Unconjugated bilirubin and an increased proportion of bilirubin monoconjugates in the bile of patients with Gilbert's syndrome and Crigler-Najjar disease

Systems pharmacology modeling of drug-induced hyperbilirubinemia: Differentiating hepatotoxicity and inhibition of enzymes/transporters

Elevations in serum bilirubin during drug treatment may indicate global liver dysfunction and a high risk of liver failure. However, drugs also can increase serum bilirubin in the absence of hepatic injury by inhibiting specific enzymes/transporters. We constructed a mechanistic model of bilirubin disposition based on known functional polymorphisms in bilirubin metabolism/transport. Using physiologically based pharmacokinetic (PBPK) model-predicted drug exposure and enzyme/transporter inhibition constants determined in vitro, our model correctly predicted indinavir-mediated hyperbilirubinemia in humans and rats. Nelfinavir was predicted not to cause hyperbilirubinemia, consistent with clinical observations. We next examined a new drug candidate that caused both elevations in serum bilirubin and biochemical evidence of liver injury in rats. Simulations suggest that bilirubin elevation primarily resulted from inhibition of transporters rather than global liver dysfunction. We conclude that mechanistic modeling of bilirubin can help elucidate underlying mechanisms of drug-induced hyperbilirubinemia, and thereby distinguish benign from clinically important elevations in serum bilirubin.

Acute cholangitis in an old patient with Crigler-Najjar syndrome type II - a case report

Background

Crigler-Najjar syndrome (CN) is a very rare genetic disorder characterized by an inability to conjugate bilirubin. Contrary to CN type I, patients with CN II exhibit residual capacity to conjugate bilirubin and may present a normal life expectancy.

Case presentation

We report an unusual late diagnosis of CN type II in an 80-year-old female admitted with severe acute cholangitis. While the patient present typical clinical and radiologic signs of bile duct obstruction and cholangitis, her blood analysis showed severe unconjugated hyperbilirubinemia. Endoscopic retrograde cholangiopancreatography confirmed the diagnosis and allowed therapeutic intervention. The anatomopathologic examination of her gallbladder following cholecystectomy showed signs of chronic cholecystitis.

Conclusion

The risk of gallstone disease may be increased in patients with CN syndrome. While unusual, we alert to this curious and potential life-threatening presentation.

Restriction fragment length polymorphism effectively identifies exon 1 mutation of UGT1A1 gene in patients with Gilbert's Syndrome

BACKGROUND & AIMS

Gilbert's syndrome causes pharmacological variation in drug glucuronidation and unexpected toxicity from therapeutic agents. The two common genotypes of Gilbert's syndrome are a dinucleotide polymorphism (TA)7 in TATA-Box as well as the 211G>A mutation in the coding exon 1, particularly in Asians, of human UGT1A1 gene. In this study, we aimed to establish an effective method to detect the 211G>A mutation.

METHODS

The coding exon 1 sequence of human UGT1A1 gene was analysed by Vector NTI software. The 211G>A mutation in the coding exon 1 of UGT1A1 gene was determined by restriction fragment length polymorphism (RFLP) method. Serum total bilirubin level was measured as well.

RESULTS

A newly identified BsmBI site was located in the coding exon 1 of UGT1A1 gene. The 211G>A mutation in the coding exon 1 of UGT1A1 gene was determined by DNA RFLP. Furthermore, we reported our present work on genetic analysis of mutations of UGT1A1 gene, and the correlation of UGT1A1 mutations with serum total bilirubin levels in Taiwanese population. The results showed that 15 subjects carried 211G>A mutation in 23 subjects related with Gilbert's syndrome. The homozygous 211G>A mutant as well as simultaneously heterozygous mutants both in TATA-Box and 211G>A significantly increased the risk of Gilbert's syndrome similar to subjects carrying homozygous TATA-Box mutant.

CONCLUSIONS

BsmBI RFLP is an effective method to detect 211G>A mutation in the coding exon 1 of UGT1A1 gene. The common 211G>A mutation is one of the causes of Gilbert's syndrome in Taiwanese population.

Quantitative assessment of the multiple processes responsible for bilirubin homeostasis in health and disease

Serum bilirubin measurements are commonly obtained for the evaluation of ill patients and to screen for liver disease in routine physical exams. An enormous research effort has identified the multiple mechanisms involved in the production and metabolism of conjugated (CB) and unconjugated bilirubin (UB). While the qualitative effects of these mechanisms are well understood, their expected quantitative influence on serum bilirubin homeostasis has received less attention. In this review, each of the steps involved in bilirubin production, metabolism, hepatic cell uptake, and excretion is quantitatively examined. We then attempt to predict the expected effect of normal and defective function on serum UB and CB levels in health and disease states including hemolysis, extra- and intrahepatic cholestasis, hepatocellular diseases (eg, cirrhosis, hepatitis), and various congenital defects in bilirubin conjugation and secretion (eg, Gilbert's, Dubin-Johnson, Crigler-Najjar, Rotor syndromes). Novel aspects of this review include: 1) quantitative estimates of the free and total UB and CB in the plasma, hepatocyte, and bile; 2) detailed discussion of the important implications of the recently recognized role of the hepatic OATP transporters in the maintenance of CB homeostasis; 3) discussion of the differences between the standard diazo assay versus chromatographic measurement of CB and UB; 4) pharmacokinetic implications of the extremely high-affinity albumin binding of UB; 5) role of the enterohepatic circulation in physiologic jaundice of newborn and fasting hyperbilirubinemia; and 6) insights concerning the clinical interpretation of bilirubin measurements.

Single liver lobe repopulation with wildtype hepatocytes using regional hepatic irradiation cures jaundice in Gunn rats

BACKGROUND AND AIMS

Preparative hepatic irradiation (HIR), together with mitotic stimulation of hepatocytes, permits extensive hepatic repopulation by transplanted hepatocytes in rats and mice. However, whole liver HIR is associated with radiation-induced liver disease (RILD), which limits its potential therapeutic application. In clinical experience, restricting HIR to a fraction of the liver reduces the susceptibility to RILD. Here we test the hypothesis that repopulation of selected liver lobes by regional HIR should be sufficient to correct some inherited metabolic disorders.

METHODS

Hepatocytes (10(7)) isolated from wildtype F344 rats or Wistar-RHA rats were engrafted into the livers of congeneic dipeptidylpeptidase IV deficient (DPPIV(-)) rats or uridinediphosphoglucuronateglucuronosyltransferase-1A1-deficient jaundiced Gunn rats respectively by intrasplenic injection 24 hr after HIR (50 Gy) targeted to the median lobe, or median plus left liver lobes. An adenovector expressing hepatocyte growth factor (10(11) particles) was injected intravenously 24 hr after transplantation.

RESULTS

Three months after hepatocyte transplantation in DPPIV(-) rats, 30-60% of the recipient hepatocytes were replaced by donor cells in the irradiated lobe, but not in the nonirradiated lobes. In Gunn rats receiving median lobe HIR, serum bilirubin declined from pretreatment levels of 5.17 ± 0.78 mg/dl to 0.96 ± 0.30 mg/dl in 8 weeks and remained at this level throughout the 16 week observation period. A similar effect was observed in the group, receiving median plus left lobe irradiation.

CONCLUSIONS

As little as 20% repopulation of 30% of the liver volume was sufficient to correct hyperbilirubinemia in Gunn rats, highlighting the potential of regiospecific HIR in hepatocyte transplantation-based therapy of inherited metabolic liver diseases.

Is Ceftriaxone-Induced Biliary Pseudolithiasis Influenced by UDP-Glucuronosyltransferase 1A1 Gene Polymorphisms?

Ceftriaxone (cfx), a third-generation cephalosporin antibiotic, leads to transient cholelithiasis in some children, also known as pseudolithiasis. However, the underlying pathogenetic mechanism of this adverse effect has not yet been elucidated. We describe 3 children with ceftriaxone-induced pseudolithiasis, who were also carriers of the A(TA)₇TAA polymorphism of the UGT1A1 gene, implying that a cause and effect relation may exist.

Assessment of UGT polymorphisms and neonatal jaundice

Elevation of the serum bilirubin level is a common, if not universal, finding during the first week of life. This can be a transient phenomenon that resolves spontaneously or can signify a serious or even life-threatening condition. There are many causes of hyperbilirubinemia and related therapeutic and prognostic implications. The diseases in which there is a primary disorder of the metabolism of bilirubin will be reviewed regarding their clinical presentation, pathophysiology, diagnosis, and treatment. These disorders-Gilbert's syndrome and Crigler-Najjar Syndrome-both involve abnormalities in bilirubin conjugation secondary to deficiency of bilirubin uridine diphosphate glucuronosyltransferase. The purpose of this article is to review the current understanding of the genetic polymorphisms that result in these diseases and discuss recent advances in diagnosis and treatment.

Loci from a genome-wide analysis of bilirubin levels are associated with gallstone risk and composition

BACKGROUND & AIMS

Genome-wide association studies have mapped loci that are associated with serum levels of bilirubin. Bilirubin is a major component of gallstones so we investigated whether these variants predict gallstone bilirubin content and overall risk for gallstones.

METHODS

Loci that were identified in a meta-analysis to attain a genome-wide significance level of a P value less than 1.0×10(-7) (UGT1A1, SLCO1B1, LST-3TM12, SLCO1A2) were analyzed in 1018 individuals with known gallstone composition. Gallstone risk was analyzed in 2606 German choleystecomized individuals and 1121 controls and was replicated in 210 cases and 496 controls from South America.

RESULTS

By using the presence of bilirubin as a phenotype, variants rs6742078 (UGT1A1; P = .003), rs4149056 (SLCO1B1; P = .003), and rs4149000 (SLCO1A2; P = .015) were associated with gallstone composition. In regression analyses, only UGT1A1 and SLCO1B1 were independently retained in the model. UGT1A1 (rs6742078; P = .018) was associated with overall gallstone risk. In a sex-stratified analysis, only male carriers of rs6742078 had an increased risk for gallstone disease (P = 2.1×10(-7); odds ratio(recessive), 2.34; P(women) = .47). The sex-specific association of rs6742078 was confirmed in samples from South America (P(men) = .046; odds ratio(recessive), 2.19; P(women) = .96).

CONCLUSIONS

The UGT1A1 Gilbert syndrome variant rs6742078 is associated with gallstone disease in men; further studies are required regarding the sex-specific physiology of bilirubin and bile acid metabolism. Variants of ABCG8 and UGT1A1 are the 2 major risk factors for overall gallstone disease, they contribute a population attributable risk of 21.2% among men.

Hereditary liver disease: gallstones

Gallstones are common in Western countries and due to pain and complications pose a substantial burden on health care systems. In general, cholesterol gallstones are distinguished from bilirubin gallstones. Bilirubin gallstones form if the ion product of unconjugated bilirubin and calcium in gallbladder bile exceeds the solubilisation capacities of mixed micelles and vesicles. Cholesterol gallstones develop if the amount of cholesterol in gallbladder bile exceeds the maximum concentration that is soluble at the given concentration of bile salts and phospholipids. In addition, cholesterol gallstone formation requires hypomotility of the gallbladder and a mucin gel as nucleation matrix for monohydrate crystals. The individual risk of gallstone formation is determined by interactions of lithogenic alleles of gallstone susceptibility genes and multiple environmental factors. For asymptomatic gallstones, expectant management is recommended, whereas an episode of gallstone-associated pain substantially increases the risk of complications such as cholecystitis, cholangitis and pancreatitis and therefore necessitates cholecystectomy.

Gilbert syndrome as a predisposing factor for cholelithiasis risk in the Greek adult population

We investigated the hypothesis that coinheritance of the common A(TA)(n)TAA promoter mutation at the UGT1A1 locus associated with Gilbert syndrome is a risk factor for gallstone formation in a homogeneous adult population, by conducting a case-control study that included 198 adult patients with cholelithiasis and 152 healthy controls both of Greek origin. Three genotypes were found: 7/7 (17.8% in controls and 23.3% in patients), 6/7 (33.5% in controls and 46.5% in patients), and normal homozygous 6/6 (48.7% in controls and 30.3% in patients). The Gilbert UGT1A1 genotypes 6/7 and 7/7 show significant association (odds ratio 2.225, 95% confidence interval 1.373-3.605, p=0.001, and odds ratio 2.101, 95% confidence interval 1.171-3.770, p=0.013, respectively) with cholelithiasis risk. This association supports the theory that genetic factors are responsible for a fraction of symptomatic gallstone disease; however, further studies are required in different ethnic groups to fully elucidate the involvement of Gilbert syndrome in gallstone disease.

Drug-induced acute cholestatic liver damage in a patient with mutation of UGT1A1

BACKGROUND

A 54-year-old woman presented with a 3-week history of fatigue and with jaundice that began 2 days before admission. She had been undergoing treatment with flavoxate for urinary incontinence (for 2 months before admission) and with tibolone for climacteric syndrome (for 6 months before admission). Laboratory tests revealed elevated concentrations of aminotransferases, bilirubin, gamma-glutamyltransferase and alkaline phosphatase. Liver biopsy revealed histological evidence of subacute, drug-induced liver damage.

INVESTIGATIONS

Physical examination, liver function tests, serology tests, autoantibody tests, genetic analysis of the TATA box of the UGT1A1 gene, ultrasonography and CT scan; MRI cholangiography; liver biopsy.

DIAGNOSIS

Drug-related hepatitis in a patient with Gilbert's syndrome.

MANAGEMENT

Flavoxate and tibolone were discontinued. Liver function test results improved progressively and normalized after almost 2 months.

[Crigler-Najjar syndrome: diagnosis and treatment]

INTRODUCTION

Crigler-Najjar syndrome (CNS) is a very rare disease characterized by severe indirect hyperbilirubinemia from birth with normal liver function. It may cause kernicterus at any age. This disease is due to a total or partial deficiency of the UDP-glucuronosyltransferase enzyme caused by a mutation of the five exons of the ULT1A1 gene.

PATIENTS AND METHODS

We reviewed the clinical outcomes of 7 children diagnosed with CNS between 1987 and 2004.

RESULTS

There were three boys and four girls (two of which were homozygote twins). Two children had familial consanguinity. Three out of the six families had another healthy child. The mean follow-up was 8.3 years (14 months-17 years). In all patients, jaundice was detected in the first 3 days of life. The children were admitted to hospital between the fourth and the sixtieth day of life with jaundice and indirect bilirubin levels of between 12.5 and 32 mg/dl. In all patients, hemolysis was ruled out and hepatic function was normal. The diagnosis was based on genetic study in 4 patients, on inactive UGT enzyme in liver in 1 patient, and on clinical features exclusively in 2 patients. Treatment consisted of phenobarbital and phototherapy from 8 to 16 hours a day in all patients except three. Associated calcium salts were found in 5 patients and cholestyramine was found in two. Two patients developed kernicterus. Two underwent liver transplantation and bilirubin levels became normal. The remaining patients maintained indirect bilirubin from 15 to 25 mg/dl with no associated neurological alterations.

CONCLUSIONS

Patients with CNS are at greater risk of developing kernicterus, mostly associated with indirect bilirubin levels of around 25 mg/dl. Phototherapy is very useful in these patients but the only definitive treatment is liver transplantation.

Coinheritance of Gilbert syndrome-associated UGT1A1 mutation increases gallstone risk in cystic fibrosis

The prevalence of "black" pigment gallstones is increased in patients with cystic fibrosis (CF). Bile acid malabsorption with augmented bilirubin uptake from the intestine and the development of "hyperbilirubinbilia" have been proposed as key factors in gallstone formation in CF patients. We have now tested the hypothesis that the coinheritance of the common UGT1A1 promoter mutation associated with Gilbert syndrome is an additional lithogenic risk factor for gallstone formation in CF. Our results show that patients with CF and gallstones are significantly more likely to carry at least one Gilbert UGT1A1 allele compared with stone-free patients (OR 7.3; P = .042) and that these carriers display significantly higher serum levels of unconjugated bilirubin (P = .002). In conclusion, the Gilbert UGT1A1 allele increases the risk of gallstone formation in CF. This genetic association supports the current concept for gallstone formation in CF and suggests that genetic and exogenous sources contributing to hyperbilirubinbilia might be lithogenic in CF patients.

Enterohepatic cycling of bilirubin as a cause of 'black' pigment gallstones in adult life

In contrast to bile salts, which undergo a highly efficient enterohepatic circulation with multiple regulatory and physiologic functions, glucuronic acid conjugates of bilirubin are biliary excretory molecules that in health do not have a continuing biologic life. Intestinal absorptive cells are devoid of recapture transporters for bilirubin conjugates, and their large size and polarity prevent absorption by passive diffusion. However, unconjugated bilirubin, the beta-glucuronidase hydrolysis product of bilirubin glucuronides can be absorbed passively from any part of the small and large intestines. This can occur only if unconjugated bilirubin is kept in solution and does not undergo rapid bacterial reduction to form urobilinoids. Here we collect, and in some cases reinterpret, experimental and clinical evidence to show that in addition to the well-known occurrence in newborns, enterohepatic cycling of unconjugated bilirubin can reappear in adult life. This happens as a result of several common conditions, particularly associated with bile salt leakage from the small intestine, the most notable ileal dysfunction resulting from any medical or surgical cause. We propose that when present in excess, colonic bile salts solubilize unconjugated bilirubin, delay urobilinoid formation, prevent calcium complexing of unconjugated bilirubin and promote passive absorption of unconjugated bilirubin from the large intestine. Following uptake, reconjugation, and resecretion into bile, this source of 'hyperbilirubinbilia' may be the important pathophysiological risk factor for 'black' pigment gallstone formation in predisposed adult humans.

Zinc sulfate inhibits the enterohepatic cycling of unconjugated bilirubin in subjects with Gilbert's syndrome

We have previously observed that UCB binds to ZnSO4 in vitro, and suppressed the biliary bilirubin secretion in the hamster. The aim of this study was designed to investigate whether Zn salts might inhibit the enterohepatic cycling of UCB in subjects with Gilbert s syndrome. Fifteen patients with Gilbert s syndrome and 5 normal healthy volunteers were included in this study according to the following criteria: fasting hyperbilirubinemia, no hemolysis, and free of any medication. Patients were randomly assigned to receive acute o chronic treatment. Subjects treated in acute form and normal healthy volunteers were treated with 40 mg of ZnSO4 in a single dose, where as patients treated in chronic form received 100 mg ZnSO4 in a single dose daily for 7 days. The serum UCB levels (mg/dL) decreased from 2.64 +/- 1.04 to 2.02 +/- 0.87 (p < 0.001) and 1.8 +/- 0.36 to 1.48 +/- 0.32 (p < 0.005) in subjects treated in acute an chronic form respectively, but not in the control group. Whereas, the serum Zn levels (mg/dL) increased from 96.3 +/- 16.8 to 118.8 +/- 19. 5, (p < 0.01) and from 117.6 +/- 8.5 to 130.7 +/- 6.6 (p < 0.03) in subjects treated in acute an chronic form and also in subjects in the control group (98.0 +/- 7.3 to 128.0 +/- 21.9) p < 0.03. This study showed that acute and chronic oral administration of ZnSO4 decreased serum UCB levels significantly in subjects with Gilbert s syndrome. Most likely by the inhibition of the "normal" enterohepatic cycling of UCB.

Zinc salts precipitate unconjugated bilirubin in vitro and inhibit enterohepatic cycling of bilirubin in hamsters

BACKGROUND

We have evidence for enterohepatic cycling of bilirubin experimentally and in vivo in humans. This study was designed to investigate whether Zn salts might inhibit such cycling of bilirubin.

MATERIALS AND METHODS

Micellar bile salt solutions with unconjugated bilirubin were prepared, appropriate concentrations of Zn salts were added, and unconjugated bilirubin precipitation was measured. Hamsters and Wistar rats were fed a chow diet or a chow diet enriched with 1% ZnSO4, and bilirubin secretion rates were monitored.

RESULTS

Unconjugated bilirubin was precipitated maximally (90%) after a 10-min incubation with 5 mM Zn salts in the pH range of 6.8-9.0. In control hamsters, total bilirubin secretion rates into bile were 36.0 +/- 2.8 nmol h(-1) 100g(-1) body weight, whereas they were 25.0 +/- 3.3 nmol h-1 100(-1) g in the ZnSO4 group (P < 0.05).

CONCLUSIONS

Zn salts that flocculate at physiological pH adsorb unconjugated bilirubin almost completely from unsaturated micellar BS solutions. In addition, Zn salts administered orally suppress biliary bilirubin secretion rates in hamsters. These findings suggest that the administration of Zn salts may inhibit the enterohepatic cycling of unconjugated bilirubin in humans who are predisposed to pigment gallstone formation due to diet, disease or drugs.

A replication-deficient rSV40 mediates liver-directed gene transfer and a long-term amelioration of jaundice in gunn rats

BACKGROUND & AIMS

In the quest for a recombinant viral vector for liver-directed gene therapy that would permit both prolonged and efficient transgene expression in quiescent hepatocytes in vivo and repeated administration, we evaluated a recombinant simian virus 40 (rSV40).

METHODS

The rSV40 was generated through replacement of the DNA encoding for the T antigens (Tag) by the coding region of human bilirubin-uridine 5'-diphosphate-glucuronosyl-transferase (BUGT) complementary DNA (SV-hBUGT). Helper-free rSV40 units were generated at infectious titers of 5 x 10(9) to 1 x 10(10) infectious units (IU)/mL in a Tag-producing packaging cell line (COS-7 cells).

RESULTS

After 1, 3, or 7 daily infusions of 3 x 10(9) IU of SV-hBUGT through an indwelling portal vein catheter in bilirubin-UGT-deficient jaundiced Gunn rats, mean serum bilirubin concentrations decreased by 40%, 60% and 70%, respectively, in 3 weeks and remained at those levels throughout the duration of the study (40 days). Results of liver biopsies from SV-hBUGT-treated Gunn rats, but not from controls, were positive for human BUGT DNA, messenger RNA, and protein. Bilirubin-UGT activity in liver homogenates was 8%-12% of normal, and bilirubin glucuronides were excreted in bile. Immunostaining showed that >50%-60% of hepatocytes stably expressed the transgene. Portal vein infusion of an rSV40 expressing hepatitis B surface antigen (HBsAg) in a naive Gunn rat and a Gunn rat that had received 7 injections of SV-BUGT resulted in approximately equal levels of hepatic expression of HBsAg, indicating that multiple inoculations of SV-BUGT did not elicit neutralizing antibodies. Plasma alanine aminotransferase levels and liver histology remained normal despite repeated injections of rSV40.

CONCLUSIONS

rSV40 vectors may represent a significant advance toward gene therapy for metabolic diseases.

Genetic lesions of bilirubin uridine-diphosphoglucuronate glucuronosyltransferase (UGT1A1) causing Crigler-Najjar and Gilbert syndromes: correlation of genotype to phenotype

Uridine-diphosphoglucuronate glucuronosyltransferases (UGTs) are a family of enzymes that conjugate various endogenous and exogenous compounds with glucuronic acid and facilitate their excretion in the bile. Bilirubin-UGT(1) (UGT1A1) is the only isoform that significantly contributes to the conjugation of bilirubin. Lesions in the gene encoding bilirubin-UGT(1), lead to complete or partial inactivation of the enzyme causing the rare autosomal recessively inherited conditions, Crigler-Najjar syndrome type-1 (CN-1) and type 2 (CN-2), respectively. Inactivation of the enzyme leads to accumulation of unconjugated bilirubin in the serum. Severe hyperbilirubinemia seen in CN-1 can cause bilirubin encephalopathy (kernicterus). Kernicterus can be fatal or may leave behind permanent neurological sequelae. Here, we have compiled more than 50 genetic lesions of UGT1A1 that cause CN-1 (including 9 novel mutations) or CN-2 (including 3 novel mutations) and have presented a correlation of structure to function of UGT1A1. In contrast to Crigler-Najjar syndromes, Gilbert syndrome is a common inherited condition characterized by mild hyperbilirubinemia. An insertional mutation of the TATAA element upstream to UGT1A1 results in a reduced level of expression of the gene. Homozygosity for the variant promoter is required for Gilbert syndrome, but not sufficient for manifestation of hyperbilirubinemia, which is partly dependent on the rate of bilirubin production. Several structural mutations of UGT1A1, for example, a G71R substitution, have been reported to cause mild reduction of UGT activity toward bilirubin, resulting in mild hyperbilirubinemia, consistent with Gilbert syndrome. When the normal allele of a heterozygote carrier for a Crigler-Najjar type structural mutation contains a Gilbert type promoter, intermediate levels of hyperbilirubinemia, consistent with the diagnosis of CN-2, may be observed.

Genetic lesions of bilirubin uridine-diphosphoglucuronate glucuronosyltransferase (UGT1A1) causing Crigler-Najjar and Gilbert syndromes: correlation of genotype to phenotype

Uridine-diphosphoglucuronate glucuronosyltransferases (UGTs) are a family of enzymes that conjugate various endogenous and exogenous compounds with glucuronic acid and facilitate their excretion in the bile. Bilirubin-UGT(1) (UGT1A1) is the only isoform that significantly contributes to the conjugation of bilirubin. Lesions in the gene encoding bilirubin-UGT(1), lead to complete or partial inactivation of the enzyme causing the rare autosomal recessively inherited conditions, Crigler-Najjar syndrome type-1 (CN-1) and type 2 (CN-2), respectively. Inactivation of the enzyme leads to accumulation of unconjugated bilirubin in the serum. Severe hyperbilirubinemia seen in CN-1 can cause bilirubin encephalopathy (kernicterus). Kernicterus can be fatal or may leave behind permanent neurological sequelae. Here, we have compiled more than 50 genetic lesions of UGT1A1 that cause CN-1 (including 9 novel mutations) or CN-2 (including 3 novel mutations) and have presented a correlation of structure to function of UGT1A1. In contrast to Crigler-Najjar syndromes, Gilbert syndrome is a common inherited condition characterized by mild hyperbilirubinemia. An insertional mutation of the TATAA element upstream to UGT1A1 results in a reduced level of expression of the gene. Homozygosity for the variant promoter is required for Gilbert syndrome, but not sufficient for manifestation of hyperbilirubinemia, which is partly dependent on the rate of bilirubin production. Several structural mutations of UGT1A1, for example, a G71R substitution, have been reported to cause mild reduction of UGT activity toward bilirubin, resulting in mild hyperbilirubinemia, consistent with Gilbert syndrome. When the normal allele of a heterozygote carrier for a Crigler-Najjar type structural mutation contains a Gilbert type promoter, intermediate levels of hyperbilirubinemia, consistent with the diagnosis of CN-2, may be observed.

Drug-mediated toxicity caused by genetic deficiency of UDP-glucuronosyltransferases

Human gene families encoding UDP-Glucuronosyltransferases (UGTs) have been identified and partially characterised. This family of enzymes catalysed the glucuronidation of drugs, xenobiotics and endobiotics. Genetic mutations and polymorphisms have been identified in several UGT genes and examples should be anticipated in all UGT genes. A common genetic defect in the TATA box promoter of the UGT1A1 gene is associated with Gilbert's Syndrome (GS) causing mild hyperbilirubinaemia. Recently, adverse effects of anticancer agents have been observed in Gilbert's patients due to reduced drug or bilirubin glucuronidation.

Correction of the UDP-glucuronosyltransferase gene defect in the gunn rat model of crigler-najjar syndrome type I with a chimeric oligonucleotide

Crigler-Najjar syndrome type I is characterized by unconjugated hyperbilirubinemia resulting from an autosomal recessive inherited deficiency of hepatic UDP-glucuronosyltransferase (UGT) 1A1 activity. The enzyme is essential for glucuronidation and biliary excretion of bilirubin, and its absence can be fatal. The Gunn rat is an excellent animal model of this disease, exhibiting a single guanosine (G) base deletion within the UGT1A1 gene. The defect results in a frameshift and a premature stop codon, absence of enzyme activity, and hyperbilirubinemia. Here, we show permanent correction of the UGT1A1 genetic defect in Gunn rat liver with site-specific replacement of the absent G residue at nucleotide 1206 by using an RNA/DNA oligonucleotide designed to promote endogenous repair of genomic DNA. The chimeric oligonucleotide was either complexed with polyethylenimine or encapsulated in anionic liposomes, administered i.v., and targeted to the hepatocyte via the asialoglycoprotein receptor. G insertion was determined by PCR amplification, colony lift hybridizations, restriction endonuclease digestion, and DNA sequencing, and confirmed by genomic Southern blot analysis. DNA repair was specific, efficient, stable throughout the 6-month observation period, and associated with reduction of serum bilirubin levels. Our results indicate that correction of the UGT1A1 genetic lesion in the Gunn rat restores enzyme expression and bilirubin conjugating activity, with consequent improvement in the metabolic abnormality.

Contribution of two missense mutations (G71R and Y486D) of the bilirubin UDP glycosyltransferase (UGT1A1) gene to phenotypes of Gilbert's syndrome and Crigler-Najjar syndrome type II

In our mutation analyses of bilirubin UDP glycosyltransferase (UGT1A1) gene, we encountered six patients with Crigler-Najjar syndrome type II who were double homozygotes for G71R and Y486D, a patient with Gilbert's syndrome who was a single homozygote for G71R and six patients with Gilbert's syndrome who were single heterozygote for G71R. To clarify the role of each mutation in the occurrence of the two syndromes, we made four mutant expression models. Relative UGT1A1 activity of a single homozygous model of G71R was 32.2+/-1.6% of normal, that of a single homozygous model of Y486D was 7.6+/-0.5%, that of a double homozygous model of G71R and Y486D was 6.2+/-1.6% and that of a heterozygous model of G71R was 60.2+/-3.5%. The decreased activities of the single homozygous model of G71R and the double homozygous model were at an appropriate level to be diagnosed as Gilbert's syndrome and CN-II, respectively. The activity of a single heterozygous model of G71R was somewhat high to develop to the phenotype of Gilbert's syndrome, suggesting the presence of additional factors for the etiology of Gilbert's syndrome.

Intestinal excretion of unconjugated bilirubin in man and rats with inherited unconjugated hyperbilirubinemia

Patients with Crigler-Najjar syndrome and Gunn rats cannot form bilirubin glucuronides owing to a lack of bilirubin UDP-glucuronosyltransferase activity. Because increased serum and tissue bilirubin levels remain constant, an alternative excretory route has to substitute for this deficiency. Gunn rats excrete in bile only 2-13% of the bilirubins eliminated in Wistar rats. In contrast, the biliary excretion rate of urobilinogen in Gunn and Wistar rats is comparable. The sum of bilirubins and urobilinogen excreted in the bile of Gunn rats amounts to 10-30% of pigments excreted in Wistar rats. Despite this low biliary excretion, the intestinal content and fecal excretion of bile pigments in Gunn and Wistar rats were similar. These data support an extrabiliary entrance of unconjugated bilirubin into the intestine. Additional proof for this was found in that the intestinal lumen of Gunn rats still contains a high amount of bilirubins and urobilinogen after 3 d of external biliary drainage. A similar procedure in Wistar rats resulted in the complete disappearance of bile pigments from the intestine. The direct transmural transport of bilirubin from blood to all parts of the intestinal lumen was demonstrated by injecting 14C-bilirubin i.v. into Gunn rats with isolated parts of small and large intestine. In Crigler-Najjar and Gilbert's syndrome patients, the biliary excretion of bile pigments has previously been shown to be strongly reduced. Their stools, however, contained approximately the same amount of bile pigments as in normal subjects. Although only traces of unconjugated bilirubin were detected in the stool of normal persons (4 +/- 3% of total bile pigments), higher amounts were found in patients with Crigler-Najjar disease (20 +/- 12&). These results suggest a direct intestinal permeation of unconjugated bilirubin in severe unconjugated hyperbilirubinemia both in man and rats.

Genetic and environmental factors associated with variation of human xenobiotic glucuronidation and sulfation

Glucuronidation and sulfation are phase 2 metabolic reactions catalyzed by large families of different isoenzymes in man. The textbook view that glucuronidation and sulfation lead to the production of harmless conjugates for simple excretion is not valid. Biologically active and toxic sulfates and glucuronides are produced and leed to adverse drug reactions, including immune hypersensitivity. Considerable variation in xenobiotic conjugation is observed as a result of altered expression of UDP-glucuronosyltransferases (UGTs) and sulfotransferases (STs). Recent cloning and expression of human cDNA encoding UGTs and STs has facilitated characterization of isoform substrate specificity, which has been further validated using specific antibodies and human tissue fractions. The availability of cloned/expressed human enzymes and specific antibodies has enabled the investigation of xenobiotic induction and metabolic disruption leeding to adverse responses. Genetic polymorphisms of glucuronidation and sulfation are known to exist although the characterization and assessment of the importance of these variations are hampered by appropriate ethical studies in men with suitable safe model compounds. Genetic analysis has allowed molecular identification of defects in well-known hyperbilirubinemias. However, full characterization of the specific functional roles of human UGTs and STs requires rigorous kinetic and molecular analyses of the role of each enzyme in vivo through the use of specific antibodies and inhibitors. This will leed to the better prediction of variation of xenobiotic glucuronidation and sulfation in man.

Current therapy for Crigler-Najjar syndrome type 1: report of a world registry

This study represents a multicenter survey on the management of patients with Crigler-Najjar syndrome (CNS) type 1. The aim of the survey was to find guiding principles for physicians in the care of these patients. Fifty-seven patients were included. At the time of inclusion, 21 patients had received a liver transplant (37%). The average age at transplantation was 9.1 +/- 6.9 years (range, 1-23 years); the age of the patients who had not been transplanted at the time of inclusion was 6.9 +/- 6.0 years (range, 0-23 years). Brain damage had developed in 15 patients (26%). Five patients died, and 10 are alive with some degree of mental or physical handicap. In 2 patients, ages 22 and 23 years, early signs of bilirubin encephalopathy could be reversed, in 1 by prompt medical intervention followed by liver transplantation and in the other by prompt liver transplantation. Seven patients underwent transplantation with some degree of brain damage at the time of the surgery; 1 of these died after retransplantation, 2 improved neurologically, and 4 remained neurologically impaired. The age of 8 patients with and 13 without brain damage at or before transplantation was 14.3 +/- 5.9 and 5.9 +/- 5.4 years (P < .01), respectively. Therapy of CNS type 1 consists of phototherapy (12 h/d), followed by liver transplantation. Phototherapy, although initially very effective, is socially inconvenient and becomes less efficient in the older age group, thus also decreasing compliance. Currently, liver transplantation is the only effective therapy. This survey shows that, in a significant number of patients, liver transplantation is performed after some form of brain damage has already occurred. From this, one must conclude that liver transplantation should be performed at a young age, particularly in situations in which reliable administration of phototherapy cannot be guaranteed.

UDP-glucuronosyltransferase in Gilbert's syndrome

The diagnosis of Gilbert's syndrome, a condition characterised by mild jaundice related to chronic unconjugated hyperbilirubinemia, is often presumptive and the pathogenesis is incompletely understood. It would be of interest to develop an immunohistochemical staining method to confirm a diagnosis of Gilbert's syndrome. To this end liver tissues from ten patients with a presumed diagnosis of Gilbert's syndrome and six normal controls were examined by immunohistochemistry with polyclonal antibodies raised to UDP-glucuronosyltransferase (UGT). All subjects had normal liver biopsies by hemotoxylin and eosin staining. In normal human liver specific staining for UGT was seen diffusely in all hepatocytes of the hepatic lobule with zone 3 accentuation. There was a reduction of immunostaining throughout the hepatic lobule in all specimens from patients with Gilbert's syndrome and faint residual staining was seen in zone 3. This thus proved a useful method to confirm a clinical diagnosis of Gilbert's syndrome. Raising monospecific antibodies to UGT may give an insight into polypmorphisms of phase II drug metabolism. Bosma et al.* have recently provided evidence from in vitro studies that subjects with Gilbert's syndrome have a putative defect in the promoter region of the gene encoding UDP-glucuronosyltransferase 1, resulting in reduced transcription. These studies have yet to be confirmed from human biopsy specimens and the possibility of second mutations in intronic sequences affecting the stability of UDP-glucuronosyltransferase 1 m RNA are being explored. *Bosma PJ, Chowdhury JR, Bakker C et al. The genetic basis of the reduced expression of bilirubin UDP-glucuronosyltransferase 1 in Gilbert's syndrome. N Engl J Med 1995; 333: 1171-5.

UDP glucuronosyltransferase in the cirrhotic rat liver

In patients with cirrhosis, the elimination of drugs metabolized by glucuronidation is relatively preserved, in comparison with the metabolism of drugs by oxidation. This study explores this phenomenon at a molecular level. In cirrhotic rat livers the content of UDP-glucuronosyltransferase (UGT) was examined by immunohistochemistry and immunoblotting using three antibodies: (i) a polyclonal antibody directed against a broad number of UGT isoforms from both family 1 and family 2; (ii) a family 2-specific antibody; and a (iii) family 1-specific antibody. The steady state mRNA level of UGT of a family 2 isoform was also detected by northern blot analysis. The results demonstrate normal or increased UGT protein by immunohistochemistry and immunoblot in cirrhotic livers compared with controls. This was accompanied by increased steady state mRNA encoding the UGT isoform UGT2B1. In contrast, an isoform of cytochrome P450 (CYP2C11) was reduced markedly in both immunohistochemical staining and immunoblot analysis. These results suggest that in cirrhosis there is a comparative increase or at least a maintenance of UGT enzyme content and that this most likely occurs at a pretranslational level.

The genetic basis of the reduced expression of bilirubin UDP-glucuronosyltransferase 1 in Gilbert's syndrome

BACKGROUND

People with Gilbert's syndrome have mild, chronic unconjugated hyperbilirubinemia in the absence of liver disease or overt hemolysis. Hepatic glucuronidating activity, essential for efficient biliary excretion of bilirubin, is reduced to about 30 percent of normal.

METHODS

We sequenced the coding and promoter regions of the gene for bilirubin UDP-glucuronosyltransferase 1 (bilirubin/uridine diphosphoglucuronate-glucuronosyltransferase 1)--the only enzyme that contributes substantially to bilirubin glucuronidation--in 10 unrelated patients with Gilbert's syndrome, 16 members of a kindred with a history of Crigler-Najjar syndrome type II, and 55 normal subjects.

RESULTS

The coding region of the gene for the enzyme was normal in the 10 patients with Gilbert's syndrome. These patients were homozygous for two extra bases (TA) in the TATAA element of the 5' promoter region of the gene (A(TA)7TAA rather than the normal A(TA)6TAA). The presence of the longer TATAA element resulted in the reduced expression of a reporter gene, encoding firefly luciferase, in a human hepatoma cell line. The frequency of the abnormal allele was 40 percent among the normal subjects. The 3 men in the control group who were homozygous for the longer TATAA element had significantly higher serum bilirubin levels than the other 52 normal subjects (P = 0.009). Among the kindred with a history of Crigler-Najjar syndrome type II, only the six heterozygous carriers who had a longer TATAA element on the structurally normal allele had mild hyperbilirubinemia, characteristic of Gilbert's syndrome.

CONCLUSIONS

Reduced expression of bilirubin UDP-glucuronosyltransferase 1 due to an abnormality in the promoter region of the gene for this enzyme appears to be necessary for Gilbert's syndrome but not sufficient for the complete manifestation of the syndrome.

Specificity of human UDP-glucuronosyltransferases and xenobiotic glucuronidation

Several human liver UDP-Glucuronosyltransferases (UGTs) have been cloned and the cDNAs expressed in heterologous cell lines. This technological advance has allowed the assessment of the functional substrate specificity of these UGTs. The problems which may be encountered with the latency and assay of UGTs are briefly described. The data accumulated to date indicate that the Km, and possibly the Vmax/Km, for individual substrates are the best parameters to assess the specificity of the enzymes towards xenobiotic molecules. The substrate specificity of seven UGTs has been summarised from the currently available information. Of these, UGT1*02 and UGT2B8 appear to be key isoforms in the glucuronidation of a wide range of xenobiotic substrates. Additional UGTs have yet to be identified and characterised and their future inclusion may provide further insights. Finally, the functional role of each UGT in vivo has to be determined.

Discrimination between Crigler-Najjar type I and II by expression of mutant bilirubin uridine diphosphate-glucuronosyltransferase

Crigler-Najjar (CN) disease is classified into two subtypes, type I and II. The molecular basis for the difference between these types is not well understood. Several mutations in the bilirubin UDP-glucuronosyl-transferase (B-UGT) gene of six CN type I and two CN type II patients were identified. Recombinant cDNAs containing these mutations were expressed in COS cells. B-UGT activity was measured using HPLC and the amount of expressed protein was quantitated using a sandwich ELISA. This enabled us to determine the specific activities of the expressed enzymes. All type I patients examined had mutations in the B-UGT1 gene that lead to completely inactive enzymes. The mutations in the B-UGT1 gene of patients with CN type II only partially inactivated the enzyme. At saturating concentrations of bilirubin (75 microM) CN type II patient A had 4.4 +/- 2% residual activity and CN type II patient B had 38 +/- 2% residual activity. Kinetic constants for the glucuronidation of bilirubin were determined. The affinities for bilirubin of B-UGT1 expressed in COS cells and B-UGT from human liver microsomes were similar with Km of 5.1 +/- 0.9 microM and 7.9 +/- 5.3 microM, respectively. B-UGT1 from patient B had a tenfold decreased affinity for bilirubin, Km = 56 +/- 23 microM. At physiological concentrations of bilirubin both type II patients will have a strongly reduced conjugation capacity, whereas type I patients have no B-UGT activity. We conclude that CN type I is caused by a complete absence of functional B-UGT and that in CN type II B-UGT activity is reduced.

Genetic heterogeneity of Crigler-Najjar syndrome type I: a study of 14 cases

Crigler-Najjar syndrome type I (CN-I) is an autosomal recessive condition characterized by severe unconjugated hyperbilirubinemia caused by the lack of bilirubin-UDP-glucuronosyltransferase (B-UGT) activity in the liver. Two B-UGTs are coded for by a gene complex (UGT1) that maps to chromosome 2q37 and that also encodes two phenol-UDP-glucuronosyltransferases. Here, we report eleven mutations (including nine novel mutations) of the B-UGT1 gene in a large series of 14 unrelated CN-I children of various geographic origins: France (seven patients: A401P, Q357X, W335X, A368T, 1223insG, A291V, K426E, K437X); Portugal (two patients: G308E); Tunisia (two patients; Q357R); Turkey (one patient: S381R); italy (two siblings: S381R). Interestingly, 6/14 mutant alleles carried by unrelated probands of French ancestry bore the A401P mutation, indicating a founder effect; this effect is probably also present in Portugal, Turkey, and Tunisia. Since mutations occurred in exons 2-5 shared by all mRNAs species of the gene, a combined deficiency of B-UGT and P-UGT was observed in the liver of five patients in whom these activities were measured. The present study confirms that CN-I is genetically heterogeneous and suggests that different founder effects are involved in Western Europe, the Middle East, and North Africa.

Prenatal diagnosis of bilirubin-UDP-glucuronosyltransferase deficiency in rats by genomic DNA analysis

Hepatic bilirubin excretion requires UDP-glucuronosyltransferase-mediated glucuronidation. Patients with type I Crigler-Najjar syndrome and mutant rats (Gunn strain) inherit deficiency of UDP-glucuronyltransferase activity toward bilirubin as an autosomal recessive trait and, as a result, exhibit marked nonhemolytic unconjugated hyperbilirubinemia throughout postnatal life. Heterozygous carriers of the trait have normal serum bilirubin levels. Because of placental excretion of unconjugated bilirubin, type 1 Crigler-Najjar syndrome patients and Gunn rats are not jaundiced in utero, making prenatal diagnosis difficult. Here we report a diagnostic method in Gunn rats based on genomic DNA analysis for prenatal recognition of deficiency of UDP-glucuronyltransferase activity toward bilirubin in Gunn rats and identification of heterozygous carriers. We and others have shown that two distinct messenger RNA species (UDP-glucuronyltransferase activity toward bilirubin and the 3-methylcholanthrene-inducible phenol-UDP-glucuronyltransferase messenger RNA) in Gunn rat liver contain identical deletions of a single guanosine residue in their common 3' regions. Loss of the restriction site for the endonuclease BstNI, which results from this deletion, was used as the basis for a diagnostic test. Female heterozygous Gunn rats were mated with male homozygous Gunn rats. Genomic DNA was extracted from the chorionic aspect of placenta of 17-day fetuses or from leukocytes from normal rats, obligate heterozygotes and homozygous Gunn rats. The DNA was sequentially digested with the restriction enzymes EcoRI and BstNI and subjected to Southern-blot analysis with a double-stranded DNA probe for the common region of UDP-glucuronyltransferase activity toward bilirubin and the 3-methylcholanthrene-inducible UDP-glucuronyltransferase messenger RNAs.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterisation of a human bilirubin UDP-glucuronosyltransferase stably expressed in hamster lung fibroblast cell cultures

A cDNA encoding a human bilirubin UDP-glucuronosyltransferase has been isolated and stably expressed in Chinese hamster V79 lung fibroblast cell line. Western blotting of cell homogenates with anti-UGT antibody revealed a highly expressed protein of approx. 55.5 kDa in size. The expressed enzyme specifically catalysed the formation of bilirubin mono- and diglucuronides, and also catalysed the glucuronidation of two phenolic compounds, which are good substrates for other human UGT isoenzymes, at low rates.

The differential diagnosis of Crigler-Najjar disease, types 1 and 2, by bile pigment analysis

Phenobarbital response, bile pigment composition, and the fractional biliary excretion ratio of bilirubin were studied in nine children with Crigler-Najjar disease. In five children, serum bilirubin levels decreased during phenobarbital treatment by 26% or more and the pigment composition in bile changed with a decrease in the proportion of unconjugated bilirubin from 33% +/- 12% to 13% +/- 1% and an increase in monoconjugates and diconjugates from 57% +/- 14% and 10% +/- 2%, respectively, to 72% +/- 4% and 16% +/- 3%. In four children, serum bilirubin levels did not change significantly during phenobarbital treatment. In these patients, bile pigments comprised 91% +/- 10% unconjugated bilirubin, 9% +/- 11% monoconjugates, and 1% +/- 1% diconjugates. On the basis of these differences, the former group can be classified as having type 2 Crigler-Najjar disease and the latter, type 1. Bile pigment analysis in parents of patients with Crigler-Najjar disease showed an increased proportion of monoconjugates in at least one of the partners in three of four couples tested, despite normal serum bilirubin levels. Serum bilirubin levels were about the same in type 1 and 2 patients and amounted to 236 +/- 62 mumol/L and 214 +/- 82 mumol/L, respectively. In addition the fractional bilirubin excretion ratio, calculated as the ratio ([bilirubin in bile]/[bilirubin in serum])/([bile acid in bile]/[bile acid in serum]) could not differentiate between these two groups. However, there was a 10-fold and 100-fold difference of this ratio between patients with Crigler-Najjar disease and those with Gilbert's syndrome and between patients with Crigler-Najjar disease and controls. The fractional bilirubin excretion ratio proved an excellent tool to differentiate between Gilbert's syndrome and Crigler-Najjar disease, whereas Crigler-Najjar disease types 1 and 2 could be differentiated on the basis of bile pigment analysis.

The influence of gestational age on bilirubin conjugation in newborns

Unconjugated, mono- and diconjugated bilirubin levels were determined in serum soon after birth, and followed up for several days. Fourteen preterm neonates were studied with a gestational age below 33 weeks (n = 7) or between 34 and 37 weeks (n = 7), respectively, as well as 19 full-term newborns either untreated (n = 9) or treated by phototherapy (n = 10). Bilirubin and its derivatives were analysed by alkaline methanolysis and spectrometry after separation by thin-layer chromatography. In normal full-term neonates total and unconjugated bilirubin reached peak levels at days 2-4. Thereafter, a decline of 11% per day was detectable. Monoconjugates in serum amounted to 3.1 +/- 1.1% of total pigment and remained at that level. The relative amount of diconjugates increased from 0.55 +/- 0.25% (2-4th postnatal day) to 1.62 +/- 0.99% (9-13th day of life). The rapid decline of unconjugated bilirubin paralleled by an increase of diconjugates are an expression of the maturation process for bilirubin conjugation. The premature neonates with less than 33 weeks gestation exhibited an increase of unconjugated serum bilirubin up to the 4-5th postnatal day, the decline thereafter amounted 2% per day. The fraction of 2.3 +/- 1.1% monoconjugates was small and exhibited only a moderate increase in the follow up. In contrast diconjugates were undetectable or very low and remained at this level. These results suggest the presence of a more severe immaturity as well as a slower maturation process of bilirubin conjugation in preterm newborns.(ABSTRACT TRUNCATED AT 250 WORDS)

Subfractionation of serum bilirubins by alkaline methanolysis and thin-layer chromatography. An aid in the differential diagnosis of icteric diseases

The determination of direct and indirect-reacting bilirubin fractions by diazo procedures does not allow a definite diagnosis of icteric diseases. Therefore, the clinical relevance of serum bilirubin subfractionation by alkaline methanolysis and subsequent thin-layer chromatography (AM-TLC) was evaluated. Esterified bilirubins could be detected and quantitated in all serum samples investigated. The ratio of serum esterified to total bilirubin was 10-28% in 60 healthy adults (mean 17 +/- 5% S.D.), 1-11% in 77 patients with Gilbert's syndrome (mean 6 +/- 2%), and 2 and 3%, respectively, in two patients with Crigler-Najjar disease type II. The difference was highly significant (p less than 0.001) and the overlap was restricted to three of 139 individuals. The ratio of esterified to total bilirubin was similar to that obtained with HPLC when corrected for with a blank run. The absolute concentration of bilirubin esters in serum from Gilbert's syndrome patients was similar to that from healthy controls, but the unconjugated pigment was increased. In patients with chronic haemolysis (n = 9) and chronic persistent hepatitis (n = 12), the hyperbilirubinaemia consisted of a proportional increase of both unconjugated and esterified bilirubin. As such, the ratio of conjugated to total bilirubin was not significantly different from control values. Patients with acute hepatitis during the first (n = 18) and third ('remission') week of the disease (n = 15), liver cirrhosis (n = 34), and extrahepatic cholestasis (n = 20) predominantly showed an increase in bilirubin conjugates.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of uridine diphosphate glucuronosyltransferase activity on the maximal secretion rate of bilirubin conjugates in the rat

It is generally accepted that biliary secretion is rate-limiting for the plasma-to-bile transport of a saturating load of bilirubin. Previous studies from this laboratory have suggested a relationship between the hepatic bilirubin uridine diphosphate glucuronosyltransferase activity and the apparent maximal rate of bilirubin secretion (excretory transport maximum). The present study was undertaken to further investigate this relationship in rats with a wide range of transferase levels and to analyze the effects of transferase activity on the formation of bilirubin monoglucuronides and diglucuronides in vivo and in vitro. Animals with moderately decreased enzyme activity still showed a near-normal excretory transport maximum but decreased fractional amounts of conjugated bilirubins in plasma and liver and a decreased ratio of bilirubins diconjugates to monoconjugates in bile, liver, and plasma. A more pronounced decrease in the level of transferase activity yielded lower excretory transport maximum values as well. Enhanced transferase activities were obtained by pretreatment with a series of enzyme-inducing agents, usually at dosages not influencing bile flow. An enhanced transferase activity produced significant increases in bilirubin excretory transport maximum, decreased concentrations of both unconjugated and conjugated bilirubins in plasma, and an increased ratio of diconjugates to monoconjugates in bile and in liver. In all animals, the ratio of disconjugates to monoconjugates observed in bile was correlated with the ratio found after incubation of liver homogenates in vitro at low concentrations of bilirubin substrate. Overall, the relationship between bilirubin excretory transport maximum and uridine diphosphate glucuronosyltransferase activity can best be described by a hyperbolic curve. At "saturation" of the excretory transport maximum values, no increase of conjugates in liver or in plasma could be observed, suggesting that not the biliary secretion, per se, but rather the conjugation rate in vivo is rate-limiting. The latter seems lower than the in vitro assayed enzyme activity. The present investigations point to an equilibration of bilirubin pigments between plasma, liver, and bile. The in vivo conjugation rate can be estimated from the in vitro assay (but not at high enzyme activities) and determines the maximal biliary secretion rate, the ratio of diconjugates to monoconjugates in the three compartments studied, and the relative amount of conjugates in plasma. Our studies suggest that the conjugation rate in vivo rather than the biliary secretion step is the major determinant of the maximal bilirubin secretion rate.

High-performance liquid chromatographic separation of bilirubin conjugates: the effects of change in molarity and pH

A fast, sensitive high-performance liquid chromatographic method has been developed for the separation and quantitation of biliary bile pigments; this utilizes a C18 reversed-phase column with two solvents, a buffer and an organic solvent, which were changed in a linear gradient from a polar to a less polar combination. Nine glycosidic conjugates of bilirubin as well as unconjugated bilirubin and a suitable internal standard, unconjugated mesobilirubin IX alpha, were all separated to baseline by gradient elution; the species eluted in a polar to less polar fashion. Increasing the molarity of the solvent decreased the binding of non-glucuronide pigments to the column, with a decrease in their retention times, whereas for bilirubin monoglucuronide they increased. Decrease in pH, similarly, preferentially increased bilirubin monoglucuronide retention times.