilyl ethers and then separated by GC. The detection limit was 2 fg when the characteristic anion was monitored in the NICI mode. The preparation of 18O-labelled bile acids, as the internal standard for the trace analysis or the tracer for the metabolic study, was developed. Finally, the clean-up procedure for bile acids in biological fluids was investigated. The combined use of solid-phase extraction with a Sep-pak C18 or Bond Elut cartridge and group separation on a lipophilic ion-exchange gel, piperidinohydroxypropyl Sephadex LH-20, was found most effective for this purpose.     

Interactions of combined bile acids on hepatocyte viability: cytoprotection or synergism

Cholestasis results from hepatocyte dysfunction due to the accumulation of bile acids in the cell, many of which are known to be cytotoxic. Recent evidence implicates competitive antagonism of key cytotoxic responses as the mechanism by which certain therapeutic bile acids might afford cytoprotection against cholestasis. In this work, we compare the relative cytotoxicity of bile acids in terms of dose- and time-dependence. To better elucidate the controversy related to the therapeutic use of ursodeoxycholate (UDCA) in cholestatic patients, we also evaluated the effects of bile acid combinations. Viability of Wistar rat hepatocytes in primary culture was measured by LDH leakage after 12 and 24 h exposure of cells to the various bile acids. All unconjugated bile acids caused a dose-dependent decrease in cell viability. The tauro- and glyco-conjugates of chenodeoxycholate (CDCA) and UDCA were all less toxic than the corresponding unconjugated form. Although relatively non-toxic, UDCA caused synergistic cell killing by lithocholate (LCA), CDCA, glyco-CDCA (GCDC) and tauro-CDCA (TCDC). Glycoursodeoxycholate decreased the toxicity of GCDC, but potentiated the toxicity of unconjugated CDCA and LCA. The tauro-conjugate of UDCA had no significant effect. These data suggest that at cholestatic concentrations, bile acid-induced cell death correlates with the degree of lipophilicity of individual bile acids. However, these results indicate that the reported improvement of biochemical parameters in cholestatic patients treated with UDCA is not due to a direct effect of UDCA on hepatocyte viability. Therefore, any therapeutic effect of UDCA must be secondary to some other process, such as altered membrane transport or nonparenchymal cell function.     

Roles of endogenous and exogenous taurine and glycine in the formation of conjugated bile acids: analyses using freshly isolated and primary cultured rat hepatocytes

The regulatory roles of intracellular taurine and glycine, transported and biosynthesized in hepatocytes, on the formation of conjugated bile acids were studied using freshly isolated hepatocytes (fresh hepatocytes) and hepatocytes in primary culture (cultured hepatocytes). Transported taurine significantly increased the rate of taurocholic acid formation both in fresh hepatocytes and cultured hepatocytes. Similarly, the addition of cysteine and hypotaurine, which were metabolically converted to taurine in hepatocytes, facilitated the formation of taurocholic acid in these cells. On the other hand, exogenous glycine into the incubation medium had no effect on the formation of glycocholic acid both in fresh and cultured hepatocytes. In contrast, the addition of serine and threonine, which are metabolically converted to glycine in hepatocytes, significantly increased the formation of glycocholic acid in fresh hepatocytes, although little effect of the additions of serine and threonine on the formation of glycocholic acid was noted in the case of cultured hepatocytes. The present results indicate that the formation of taurine-conjugated bile acids in hepatocytes is maintained by both transported and intracellularly formed taurine in hepatocytes, while that of glycine-conjugated bile acids is regulated by glycine formed within hepatocytes, but not by transported glycine.     

Inhibition of the rat hepatic microsomal flurbiprofen acyl glucuronidation by bile acids

Glucuronidation of carboxylic acids, primarily catalyzed by hepatic UDP-glucuronosyltransferases, is an important phase II metabolic pathway functioning in detoxification. Acyl glucuronides of 2-aryl propionates, however, can form covalently bound protein adducts, which may generate hypersensitive reactions. We previously identified and quantified R- and S-flurbiprofen acyl glucuronides in human urine following the oral administration of flurbiprofen by liquid chromatography/electrospray ionization mass spectrometry. Recent studies also demonstrated the inhibitory effect of bile acids and their metabolites toward rat hepatic bile acid acyl glucuronidation, which may also be the target of the flurbiprofen isoenzyme. We therefore performed a kinetic analysis of rat hepatic flurbiprofen UDP-glucuronosyltransferase using bisubstrate kinetic analysis and inhibition studies. The results indicated that both bile acid and its metabolites clearly inhibited flurbiprofen acyl glucuronidation. The inhibitory effect on flurbiprofen was more efficient than the effect seen on bile acid acyl glucuronidation. Unconjugated, glycine- and taurine-conjugated chenodeoxycholic acids inhibited glucuronidation using a noncompetitive mechanism, whereas the inhibition by chenodeoxycholic acid 24-acyl glucuronide occurred according to a mixed type mechanism. The inhibition by bile acids and their metabolites may be responsible for the suppression of the toxicity of carboxy-linked glucuronides.     

Differential activation of the human farnesoid X receptor depends on the pattern of expressed isoforms and the bile acid pool composition

The farnesoid X receptor (FXR) is a key sensor in bile acid homeostasis. Although four human FXR isoforms have been identified, the physiological role of this diversity is poorly understood. Here we investigated their subcellular localization, agonist sensitivity and response of target genes. Measurement of mRNA revealed that liver predominantly expressed FXRα1(+/−), whereas FXRα2(+/−) were the most abundant isoforms in kidney and intestine. In all cases, the proportion of FXRα(1/2)(+) and FXRα(1/2)(−) isoforms, i.e., with and without a 12 bp insert, respectively, was approximately 50%. When FXR was expressed in liver and intestinal cells the magnitude of the response to GW4064 and bile acids differs among FXR isoforms. In both cell types the strongest response was that of FXRα1(−). Different efficacy of bile acids species to activate FXR was found. The four FXR isoforms shared the order of sensitivity to bile acids species. When in FXR-deficient cells FXR was transfected, unconjugated, but not taurine- and glycine-amidated bile acids, were able to activate FXR. In contrast, human hepatocytes and cell lines showing an endogenous expression of FXR were sensitive to both unconjugated and conjugated bile acids. This suggests that to activate FXR conjugated, but not unconjugated, bile acids require additional component(s) of the intracellular machinery not related with uptake processes, which are missing in some tumor cells. In conclusion, cell-specific pattern of FXR isoforms determine the overall tissue sensitivity to FXR agonists and may be involved in the differential response of FXR target genes to FXR activation.     

High sensitive analysis of rat serum bile acids by liquid chromatography/electrospray ionization tandem mass spectrometry

Sensitive liquid chromatography (LC)/electrospray ionization (ESI) tandem mass spectrometry (MS) can be used to analyze the bile acid composition of rat serum. This method can analyze eight common bile acids and their glycine and taurine conjugates in 100 μl rodent serum by gradient elution on a reversed-phase column using a mixture of 20 mM ammonium acetate buffer (pH 8.0), acetonitrile and methanol as a mobile phase. Selected reaction monitoring analysis under negative ion detection mode allowed the achievement of a high sensitive assay with a simple solid phase extraction using an ODS cartridge column. We used this method to investigate the effect of a one-day fast on the concentration and composition of serum bile acids in rats. The results suggested that the method described here is useful for the dynamic analysis of serum bile acids in rats.     

Interaction of sulfonylureas with the transport of bile acids into hepatocytes.

The sulfonylurea compounds glisoxepide and glibenclamide inhibit the uptake of bile acids into isolated rat hepatocytes. The Ki values for the inhibition of cholate uptake was 9 microM with glibenclamide and 200 microM with glisoxepide. The inhibition of cholate uptake by both sulfonylureas was noncompetitive. Uptake of the conjugated bile acid taurocholate was inhibited by glibenclamide, Ki = 75 microM. Again the inhibition was noncompetitive. Glisoxepide inhibited taurocholate uptake only in the absence of sodium ions. Under sodium-free conditions glisoxepide also strongly inhibited cholate uptake. The inhibition was competitive, Ki = 42 microM. Both bile acids interfered with the hepatocellular uptake of [3H]glisoxepide, with IC50 values of 375 and 467 microM for cholate and taurocholate, respectively. The uptake of [3H]glibenclamide was inhibited by cholate, IC50 = 328 microM, but not by taurocholate. Glisoxepide uptake was further inhibited by blockers of the hepatocellular monocarboxylate transporter, by the loop diuretic bumetanide, by 4,4'-diisothiocyano-2,2'-stilbenedisulfonate (DIDS) and by sulfate. Glibenclamide uptake was weakly inhibited by DIDS and by anthracene-9-carboxylic acid (A-9-C) but not by bumetanide and sulfate. Neither bromosulfophthalein nor the fatty acid oleate inhibited glisoxepide or glibenclamide uptake. These results are consistent with the transport of glisoxepide via the transport system for the unconjugated bile acid cholate. Glibenclamide uptake is mediated by a still unknown hepatocellular transport system.     

Basic cholane derivatives. XI: Comparison between acid and basic derivatives.

A series of hydroxycholan-24-amines was synthesized; the carboxyl group of starting unconjugated bile acids was transformed into a basic moiety [-NH2, -NHCH3, -N(CH3)2, or -NHCH2C6H5] at C-24. Solubilities, acidities, partition coefficients, and critical micellar concentrations were measured and compared with those of the corresponding bile acids. Because the steroid nucleus in the amines is the same as that in the bile acids, most of the physical-chemical properties of the two compound classes were similar. The amines were more soluble than the corresponding acids; solubilities depended mainly on the number of steroid hydroxyls and, to a lesser extent, on the side chain. Amines are strong bases in water, whereas unconjugated bile acids can be classified as weak acids. N-Benzylamino derivatives have higher log P (P is partition coefficient) values, as a consequence of the bulky hydrophobic substituent; the log P values were almost the same for the amines and the bile acids and depended on the steroid hydroxyls. Amines can self-aggregate at an acidic pH and form cationic micelles; the critical micellar concentrations of amines were of the same order of magnitude as those of bile acids. The introduction of a basic function in the side chain of the cholane moiety increased the antimicrobial activity toward most gram-positive strains.     

Interaction of fusidates with bile acid uptake by isolated rat hepatocytes

Summary The interaction of fusidic acid and two of its conjugates with carrier-mediated uptake of bile acids was investigated in isolated rat hepatocytes. All three fusidates inhibited the uptake of both cholate and taurocholate competitively suggesting a direct interaction of fusidates with bile acid carrier. The inhibition constants for all three fusidates for the inhibition of cholate uptake were significantly different from the respective inhibition constants for the inhibition of taurocholate uptake. This would indicate that both cholate and taurocholate are transported by more than one carrier into hepatocytes. The results may also indicate that taurine conjugated bile acids may be transported preferentially by one transport system while unconjugated bile acids may be preferentially transported by another transport system.Part of this publication was presented in the 18. Frühjahrstagung der Deutschen Pharmakologischen Gesellschaft (1977)     

3alpha-6alpha-Dihydroxy-7alpha-fluoro-5beta-cholanoate (UPF-680), physicochemical and physiological properties of a new fluorinated bile acid that prevents 17alpha-ethynyl-estradiol-induced cholestasis in rats

3alpha-6alpha-Dihydroxy-7alpha-fluoro-5beta-cholanoate (UPF-680), the 7alpha-fluorine analog of hyodeoxycholic acid (HDCA), was synthesized to improve bioavailability and stability of ursodeoxycholic acid (UDCA). Acute rat biliary fistula and chronic cholestasis induced by 17alpha-ethynyl-estradiol (17EE) models were used to study and compare the effects of UPF-680 (dose range 0.6-6.0 micromol/kg min) with UDCA on bile flow, biliary bicarbonate (HCO(3)(-)), lipid output, biliary bile acid composition, hepatic enzymes and organic anion pumps. In acute infusion, UPF-680 increased bile flow in a dose-related manner, by up to 40.9%. Biliary HCO(3)(-) output was similarly increased. Changes were observed in phospholipid secretion only at the highest doses. Treatment with UDCA and UPF-680 reversed chronic cholestasis induced by 17EE; in this model, UDCA had no effect on bile flow in contrast to UPF-680, which significantly increased bile flow. With acute administration of UPF-680, the biliary bile acid pool became enriched with unconjugated and conjugated UPF-680 (71.7%) at the expense of endogenous cholic acid and muricholic isomers. With chronic administration of UPF-680 or UDCA, the main biliary bile acids were tauro conjugates, but modification of biliary bile acid pool was greater with UPF-680. UPF-680 increased the mRNA for cytochrome P450 7A1 (CYP7A1) and cytochrome P450 8B (CYP8B). Both UDCA and UPF-680 increased the mRNA for Na(+) taurocholate co-transporting polypeptide (NCTP). In conclusion, UPF-680 prevented 17EE-induced cholestasis and enriched the biliary bile acid pool with less detergent and cytotoxic bile acids. This novel fluorinated bile acid may have potential in the treatment of cholestatic liver disease.     

The effect of drugs on bile flow and composition. An overview

Many drugs are eliminated via the hepatobiliary route, after biotransformation in the liver. Some of them may affect bile flow and/or the hepatic secretion of biliary lipids such as bile acids, cholesterol and phospholipids. Bile acids are the most potent agents which increase bile flow, especially unconjugated bile acids. Other drugs which increase bile flow include phenobarbitone (phenobarbital), theophylline, glucagon and insulin. In contrast, ethacrynic acid, amiloride, ouabain, oestrogens and chlorpromazine are among those agents which decrease bile flow. Biliary bile acid secretion is altered by a variety of drugs, including cheno- and ursodeoxycholic acids (CDCA and UCDA), the bile acid sequestrants cholestyramine and colestipol, and ethinyloestradiol. The composition of bile can also be altered by drug therapy. Thus, clofibrate increases biliary cholesterol secretion, and reduces bile acid concentrations, without altering biliary phospholipid concentrations. However, other clofibrate derivatives may produce changes of a different pattern, suggesting that the risk of developing gallstones may differ for each derivative. Nicotinic acid and d-thyroxine also increase biliary cholesterol saturation, while CDCA and UDCA reduce biliary cholesterol concentration. The potential consequences of drug-induced changes in bile flow and composition extend to the liver, the gallbladder and the intestine. If adverse effects are to be avoided, further study in this often overlooked area is required.     

Comparison of the choleretic properties of bile acids

The choleretic properties of cholic, chenodeoxycholic, and deoxycholic acid and their taurine and glycine conjugates were compared to their ability to form micelles. It has previously been concluded that deoxycholate has the lowest critical micellar concentration; chenodeoxycholate is slightly higher and cholic is much higher. Conjugation with glycine and taurine has little or no effect on the critical micelle concentration. Since the choleretic properties of bile salts are thought to be directly proportional to their osmotic activities, one might suspect that deoxycholic acid would be the least choleretic, chenodeoxycholic slightly more choleretic and cholic much more choleretic, with little difference between the conjugated and unconjugated forms. However, in the present study, cholic, chenodeoxycholic and taurocholic acid produced similar increases in bile flow (450–700 μl/kg) after an equimolar dose (55 μM/kg). Except for the conjugation of deoxycholic acid with taurine, conjugation of these bile acids with glycine or taurine always decreased the choleretic properties of the bile acids. Therefore, it has been concluded that there is not a good correlation between the in vitro osmotic properties of bile acids and their ability to increase bile flow.     

Quantitative relationship between bile acid structure and biliary lipid secretion in rats

A series of unconjugated and taurine conjugated bile acids (BAs) differing in water solubility (SWo), critical micellar concentration (CMC), and hydrophilicity (K') were infused iv to rats at a tracer dose and a dose of 6 mumol/min/kg over a 1-h period. Bile was collected for 3 h to evaluate the role of BA structure on cholesterol, phospholipids secretions, and bile flow. The BAs studied differ in the number (2-3), position (-3, -6, -7, -12), and orientation of the hydroxyls (alpha/beta); the side chain structure was modified by shortening (C-23, nor-BA) and by lengthening (C-25, homo-BA), while maintaining the same structure of nuclear hydroxyls (3 alpha 7 beta). At a "tracer" dose, all C-24 natural BAs are efficiently recovered in bile when administered in both unconjugated and taurine conjugated forms. At a "high dose", all taurine conjugated BAs are efficiently recovered in bile (80-100%). However, a variable recovery was observed among unconjugated BAs: trihydroxy BAs are efficiently recovered (85-100%), while dihydroxy BAs are only partially recovered (25-40%). The side chain-modified BAs [i.e., C-23 nor and C-25 homo analogs of ursodeoxycholic acid (UDCA)] are partially recovered at a tracer dose (20-30%), but less at a high dose (10-20%) when administered in the unconjugated form. In contrast, the corresponding taurine conjugates are more efficiently recovered in bile (60-80%). Conjugation with taurine increases total recovery of unconjugated BAs in bile by not more than 30-40%. Highly hydrophilic and water-soluble BAs, such as ursocholic acid (SWo = 1.67 mM) and cholic acid (SWo = 0.27 mM), can also be secreted as unconjugates, and this accounts for their complete recovery. The conjugation step is rate limiting for poorly soluble BAs such as ursodeoxycholic acid (SWo = 0.009 mM) when administered at a high dose, and critical for nor and homo analogs which are poorly soluble and whose side chain modification partially suppresses their conjugation with taurine or glycine and thereby induces alternative pathways such as glucuronidation or sulfation. The induced bile flow is directly related to the hydrophilicity of the natural C-24 bile acid.(ABSTRACT TRUNCATED AT 400 WORDS)