3 beta-hydroxy-delta 5-C27-steroid dehydrogenase deficiency; effect of chenodeoxycholic acid therapy on liver histology

Genetic spectrum and clinical characteristics of 3β-hydroxy-Δ5-C27-steroid oxidoreductase (HSD3B7) deficiency in China

BACKGROUND

Biallelic variants in HSD3B7 cause 3β-hydroxy-Δ5-C27-steroid oxidoreductase (HSD3B7) deficiency, a life-threatening but treatable liver disease. The goal of this study was to obtain detailed information on the correlation between the genotype and phenotype of HSD3B7 deficiency and to report on responses to primary bile acid therapy.

METHODS

The medical records of a cohort of 39 unrelated patients with genetically and biochemically confirmed HSD3B7 deficiency were examined to determine whether there exist genotype-phenotype relationships in this bile acid synthesis disorder.

RESULTS

In all, 34 of the 44 variants identified in HSD3B7 were novel. A total of 32 patients presented early with neonatal cholestasis, and 7 presented after 1-year of age with liver failure (n = 1), liver cirrhosis (n = 3), cholestasis (n = 1), renal cysts and abnormal liver biochemistries (n = 1), and coagulopathy from vitamin K1 deficiency and abnormal liver biochemistries (n = 1). Renal lesions, including renal cysts, renal stones, calcium deposition and renal enlargement were observed in 10 of 35 patients. Thirty-three patients were treated with oral chenodeoxycholic acid (CDCA) resulting in normalization of liver biochemistries in 24, while 2 showed a significant clinical improvement, and 7 underwent liver transplantation or died. Remarkably, renal lesions in 6 patients resolved after CDCA treatment, or liver transplantation. There were no significant correlations between genotype and clinical outcomes.

CONCLUSIONS

In what is the largest cohort of patients with HSD3B7 deficiency thus far studied, renal lesions were a notable clinical feature of HSD3B7 deficiency and these were resolved with suppression of atypical bile acids by oral CDCA administration.

Δ4-3-oxosteroid-5β-reductase deficiency: Responses to oral bile acid therapy and long-term outcomes

BACKGROUND

Disorders of primary bile acid synthesis may be life-threatening if undiagnosed, or not treated with primary bile acid replacement therapy. To date, there are few reports on the management and follow-up of patients with Δ4-3-oxosteroid 5β-reductase (AKR1D1) deficiency. We hypothesized that a retrospective analysis of the responses to oral bile acid replacement therapy with chenodeoxycholic acid (CDCA) in patients with this bile acid synthesis disorder will increase our understanding of the disease progression and permit evaluation of this treatment regimen as an alternative to the Food and Drug Administration (FDA) approved drug cholic acid, which is currently unavailable in China.

AIM

To evaluate the therapeutic responses of patients with AKR1D1 deficiency to oral bile acid therapy, specifically CDCA.

METHODS

Twelve patients with AKR1D1 deficiency, confirmed by fast atom bombardment ionization-mass spectrometry analysis of urine and by gene sequencing for mutations in AKR1D1, were treated with differing doses of CDCA or ursodeoxycholic acid (UDCA). The clinical and biochemical responses to therapy were monitored over a period ranging 0.5-6.4 years. Dose adjustment, to optimize the therapeutic dose, was based on changes in serum biochemistry parameters, notably liver function tests, and suppression of the urinary levels of atypical hepatotoxic 3-oxo-Δ4-bile acids measured by mass spectrometry.

RESULTS

Physical examination, serum biochemistry parameters, and sonographic findings improved in all 12 patients during bile acid therapy, except one who underwent liver transplantation. Urine bile acid analysis confirmed a significant reduction in atypical hepatotoxic 3-oxo-Δ4 bile acids concomitant with clinical and biochemical improvements in those patients treated with CDCA. UDCA was ineffective in down-regulating endogenous bile acid synthesis as evidenced from the inability to suppress the urinary excretion of atypical 3-oxo-Δ4-bile acids. The dose of CDCA required for optimal clinical and biochemical responses varied from 5.5-10 mg/kg per day among patients based on maximum suppression of the atypical bile acids and improvement in serum biochemistry parameters, and careful titration of the dose was necessary to avoid side effects from CDCA.

CONCLUSION

The primary bile acid CDCA is effective in treating AKR1D1 deficiency but the therapeutic dose requires individualized optimization. UDCA is not recommended for long-term management.

Tandem Mass Spectrometric Determination of Atypical 3β-Hydroxy-Δ5-Bile Acids in Patients with 3β-Hydroxy-Δ5-C27-Steroid Oxidoreductase Deficiency: Application to Diagnosis and Monitoring of Bile Acid Therapeutic Response

BACKGROUND

3β-Hydroxy-Δ5-C27-steroid oxidoreductase (HSD3B7) deficiency, a progressive cholestatic liver disease, is the most common genetic defect in bile acid synthesis. Early diagnosis is important because patients respond to oral primary bile acid therapy, which targets the negative feedback regulation for bile acid synthesis to reduce the production of hepatotoxic 3β-hydroxy-Δ5-bile acids. These atypical bile acids are highly labile and difficult to accurately measure, yet a method for accurate determination of 3β-hydroxy-Δ5-bile acid sulfates is critical for dose titration and monitoring response to therapy.

METHODS

We describe a electrospray ionization LC-MS/MS method for the direct measurement of atypical 3β-hydroxy-Δ5-bile acid sulfates in urine from patients with HSD3B7 deficiency that overcomes the deficiencies of previously used GC-MS methods.

RESULTS

Separation of sulfated 3β-hydroxy-Δ5-bile acids was achieved by reversed-phase HPLC in a 12-min analytical run. The mean (SE) urinary concentration of the total 3β-sulfated-Δ5-cholenoic acids in patients with HSD3B7 deficiency was 4650 (1711) μmol/L, approximately 1000-fold higher than in noncholestatic and cholestatic patients with intact primary bile acid synthesis. GC-MS was not reliable for measuring 3β-hydroxy-Δ5-bile acid sulfates; however, direct analysis of urine by fast atom bombardment mass spectrometry yielded meaningful semiquantitative assessment of urinary excretion.

CONCLUSIONS

The tandem mass spectrometry method described here for the measurement of 3β-hydroxy-Δ5-bile acid sulfates in urine can be applied to the diagnosis and accurate monitoring of responses to primary bile acid therapy in HSD3B7 patients.

Homozygosity mapping identifies a bile acid biosynthetic defect in an adult with cirrhosis of unknown etiology

The most common inborn error of bile acid metabolism is 3β-hydroxy-Δ(5)-C(27)-steroid oxidoreductase (3β-HSD) deficiency, a disorder that usually presents in early childhood with hepatic dysfunction. Timely diagnosis of this disorder is crucial because it can be effectively treated with primary bile acid replacement. Here we describe a 24-year-old woman from Iran with cirrhosis of unknown etiology. Her sister and a first cousin died of cirrhosis (ages 19 and 6 years) and another 32-year-old first cousin had a self-limited liver disorder in childhood that resolved at age 9 years. The family history suggested that the affected family members were homozygous for a mutant allele inherited identical-by-descent. A genome-wide analysis of 2.4 million single nucleotide polymorphisms was performed to identify regions of homozygosity that were present in the proband and the 32-year-old first cousin, but not in a healthy relative. One of these regions contained the gene encoding 3β-HSD (HSD3B7). Sequence analysis of HSD3B7 revealed that the proband and her 32-year-old cousin were homozygous for a frameshift mutation (c.45_46del AG, p.T15Tfsx27) in exon 1. The diagnosis of 3β-HSD deficiency was confirmed by documenting high levels of 3β-hydroxy-Δ(5) bile acids in the serum of the proband and the 32-year-old first cousin using mass spectrometry. To our knowledge, the 32-year-old relative in this family represents the oldest asymptomatic patient with this disorder.

CONCLUSION

This study highlights the clinical utility of homozygosity mapping in diagnosing autosomal recessive metabolic disorders. This family illustrates the wide variation in expressivity that occurs in 3β-HSD deficiency and underscores the need to consider a bile acid synthetic defect as a possible cause of liver disease in adults.

Oral cholic acid for hereditary defects of primary bile acid synthesis: a safe and effective long-term therapy

BACKGROUND & AIMS

Oral bile acid replacement has been shown to be an effective therapy in primary bile acid synthesis defects, but to date there have been no reports of the long-term effects of this therapy. The aim of the study was to evaluate the long-term effectiveness and safety of cholic acid (CA) therapy.

METHODS

Fifteen patients with either 3beta-hydroxy-Delta(5)-C(27)-steroid oxidoreductase (3beta-HSD) (n = 13) or Delta(4)-3-oxosteroid 5beta-reductase (Delta(4)-3-oxo-R) (n = 2) deficiency confirmed by mass spectrometry and gene sequencing received oral CA and were followed up prospectively.

RESULTS

CA therapy was started at a median age of 3.9 years (range, 0.3-13.1 years). The median follow-up with treatment was 12.4 years (range, 5.6-15 years). The mean daily dose of CA was initially 13 mg/kg and was 6 mg/kg at last evaluation. During CA therapy, physical examination findings, laboratory test results, and findings on sonography normalized. Mass spectrometry analysis of urine showed that excretion of the atypical metabolites was reduced by 500-fold and 30-fold in 3beta-HSD and Delta(4)-3-oxo-R deficiency, respectively, and total urinary bile acid excretion decreased dramatically. Liver biopsies performed in 14 patients after at least 5 years of CA therapy showed marked improvement, especially in patients with the 3beta-HSD deficiency. CA was well tolerated with all children developing normally, including 2 women having 4 normal pregnancies during treatment.

CONCLUSIONS

Oral CA therapy is a safe and effective long-term treatment of the most common primary bile acid synthesis defects.

Mechanisms of disease: Inborn errors of bile acid synthesis

Inborn errors of bile acid synthesis are rare genetic disorders that can present as neonatal cholestasis, neurologic disease or fat-soluble-vitamin deficiencies. There are nine known defects of bile acid synthesis, including oxysterol 7alpha-hydroxylase deficiency, Delta(4)-3-oxosteroid-5beta-reductase deficiency, 3beta-hydroxy-Delta(5)-C(27)-steroid dehydrogenase deficiency, cerebrotendinous xanthomatosis (also known as sterol 27-hydroxylase deficiency), alpha-methylacyl-CoA racemase deficiency, and Zellweger syndrome (also known as cerebrohepatorenal syndrome). These diseases are characterized by a failure to produce normal bile acids and an accumulation of unusual bile acids and bile acid intermediaries. Individuals with inborn errors of bile acid synthesis generally present with the hallmark features of normal or low serum bile acid concentrations, normal gamma-glutamyl transpeptidase concentrations and the absence of pruritus. Failure to diagnose any of these conditions can result in liver failure or progressive chronic liver disease. If recognized early, many patients can have a remarkable clinical response to oral bile acid therapy.

Review article: The function and regulation of proteins involved in bile salt biosynthesis and transport

BACKGROUND

Bile salts are produced and secreted by the liver and are required for intestinal absorption of fatty food components and excretion of endobiotics and xenobiotics. They are reabsorbed in the terminal ileum and transported back to the liver via the portal tract. Dedicated bile salt transporters in hepatocytes and enterocytes are responsible for the unidirectional transport of bile salts in the enterohepatic cycle.

AIM

To give an overview of the function and regulations of proteins involved in bile salt synthesis and transport.

METHODS

Data presented are obtained from PubMed-accessible literature combined with our own recent research.

RESULT

Hepatocytes and enterocytes contain unique bile salt importers (sodium-taurocholate cotransporting polypeptide and apical sodium-dependent bile acid transporter, respectively) and exporters (bile salt export pump and organic solute transporter alpha-beta, respectively). Enzymes involved in bile salt biosynthesis reside in different subcellular locations, including the endoplasmic reticulum, mitochondria, cytosol and peroxisomes. Defective expression or function of the transporters or enzymes may lead to cholastasis. The bile salt-activated transcription factor Farnesoid X receptor controls expression of genes involved in bile salt biosynthesis and transport.

CONCLUSIONS

Detailed knowledge is available about the enzymes and transporters involved in bile salt homeostasis and how their defective function is associated with cholestasis. In contrast, the process of intracellular bile salt transport is largely unexplored.

Cholestatic liver disease in adults may be due to an inherited defect in bile acid biosynthesis

BACKGROUND

An increasing number of treatable inborn errors of bile acid synthesis have been described, primarily in infants with severe cholestatic liver disease.

RESULTS

The present patient, whose two older siblings had died from progressive cholestatic liver disease, developed neonatal cholestasis and rickets but recovered during the childhood years and follow-up was terminated at 12 years of age. The patient presented again at 26 years of age with jaundice and pathological liver function tests. This was normalized upon treatment with ursodeoxycholic acid. Electrospray mass spectrometry of urine showed predominance of unsaturated bile acids, characteristic of 3beta-hydroxy-Delta5-C27-steroid dehydrogenase/isomerase (HSD3B7) deficiency. The activity of HSD3B7 in cultured fibroblasts was less than 5% of normal. A single homozygous mutation was found in exon 4 leading to an amino acid exchange (S162R) and loss of enzyme activity.

CONCLUSION

This case illustrates that infants with an inherited absence of HSD3B7 may survive the neonatal period of life and childhood without treatment with bile acids. A low level of sulphation of the abnormal trihydroxy bile acid formed as a result of enzyme deficiency may be of importance for survival. The possibility that liver disease presenting in the adult may be due to a mutation in the HSD3B7 gene should be considered, especially in cases with familial occurrence of liver disease and earlier periods of liver dysfunction.

Defects in bile acid biosynthesis--diagnosis and treatment

Bile acid synthetic defects represent a specific category of metabolic liver disease. This article highlights the history and summarizes our analytical approach to the diagnosis and treatment of genetic defects in bile acid synthesis. By the application of mass spectrometry as a screening tool, it is possible to perform rapid diagnosis of potential inborn errors in bile acid synthesis from urinary bile acid analysis. Molecular techniques then afford the identification of specific mutations in genes encoding the enzymes responsible for bile acid synthesis. Using this approach, 6 of the 7 known genetic defects that are causes of progressive cholestatic liver disease, syndromes of fat-soluble vitamin malabsorption, or neurological disease, have been characterized. Bile-acid therapy using oral cholic acid has proven effective in most of these bile acid synthetic defects making early diagnosis crucial to optimum clinical prognosis.

Genetics of familial intrahepatic cholestasis syndromes

Bile acids and bile salts have essential functions in the liver and in the small intestine. Their synthesis in the liver provides a metabolic pathway for the catabolism of cholesterol and their detergent properties promote the solubilisation of essential nutrients and vitamins in the small intestine. Inherited conditions that prevent the synthesis of bile acids or their excretion cause cholestasis, or impaired bile flow. These disorders generally lead to severe human liver disease, underscoring the essential role of bile acids in metabolism. Recent advances in the elucidation of gene defects underlying familial cholestasis syndromes has greatly increased knowledge about the process of bile flow. The expression of key proteins involved in bile flow is tightly regulated by transcription factors of the nuclear hormone receptor family, which function as sensors of bile acids and cholesterol. Here we review the genetics of familial cholestasis disorders, the functions of the affected genes in bile flow, and their regulation by bile acids and cholesterol.

Bile acid synthetic defects and liver disease: a comprehensive review

Bile acid synthetic defects (BASD), uncommon genetic disorders that are responsible for approximately 2% of persistent cholestasis in infants, are reviewed with emphasis on morphology of associated liver disease. The associated liver diseases may be life threatening, and are treatable, usually by replacement of deficient primary bile acids. Specific diagnosis is made by analysis of body fluids (bile, blood, and urine) using fast atom bombardment-mass spectroscopy (FAB-MS) and gas chromatography-mass spectroscopy (GC-MS). Inborn errors have been demonstrated for four single enzymes involved in modification of the sterol nucleus and in five steps in modification of the side-chain to form cholic and chenodeoxycholic acids, the primary bile acids. With few exceptions, BASD cause liver diseases that vary from severe to mild depending on the defect. In three of four known defects of sterol nucleus modification, liver disease is progressive. Progression of liver disease is most rapid when the defect results in accumulation of toxic monohydroxy and unsaturated oxo-bile acids. Liver disease may be transient, delayed in onset and mild. Reduced bile flow caused by atypical bile acids contributes to cholestasis and may be the dominant factor in defects of side-chain synthesis, peroxisomal abiogenesis and S-L-O syndrome. Pathological findings may include intralobular cholestasis with giant cell transformation, prevalence of necrotic hepatocytes including giant cell forms, and hepatitic injury confined to the portal limiting plate where the smallest bile ductules may be injured and where fibrosis typically develops. Interlobular bile ducts are usually spared. Ultrastructure of liver reveals nonspecific changes with the possible exception of unusual canalicular morphology in some defects. The course of BASD may be modified by replacement of deficient primary bile acids, which produces beneficial feedback inhibition of abnormal bile acid production and enhances choluresis. Giant cell transformation is present in all symptomatic infants with BASD and seems to have a more consistent association with BASD than with the many other liver diseases in infants where it occurs. We hypothesize that immature hepatocytes of infants may fuse to form multinucleate hepatocytes whenever atypical or toxic bile acids are present and the pool of normal bile acids is critically reduced.

The bile acid synthetic gene 3beta-hydroxy-Delta(5)-C(27)-steroid oxidoreductase is mutated in progressive intrahepatic cholestasis

We used expression cloning to isolate cDNAs encoding a microsomal 3beta-hydroxy-Delta(5)-C(27)-steroid oxidoreductase (C(27) 3beta-HSD) that is expressed predominantly in the liver. The predicted product shares 34% sequence identity with the C(19) and C(21) 3beta-HSD enzymes, which participate in steroid hormone metabolism. When transfected into cultured cells, the cloned C(27) 3beta-HSD cDNA encodes an enzyme that is active against four 7alpha-hydroxylated sterols, indicating that a single C(27) 3beta-HSD enzyme can participate in all known pathways of bile acid synthesis. The expressed enzyme did not metabolize several different C(19/21) steroids as substrates. The levels of hepatic C(27) 3beta-HSD mRNA in the mouse are not sexually dimorphic and do not change in response to dietary cholesterol or to changes in bile acid pool size. The corresponding human gene on chromosome 16p11.2-12 contains six exons and spans 3 kb of DNA, and we identified a 2-bp deletion in the C27 3beta-HSD gene of a patient with neonatal progressive intrahepatic cholestasis. This mutation eliminates the activity of the enzyme in transfected cells. These findings establish the central role of C(27) 3beta-HSD in the biosynthesis of bile acids and provide molecular tools for the diagnosis of a third type of neonatal progressive intrahepatic cholestasis associated with impaired bile acid synthesis.

Liver disease caused by disorders of bile acid synthesis

Bile acid synthetic defects are uncommon disorders that cause progressive cholestatic liver disease that is often lethal in infancy or early childhood. Five specific primary defects have been described. Diagnosis is based on mass spectrometry of urine and serum. Pathogenesis of liver injury is related to persistent reduction in levels of normal bile acids and accumulation of abnormal, potentially hepatotoxic, intermediaries. Sites of injury are the liver cell, the bile canaliculus, and the smallest bile ductules. The interlobular bile ducts are normal. The liver lesion is progressive chronic hepatitis with an especially high incidence of GCT in patients who present in infancy. Bile acid replacement therapy is usually effective in arresting the liver injury. Regression of liver damage has been documented during treatment of patients who were diagnosed early in life. Because bile acid synthetic disorders are the only cholestatic diseases of infancy in which GCT of hepatocytes is consistently present, the author suggest that the injury responsible for GCT may be specific for toxic bile acids. Accordingly, immaturity of the bile acid synthetic pathway may render many otherwise normal infants vulnerable to transient "neonatal hepatitis" with GCT in a broad range of cholestatic disorders.

An inborn error of bile acid synthesis (3beta-hydroxy-delta5-C27-steroid dehydrogenase deficiency) presenting as malabsorption leading to rickets

Deficiency of 3beta-hydroxy-delta5-C27-steroid dehydrogenase (3beta-HSDH), the enzyme that catalyses the second reaction in the principal pathway for the synthesis of bile acids, has been reported to present with prolonged neonatal jaundice with the biopsy features of neonatal hepatitis. It has also been shown to present between the ages of 4 and 46 months with jaundice, hepatosplenomegaly, and steatorrhoea (a clinical picture resembling progressive familial intrahepatic cholestasis). This paper reports two children with 3beta-HSDH deficiency who developed rickets during infancy and did not develop clinically evident liver disease until the age of 3 years. Bile acid replacement resulted in considerable clinical and biochemical improvement. The importance of thorough investigation of fat soluble vitamin deficiencies in infancy is emphasised.

Inborn errors of bile acid biosynthesis and transport. Novel forms of metabolic liver disease

Defective bile acid biosynthesis, metabolism, and transport can now be delineated in a wide variety of disease states. This ability to recognize specific aberrations, such as the documented inborn errors in bile acid biosynthesis manifesting as neonatal cholestasis, offers new opportunities for therapeutic intervention. Future studies should determine the incidence of bile acid biosynthetic and transport defects in patients with enigmatic and unexplained liver diseases.

A method for the quantitation of conjugated bile acids in dried blood spots using electrospray ionization-mass spectrometry

Bile acid concentrations are elevated in the blood of neonates with cholestatic hepatobiliary disorders providing a possible means of screening for treatable conditions including biliary atresia. A method is described for the determination of concentrations of conjugated bile acids in dried blood spots using electrospray ionization mass spectrometry. Bile acids were eluted from the blood spots using methanol containing, as internal standards, the taurine and glycine conjugates of D4-chenodeoxycholic acid and D4-cholic acid. The samples were then reconstituted in acetonitrile/water and injected by autosampler into the electrospray source operating in negative ion mode. Optimal conditions were determined for both single quadrupole and tandem mass spectrometry analysis. Blood spot bile acid profiles were studied in two groups of infants (< 1 y), a cholestatic group (conjugated bilirubin > 25 mumol/L; n = 49), and a control group (n = 96). The best discrimination between the two groups was provided by measurements of taurodihydroxycholanoates (normal < 5 mumol/L; cholestatic group 18-94 mumol/L) and glycodihydroxycholanoates (normal < 5 mumol/L; cholestatic group 11-66 mumol/L). The method can also be adapted to detect unusual bile acids which are diagnostic of inborn errors of bile acid synthesis and peroxisomal disorders. The method is fast, reliable, reproducible, and relatively cheap; however, much more work is required to determine whether it can be used for mass screening for cholestasis. It will be necessary to show that measurement of bile acid concentrations in blood spots obtained at 7-10 d can be used to detect infants who currently present with jaundice, pale stools, and dark urine during the first 6 mo of life.

Differential interaction of bile acids from patients with inborn errors of bile acid synthesis with hepatocellular bile acid transporters

People with genetic or acquired defects in the biosynthesis of bile acids may suffer from cholestasis. Patients with a deficiency of 3 beta-hydroxy-delta 5-C27-steroid dehydrogenase/isomerase from 3 beta, 7 alpha-dihydroxy- and 3 beta, 7 alpha, 12 alpha-trihydroxy-5-cholenoic acids, the sulfated and partially glycine-conjugated forms of which are found in their urine and bile. 3-Oxo-delta 4 bile acids are detected in the urine of patients with a deficiency of 5 beta-reductase. It has been postulated that these unusual bile acids might act as cholestatic agents in these patients. The aim of the present study was to test this hypothesis in an in vitro system, since the abnormal bile acids would be metabolized in in vivo experiments. Basolateral (sinusoidal) and canalicular plasma membrane vesicles were isolated from rat liver. A rapid filtration method was used to determine transport of cholyltaurine in the presence of model bile acids into the isolated vesicles. It was found that 3 beta, 7 alpha-dihydroxy-5-cholenoic acid and 7 alpha-hydroxy-3-oxo-4-cholenoic acid both inhibited the apical, ATP-dependent transport system for cholyltaurine in a competitive manner with K(m) values of 15 microM and 16 microM, respectively. Radioactively labeled 3 beta, 7 alpha-dihydroxy-5-cholenoyltaurine and 7 alpha-hydroxy-3-oxo-4-cholenoyltaurine were not transported by the same transport system. The same types of experiments were performed with basolateral plasma membrane vesicles. It was found that, in contrast to the canalicular ATP-dependent bile acid transport system, only 7 alpha-hydroxy-3-oxo-4-cholenoyltaurine was a competitive inhibitor of the sodium-dependent transport system for cholyltaurine with a K(m) of 16 microM. Studies with radioactively labeled 7 alpha-hydroxy-3-oxo-4-cholenoyltaurine and 3 beta, 7 alpha-dihydroxy-5-cholenoyltaurine revealed that 7 alpha-hydroxy-3-oxo-4-cholenoyltaurine was transported in a sodium-dependent manner into basolateral rat liver plasma membrane vesicles, whereas 3 beta, 7 alpha-dihydroxy-5-cholenoyltaurine was not transported in a sodium-dependent way. These results support the hypothesis that the unusual bile acids found in patients with defects in bile acid biosynthesis might act as cholestatic agents by inhibiting the canalicular ATP-dependent transport system for bile acids which constitutes the rate-limiting step in the overall process of bile acid transport across hepatocytes. Furthermore, the experiments demonstrated that, despite similar substrate specificities, the basolateral sodium-dependent and the apical ATP-dependent transport system for cholyltaurine might have different recognition sites for bile acids.