Uridine diphosphate-glucuronic acid-independent conversion of bilirubin monoglucuronides to diglucuronide in presence of plasma membranes from rat liver is nonenzymic

Membranous nephropathy

Membranous nephropathy (MN) is a glomerular disease that can occur at all ages. In adults, it is the most frequent cause of nephrotic syndrome. In ~80% of patients, there is no underlying cause of MN (primary MN) and the remaining cases are associated with medications or other diseases such as systemic lupus erythematosus, hepatitis virus infection or malignancies. MN is an autoimmune disease characterized by a thickening of the glomerular capillary walls due to immune complex deposition. Identification of the phospholipase A2 receptor (PLA2R) as the major antigen in adults in 2009 induced a paradigm shift in disease diagnosis and monitoring and several other antigens have since been characterized. Disease outcome is difficult to predict and around one-third of patients will undergo spontaneous remission. In those at high risk of progression, immunosuppressive therapy with cyclophosphamide plus corticosteroids has substantially reduced the need for kidney replacement therapy. Owing to carcinogenic risk, other treatments (calcineurin inhibitors and CD20-targeted B cell depletion therapy (rituximab)) have been developed. However, disease relapses are frequent when calcineurin inhibitors are stopped and the remission rate with rituximab is lower than with cyclophosphamide, particularly in patients with high PLA2R antibody titres. Other new drugs are already available and antigen-specific immunotherapies are being developed.

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.

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.

Proportion of conjugated bilirubin in bile in relation to hepatic bilirubin UDP-glucuronyltransferase activity

To elucidate the diagnostic relevance of biliary conjugated bilirubin, biliary bilirubin from normal volunteers (NV), patients with Gilbert's syndrome (GS) and Crigler-Najjar syndrome type II (C-N II), and from various rat strains was fractionated. Biliary bilirubin diglucuronide (BDG) was present at lower levels, and bilirubin monoglucuronide (BMG) and unconjugated bilirubin were present at higher levels in GS and C-N II compared with NV, which is consistent with decreased hepatic bilirubin UDP-glucuronyltransferase activity (BGTA). The level of biliary BDG was higher in Wistar-Kyoto rats and lower in heterozygous (Jj) Gunn rats than in SD and Wistar rats. The hepatic BGTA level in heterozygous (Jj) Gunn rats was decreased to 60% of that in Wistar rats, in accordance with decreased biliary BDG. On the other hand, BGTA in Wistar-Kyoto rats whose biliary BDG level was high, was not different from that of Wistar and SD rats. Thus, a correlation between BGTA and biliary bilirubin fractions may not exist on some occasions.

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.

Lincomycin treatment of guinea pigs causes formation of pigmented phosphate containing gallbladder sludge and stones

We studied the mechanism of gallbladder sludge formation in guinea pigs (n = 30) treated with lincomycin (80 mg/kg/day) for 7 consecutive days. At sacrifice (day 8) gallbladders of treated animals contained turbid bile, sludge and in one animal a single gallstone. The precipitates were amorphous on X-ray diffraction. Infra-red spectroscopy revealed calcium phosphate as the major component. Compared to saline-treated controls (n = 15) concentrations of total protein, total phosphate and total bilirubin in gallbladder bile were significantly increased (P less than 0.05). The increase in total phosphate was due to the inorganic component, since phospholipid phosphorus was unchanged. The relative amounts of unconjugated bilirubin and of bilirubin mono- and diconjugates in gallbladder bile were unaffected by treatment as was beta-glucuronidase activity. However, sludge was enriched in unconjugated bilirubin compared to gallbladder bile. This was most probably caused by alkaline hydrolysis of bilirubin monoconjugates. To some extent, disproportionation of bilirubin monoconjugates in bile or sludge, either in vivo or during sample preparation, might also have led to increased unconjugated pigment.

Similarities in maximal biliary bilirubin output in the normal rat after administration of unconjugated bilirubin or bilirubin diglucuronide

The rate-limiting step in the overall plasma-to-bile transport of a saturating load of bilirubin is still a matter of controversy. We reassessed the apparent maximal biliary bilirubin excretion following i.v. infusion of unconjugated bilirubin and--for the first time--of highly purified bilirubin diglucuronide in the rat. The bilirubin diglucuronide preparation could be kept in a stable form at -20 degrees C for at least 2 months after addition of 3 mM sodium ascorbate. The biliary bilirubin excretion rates in animals with and without bile depletion in order to induce different flow rates were comparable after infusion of unconjugated bilirubin and of bilirubin diglucuronide. No significant hydrolysis of bilirubin diglucuronide seemed to occur during the hepatic transport of the pigment. Injection of bilirubin diglucuronide into rats which were already being infused with saturating doses of unconjugated bilirubin did not result in increased biliary bilirubin excretion. In contrast, a reversible inhibition of bilirubin output and bile acid-dependent bile flow was observed. If unconjugated and diglucuronidated bilirubin follow the same intracellular routes, the present results would suggest that conjugation did not restrict maximal biliary excretion. However, if exogenously administered diglucuronide utilizes a separate pathway, as was recently proposed, the biliary secretion of this exogenous conjugate might be restricted, presumably due to a toxic effect of the high local concentration of diglucuronide. The pathways utilized by the unconjugated pigment, on the other hand, could be primarily determined by the conjugating capacity.

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

We have developed an extremely rapid and efficient reverse-phase h.p.l.c. method for the measurement of bilirubin and its conjugates in human bile and in model bile systems. Our method involves the use of a Perkin-Elmer 3 mu C18 column and a methanol/sodium acetate/aq. ammonium acetate buffer system. Three isomers of bilirubin diglucuronide (BDG), two isomers of bilirubin monoglucuronide (BMG), three isomers of unconjugated bilirubin (UCB) and minor conjugates containing glucose and xylose were separated in 12 min. Initial quantification of BDG and BMG was based on the use of the ethyl anthranilate azo derivative of bilirubin (AZO UCB); however, the standard curves for BDG, BMG and UCB were similar enough to permit quantification to be later based on the UCB standard curve only, thereby simplifying the quantification process. Routine direct injection of 6 or 10 microliter of crude undiluted or diluted (1:1) bile sample was sufficient for analysis. The method was helpful in diagnosing biliary-tract obstruction in a newborn and a partial deficiency state of bilirubin conjugation (Crigler-Najjar syndrome) in a 10-year-old male. When the method was applied to biles of patients both with and without gallstones, levels of UCB were less than 2% of total pigment, consistent with previous reports. Because of its speed and efficiency, this method has the potential for a broad range of applications including enzymic, kinetic and bile sample analyses.

Microsomal UDP-glucuronyltransferase-catalyzed bilirubin diglucuronide formation in human liver

Human liver microsomal bilirubin UDP-glucuronyltransferase catalyzes formation of bilirubin mono- and diglucuronide. KmUDPGA and Vmax of the enzyme are 0.6 mM and 1.69 nmol/mg protein X min. In vitro, bilirubin readily dissolves in the microsomal lipid phase. Taking this into account a Kmbilirubin of 60.6 microM was found, which is much higher than the in vivo microsomal UCB concentration of human liver (2.9-11.4 microM). The total capacity of human liver to form bilirubin mono- and diglucuronide in vitro exceeds the in vivo mono- and diglucuronide production rates by a factor 8 to 10. Radiation-inactivation studies reveal that human liver microsomal bilirubin UDP-glucuronyltransferase is a tetrameric enzyme with a molecular mass of 209 000 +/- 20 000 Da. The complete tetrameric enzyme catalyzes both glucuronidation steps, formation of bilirubin monoglucuronide and conversion of mono- to diglucuronide. In its monomeric form, the enzyme with molecular mass of 55 000 +/- 1 500 Da catalyzes only the first step of bilirubin glucuronidation, the formation of bilirubin monoglucuronide.

Substrates and products of purified rat liver bilirubin UDP-glucuronosyltransferase

To determine whether the isoform of UDP-glucuronosyltransferase which catalyzes the formation of bilirubin monoglucuronide also mediates the formation of bilirubin diglucuronide and other specific sugar conjugates of bilirubin, Wistar rats were treated with clofibrate (300 mg per kg i.p. X 7 days); this resulted in a 200% increase in hepatic transferase specific activity for bilirubin. Proteins from hepatic microsomal fractions were solubilized, and the transferase isoform with activity toward bilirubin was purified by a combination of chromatofocusing, affinity chromatography and hydrophobic chromatography, to apparent homogeneity as judged by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The purified isoform catalyzed the formation of monoglucuronide and diglucuronide (with UDP-glucuronic acid as a cosubstrate), and glucoside and xyloside (with UDP-glucose and UDP-xylose as respective cosubstrates) of bilirubin and glucuronidation of the carcinogen metabolite 4'-hydroxydimethylaminoazobenzene. It also catalyzed the conversion of bilirubin monoglucuronide to diglucuronide (with UDP-glucuronic acid as cosubstrate, pH optimum 7.8), to mixed glucuronide-glucoside conjugate (with UDP-glucose as a cosubstrate) and to unconjugated bilirubin (with UDP as a cosubstrate, pH optimum 5.5). Each transferase activity was copurified at each purification step. Results of enzyme kinetic studies suggest that UDP-glucuronic acid, UDP-glucose and UDP-xylose recognize a common site. Transferase activities toward bilirubin were not detectable in homozygous Gunn rats liver microsomal fractions; in heterozygous Gunn rats, these activities were reduced by 40 to 60%. The results suggest that conjugation of bilirubin with glucuronic acid, glucose or xylose is catalyzed by a single transferase isoform.

Study of bilirubin metabolism by high-performance liquid chromatography: stability of bilirubin glucuronides

The stabilities of bilirubin (BR) glucuronide, monoglucuronide (BMG), and diglucuronide (BDG) were studied under various conditions by HPLC. In aqueous media, BMG showed a pronounced lability and was easily transformed into equimolar BDG and BR. It was proved by direct analysis of tetrapyrrole isomers that BDG and BR were formed from dipyrrole exchange of BMG molecules. All reducing agents examined (sodium ascorbate, cysteine, GSH, dithiothreitol, NADH, and NADPH) suppressed the transformation of BMG into BDG and BR. Bovine serum albumin and rat liver cytosol fractions also stabilized BMG strongly. BDG was fairly stable in aqueous media as compared with BMG. When BMG was incubated both with and without liver plasma membranes (N2 fraction) from Wistar rats, the formation rates of BDG and BR in both incubation mixtures were exactly the same. The composition of BDG and BR isomers was the same in both mixtures. Also, heat denaturation of the plasma membranes did not affect formation rates. Moreover, the reaction was completely inhibited by sodium ascorbate. These findings indicate that rat liver plasma membranes have no enzyme activity for BDG formation from BMG.

Formation of benzo[a]pyrene-3,6-quinol mono- and diglucuronides in rat liver microsomes

The formation of benzo[a]pyrene (BP)-3,6 quinol glucuronides in liver microsomes in the presence of UDP-glucuronic acid and NAD(P)H appears to occur by a sequence of three reactions: BP-3,6-quinone----BP-3,6 hydroquinone----BP-3,6-quinol monoglucuronide----BP-3,6-quinol diglucuronide. This conclusion is based on the following results. Incubations with [14C]BP-3,6-quinone or UDP-[14C]glucuronic acid and analysis of the samples by TLC established the existence and identity of the two BP-3,6-quinol glucuronides which exhibit different fluorescence spectra. The nature of the monoglucuronide, i.e., a quinol and not a semiquinone glucuronide, was suggested by the finding that the rate of diglucuronide formation was the same with or without NAD(P)H provided that a sufficient amount of monoglucuronide had been formed prior to oxidation of the nucleotides. Furthermore, BP-3,6-quinol monoglucuronides can serve as substrates in the formation of diglucuronides. The ratio between the decrease in monoglucuronides and the formation of diglucuronides was found to be close to 1, suggesting that the conversion of the monoglucuronide of BP-3,6-quinol to the diglucuronide is also catalyzed by UDP-glucuronosyltransferase. However, great differences in the pattern of induction of mono- and diglucuronide formation indicate that two different UDP-glucuronosyltransferases are involved. The yield of BP-3,6-quinol glucuronides with NADH relative to NADPH and the increase in glucuronide formation observed in the presence of cytosolic DT-diaphorase (NAD(P)H-quinone oxidoreductase) are discussed with regards as to whether DT-diaphorase plays an important role as a BP-3,6-quinone reductase in the formation of BP-3,6-quinol glucuronides compared to other NAD(P)H-oxidizing flavoproteins.

Reverse-phase h.p.l.c. separation, quantification and preparation of bilirubin and its conjugates from native bile. Quantitative analysis of the intact tetrapyrroles based on h.p.l.c. of their ethyl anthranilate azo derivatives

We describe a facile and sensitive reverse-phase h.p.l.c. method for analytical separation of biliary bile pigments and direct quantification of unconjugated bilirubin (UCB) and its monoglucuronide (BMG) and diglucuronide (BDG) conjugates in bile. The method can be 'scaled up' for preparative isolation of pure BDG and BMG from pigment-enriched biles. We employed an Altex ultrasphere ODS column in the preparative steps and a Waters mu-Bondapak C18 column in the separatory and analytical procedures. Bile pigments were eluted with ammonium acetate buffer, pH 4.5, and a 20 min linear gradient of 60-100% (v/v) methanol at a flow rate of 2.0 ml/min for the preparative separations and 1.0 ml/min for the analytical separations. Bile pigments were eluted in order of decreasing polarity (glucuronide greater than glucose greater than xylose conjugates greater than UCB) and were chemically identified by t.l.c. of their respective ethyl anthranilate azo derivatives. Quantification of UCB was carried out by using a standard curve relating a range of h.p.l.c. integrated peak areas to concentrations of pure crystalline UCB. A pure crystalline ethyl anthranilate azo derivative of UCB (AZO . UCB) was employed as a single h.p.l.c. reference standard for quantification of BMG and BDG. We demonstrate that: separation and quantification of biliary bile pigments are rapid (approximately 25 min); bile pigment concentrations ranging from 1-500 microM can be determined 'on line' by using 5 microliters of bile without sample pretreatment; bilirubin conjugates can be obtained preparatively in milligram quantities without degradation or contamination by other components of bile. H.p.l.c. analyses of a series of mammalian biles show that biliary UCB concentrations generally range from 1 to 17 microM. These values are considerably lower than those estimated previously by t.l.c. BMG is the predominant, if not exclusive, bilirubin conjugate in the biles of a number of rodents (guinea pig, hamster, mouse, prairie dog) that are experimental models of both pigment and cholesterol gallstone formation. Conjugated bilirubins in the biles of other animals (human, monkey, pony, cat, rat and dog) are chemically more diverse and include mono-, di- and mixed disconjugates of glucuronic acid, xylose and glucose in proportions that give distinct patterns for each species.

Bilirubin mono- and di-glucuronide formation by purified rat liver microsomal bilirubin UDP-glucuronyltransferase

Highly purified bilirubin UDP-glucuronyltransferase from Wistar-rat liver, when reconstituted with Gunn-rat liver microsomes (microsomal fraction), was able to catalyse the conversion of unesterified bilirubin into both bilirubin monoglucuronide and diglucuronide. Under zero-order kinetic conditions for monoglucuronide formation, the fraction of bilirubin diglucuronide formed by incubation of bilirubin with the reconstituted highly purified transferase accounted for 18% of total bilirubin glucuronides, which was only slightly lower than the fraction of diglucuronides (23% of total bilirubin glucuronides) formed by incubation with hepatic microsomes in the presence of UDP-N-acetylglucosamine or Lubrol. The reconstituted purified enzyme also catalysed the UDP-glucuronic acid-dependent conversion of bilirubin monoglucuronide into diglucuronide and, when bilirubin was incubated with UDP-glucose or UDP-xylose, the formation of bilirubin glucosides and xylosides respectively. These results suggest that a single microsomal bilirubin UDP-glycosyltransferase may be responsible for the formation of bilirubin mono- and di-glycosides.

UDP-glucuronyltransferase-catalyzed deconjugation of bilirubin monoglucuronide

Bilirubin monoglucuronide is rapidly deconjugated when incubated with UDP and rat liver microsomal preparations at pH 5.1. The following evidence was found that this reaction is catalyzed by UDP-glucuronyltransferase: (i) unconjugated bilirubin and UDP-glucuronic acid were identified as the reaction products; (ii) Gunn rat microsomal preparations lack bilirubin UDP-glucuronyltransferase deficiency and do not catalyze the deconjugation reaction, and (iii) neither saccharo-1,4-lactone, a beta-glucuronidase inhibitor, nor butylated hydroxytoluene, an inhibitor of spontaneous isomerisation, affect the rate of the deconjugation reaction. Deconjugation appears to be the reverse of UDP-glucuronyltransferase-catalyzed glucuronidation. The conditions for the reverse reaction differ in the following aspects from those of the forward reaction: (i) nucleotide triphosphates stimulate the reverse reaction probably allosterically; (ii) UDP-N-acetylglucosamine stimulates the forward reaction but has no effect on the reverse reaction; (iii) the optimal pH for the reverse reaction is pH 5.1 and for the forward reaction is pH 7.8, and (iv) Mg++ ion is not required for the reverse reaction but stimulates the forward reaction. Detergents stimulate both reactions. Stimulation of the reverse reaction by nucleotide triphosphates and detergents is mutually independent and additive which suggests different mechanisms of action. Deconjugation reactions may become important during parenchymatous liver disease when, as a result of anaerobic glycolysis, intracellular pH decreases. Elevated levels of unconjugated bilirubin in the serum of patients with parenchymatous liver disease may be a sign of sick liver cells rather than decreased UDP-glucuronyltransferase activity.

Bilirubin analysis--the state of the art and future prospects

Progress in separating and identifying different bile pigments in serum has led to the recognition of a bilirubin fraction (delta) distinct from unconjugated bilirubin and its (mono- and di-) sugar conjugates. Delta bilirubin reacts directly diazo-positive and is strongly linked to an albumin-like protein, presumably via an amide bond between a propionic acid side-chain of the tetrapyrrole and a functional group (e.g., epsilon-amino group of lysine) on the protein backbone. Because of its unusual properties and its wide occurrence in icteric sera, the delta fraction may have important analytical and clinical implications. We examine here some of these implications and discuss the prospects for a better understanding of the molecular basis of jaundice.

New developments in the regulation of heme metabolism and their implications

Various endogenous and exogenous chemicals, such as hormones, drugs, and carcinogens and other environmental pollutants are enzymatically converted to polar metabolites as a result of their oxidative metabolism by the mixed-function oxidase system. This enzyme complex constitutes the major detoxifying system of man and utilizes the hemoprotein--cytochrome P-450--as the terminal oxidase. Recent studies with trace metals have revealed the potent ability of these elements to alter the synthesis and to enhance the degradation of heme moiety of cytochrome P-450. An important consequence of these metal actions is to greatly impair the ability of cells to oxidatively metabolize chemicals because of the heme dependence of this metabolic process. In this report the effects of exposure to trace metals on drug oxidations is reviewed within the framework of metal alterations of heme metabolism, including both its synthesis and degradation, since these newly discovered properties of metals have made it possible to define a major dimension of metal toxicity in terms of a unified cellular mechanism of action.

Bilirubin diglucuronide formation by rat liver microsomes: demonstration by affinity and thin layer chromatography of bile pigment tetrapyrroles

The conjugates formed in vitro by bilirubin UDP-glucuronyl transferase were studied by examining reaction products as intact tetrapyrroles, rather than as dipyrrolic azoderivatives. Bile pigments were extracted from conventional microsomal enzyme reaction mixtures by affinity chromatography over albumin-agarose, eluted with 50% ethanol, and separated by a silica gel thin layer chromatographic system. In the presence of UDPGA, native and activated microsomal preparations all formed both bilirubin mono- and diglucuronides from unconjugated bilirubin, and bilirubin diglucuronide from bilirubin monoglucuronide. No significant non-enzymatic conversion of mono- to diglucuronide occurred without UDPGA, or in the presence of denatured enzyme. Hence, bilirubin diglucuronide is a major product of bilirubin-UDP-glucuronyl transferase.

Formation of bilirubin monoglucuronide and diglucuronide in isolated rat hepatocytes. Effect of spironolactone

The formation of bilirubin monoglucuronide (BMG) and diglucuronide (BDG) was studied in isolated rat hepatocytes with appropriate viability. Isolated cells were obtained from normal rats and from rats pretreated with spironolactone (SP). A fixed number of cells (4.8 X 10(6)) was incubated in a medium containing uridine diphosphoglucuronic acid (UDPGA, 3.4 mM) and bilirubin (11.3 microM, 29 microM, 50 microM and 81 microM) for different time intervals (from 0 to 25 min). The pellet of cells and the supernatant fraction were subjected to alkaline methanolysis, and the proportions of BMG and BDG were estimated by thin-layer chromatography. No conjugates were detected at time O or in the absence of UDPGA in the incubation system. BMG and BDG were detected after 2 min of incubation and then they increased up to 15 min of incubation. Both conjugates were mostly found in the supernatant fraction, and a predominance of BMG was apparent. Normal cells also synthesized increasing amounts of BMG and BDG with the increase of bilirubin substrate concentration up to 50 microM. When hepatocytes from SP-treated rats were used, a more rapid rate of glucuronidation, that was mainly produced at the expense of BMG found in the supernatant fraction, was clear. The results probably indicate that enzymic conversion of BMG to BDG may be rate limiting in isolated hepatocytes although other possible mechanisms were not excluded.

Analysis of bilirubins in biological fluids by extraction and thin-layer chromatography of the intact tetrapyrroles: application to bile of patients with Gilbert's syndrome, hemolysis, or cholelithiasis

A method was developed to extract quantitatively the bilirubins from bile, urine, serum, stool, and preparations from liver with a chloroform-ethanol mixture at pH 1.8 in the presence of ascorbic acid and NaCl. Extracted pigment was submitted to thin-layer chromatography, and the separated bilirubins were either immediately eluted and determined spectrophotometrically or individually converted to ethyl anthranilate azo derivatives for thin-layer chromatographic analysis of each isolated pigment band. Bilirubins in duodenal bile of eight healthy adults comprised 1.5 +/- 1.3% unconjugated bilirubin-IX alpha, 69 +/- 6% bilirubin diglucuronide, and 16 +/- 4% bilirubin monoglucuronides. Mixed diconjugates containing one glucuronosyl moiety and either one xylosyl or one glucosyl group amounted to 10 +/- 3%. Most samples (6 of 8) contained trace amounts (0.6 +/- 0.6%) of unconjugated bilirubin-IX beta, in agreement with nearly exclusive cleavage of heme at the alpha-meso position. The composition of the bilirubins in bile was normal in 6 patients with cholesterol gallstones, 4 with chronic hepatitis, and 3 with hemolysis. In duodenal bile of individuals with Gilbert's syndrome (n = 10), the concentration of bilirubin conjugates was comparable to that in healthy adults, but the proportion of bilirubin diglucuronides (52 +/- 8%) was decreased. The concentration of unconjugated bilirubin-IX alpha showed a fair positive correlation with that of bilirubin monoglucuronide and was increased in half of the patients with Gilbert's syndrome.