Application of a rapid and efficient h.p.l.c. method to measure bilirubin and its conjugates from native bile and in model bile systems. Potential use as a tool for kinetic reactions and as an aid in diagnosis of hepatobiliary disease

Diagnostic methods for neonatal hyperbilirubinemia: benefits, limitations, requirements, and novel developments

Invasive bilirubin measurements remain the gold standard for the diagnosis and treatment of infants with severe neonatal hyperbilirubinemia. The present paper describes different methods currently available to assess hyperbilirubinemia in newborn infants. Novel point-of-care bilirubin measurement methods, such as the BiliSpec and the Bilistick, would benefit many newborn infants, especially in low-income and middle-income countries where the access to costly multi-analyzer in vitro diagnostic instruments is limited. Total serum bilirubin test results should be accurate within permissible limits of measurement uncertainty to be fit for clinical purposes. This implies correct implementation of internationally endorsed reference measurement systems as well as participation in external quality assessment programs. Novel analytic methods may, apart from bilirubin, include the determination of bilirubin photoisomers and bilirubin oxidation products in blood and even in other biological matrices. IMPACT: Key message: Bilirubin measurements in blood remain the gold standard for diagnosis and treatment of severe neonatal hyperbilirubinemia (SNH). External quality assessment (EQA) plays an important role in revealing inaccuracies in diagnostic bilirubin measurements. What does this article add to the existing literature? We provide analytic performance data on total serum bilirubin (TSB) as measured during recent EQA surveys. We review novel diagnostic point-of-care (POC) bilirubin measurement methods and analytic methods for determining bilirubin levels in biological matrices other than blood. Impact: Manufacturers should make TSB test results traceable to the internationally endorsed total bilirubin reference measurement system and should ensure permissible limits of measurement uncertainty.

Impaired cholecystokinin-induced gallbladder emptying incriminated in spontaneous "black" pigment gallstone formation in germfree Swiss Webster mice

"Black" pigment gallstones form in sterile gallbladder bile in the presence of excess bilirubin conjugates ("hyperbilirubinbilia") from ineffective erythropoiesis, hemolysis, or induced enterohepatic cycling (EHC) of unconjugated bilirubin. Impaired gallbladder motility is a less well-studied risk factor. We evaluated the spontaneous occurrence of gallstones in adult germfree (GF) and conventionally housed specific pathogen-free (SPF) Swiss Webster (SW) mice. GF SW mice were more likely to have gallstones than SPF SW mice, with 75% and 23% prevalence, respectively. In GF SW mice, gallstones were observed predominately in heavier, older females. Gallbladders of GF SW mice were markedly enlarged, contained sterile black gallstones composed of calcium bilirubinate and <1% cholesterol, and had low-grade inflammation, edema, and epithelial hyperplasia. Hemograms were normal, but serum cholesterol was elevated in GF compared with SPF SW mice, and serum glucose levels were positively related to increasing age. Aged GF and SPF SW mice had deficits in gallbladder smooth muscle activity. In response to cholecystokinin (CCK), gallbladders of fasted GF SW mice showed impaired emptying (females: 29%; males: 1% emptying), whereas SPF SW females and males emptied 89% and 53% of volume, respectively. Bilirubin secretion rates of GF SW mice were not greater than SPF SW mice, repudiating an induced EHC. Gallstones likely developed in GF SW mice because of gallbladder hypomotility, enabled by features of GF physiology, including decreased intestinal CCK concentration and delayed intestinal transit, as well as an apparent genetic predisposition of the SW stock. GF SW mice may provide a valuable model to study gallbladder stasis as a cause of black pigment gallstones.

Effect of bile pigments on the compromised gut barrier function in a rat model of bile duct ligation

Background

Studies have shown that the absence of bile in the gut lumen, either by bile duct ligation or bile diversion, induces mucosal injury. However, the mechanism remains elusive. In this study, the role of bile pigments in gut barrier function was investigated in a rat model of bile duct ligation.

Methods

Male Sprague Dawley (SD) rats were used in this study. After ligation of bile duct, the animals were administrated with free bilirubin, bilirubin ditaurate, or biliverdin by intragastric gavage. 1, 2, or 3 days later, the animals were sacrificed and the damage of mucosa was assessed by histological staining as well as biochemical parameters such as changes of diamine oxidase (DAO) and D-lactate (D-Lac) in the blood. Trypsin and chymotrypsin of the gut were also measured to determine how these digestive proteases may relate to the observed effects of bile pigments.

Results

Bile duct ligation (BDL) caused significant increases in gut trypsin and chymotrypsin along with damage of the mucosa as demonstrated by the histological findings under microscope, the reduced expression of tight junction molecules like occludin, and significant changes in DAO and D-lac in the blood. Free bilirubin but not bilirubin ditaurate or biliverdin showed significant inhibitions on trypsin and chymotrypsin as well as alleviated changes of histological and biochemical parameters related to gut barrier disruption.

Conclusion

Bile may protect the gut from damage through inhibiting digestive proteases like trypsin and chymotrypsin by free bilirubin.

Animal pigment bilirubin discovered in plants

The bile pigment bilirubin-IXalpha is the degradative product of heme, distributed among mammals and some other vertebrates. It can be recognized as the pigment responsible for the yellow color of jaundice and healing bruises. In this paper we present the first example of the isolation of bilirubin in plants. The compound was isolated from the brilliant orange-colored arils of Strelitzia nicolai, the white bird of paradise tree, and characterized by HPLC-ESMS, UV-visible, (1)H NMR, and (13)C NMR spectroscopy, as well as comparison with an authentic standard. This discovery indicates that plant cyclic tetrapyrroles may undergo degradation by a previously unknown pathway. Preliminary analyses of related plants, including S. reginae, the bird of paradise, also revealed bilirubin in the arils and flowers, indicating that the occurrence of bilirubin is not limited to a single species or tissue type.

Pathophysiological preconditions promoting mixed "black" pigment plus cholesterol gallstones in a DeltaF508 mouse model of cystic fibrosis

Gallstones are frequent in patients with cystic fibrosis (CF). These stones are generally "black" pigment (i.e., Ca bilirubinate) with an appreciable cholesterol admixture. The pathophysiology and molecular mechanisms for this "mixed" gallstone in CF are unknown. Here we investigate in a CF mouse model with no overt liver or gallbladder disease whether pathophysiological changes in the physical chemistry of gallbladder bile might predict the occurrence of "mixed" cholelithiasis. Employing a DeltaF508 mouse model with documented increased fecal bile acid loss and induced enterohepatic cycling of bilirubin (Am J Physiol Gastrointest Liver Physiol 294: G1411-G1420, 2008), we assessed gallbladder bile chemistry, morphology, and microscopy in CF and wild-type mice, with focus on the concentrations and compositions of the common biliary lipids, bilirubins, Ca(2+), and pH. Our results demonstrate that gallbladder bile of CF mice contains significantly higher levels of all bilirubin conjugates and unconjugated bilirubin with lower gallbladder bile pH values. Significant elevations in Ca bilirubinate ion products in bile of CF mice increase the likelihood of supersaturating bile and forming black pigment gallstones. The risk of potential pigment cholelithogenesis is coupled with higher cholesterol saturations and bile salt hydrophobicity indexes, consistent with a proclivity to cholesterol phase separation during pigment gallstone formation. This is an initial step toward unraveling the molecular basis of CF gallstone disease and constitutes a framework for investigating animal models of CF with more severe biliary disease, as well as the human disease.

Pathophysiological basis of liver disease in cystic fibrosis employing a DeltaF508 mouse model

The molecular pathogenesis of cystic fibrosis (CF) liver disease is unknown. This study investigates its earliest pathophysiological manifestations employing a mouse model carrying DeltaF508, the commonest human CF mutation. We hypothesized that, if increased bile salt spillage into the colon occurs as in the human disease, then this should lead to a hydrophobic bile salt profile and to "hyperbilirubinbilia" because of induced enterohepatic cycling of unconjugated bilirubin. Hyperbilirubinbilia may then lead to an increased bile salt-to-phospholipid ratio in bile and, following hydrolysis, precipitation of divalent metal salts of unconjugated bilirubin. We document in CF mice elevated fecal bile acid excretion and biliary secretion of more hydrophobic bile salts compared with control wild-type mice. Biliary secretion rates of bilirubin monoglucuronosides, bile salts, phospholipids, and cholesterol are increased significantly with an augmented bile salt-to-phospholipid ratio. Quantitative histopathology of CF livers displays mild early cholangiopathy in approximately 53% of mice and multifocal divalent metal salt deposition in cholangiocytes. We conclude that increased fecal bile acid loss leads to more hydrophobic bile salts in hepatic bile and to hyperbilirubinbilia, a major contributor in augmenting the bile salt-to-phospholipid ratio and endogenous beta-glucuronidase hydrolysis of bilirubin glucuronosides. The confluence of these perturbations damages intrahepatic bile ducts and facilitates entrance of unconjugated bilirubin into cholangiocytes. This study of the earliest stages of CF liver disease provides a framework for investigating the molecular pathophysiology of more advanced disease in murine models and in humans with CF.

Solvent partition of 14C-unconjugated bilirubin to remove labeled polar contaminants

Polar derivatives contaminating unconjugated bilirubin (UCB) are customarily extracted with weak alkali. As UCB degrades rapidly at alkaline pH, regeneration of polar derivatives may limit purification. This problem is especially important when trying to eliminate radiolabeled contaminants from 14C-UCB. As polar derivatives of UCB should have a much greater aqueous to CHCl3 partition ratio (PR) than UCB even at neutral pH, where degradation of UCB is minimal, 14C-UCB in CHCl3 was serially extracted with an aqueous buffer at pH 7.0 to determine whether labeled derivatives could be preferentially removed. A single extraction of customarily purified 14C-UCB removed 0.18+/-0.06% of the radioactivity as labeled derivatives. Subsequent serial extractions yielded a stable, 67% lower 14C-PR with only 0.03% of radioactivity as labeled derivatives. Reverse-phase high-performance liquid chromatography (HPLC) of phases from later extractions revealed, however, that up to 1.1% of the disintegration per minute (dpm) in CHCl3 phases and up to 50% in aqueous phases were polar impurities. HPLC of partition phases spiked with purified 14C-UCB revealed that these impurities derived from incomplete extraction of the least polar impurities and their regeneration during HPLC. In the dark under argon, 14C-UCB in CHCl3or dimethyl sulfoxide (DMSO) solution degraded very slowly to polar derivatives. Extraction of impurities from a solution of 14C-UCB in CHCl3 is best done using pH 7.0 buffer, with removal of over 80% of the labeled contaminants remaining after customary purification by alkaline extraction.

Single Hepatic Venous Injection of Liver-Specific Naked Plasmid Vector Expressing Human UGT1A1 Leads to Long-Term Correction of Hyperbilirubinemia and Prevention of Chronic Bilirubin Toxicity in Gunn Rats

Previously, we have demonstrated that hepatic venous injection of pcDNA3hUGT1A1 expressing human bilirubin glucuronosyl transferase 1A1 (hUGT1A1) under the control of the cytomegalovirus promoter results in excretion of bilirubin glucuronides in bile and significant decrease in serum bilirubin for at least 2 weeks in the Gunn rat, an animal model of Crigler-Najjar syndrome type I. In this study we compared repeat delivery of pcDNA3hUGT1A1 with single injection of pBShUGT1A1 expressing hUGT1A1 under liver-specific regulatory control, for treatment of hyperbilirubinemia in the Gunn rat. Although repeat injections of pcDNA3hUGT1A1 consistently reduced serum bilirubin levels, the effect did not exceed 2 weeks; hUGT1A1 was detectable in livers only for 2 weeks, despite the presence of vector and transcript for at least 1 month. In contrast, injection of pBShUGT1A1 resulted in persistence of vector, transcript, and recombinant protein and sustained correction of hyperbilirubinemia for at least 8 months; furthermore, renal tubular damage, the principal manifestation of chronic bilirubin toxicity in the Gunn rat, was prevented. Sera from animals treated with pBShUGT1A1 consistently contained anti-hUGT1A1 antibodies, but a significant increase in the number of hepatic CD4(+) and CD8(+) cells was seen only in the pcDNA3hUGT1A1 group; thus liver-specific expression of hUGT1A1 may attenuate immune response. Our results provide further evidence of the feasibility of long-term correction of hepatic enzyme deficiencies with plasmid vectors optimized for expression in the liver.

A controlled, randomized, double-blind trial of prophylaxis against jaundice among breastfed newborns

OBJECTIVES

Neonatal jaundice is a greater problem for infants fed breast milk, compared with formula. This study tested the hypotheses that feeding breastfed newborns beta-glucuronidase inhibitors during the first week after birth would increase fecal bilirubin excretion and would reduce jaundice without affecting breastfeeding deleteriously.

METHODS

Sixty-four breastfed newborns were randomized to 4 groups, ie, control or receiving 6 doses per day (5 mL per dose) of L-aspartic acid, enzymatically hydrolyzed casein (EHC), or whey/casein (W/C) for the first week. L-aspartic acid and EHC inhibit beta-glucuronidase. Transcutaneous bilirubin levels (primary outcome) were measured daily (Jaundice Meter [Minolta/Air Shields, Hatboro, PA] and Bilicheck [Respironics, Pittsburgh, PA]). All stools were collected, and fecal bile pigments, including bilirubin diglucuronide, bilirubin monoglucuronides, and bilirubin, were analyzed with high-performance liquid chromatography. Follow-up assessments included day 7 body weight, day 6/7 prebreastfeeding/postbreastfeeding weights, maternal ratings, and ages at formula introduction and breastfeeding cessation.

RESULTS

The groups were comparable at entry. Overall, the L-aspartic acid, EHC, and W/C groups had significantly lower transcutaneous bilirubin levels than did the control group (75.8%, 69.6%, and 69.2%, respectively, of the control mean, 8.53 mg/dL, at the bilirubin peak on day 4). The L-aspartic acid, EHC, and W/C groups had significantly lower transcutaneous bilirubin levels on days 3 to 7. Fecal bile pigment excretion was greatest in the L-aspartic acid group, significantly greater than control values. There were no significant differences in dosages, follow-up measurements, and maternal ratings.

CONCLUSIONS

Use of minimal aliquots of L-aspartic acid and EHC for beta-glucuronidase inhibition results in increased fecal bilirubin excretion and less jaundice, without disruption of the breastfeeding experience. Decreased jaundice in the W/C group, which lacked a beta-glucuronidase inhibitor, suggests a different mechanism.

Long-term correction of hyperbilirubinemia in the Gunn rat by repeated intravenous delivery of naked plasmid DNA into muscle

We evaluated nonviral gene delivery into skeletal muscle via femoral artery and great saphenous vein for correction of hyperbilirubinemia in the Gunn rat, the animal model of Crigler-Najjar syndrome type I. A single injection of pDNA expressing hUGT1A1 under the CMV promoter resulted in excretion of bilirubin glucuronides in bile and a significant decrease in serum bilirubin for at least 2 or 4 weeks, respectively. Loss of metabolic effect was associated with a decrease in recombinant protein in muscle, while pDNA and transcript were detectable 4 weeks after gene delivery. Monthly intravenous gene delivery maintained metabolic correction for at least 5 months. Fibrosis around vessels in the arterial group limited the number of successful repeat gene transfer sessions to 3. Animals expressing hUGT1A1 developed anti-hUGT1A1 antibodies and lymphocytic infiltrate in muscle. Immunosuppression abrogated antibody response, ameliorated lymphocytic inflammation, and enhanced metabolic correction but did not prevent a decrease in the amount of recombinant protein. In conclusion, repeated intravenous delivery of pDNA into muscle enables long-term correction of hyperbilirubinemia in the Gunn rat. The procedure is safe and simple, with great clinical potential. Further studies are needed to explain the mechanisms of loss and improve the stability of recombinant hUGT1A1 in muscle.

Nonviral gene transfer into liver and muscle for treatment of hyperbilirubinemia in the gunn rat

We evaluated naked plasmid DNA (pDNA)-mediated expression of human hepatic bilirubin UDP-glucuronosyltransferase (hUGT1A1) in skeletal muscle to correct hyperbilirubinemia in the UGT1A1-deficient Gunn rat, an animal model of Crigler-Najjar syndrome type I (CN-I). After delivery of pDNA encoding hUGT1A1 via hepatic vein or femoral artery, in vitro bilirubin glucuronidation activity was detectable in Gunn rat liver and muscle extracts. Expression of hUGT1A1 in Gunn rat liver or muscle resulted in excretion of bilirubin glucuronides in bile. Total biliary bilirubin concentrations increased from a pretreatment average of 10.5 +/- 2.1 microM to 29.2 +/- 4.2 microM after gene transfer into the liver, and to 28.6 +/- 3.8 microM after gene transfer into muscle. Total serum bilirubin decreased by up to 31.2 +/- 6.9 and 29.2 +/- 3.7% and remained significantly lower for at least 1 and 2 weeks, respectively. Tissue damage associated with the procedure was minimal and reversible. Our results demonstrate that muscle can be genetically modified to glucuronidate bilirubin, leading to elimination in bile. A 30% decrease in serum bilirubin, if sustained, would provide meaningful clinical benefit for CN-I patients. However, to be clinically useful, this method needs further optimization and stable gene expression must be achieved.

Measurements of UDP- glucuronosyltransferase (UGT) activities

Mammalian UDP-glucuronosyltransferases are a family of isoenzymes that catalyze the reaction of endobiotics and xenobiotics with glucuronic acid resulting in the formation of hydrophilic glucuronides. This pathway is an important step in the metabolism and subsequent excretion of many compounds that would otherwise have toxic effects. This unit describes three methods for measuring UGT activity. Thin layer chromatography is a powerful screening method and may be used to analyze multiple substrates simultaneously. The Sep-Pak C18 cartridge extraction method has been developed to specifically separate opioid glucuronides from UDP-glucuronic acid. Finally, the ethyl acetate extraction method is used to separate the glucuronides of bilirubin, sterols, and vile acids from UDP-glucuronic acid. These methods may be applied to a microsomal fraction or to cultured cells transformed with cDNA for UGT.

Pitfalls in preparation of (3)H-unconjugated bilirubin by biosynthetic labeling from precursor (3)H-5-aminolevulinic acid in the dog

We report problems encountered during preparation of tritium-labeled unconjugated bilirubin ((3)H-UCB) from precursor (3)H-5-aminolevulinic acid ((3)H-ALA) in 2 dogs with external biliary drainage installed into the animals under general anesthesia. Under prolonged sedation, 12.9 or 14.0 mCi of (3)H-ALA was administered intravenously in two divided doses, and bile was collected for 9 hours. In one animal, taurocholate (TC) infusion was needed to maintain bile flow. (3)H-UCB was isolated from the bile and recrystallized with the improved method of Webster et al (Webster CC, Tiribelli C, Ostrow JD. J Lab Clin Med 2001;137:370-3). Based on radioactivity and pigment content, hourly bile collections were pooled to optimize specific activities. Surprisingly, in the first dog, only 2.9% of injected radioactivity was recovered in bile and only 14.1% in urine, and the specific activities of the crystalline (3)H-UCB from the two pools were only 39.5 and 30.0 x 10(3) dpm/microg. High-performance liquid chromatography analysis revealed that only 4% of ALA degraded during 5 minutes in injection solution at pH 6.8. The low incorporation of (3)H-ALA and low specific activity of (3)H-UCB was apparently caused mainly by prior degradation and exchange of labile tritium of the (3)H-ALA and probably by enhanced endogenous ALA synthesis caused by the anesthetic/sedative agents. Revised procedures in the second dog improved the incorporation of (3)H-ALA to 11.9% excreted in bile and the specific activity of the crystalline (3)H-UCB to 122.0 and 50.8 x 10(3) dpm/microg, while urinary excretion of tritium increased to 28.5%. These experiences emphasize possible pitfalls in preparing (3)H-UCB by biosynthetic labeling from (3)H-ALA administered to dogs.

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.

Noninvasive measurement of total serum bilirubin in a multiracial predischarge newborn population to assess the risk of severe hyperbilirubinemia

BACKGROUND

Jaundice in near-term and term newborns is a frequent diagnosis that may prompt hospital readmission in the first postnatal week. Hyperbilirubinemia, when excessive, can lead to potentially irreversible bilirubin-induced neurotoxicity. Predischarge risk assessment (at 24-72 hours of age) for subsequent excessive hyperbilirubinemia is feasible by a laboratory-based assay of total serum bilirubin (TSB). Hypothesis. Noninvasive, transcutaneous, point-of-care measurement of transcutaneous bilirubin (TcB) predischarge by multiwavelength spectral analysis, using a portable BiliCheck device (SpectRx Inc, Norcross, GA), is clinically equivalent to measurement of TSB in a diverse, multiracial term and near-term newborn population and predictive of subsequent hyperbilirubinemia.

METHODOLOGY

We evaluated a hand-held device that uses multiwavelength spectral reflectance analysis to measure TcB (BiliCheck). The study population (490 term and near-term newborns) was racially diverse (59.1% white, 29.5% black, 3.46% Hispanic, 4.48% Asian, and 3.46% other) and was evaluated at 2 separate institutions using multiple (11) devices. The postnatal age ranged from 12 to 98 hours and the ranges of birth weights and gestational ages were 2000 to 5665 g and 35 to 42 weeks, respectively. All transcutaneous evaluations were performed contemporaneously and paired with a heelstick TSB measurement. All TSB assays were performed by high performance liquid chromatography, as well as by diazo dichlorophenyldiazonium tetrafluoroborate techniques.

RESULTS

TSB values ranged from .2 to 18.2 mg/dL (mean +/- standard deviation: 7.65 +/- 3.35 mg/dL). The overall correlation of TSB (by high performance liquid chromatography technique) to TcB (by BiliCheck devices) was linear and statistically significant (r =.91; r(2) =.83; TcB =.84; TSB = +.75; standard error of regression line = 1.38; P <.001; n = 490 infants; 1788 samples). Similar regression statistics were evident in subset populations categorized by race (white: r =.91 [n = 289 infants]; black: r =.91 [n = 145 infants]) as well as by gestation (term: r =. 91 [n = 1625 samples]; near-term: r =.89 [n = 163 samples]). Intradevice precision was determined to be.59 mg/dL (2-3 measurements per infant with 1 device; n = 210 infants; 510 samples in a separate subset). Interdevice evaluation of 11 devices determined the precision to be.68 mg/dL (2-4 devices used for measurements per patient). In 23 of 419 of the study population infants who were in the 24- to 72-hour age range, the predischarge TSB values designated them to be at high risk for subsequent excessive hyperbilirubinemia (above the 95th percentile track on the hour-specific bilirubin nomogram). For these infants, the paired BiliCheck TcB values were all above the 75th percentile track (negative predictive value = 100%; positive predictive value = 32. 86%; sensitivity = 100%; specificity = 88.1%; likelihood ratio = 8. 43).

CONCLUSIONS

Our data demonstrate the accuracy and reproducibility of the predischarge BiliCheck measurements in term and near-term newborn infants of diverse races and ethnicities. Infants with predischarge BiliCheck values above the 75th percentile of hour-specific TSB values on the bilirubin nomogram may be considered to be at high risk for subsequent excessive hyperbilirubinemia. Further studies are needed to assess the efficacy of this technique in preterm infants, those undergoing phototherapy, and those with TSB values of >/=15 mg/dL (>/=256 micromol/L).

Measurement of bilirubin partition coefficients in bile salt micelle/aqueous buffer solutions by micellar electrokinetic chromatography

The partition coefficients for the distribution of bilirubin between aqueous phosphateborate buffer and cholic, taurocholic, taurodeoxycholic, and taurochenodeoxycholic micelles have been measured by micellar electrokinetic chromatography at pH 8.5. Determination of the partition coefficients required that the critical micelle concentration and partial specific volumes be determined for each bile salt. Critical micelle concentrations were slightly higher for the trihydroxy bile salts. Partial specific volumes of the bile salt micelles differed very little from each other, and for each bile salt they were constant over the concentration range studied, which was typically from slightly above the critical micelle concentration to 35 mM. Capacity factors were corrected for the effects of applied voltage by extrapolation of the capacity factor to zero applied volts. The free solution mobility of bilirubin, determined in the absence of bile salt, was also corrected for the effects of applied voltage. Plots of extrapolated capacity factor versus phase ratio yield the partition coefficient as the slope of a linear fit to the data. Partition coefficients for bilirubin were significantly higher for dihydroxy bile salts than for trihydroxy bile salts.

Physical and metabolic factors in gallstone pathogenesis

Gallstones form when the tenuous balance of solubility of biliary lipids tips in favor of precipitation of cholesterol, unconjugated bilirubin, or bacterial degradation products of biliary lipids. For cholesterol gallstones, metabolic alterations in hepatic cholesterol secretion combine with changes in gallbladder motility and intestinal bacterial degradation of bile salts to destabilize cholesterol carriers in bile and produce cholesterol crystals. For black pigment gallstones, changes in heme metabolism or bilirubin absorption lead to increased bilirubin concentrations and precipitation of calcium bilirubinate. In contrast, mechanical obstruction of the biliary tract is the major factor leading to bacterial degradation and precipitation of biliary lipids in brown pigment stones. Further understanding of the physical and metabolic factors of cholesterol and black pigment formation is likely to provide interventions to interrupt the earliest stages of gallstone formation.

Ursodeoxycholic acid and cholesterol induce enterohepatic cycling of bilirubin in rodents

BACKGROUND & AIMS

Oral administration of ursodeoxycholic acid (UDCA) and cholesterol causes bile salt malabsorption; the former by competition for and the latter by down-regulation of ileal bile acid transporters. Because ileectomy in rats induces enterohepatic cycling of bilirubin, the hypothesis that dietary steroids might have the same effect was tested.

METHODS

Male inbred C57L/J mice and Sprague-Dawley rats were fed low doses of UDCA, chenodeoxycholic acid (CDCA), or cholesterol added to laboratory chow with simultaneous chow-fed controls. After 1 week (mice) or 2 weeks (rats), indices of bile salt malabsorption and enterohepatic cycling of bilirubin were measured, including bilirubin secretion rates into bile, serum and intestinal bilirubin and bile salt levels, and urobilinogen levels in cecum, large intestine, and feces.

RESULTS

Dietary UDCA and cholesterol, but not CDCA, significantly increased bilirubin secretion rates into bile. In UDCA-fed mice, gallbladder biles contained increased levels of bilirubin conjugates and unconjugated bilirubin, and in 60%, granules of amorphous calcium bilirubinate precipitated. Dietary cholesterol and bile acids, particularly UDCA, increased cecal bile salt levels, unconjugated bilirubin and urobilinogen concentrations, and decreased fecal bilirubin outputs, consistent with colonic absorption.

CONCLUSIONS

By causing bile salt malabsorption, dietary UDCA and cholesterol induce enterohepatic cycling of bilirubin.

Deconjugation of bilirubin accelerates coprecipitation of cholesterol, fatty acids, and mucin in human bile--in vitro study

To examine the initial step of brown pigment gallstone formation, sterile human gallbladder bile samples were incubated with or without beta-glucuronidase in vitro. Enhanced bilirubin deconjugation achieved by adding beta-glucuronidase significantly accelerated the formation of a precipitate that contained bilirubin (28.2 +/- 3.8% of dry weight), cholesterol (14.3 +/- 5.2%), free fatty acids (12.0 +/- 1.3%), and glycoprotein (10.0 +/- 6.7%). Both the composition and scanning electron microscopic appearance of the precipitate were similar to these features in brown pigment gallstones. The cholesterol saturation index and nucleation time in the supernatant did not change with various incubation periods. The weight ratios of bilirubin to cholesterol in the precipitates correlated with those in bile (r = 0.76; P = 0.017). Gel chromatography of the precipitate showed high molecular weight glycoprotein to be the major constituent. Bilirubin, cholesterol, fatty acids, and mucin were found to coprecipitate in accordance with bilirubin deconjugation, which process may play an important role in an early stage of the formation of brown pigment gallstones.

Influence of glutathione S-transferase B (ligandin) on the intermembrane transfer of bilirubin. Implications for the intracellular transport of nonsubstrate ligands in hepatocytes

To examine the hypothesis that glutathione S-transferases (GST) play an important role in the hepatocellular transport of hydrophobic organic anions, the kinetics of the spontaneous transfer of unconjugated bilirubin between membrane vesicles and rat liver glutathione S-transferase B (ligandin) was studied, using stopped-flow fluorometry. Bilirubin transfer from glutathione S-transferase B to phosphatidylcholine vesicles was best described by a single exponential function, with a rate constant of 8.0 +/- 0.7 s-1 (+/- SD) at 25 degrees C. The variations in transfer rate with respect to acceptor phospholipid concentration provide strong evidence for aqueous diffusion of free bilirubin. This finding was verified using rhodamine-labeled microsomal membranes as acceptors. Bilirubin transfer from phospholipid vesicles to GST also exhibited diffusional kinetics. Thermodynamic parameters for bilirubin dissociation from GST were similar to those for human serum albumin. The rate of bilirubin transfer from rat liver basolateral plasma membranes to acceptor vesicles in the presence of glutathione S-transferase B declined asymptotically with increasing GST concentration. These data suggest that glutathione S-transferase B does not function as an intracellular bilirubin transporter, although expression of this protein may serve to regulate the delivery of bilirubin, and other nonsubstrate ligands, to sites of metabolism within the cell.

Micellar electrokinetic capillary chromatography analysis of the behavior of bilirubin in micellar solutions

The capacity factor of bilirubin is determined by micellar electrokinetic capillary chromatography (MECC) techniques in three different surfactant systems. The capacity factor of bilirubin in cholic acid, taurocholic acid, and taurochenodeoxycholic acid solutions are compared to each other as a function of pH. The pH range studied is 6.5 to 9.5 which includes the pH range of bile, and includes the most likely pKa values of bilirubin carboxyl groups. MECC techniques are used to estimate these apparent pKa values for bilirubin as well as to determine the capacity factors for the separate ionization states of bilirubin in the three different surfactants. Due to the complexity of the bilirubin-bile salt system, it appears as though it is not possible to use MECC to accurately determine the bilirubin apparent pKa values. Separations are performed in 75 microns capillaries, typically 36 to 52 cm in length. UV detection, electrokinetic injection, and run voltages of 7 kV are typical. Solutions of 25 mM of each bile salt are prepared in a 20 mM phosphate-borate buffer system.

Effect of cholecystectomy on gallbladder bile composition

Human bile samples are commonly used in biliary research; however, the optimal sampling technique is not known. The current study examines whether bile obtained prior to operative manipulation of the gallbladder differs in composition from samples obtained after cholecystectomy. Gallbladder bile samples were obtained from 26 patients undergoing cholecystectomy. An initial sample was obtained prior to manipulation or devascularization of the gallbladder, and a second sample was obtained after the removal of the gallbladder from the operative field. Gallbladder bile pH and total protein were significantly increased in the postcholecystectomy samples. Bile obtained after cholecystectomy also contained significantly less phospholipid. Gallbladder bile cholesterol, total bile acids, bilirubin, ionized and total calcium, cholesterol saturation index, and total lipids were similar between techniques. These results indicate that manipulation of the gallbladder during cholecystectomy produces alterations in gallbladder bile composition. These results also emphasize the need for consistent sampling technique when obtaining samples for biliary research.

Defective biliary excretion of epinephrine metabolites in mutant (TR-) rats: relation to the pathogenesis of black liver in the Dubin-Johnson syndrome and Corriedale sheep with an analogous excretory defect

Dubin-Johnson patients, mutant Corriedale sheep and TR- and EHBR mutant rats have recessively inherited defective bile canalicular secretion of many nonbile acid organic anions. The human and ovine mutants have black livers and lysosomal pigment accumulation. The livers in TR- and EHBR mutant rats are not black, and sparse lysosomal pigment accumulation is seen. Previously, we postulated that the unidentified pigment in the Dubin-Johnson syndrome results from the accumulation of tyrosine, phenylalanine and tryptophan metabolites, such as metanephrine, which are normally secreted in bile as organic anions. We tested this hypothesis in TR- rats. 3H-epinephrine was injected intravenously; control rats secreted 2.80% +/- 0.52% of the injected dose in bile as compared with 0.19% +/- 0.07% in TR- rats. From 82% to 90% of biliary radioactivity was due to polar conjugates in control rats and mutant rats. TR- rats retained more of the injected dose in the liver, particularly in lysosomes, and secreted more in urine than did control rats. After feeding control and TR- rats for 4 mo with a rat chow diet supplemented with 4% tyrosine, tryptophan and phenylalanine, the liver did not become grossly black; however, histological and electron microscopic study revealed dense lysosomal pigment accumulation in TR- rats. Intraportal injection of metanephrine resulted in the appearance of black liver in TR- rats that persisted for at least 2 hr and was not associated with pigment accumulation by light or electron microscopic examination.(ABSTRACT TRUNCATED AT 250 WORDS)

Significance of different conjugate forms of bilirubin in the formation of pigment gallstones

The aim of the present study was to analyse bile samples from cases with gallstones by high performance liquid chromatography according to the type of stones present, with special reference to the glucoside and xyloside conjugates of bilirubin, and to investigate their deconjugation. The composition of bilirubin conjugates in bile was similar between cholesterol and black pigment stones except that the total bilirubin concentration was about 5 times higher in black pigment stone cases with haemolysis. Unconjugated bilirubin was higher in brown pigment stone cases than in cholesterol stone cases, although total bilirubin concentration was lower in the former. In addition, in brown pigment stone cases, bile contained statistically less bilirubin diglucuronide and more bilirubin diglucoside and monoglucoside than in bile with cholesterol stones (P less than 0.05). Glucoside and xyloside conjugates are also major components, regardless of the types of gallstones present, accounting for as much as 18 to 25%. Incubation experiment revealed that bilirubin diglucuronide was more readily deconjugated than bilirubin diglucoside or bilirubin monoglucoside monoxyloside. Therefore, glucuronide conjugates were likely to be the main source of unconjugated bilirubin in the formation of pigment gallstones.

Rat liver canalicular membrane vesicles contain an ATP-dependent bile acid transport system

The secretion of bile by the liver is primarily determined by the ability of the hepatocyte to transport bile acids into the bile canaliculus. A carrier-mediated process for the transport of taurocholate, the major bile acid in humans and rats, was previously demonstrated in canalicular membrane vesicles from rat liver. This process is driven by an outside-positive membrane potential that is, however, insufficient to explain the large bile acid concentration gradient between the hepatocyte and bile. In this study, we describe an ATP-dependent transport system for taurocholate in inside-out canalicular membrane vesicles from rat liver. The transport system is saturable, temperature-dependent, osmotically sensitive, specifically requires ATP, and does not function in sinusoidal membrane vesicles and right side-out canalicular membrane vesicles. Transport was inhibited by other bile acids but not by substrates for the previously demonstrated ATP-dependent canalicular transport systems for organic cations or nonbile acid organic anions. Defects in ATP-dependent canalicular transport of bile acids may contribute to reduced bile secretion (cholestasis) in various developmental, inheritable, and acquired disorders.

Why does somatostatin cause gallstones?

Long-term administration of the somatostatin analogue, octreotide, is complicated by gallstone formation. Somatostatin is known to inhibit hepatic bile secretion and gallbladder emptying. However, the effect of octreotide on gallbladder bile composition remains unknown. Therefore, we tested the hypothesis that octretide would alter hepatic bile composition and cause gallbladder stasis, thereby increasing gallbladder bile solute concentrations. Fourteen control prairie dogs received daily saline injections, whereas 10 animals received 1 micrograms of octreotide subcutaneously three times per day for 5 days. Cholecystectomy and common bile duct cannulation were then performed. Octreotide increased hepatic bile concentrations of bilirubin monoglucuronide (p less than 0.05), total bilirubin (p less than 0.05), and total protein (p less than 0.01). Rsa, an index of gallbladder stasis, was decreased (p less than 0.01) in the octreotide group. Gallbladder bile total calcium (p less than 0.05), bilirubin monoglucuronide (p less than 0.05), total bilirubin (p less than 0.01), total protein (p less than 0.05), and total lipids (p less than 0.05) were increased in the octreotide group. Animals receiving octreotide also had decreased hepatic (p less than 0.05) and gallbladder (p less than 0.001) bile pH. No differences in cholesterol saturation index were observed. These data suggest that in the prairie dog, octreotide (1) alters hepatic bile composition, (2) causes gallbladder stasis, and (3) increases gallbladder bile calcium, bilirubin, protein, lipid, and hydrogen ion concentrations. We conclude that octreotide causes alterations in gallbladder bile composition that increase the likelihood of cholesterol and calcium bilirubinate precipitation.

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.

Urobilinogen-i is a major derivative of bilirubin in bile of homozygous Gunn rats

Gunn rats lack bilirubin UDP-glycosyltransferases, but diazo-negative derivatives of bilirubin have been described in their bile. In order to investigate this alternative disposal of bilirubin, crude bile samples from Gunn and Wistar rats were directly analysed by h.p.l.c. Besides bilirubin (in Gunn rats) or its glycosides (in Wistar rats), two major compounds were detected. A yellow one corresponded to the previously documented vitamin B-2 and was equally prominent in Gunn rats or Wistar-rat bile. The other compound was colourless, but on standing in contact with air it was spontaneously oxidized to a pinkish-yellow pigment. It was far more prominent in Gunn-rat bile. Analysis of bile obtained after intravenous injection of [14C]bilirubin to Gunn rats demonstrated that this compound was highly labelled. Freezing and thawing of the bile resulted in the formation of a series of diazo-negative derivatives, demonstrating that the original compound was quite labile. Spectral (adsorption and fluorescent) and chromatographic (h.p.l.c., t.l.c. and paper chromatography) analysis of the oxidized form of the labelled compound allowed its identification as urobilin-i. The colourless compound secreted in bile was urobilinogen-i. Administration of neomycin and bacitracin to Gunn rats or gut resection suppressed the biliary excretion of urobilinogen and thus confirmed its intestinal origin. Urobilinogen seems thus to represent the major bilirubin derivative present in Gunn-rat bile. Its breakdown products might represent the so-far-unidentified diazo-negative polar bilirubin derivatives. Since only a small amount of bilirubin is present in Gunn-rat bile, the urobilinogen formed in the intestinal lumen seems to be derived from bilirubin reaching the gut via routes other than the biliary one.

Spontaneous formation of pigmentary precipitates in bile salt-depleted rat bile and its prevention by micelle-forming bile salts

During studies on the effect of bile salt-pool depletion in the bile-fistula rat (adult male Sprague-Dawley), the spontaneous formation of an orange-brown precipitate was noted. The nature of this phenomenon and its relationship to BS and calcium concentration was investigated in depth. Bile from 18 animals was collected in the dark into transparent tubes containing sodium azide, ascorbic acid, and glucaro-1,4-lactone. The tubes were flushed with nitrogen, sealed, and incubated at 37 degrees C. The pigmentary precipitate formed in all the bile salt-depleted (less than 3-5 mM) bile samples (i.e., those collected after 5-7 h of external biliary drainage), but not in bile salt-rich biles. It appeared within 30-240 min after collection, both in bile samples collected at room temperature and at 37 degrees C, initially as a pale flocculation and then slowly sedimenting to form, after centrifugation, a solid, dark-orange pellet. There were no pH changes during incubation, and bile cultures were negative. Under polarizing microscopy, the precipitate appeared amorphous, and there was no evidence of birefringence. High-performance liquid chromatography showed that unconjugated bilirubin was the prevalent pigmentary component, but significant amounts of monoconjugated bilirubin also coprecipitated. Lipid chemistry showed the presence of lecithin (80.1% of total lipids), which was rich in palmitoyl and linoleoyl fatty acids, and of fatty acids (predominantly palmitic and oleic). Infrared spectroscopy and x-ray diffraction showed the presence of calcium bilirubinate and palmitate. In-vivo replenishment of the bile salt pool by intravenous infusion of either taurocholate or taurochenodeoxycholate (1 mumol/min) completely prevented the pigmentary precipitation. In vitro experiments showed inhibition of the precipitate formation by the addition of individual bile salt in concentrations approximating their critical micellar concentration. Precipitate formation was hastened by the addition of calcium chloride (4-12 mM), but only in bile salt-depleted biles. As the composition of the precipitate closely resembles that of human brown-pigment stones and sludge, these findings may provide new insights into an understanding of the pathogenesis of pigment gallstone disease.

High-performance liquid chromatographic separation of bilirubin conjugates

A fast sensitive method for the isolation and quantitation of biliary bile pigments by reverse-phase high-performance liquid chromatography has been developed. Nine conjugates of bilirubin as well as unconjugated bilirubin and an internal standard, unconjugated mesobilirubin IX alpha, were all separated to baseline by gradient elution. The following sequence of eluted compounds was chemically identified by separating their ethyl anthranilate derivatives by thin-layer chromatography and by their enzymatic formation with UDP-bilirubin transferase and cosubstrate: bilirubin diglucuronide, bilirubin monoglucuronide monoglucoside, bilirubin monoglucuronide monoxyloside, bilirubin monoglucuronide (C-8, C-12), bilirubin diglucoside, bilirubin monoglucoside monoxyloside, bilirubin dixyloside, bilirubin monoglucoside (C-8, C-12), and bilirubin monoxyloside. The use of the commercially available mesobilirubin IX alpha as an internal standard was found to facilitate quantitation of the bilirubin conjugates.

Chromatographic analysis and structure determination of biliverdins and bilirubins

Recent applications of thin-layer chromatographic (TLC) and high-performance liquid chromatographic (HPLC) procedures has revealed an unexpected wide variety of naturally occurring unconjugated and conjugated bilirubins. Biliverdins seems to occur only in unconjugated forms, mainly as the IX alpha isomer. Several synthetic biliverdins and bilirubins present interesting models for biochemical and metabolic studies. Owing to recent recognition of the astounding heterogeneity of natural bilirubins and to the various artifactual changes that bile pigments can undergo, considerable confusion has existed, and still exists, with regard to the nomenclature of the bile pigments and their derivatives. To set a background for further discussion, the present review starts with a brief discussion of nomenclature and of the various characteristic forms of lability of the bile pigments. TLC and HPLC procedures for preparation and analysis of unconjugated biliverdins and bilirubins and their methyl ester and sugar ester conjugates, as well as procedures for analysis of bilirubin-protein conjugates, are then discussed. Since, in view of the lability and pronounced heterogeneity of bile pigments, it is important to assess the composition and nature of chromatographically isolated pigments, the review is concluded by a brief evaluation of various structural tests.

Gilbert's syndrome

While Gilbert's syndrome is extremely common and benign, its pathogenesis may not be as straightforward as once believed. It has been used as a model to examine aberrations of virtually every step in bilirubin metabolism. The clinical hallmarks are of a hereditary, chronic, mild unconjugated hyperbilirubinaemia. Not infrequently subclinical haemolysis may coexist. Liver histology is normal although some minor ultrastructural abnormalities may be evident. The universal defect appears to be a reduction in hepatic bilirubin-GT activity. However, other associated abnormalities in bilirubin metabolism, which occur less consistently, suggest that this may not be the sole defect in all patients. The syndrome is almost certainly part of a spectrum which includes the Crigler-Najjar syndromes; molecular biology data suggests that there is an absence of one (or even more) GT isoenzymes in these disorders. Whether one or more genes is consistently culpable remains open to speculation. Despite the complicated pathogenesis of Gilbert's syndrome, management remains simply reassurance alone.

Inhibition of glutathione efflux in the perfused rat liver and isolated hepatocytes by organic anions and bilirubin. Kinetics, sidedness, and molecular forms

Using isolated, in situ, single-pass perfused rat livers, incubations of freshly isolated hepatocytes, and sinusoidal membrane-enriched vesicles, we and others have shown the saturability of transport (efflux) of hepatic glutathione (GSH). These observations have implicated a carrier mechanism. Our present studies were designed to provide further evidence in support of a carrier mechanism for hepatic GSH efflux by demonstrating competition by liver-specific ligands which are taken up by hepatocytes. Perfusing livers with different substances, we found that: (a) sulfobromophthalein-GSH (BSP-GSH) had a dose-dependent and fully reversible inhibitory effect on GSH efflux, while GSH alone did not have any effect; (b) taurocholate had no inhibitory effect; (c) all of the organic anions studied, i.e., BSP, rose bengal, indocyanine green, and unconjugated bilirubin (UCB), manifested potent, dose-dependent inhibitory effects, with absence of toxic effects and complete reversibility of inhibition in the case of UCB. The inhibitory effects of UCB could be overcome partially by raising (CoCl2-induced) hepatic GSH concentration. Because of the physiological importance of UCB, we conducted a detailed study of its inhibitory kinetics in the isolated hepatocyte model in the range of circulating concentrations of UCB. Studies with Cl- -free media, to inhibit the uptake of UCB by hepatocytes, showed that the inhibition of GSH efflux by UCB is apparently from inside the cell. This point was confirmed by showing that the inhibition is overcome only when bilirubin-loaded cells are cleared of bilirubin (incubation with 5% bovine serum albumin). Using Gunn rat hepatocytes and purified bilirubin mono- and diglucuronides, we found that both UCB and glucuronide forms of bilirubin inhibit GSH efflux in a dose-dependent manner. We conclude that the organic anions, although taken up by a mechanism independent of GSH, may competitively inhibit the carrier for GSH efflux from inside the hepatocyte.

Monoconjugated bilirubin is a major component of hemolysis-induced gallstones in mice

The role of bilirubin conjugates in the formation of pigment gallstones is not known. In this study, we completely solubilized and then analyzed by high-performance liquid chromatography specimens of black pigment gallstones from eight nb/nb mice with hereditary hemolytic anemia. Each dried gallstone specimen of about 200 micrograms was dissolved in 5 ml of dimethyl sulfoxide/0.15 M HCI/50 mM disodium-EDTA (8:1:1 by volume) at room temperature. Stone dissolution was complete by 30 min as monitored by the A456 and direct observation, and no oxidative products of bilirubin were observed in the visible spectrum, 350 to 750 nm. By high-performance liquid chromatography, the intact tetrapyrroles were separated as diconjugated and monoconjugated bilirubins; unconjugated bilirubin was resolved as XIII, IX and III alpha-isomers. The isocratic solvent system used was 0.1 M di-n-dodecylamine acetate/0.1 M di-n-octylamine acetate (4:1, v/v) in methanol, pH 7.4, at a flow of 1 ml per min. Diconjugated bilirubin accounted for 6.0 +/- 2.4 molar % (mean +/- S.E.), monoconjugated bilirubin for 37.4 +/- 8.4% and unconjugated bilirubin for 56.3 +/- 8.9% of the solubilized pigments. The IX alpha-isomer represented 96 +/- 1.9% of the unconjugated bilirubin. The presence of bilirubin conjugates in gallstones was confirmed by ethylanthranilate diazotization: the conjugated azodipyrrole in stone had the same retention time as that of conjugated azodipyrrole from rat and mouse bile. A majority of the bilirubin conjugates was sensitive to beta-glucuronidase of liver origin, indicating that the C-1 glucuronide ester was present.(ABSTRACT TRUNCATED AT 250 WORDS)

Spectrophotometric determination of the critical micellar concentration of bile salts using bilirubin monoglucuronide as a micellar probe. Utility of derivative spectroscopy

We have developed a simple biologically non-invasive method for determining the critical micellar concentration (CMC) of bile salts using pure naturally occurring bilirubin IX alpha monoglucuronide (BMG), an important bile pigment present in virtually all mammalian biles. This methodology employs visible absorbance spectroscopy of BMG in bile salts over a range of bile salt concentrations that include the reported CMC. Using 100 microM-BMG in 0.4 M-imidazole buffer at pH 7.8, we calculated that the CMC for sodium taurochenodeoxycholate is between 2.5 and 3.0 mM based on: (1) an abrupt change in lambda max. in this concentration range, (2) a precipitous decrease in the amplitude of the absorbance shoulder at 450 nm, (3) a sudden decrease in the second derivative absorbance of BMG at 400 nm and an increase in absorbance at 470 nm, (4) a sharp change in the 4th derivative absorbance at 375 and 395 nm. In contrast, sodium taurocholate, a bile salt that reportedly does not have a CMC but continuously self-associates over a wide concentration range, exhibited none of these changes. The use of derivative spectroscopy enhances the ability to detect the CMC changes and also indicates the number of BMG species in solution and their relative energy states.

Nonbacterial transformation of bilirubin in bile

Bilirubin present in gallstones is mainly in the unconjugated form despite the frequent absence of bacteria in bile. The aim of the present study was to determine if nonbacterial beta-glucuronidase activity and/or nonenzymatic hydrolysis is responsible. Inflammatory cells such as polymorphonuclear leukocytes and lymphocytes appearing with the presence of brown pigment gallstones and inflammation in biliary tract was shown to effect deconjugation of bilirubin conjugates in bile and contribute to their formation in addition to that of bacterial beta-glucuronidase. Gallbladder bile (mean +/- SD, 4.0 +/- 1.6%, N = 29) contained more unconjugated bilirubin than hepatic bile (mean +/- SD, 2.7 +/- 1.1%). In vitro experiments showed the deconjugation to take place during incubation at 37 degrees C without the presence of bacteria. Therefore, transformation of conjugated to unconjugated bilirubin is likely to take place in vivo during the storage in gallbladder, and nonbacterial beta-glucuronidase activity and/or nonenzymatic hydrolysis may be responsible for such transformation.

Non-enzymic hydrolysis of bilirubin mono- and diglucuronide to unconjugated bilirubin in model and native bile systems. Potential role in the formation of gallstones

Pigment gallstones contain considerable amounts of unconjugated bilirubin (UCB) in the form of calcium bilirubinate and/or bilirubin polymers. Since more than 98% of bile pigments are excreted as conjugates of bilirubin, the source of this UCB needs to be identified. By using a rapid h.p.l.c. method, we compared the non-enzymic hydrolysis of bilirubin monoglucuronide (BMG) and bilirubin diglucuronide (BDG) to UCB in model bile and in native guinea-pig bile. Model biles containing 50 microM solutions of pure BMG and BDG were individually incubated in 25 mM-sodium taurocholate (NaTC) and 0.4 M-imidazole/5 mM-ascorbate buffer (TC-BUF) at 37 degrees C. Over an 8 h period, BMG hydrolysis produced 4-6 times more UCB than BDG hydrolysis. At pH 7.4, 25% of the BMG was converted into UCB, whereas only 4.5% of BDG was converted into UCB. Hydrolysis rates for both BMG and BDG followed the pH order 7.8 greater than 7.6 approximately equal to 7.4 greater than 7.1 Incubation with Ca2+ (6.2 mM) at pH 7.4 in TC-BUF resulted in precipitated bile pigment which, at 100 X magnification, appeared similar to precipitates seen in the bile of patients with pigment gallstones. At pH 7.4, lecithin (crude phosphatidylcholine) (4.2 mM) was a potent inhibitor of hydrolysis of BMG and BDG. The addition of a concentration of cholesterol equimolar with that of lecithin eliminated this inhibitory effect. Guinea-pig gallbladder bile incubated with glucaro-1,4-lactone (an inhibitor of beta-glucuronidase) underwent hydrolysis similar to the model bile systems. The non-enzymic hydrolysis of bile pigments, especially BMG, may be an important mechanism of bile-pigment precipitation and, ultimately, of gallstone formation.