Comparison of biliary excretion and metabolism of lithocholic acid and its sulfate and glucuronide conjugates in rats

Biliary excretion and biotransformation of tracer doses of [14C]lithocholic acid and its sulfate and glucuronide intravenously injected into bile-drainaged rats were compared. Biliary excretion efficiency was in the order of unconjugate sulfate glucuronide and all conjugates were completely excreted into bile within 60 min after injection. Only tracer doses of radioactivity were found in the liver and urine. About 90% of radiolabeled bile acids in bile were conjugated with taurine immediately after injection of lithocholic acid, whereas lithocholic acid-glucuronide was only partly conjugated with taurine all the time (less than 6%) and excreted into bile mainly as native compound. In the first 10 min, 66% of lithocholic acid-sulfate was conjugated with taurine and it gradually proceeded up to 87%. Hydroxylation at C-6 and C-7 positions of lithocholic acid proceeded time-dependently up to 45%. No hydroxylation was observed with lithocholic acid-sulfate or glucuronide. Differences of biliary excretion rate of these conjugates may be one of the reasons for the delayed decrease of sulfated and glucuronidated bile acids in serum after bile drainage to patients with obstructive jaundice of during the recovery of acute hepatitis than non-esterified bile acids.     

Synthesis of bile acid monosulphates by the isolated perfused rat kidney.

Perfusion of an isolated rat kidney with labelled bile acids, in a protein-free medium, resulted in the urinary excretion of the labelled bile acid, 3% being converted into polar metabolities in 1h. These metabolities were neither glycine nor taurine conjugates, nor bile acid glucuronides, and on solovolysis yielded the free bile acid. On t.l.c. the metabolite of [24-14C]lithocholic acid had the mobility of lithocholate 3-sulphate. The principal metabolite of [24-14C]chenodeoxycholic acid had the mobility of chenodeoxycholate 7-sulphate; trace amounts appeared as chenodeoxycholate 3-sulphate. [35S]sulphate was incorporated in chenodeoxycholic acid by the kidney, resulting in a similar pattern of sulphation. No disulphate salt of chenodeoxycholic acid was detected. These findings lend support to the hypothesis that renal synthesis may account for some of the bile acid sulphates present in urine in the cholestatic syndrome in man.     

Dynamics of the conjugate pattern during the infusion of bile acids into isolated rat liver

The conjugate pattern of biliary [14C]bile acids was investigated in isolated perfused rat livers, which were infused with either [24-14C]cholic acid or [24-14C]chenodeoxycholic acid (40 mumol/h) together with or without taurine or cysteine (80 mumol/h). [14C]Bile acids were chromatographed on a thin-layer plate and the distribution of radioactivity on the plate was measured by radioscanning. The biliary excretion of [14C]bile acids was greater in the infusion with [14C]cholic acid than in the infusion with [14C]chenodeoxycholic acid. Biliary unconjugated [14C]bile acids amounted to about 50% of the total after the infusion with [14C]cholic acid, while only about 10% with [14C]chenodeoxycholic acid. In the initial period of infusion, biliary conjugated [14C]bile acids consisted mostly of the taurine conjugate, which decreased with time and the glycine conjugate increased complementarily. When taurine was simultaneously infused, the decrease in the taurine conjugate was suppressed to some extent. Cysteine infused in place of taurine had a similar influence but was less effective than taurine. The taurine content of liver after the infusion with either of the [14C]bile acids decreased greatly compared with that before the infusion, even when taurine or cysteine was infused simultaneously. The glycine content also decreased after the infusion, but the decrease in glycine was smaller than that in taurine. The results suggest that the conjugate pattern of biliary bile acids in rats depends mainly on the amount of taurine which is supplied to hepatic cells either exogenously from plasma or endogenously within themselves.     

Ursodeoxycholic acid, chenodeoxycholic acid, and 7-ketolithocholic acid are primary bile acids of the guinea pig

Guinea pig gallbladder bile contains chenodeoxycholic acid (62 +/- 5%), ursodeoxycholic acid (8 +/- 5%), and 7-ketolithocholic acid (30 +/- 5%). All three bile acids became labeled to the same specific activity within 30 min after [3H]cholesterol was injected into bile fistula guinea pigs. When a mixture of [3H]ursodeoxycholic acid and [14C]chenodeoxycholic acid was infused into another bile fistula guinea pig, little 3H could be detected in either chenodeoxycholic acid or 7-ketolithocholic acid. But, 14C was efficiently incorporated into ursodeoxycholic and 7-ketolithocholic acids. Monohydroxylated bile acids make up 51% and ursodeoxycholic acid 38% of fecal bile acids. After 3 weeks of antibiotic therapy, lithocholic acid was reduced to 6% of the total, but ursodeoxycholic acid (5-11%) and 7-ketolithocholic (15-21%) acid persisted in bile. Lathosterol constituted 19% of skin sterols and was detected in the feces of an antibiotic-fed animal. After one bile fistula guinea pig suffered a partial biliary obstruction, ursodeoxycholic and 7-ketolithocholic acids increased to 46% and 22% of total bile acids, respectively. These results demonstrate that chenodeoxycholic acid, ursodeoxycholic acid, and 7-ketolithocholic acid can all be made in the liver of the guinea pig.     

The formation of lithocholic acid, chenodeoxycholic acid and α- and β-muricholic acids from cholesterol incubated with rat-liver mitochondria

1. When rat-liver mitochondria were incubated with [4-(14)C]cholesterol in the presence of a soluble supernatant fraction, various steroids more polar than cholesterol were formed. These included 3beta-hydroxycholest-5-en-26-oic acid, 3beta-hydroxychol-5-enoic acid, lithocholic acid, chenodeoxycholic acid and alpha- and beta-muricholic acids. 2. All the radioactive C(24) bile acids recovered were in conjugated form, probably as taurine conjugates. 3. The formation of 3beta-hydroxychol-5-enoic acid from cholesterol shows that liver mitochondria are capable of carrying out the oxidative removal of the isopropyl unit of the side chain before any modification has occurred in the ring system.     

Synthesis, intestinal absorption and metabolism of sarcosine conjugated ursodeoxycholic acid

Sarcosine conjugated ursodeoxycholic acid (SUDC) was synthesized and its intestinal absorption and metabolism were studied in rat and hamster. Intestinal absorption study using bile fistula rat shows that more than 90% of SUDC administered intraduodenally was excreted in the bile within 24 hr. No change of the administered bile acid was seen during the absorption from the intestine, the passage of the liver, and the excretion into the bile. When [24-14C]SUDC and [11,12-3H2]-ursodeoxycholic acid were administered orally to a hamster, more than 95% of both the administered 14C and 3H were recovered from the feces within 6 days. Most (77%) of the fecal 14C-labeled compound was SUDC, whereas 95% of the fecal 3H-labeled compound was unconjugated lithocholic acid. These results indicate that SUDC, unlike taurine or glycine conjugated bile acid, resists bacterial deconjugation and 7-dehydroxylation.     

Effect of diabetes on the metabolism of chenodeoxycholic acid in isolated perfused rat liver

The formation of alpha-muricholic acid and beta-muricholic acid from chenodeoxycholic acid was comparatively investigated in livers isolated from normal, streptozotocin-diabetic, and insulin-treated diabetic rats. [24-14C]Chenodeoxycholic acid or [24-14C]alpha-muricholic acid was infused into the perfused livers. There was no difference in biliary excretion of 14C among the different groups of rats after the infusion of each 14C-labelled bile acid. Biliary [14C]bile acids were chromatographed on a thin-layer plate and the distribution of radioactivity on the plate was measured by radioscanning. In the diabetic group, the formation ratio of alpha-muricholic acid and beta-muricholic acid from [24-14C]chenodeoxycholic acid and also that of beta-muricholic acid from [24-14C]alpha-muricholic acid were much smaller than in the normal group. Treatment of the diabetic group with insulin cancelled the difference in the infusion of each [24-14C]bile acid. The results indicate that not only 6 beta-hydroxylation of chenodeoxycholic acid to alpha-muricholic acid but also 7-epimerization of the latter acid to beta-muricholic acid is suppressed in an insulin-deficient state in rats.     

Interaction between bile acids and staphylococcal polysaccharide: inhibition of capsule formation in encapsulated mutant strains (taurine+, taurine-) of Staphylococcus aureus

In a previous paper, we showed that bile acid derivatives inhibit capsule formation as well as taurine biosynthesis in a taurine+ (Tau+) encapsulated strain of Staphylococcus aureus. In the present study, binding of [14C]cholic acid [( 14C]CA) and [14C]taurocholic acid [( 14C]TA) to the staphylococcal polysaccharide antigen (SPA) of the capsular fraction was examined. The bile acids were found to bind with SPA via taurine of the Tau+ cells. [14C]CA bound with the SPA fraction of the Tau+ strain within 10-30 min, whereas 60-120 min was required in the binding of [14C]TA. Various bile acids competed with cholic acid binding to Tau+ cells which was shown by the inhibition of binding with cholic acid or taurocholic acid but not with glycholic acid. Binding of bile acid derivatives to a Tau- encapsulated mutant or to capsular material from this mutant was not observed.     

Defective biliary secretion of bile acid 3-O-glucuronides in rats with hereditary conjugated hyperbilirubinemia

Biliary secretion of bile acid glucuronides was studied in control rats and in rats with a congenital defect in hepatobiliary transport of organic anions (GY rats). In control animals, hepatobiliary transport of [3H]lithocholic acid 3-O-glucuronide and [3H]cholic acid 3-O-glucuronide was efficient (greater than 95% in 1 h) and comparable to that of [14C]taurocholic acid. Secretion of both glucuronides was impaired in GY rats (24% and 71% at 1 h), whereas that of taurocholate was similar to control values. However, recovery of the glucuronides in bile was nearly complete within 24 h; virtually no radioactivity was found in urine. In control rats, biliary secretion of lithocholic acid 3-O-glucuronide, but not that of cholic acid 3-O-glucuronide or taurocholate, could be delayed by simultaneous infusion of dibromosulphthalein. In mutant rats, dibromosulphthalein infusion was also able to inhibit secretion of cholic acid 3-O-glucuronide. [3H]Hydroxyetianic acid, a C20 short-chain bile acid, was secreted by control rats as a mixture of 20% carboxyl-linked and 80% hydroxyl-linked (3-O-)glucuronide; secretion was very efficient (99% in 1 h). In GY rats, secretion was drastically impaired (16% at 1 h and 74% over a 24-h period). Initially, the mutant secreted more carboxyl- than hydroxyl-linked glucuronide, but the ratio reached that of control animals after 24 h. The rates of formation of both types of hydroxyetianic acid glucuronide by hepatic microsomes from mutant rats were similar or even slightly higher than those of control microsomes. These findings indicate that bile acid 3-O-glucuronides, but probably not carboxyl-linked glucuronides, are secreted into bile by a transport system shared with organic anions such as conjugated bilirubin and dibromosulphthalein, but different from that for amino acid-conjugated bile acids.     

Hydroxylation, conjugation and sulfation of bile acids in primary monolayer cultures of rat hepatocytes

Hydroxylation of lithocholic, chenodeoxycholic, deoxycholic and cholic acids was studied in monolayers of rat hepatocytes cultured for 76 h. The majority of added lithocholic and chenodeoxycholic acids was metabolized to beta-muricholic acid (56-76%). A small part of these bile acids (9%), however, and a considerable amount of deoxycholic and cholic acids (21%) were converted into metabolites more polar than cholic acid in the first culture period. Formation of these compounds decreased during the last day of culture. Bile acids synthesized after addition of [4-14C]-cholesterol were almost entirely (97%) sulfated and/or conjugated, predominantly with taurine (54-66%), during culture. Sulfated bile acids were mainly composed of free bile acids. The ability of hepatocytes to sulfurylate bile acids declined with culture age. Thus, rat hepatocytes in primary monolayer culture are capable to sulfurylate bile acids and to hydroxylate trihydroxylated bile acids, suggesting formation of polyhydroxylated metabolites.     

Bile acids of marsupials. 2. Hepatic formation of vulpecholic acid (1 alpha,3 alpha,7 alpha-trihydroxy-5 beta-cholan-24-oic acid) from chenodeoxycholic acid in a marsupial, Trichosurus vulpecula (Lesson).

Free vulpecholic acid (1 alpha,3 alpha,7 alpha-trihydroxy-5 beta-cholan-24-oic) is the major biliary component of the Australian opossum (Trichosurus vulpecula), accompanied only by a few percent of its taurine conjugate. In order to exclude a microbial involvement in its formation (i.e., secondary origin) four sets of experiments were performed. It was found that a) the level of vulpecholic acid remained unchanged in the bile of opossums fed with neomycin and kanamycin for 7 days prior to bile collection; b) it also remained unchanged after long bile drainage; c) in opossums prepared with biliary cannula, intraportally injected [24-14C]chenodeoxycholic acid was transformed to [24-14C]vulpecholic acid; and d) in a similar experiment, the detectable transformation of [1 alpha,2 alpha-3H2]cholesterol to vulpecholic acid was observed. In experiment c) 28-66% of the administered radioactivity was secreted in 2 h in the form of free biliary vulpecholic and chenodeoxycholic acids. Only a trace amount of the corresponding taurine conjugates (approximately 0.4%) was formed. Moreover, rapidly declining specific radioactivity of the unconjugated chenodeoxycholic acid indicated its probable participation in the native formation of vulpecholic acid.     

The metabolism of steroids in the fatty liver induced by orotic acid feeding.

1. The metabolism of 4-[4-14C]androstene-3,17-dione, 4-[4-14C]pregnene-3,20-dione, 5alpha-[4-14C]androstane-3alpha,17beta-diol, [4-14C]cholesterol, 7alpha-hydroxy-4-[6beta-3H]cholesten-3-one, 5beta-[7beta-3H]cholestane-3alpha,7alpha-diol and [3H]lithocholic acid was studied in the microsomal fraction of livers from control and orotic acid-treated male rats. 2. As a result of the treatment the orotic acid-fed rats had fatty livers and subnormal concentrations of cholesterol and triglycerides in serum. 3. The 6beta- and 7alpha-hydroxylation of 4-androstene3,17-dione, and the 2alpha-, 2beta- and 18-hydroxylation of 5alpha-androstane-3alpha,17beta-diol, and the 5alpha-reduction of 4-androstene-3,17-dione and 4-pregnene-3,20-dione were decreased by 40--50% in orotic acid-fed rats. Other oxidative and reductive reactions of the steroid hormones were not significantly affected. 4. The 12alpha-hydroxylation of 7alpha-hydroxy-4-cholesten-3-one was decreased by about 50%, whereas the 7alpha-hydroxylation of cholesterol and the 26-hydroxylation of 5beta-cholestane-3alpha,7alpha-diol were not significantly decreased. The 6beta-hydroxylation of lithocholic acid was stimulated by 40%. 5. The results are discussed in relation to present knowledge of the heapatic drug-metabolizing enzymes and to the recent findings of an abnormal bile acid metabolism in liver disease.     

Pathogenesis of lithocholate-induced intrahepatic cholestasis: role of glucuronidation and hydroxylation of lithocholate.

It has been shown that lithocholic glucuronide is more cholestatic than lithocholic acid (LCA), as well as its taurine and glycine conjugates. Furthermore, LCA hydroxylation is thought to be a major detoxifying mechanism. Therefore, the role of LCA glucuronidation and hydroxylation was investigated during the development of LCA-induced cholestasis and recovery from it. Male rats received a bolus intravenous injection of [14C]LCA (12 mumol/100 g body weight) and bile samples were collected every 30 min for 5 h. Bile flow (BF) was reduced immediately after LCA injection, dropping to 40% of basal BF at 60 min. It then started to increase, reaching normal bile flow values at 3.5 h. Morphologically, canalicular lesions were dominant at 60 min and virtually absent at 2 h. At 60 min (maximal cholestasis), 30% of the LCA injected was secreted in bile, 20% was found in plasma while the other 50% was recovered in the liver and distributed mainly in plasma membranes, microsomes and cytosol. At the end of the experiment (normal BF), 20% of the LCA injected was still in the liver but was present mainly in the cytosol. In bile, within 30 min after injection, 46% of the LCA secreted was lithocholic glucuronide, 24% was conjugated with taurine and glycine, and 21% was in the form of hydroxylated bile acids. During the recovery period, lithocholic glucuronide secretion decreased to 18-25%. Taurine and glycine conjugate secretion increased to a maximum of 43% at 60 min, after which it was reduced to 21-28%. In contrast, hydroxylated metabolites were elevated during the recovery periods, reaching a maximum (45%) at 120 min and remaining constant thereafter. These results suggest that: (i) LCA binding to plasma membranes and microsomes appeared to correlate with the development of cholestasis; (ii) LCA glucuronidation may initiate and/or contribute to LCA-induced cholestasis; and (iii) hydroxylation predominates during recovery from cholestasis.     

Suitability of primary monolayer cultures of adult rat hepatocytes for studies of cholesterol and bile acid metabolism

Monolayer cultures of hepatocytes isolated from cholestyramine-fed rats and incubated in serum-free medium converted exogenous [4-14C]cholesterol into bile acids at a 3-fold greater rate than did cultures of hepatocytes prepared from untreated rats. Cholic acid and beta-muricholic acid identified and quantitated by gas-liquid chromatography and thin-layer chromatography were synthesized by cultured cells for at least 96 h following plating. The calculated synthesis rate of total bile acids by hepatocytes prepared from cholestyramine-fed animals was approximately 0.058 micrograms/mg protein/h. beta-Muricholic acid was synthesized at approximately a 3-fold greater rate than cholic acid in these cultures. Cultured hepatocytes rapidly converted the following intermediates of the bile acid pathway; 7 alpha-hydroxy[7 beta-3H]cholesterol, 7 alpha-hydroxy-4-[6 beta-3H] cholesten-3-one, and 5 beta-[7 beta-3H]cholestane-3 alpha, 7 alpha, 12 alpha-triol into bile acids. [24-14C]Chenodeoxycholic acid and [3H]ursodeoxycholic acid were rapidly biotransformed to beta-muricholic acid. 3-Hydroxy-3-methylglutaryl-coenzyme A reductase activity measured in microsomes of cultured hepatocytes decreased during the initial 48 h following plating, but remained relatively constant for the next 72 h. In contrast, cholesterol 7 alpha-hydroxylase activity appeared to decrease during the first 48 h, followed by an increase over the next 48 h. Despite the apparent changes in enzyme activity in vitro, the rate of bile acid synthesis by whole cells during this time period remained constant. It is concluded that primary monolayer cultures of rat hepatocytes can serve as a useful model for studying the interrelationship between cholesterol and bile acid metabolism.     

Metabolism of lithocholic acid in the rat: formation of lithocholic acid 3-O-glucuronide in vivo

Milligram amounts of [3 beta-3H]lithocholic (3 alpha-hydroxy-5 beta-cholanoic) acid were administered by intravenous infusion to rats prepared with a biliary fistula. Analysis of sequential bile samples by thin-layer chromatography (TLC) demonstrated that lithocholic acid glucuronide was present in bile throughout the course of the experiments and that its secretion rate paralleled that of total isotope secretion. Initial confirmation of the identity of this metabolite was obtained by the recovery of labeled lithocholic acid after beta-glucuronidase hydrolysis of bile samples. For detailed analysis of biliary metabolites of [3H]lithocholic acid, pooled bile samples from infused rats were subjected to reversed-phase chromatography and four major labeled peaks were isolated. After complete deconjugation, the two major compounds in the combined first two peaks were identified as murideoxycholic (3 alpha, 6 beta-dihydroxy-5 beta-cholanoic) and beta-muricholic (3 alpha, 6 beta, 7 beta-trihydroxy-5 beta-cholanoic) acids and the third peak was identified as taurolithocholic acid. The major component of the fourth peak, after isolation, derivatization (to the methyl ester acetate), and purification by high pressure liquid chromatography (HPLC), was positively identified by proton nuclear magnetic resonance as lithocholic acid 3 alpha-O-(beta-D-glucuronide). These studies have shown, for the first time, that lithocholic acid glucuronide is a product of in vivo hepatic metabolism of lithocholic acid in the rat.     

Hepatocyte-hepatoma cell hybrids. Characterization and demonstration of bile acid synthesis

Hybrids were created by fusion of primary rat hepatocytes with well-differentiated Reuber H35 rat hepatoma cells. Seventeen hybrids were screened for bile acid synthesis using [26-14C]cholesterol. As [26-14C]cholesterol was converted to bile acid, 14CO2 was released. Using this assay, four hybrids (8B, 12C, 13C, and 13D) were identified which synthesized bile acid. These four hybrids also incorporated [14C]taurine into bile acid. Bile acids were identified by capillary gas chromatography/mass spectrometry, and their rates of synthesis were quantitated by isotope dilution. Reuber H35 cells synthesized little or no bile acid. However, hybrids 8B, 12C, 13C, and 13D synthesized chenodeoxycholic acid, alpha-muricholic acid, and cholic acid and secreted them into the media. The rates of synthesis of individual bile acids varied among these hybrids. For example, the relative percentage of cholic acid ranged from 11.1% (hybrid 8B) to 50.4% (hybrid 13C). The bile acids synthesized and secreted by the most active hybrid, 12C, were greater than 93% conjugated. In summary, hybrids were created that retain the capacity to synthesize, conjugate, and secrete three major rat bile acid species. Such hybrids are unique model systems that will allow the study of the biochemical and genetic regulation of bile acid synthesis.     

Biliary lipid secretion in the rat. The uncoupling of biliary cholesterol and phospholipid secretion from bile acid secretion by sulfated glycolithocholic acid

Glycolithocholic acid and its sulfated derivative are major metabolites of the secondary bile acid lithocholic acid in man. Both compounds are known to induce cholestasis in experimental animals. We compared the effects of these endogenous hepatotoxins on bile production and biliary lipid composition in rats with chronic biliary drainage. The compounds were administered enterally at relatively low rates (5-50% of the rats' endogenous bile acid secretion in these experiments) to simulate enterohepatic circulation. Both compounds were substantially secreted into bile (more than 90% of dose); sulfated glycolithocholic acid unchanged and glycolithocholic acid after hepatic hydroxylation predominantly in the form of glyco-beta-muricholic acid (cf. Kuipers et al. (1986) Am. J. Physiol. 251, G189-G194). Neither glycolithocholic acid nor its sulfated derivative affected the biliary excretion of endogenous bile acids or bile flow in these experiments. In spite of this, phospholipid and cholesterol secretion were significantly reduced by sulfated glycolithocholic acid but were not altered by glycolithocholic acid. Phospholipid and cholesterol secretion rapidly decreased to 25 and 50% of their initial values, respectively, at biliary output rates of sulfated glycolithocholic acid up to 2 mumol/h, and did not further decrease when this output was increased to 6 mumol/h. Small unilamellar liposomes consisting of cholesterol, [Me-14C]choline-labeled phosphatidylcholine, phosphatidylserine and [3H]cholesteryl oleate in a 5:4:1:0.1 molar ratio were employed to label intrahepatic lipid pools. Administration of sulfated glycolithocholic acid slightly reduced bile acid synthesis from [3H]cholesteryl oleate, but significantly reduced the biliary secretion of [14C]phospholipid. Glycolithocholic acid did not affect the hepatic processing of liposomal lipids. It is concluded that sulfated glycolithocholic acid at low doses causes the uncoupling of biliary lipid secretion from that of bile acids, which might represent in initiating event in sulfated glycolithocholic acid hepatotoxicity.     

Binding of bile acids by 100 000g supernatants from rat liver.

1. The binding of glycocholic acid, chenodeoxycholic acid and lithocholic acid to rat liver 1000 000g supernatants was studied by equilibrium dialysis. 2. The binding characteristics of the bile acids suggest that the binding components are involved in bile acid transport. 3. When mixtures of [14C]lithocholic acid and liver supernatants were eluted from columns of Sephadex G-75, a prominent peak of [14C]lithocholic acid appeared with proteins of mol.wt. approx. 40000. A second, smaller, peak of [14C]lithocholic acid was eluted with proteins of mol.wt. approx. 100000. 4. The inclusion of cholic acid, glycocholic acid or chenodeoxycholic acid in the eluting buffer decreased the amount of [14C]lithocholic acid that was eluted with the higher-molecular-weight component.     

Peroxisomal bile acid-CoA:amino-acid N-acyltransferase in rat liver

Bile acid-CoA:amino-acid N-acyltransferase activity was measured in subcellular fractions of rat liver homogenate. The conversion of [14C]choloyl-CoA and [14C]chenodeoxycholoyl-CoA into the corresponding [14C]tauro- and glyco-bile acids was calculated after isolation of the product by high performance liquid chromatography. There was an enrichment of bile acid-CoA:amino-acid N-acyltransferase activity in the light mitochondrial (L) fraction and to a lesser extent in the microsomal fraction. Surprisingly, no enrichment was found in the cytosolic fraction. Subfractionation of the L-fraction by Nycodenz gradient centrifugation, showed that the activity of the N-acyltransferase had a bimodal distribution and co-sedimented with peroxisomes (particulate catalase) and microsomes (esterase). The highest specific amidation activity of both choloyl-CoA and chenodeoxycholoyl-CoA was always found in the most peroxisome-rich fractions. [14C]Taurocholate formation in the peroxisomal fraction was 2.2 mumol/mg of protein/min. Striking differences were observed in the Km values and the saturation concentrations for glycine and taurine. The peroxisomal amidation of [14C]choloyl-CoA had a Km for taurine of 0.9 x 10(-3) M and for glycine of 17 x 10(-3) M. The results are consistent with the possibility that most of de novo synthesized bile acids conjugate to taurine by a peroxisomal bile acid-taurine N-acyltransferase in rat liver. The bile acids deconjugated in the gut and recirculating to the liver may be activated and amidated by the microsomal enzyme system prior to biliary secretion.     

Isolation and chemical synthesis of a major, novel biliary bile acid in the common wombat (Vombatus ursinus): 15alpha-hydroxylithocholic acid

The major bile acids present in the gallbladder bile of the common Australian wombat (Vombatus ursinus) were isolated by preparative HPLC and identified by NMR as the taurine N-acylamidates of chenodeoxycholic acid (CDCA) and 15alpha-hydroxylithocholic acid (3alpha,15alpha-dihydroxy-5beta-cholan-24-oic acid). Taurine-conjugated CDCA constituted 78% of biliary bile acids, and (taurine-conjugated) 15alpha-hydroxylithocholic acid constituted 11%. Proof of structure of the latter compound was obtained by its synthesis from CDCA via a Delta14 intermediate. The synthesis of its C-15 epimer, 15beta-hydroxylithocholic acid (3alpha,15beta-dihydroxy-5beta-cholan-24-oic acid), is also reported. The taurine conjugate of 15alpha-hydroxylithocholic acid was synthesized and shown to have chromatographic and spectroscopic properties identical to those of the compound isolated from bile. It is likely that 15alpha-hydroxylithocholic acid is synthesized in the wombat hepatocyte by 15alpha-hydroxylation of lithocholic acid that was formed by bacterial 7alpha-dehydroxylation of CDCA in the distal intestine. Thus, the wombat appears to use 15alpha-hydroxylation as a novel detoxification mechanism for lithocholic acid.