Bilirubin diglucuronide synthesis by a UDP-glucuronic acid-dependent enzyme system in rat liver microsomes

Bilirubin glucuronidation revisited: proper assay conditions to estimate enzyme kinetics with recombinant UGT1A1

Bilirubin, an end product of heme catabolism, is primarily eliminated via glucuronic acid conjugation by UGT1A1. Impaired bilirubin conjugation, caused by inhibition of UGT1A1, can result in clinical consequences, including jaundice and kernicterus. Thus, evaluation of the ability of new drug candidates to inhibit UGT1A1-catalyzed bilirubin glucuronidation in vitro has become common practice. However, the instability of bilirubin and its glucuronides presents substantial technical challenges to conduct in vitro bilirubin glucuronidation assays. Furthermore, because bilirubin can be diglucuronidated through a sequential reaction, establishment of initial rate conditions can be problematic. To address these issues, a robust high-performance liquid chromatography assay to measure both bilirubin mono- and diglucuronide conjugates was developed, and the incubation conditions for bilirubin glucuronidation by human embryonic kidney 293-expressed UGT1A1 were carefully characterized. Our results indicated that bilirubin glucuronidation should be assessed at very low protein concentrations (0.05 mg/ml protein) and over a short incubation time (5 min) to assure initial rate conditions. Under these conditions, bilirubin total glucuronide formation exhibited a hyperbolic (Michaelis-Menten) kinetic profile with a K(m) of ∼0.2 μM. In addition, under these initial rate conditions, the relative proportions between the total monoglucuronide and the diglucuronide product were constant across the range of bilirubin concentration evaluated (0.05-2 μM), with the monoglucuronide being the predominant species (∼70%). In conclusion, establishment of appropriate incubation conditions (i.e., very low protein concentrations and short incubation times) is necessary to properly characterize the kinetics of bilirubin glucuronidation in a recombinant UGT1A1 system.

Inhibitory effect of bilirubin on complement-mediated hemolysis

We investigated the in vitro action of the bile pigments, unconjugated bilirubin (UB) and bilirubin monoglucuronide (BMG) on complement (C) cascade reaction. Both UB and BMG inhibited hemolysis in the classical pathway (CP) in a dose-dependent manner at low micromolar concentrations, UB showing a stronger effect than BMG. The analysis of the action of UB on the hemolytic activity of the C1, C4, C2 and C-EDTA components of the C cascade revealed that the C1 step was the most inhibited. An enzyme immunoassay was developed to evaluate the effect of UB on the binding of C1q, one of the subcomponents of C1, to human IgM and IgG. The study demonstrated that the unconjugated pigment interferes both the C1q-IgM and -IgG interactions, thus tentatively explaining the inhibitory action of UB on hemolytic activity of C1. We conclude that the anti-complement effect of UB is mainly exerted on the C1 component, the recognition unit of CP. The potential clinical implication of the reported effects in hyperbilirubinemia is discussed.

Comparison of bilirubin conjugation in encapsulated hepatocytes, hepatocyte homogenate and intact hepatocytes

Homogenized rat hepatocytes, whole hepatocytes and encapsulated hepatocytes were incubated with bilirubin and UDP-glucuronic acid to test their ability to form bilirubin conjugates. Bilirubin monoconjugated and diconjugated were detected in all the three preparations by HPLC analysis. The UDP-glucuronosyltransferase (UDPGT) activity of homogenized hepatocytes was 0.002 +/- 0.00006 mM/min per million cells; that of intact hepatocytes was 0.001 +/- 0.00006 mM/min per million cells and that of encapsulated hepatocytes was 0.0005 +/- 0.00002 mM/min per million cells.

Exploring the conformation of bilirubins with natural and unnatural analogues: use of positional and bridged isomers of bilirubin IXalpha

Unlike bilirubin IXalpha (1), the isomers bilirubin IXdelta (2) and neobilirubin IXbeta (3) do not require conjugation with glucuronic acid in order to be excreted. A conformational analysis employing an optimized Monte Carlo method and a mixed Monte Carlo stochastic dynamics reveals that isomer 2 exhibits a structure more closed than the well known 'ridge-tile' conformation of 1. The change in the position of both propionic acid chains causes the loss of at least four hydrogen bonds. On the other hand, the change in the configuration of the distal dipyrrinone and the blockage of the lactamic nitrogen by the presence of a bridge in isomer 3 results in an open and more elongated structure, where the chance of hydrogen bond formation in this region is obliterated. The resulting molecular models for these compounds are consistent with 1H NM R, UV-vis, and TLC data.

Unconjugated bilirubin exhibits spontaneous diffusion through model lipid bilayers and native hepatocyte membranes

The liver is responsible for the clearance and metabolism of unconjugated bilirubin, the hydrophobic end-product of heme catabolism. Although several putative bilirubin transporters have been described, it has been alternatively proposed that bilirubin enters the hepatocyte by passive diffusion through the plasma membrane. In order to elucidate the mechanism of bilirubin uptake, we measured the rate of bilirubin transmembrane diffusion (flip-flop) using stopped-flow fluorescence techniques. Unconjugated bilirubin rapidly diffuses through model phosphatidylcholine vesicles, with a first-order rate constant of 5.3 s-1 (t(1)/(2) = 130 ms). The flip-flop rate is independent of membrane cholesterol content, phospholipid acyl saturation, and lipid packing, consistent with thermodynamic analyses demonstrating minimal steric constraint to bilirubin transmembrane diffusion. The coincident decrease in pH of the entrapped vesicle volume supports a mechanism whereby the bilirubin molecule crosses the lipid bilayer as the uncharged diacid. Transport of bilirubin by native rat hepatocyte membranes exhibits kinetics comparable with that in model vesicles, suggesting that unconjugated bilirubin crosses cellular membranes by passive diffusion through the hydrophobic lipid core. In contrast, there is no demonstrable flip-flop of bilirubin diglucuronide or bilirubin ditaurate in phospholipid vesicles, yet these compounds rapidly traverse isolated rat hepatocyte membranes, confirming the presence of a facilitated uptake system(s) for hydrophilic bilirubin conjugates.

A novel putative transporter maps to the osteosclerosis (oc) mutation and is not expressed in the oc mutant mouse

The phenotype of mice homozygous for the osteosclerosis (oc) mutation includes osteopetrosis, and a variety of studies demonstrate that osteoclasts in these mice are present but nonfunctional. We have identified a novel gene that has homology to a family of 12-transmembrane domain proteins with transport functions and maps to proximal mouse chromosome 19, in a region to which the oc mutation has been previously assigned. The putative transporter is abundant in normal kidney, but its expression is markedly reduced in kidneys from oc/oc mice when tested using Northern and Western analyses. Southern analysis of this gene, which we call Roct (reduced in oc transporter), demonstrates that it is intact and unrearranged in oc/oc mice. In situ studies show that Roct is expressed in developing bone. We propose that the absence of Roct expression results in an osteopetrosis phenotype in mice.

Elevation of the serum bilirubin diconjugate fraction provides an early marker for cholestasis in the rat

AIMS/METHODS

During cholestasis, components normally excreted into bile, e.g. bilirubin, accumulate in liver cells and biliary passages. In order to assess the conjugation of bilirubin retained in the hepatocyte during cholestasis, we analyzed the pattern of bilirubin pigments in rat serum and bile, using reversed phase alkaline methanolysis-HPLC. Cholestasis was induced by bile flow interruption for 1 to 2 h.

RESULTS

One hour after initiation of cholestasis, the serum concentration of total bilirubin rose 2-fold due to increases in bilirubin di- (BDC) and monoconjugate (BMC), while unconjugated bilirubin (UCB) decreased by 33%. As a result, the BDC/BMC ratio increased to 1.67+/-0.20 vs 0.60+/-0.10 in controls (p<0.01) and the BMC/UCB ratio to 1.0+/-0.2 vs 0.1+/-0.1 (p<0.01). After relief of biliary obstruction, biliary output rose to 8.0+/-0.5 vs 5.5+/-0.3 micromol x min(-1) x kg (p<0.01), and the biliary BDC/BMC ratio to 4.0+/-0.3 vs 1.5+/-0.2 (p<0.01). In contrast, the biliary BMC/UCB ratio remained unchanged throughout. Increasing the duration of obstruction to 2 h led to a further increase in the serum BMC/UCB ratio to 2.2+/-0.3 (p<0.01), but not in the BDC/BMC ratio. Serum aminotransferase activity and the concentration of total bile acids increased 3- and 100-fold above their respective control values. Alkaline phosphatase activity remained unaltered, and electron microscopical features of cholestasis became apparent only after 2 h of biliary obstruction.

CONCLUSIONS

We suggest that one of the initial events of cholestasis is a more efficient conjugation of bilirubin retained in the hepatocyte. This results in a shift of the equilibrium among bilirubin pigments towards BDC, the end-product of conjugation. Such a shift provides an early marker for cholestasis.

Kinetic analysis of UDP-glucuronosyltransferase in bilirubin conjugation by encapsulated hepatocytes for transplantation into Gunn rats

Kinetic analysis of the enzyme UDP-glucuronosyltransferase (UDPGT), responsible for the conjugation of bilirubin, suggests that it is a multisubunit enzyme in which there is cooperative binding of the substrate to the subunits. The binding of bilirubin to UDPGT shows positive cooperativity with an apparent Hill coefficient of 2.9. The binding of UDP-glucuronic acid (UDPGA) exhibits kinetics with mixed cooperativity with an apparent Hill coefficient of 4.028. Homogenized rat hepatocytes, intact hepatocytes, and hepatocytes encapsulated in alginate-polylysine-alginate artificial cells, when incubated with bilirubin (1.6 mM) and UDPGA (20 mM), can form monoconjugated and diconjugated bilirubin. However, the presence of the artificial membrane offers some mass transfer resistance. The intraperitoneal transplantation into the Gunn rat of free and microencapsulated Wistar rat hepatocytes shows that both are equally effective in lowering the serum bilirubin. Thus, the membrane did not contribute to a lowering of efficacy after transplantation.

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.

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.

Thyroid hormones and the hepatic handling of bilirubin. I. Effects of hypothyroidism and hyperthyroidism on the hepatic transport of bilirubin mono- and diconjugates in the Wistar rat

The effects of thyroidectomy and of thyroid hormone administration on the hepatic transport of endogenous bilirubin were investigated in the Wistar R/APfd rat. Hypothyroidism resulted in an enhanced hepatic bilirubin UDP-glucuronosyltransferase activity and in a decreased p-nitrophenol transferase activity. It caused a cholestatic condition with a 50% decrease in bile flow and bile salt excretion, and an increased proportion of conjugated bilirubin in serum. The biliary output of unconjugated and monoconjugated bilirubins decreased in parallel by about 65%, whereas the excretion rate of the diconjugate dropped by only 47%, resulting in an increased di- to monoconjugate ratio in bile. Hyperthyroidism was characterized by a decreased bilirubin and an increased p-nitrophenol transferase activity, and by an augmented bilirubin output in bile. The output of unconjugated and monoconjugated bilirubins increased in parallel by about 50 or 100%, whereas the excretion of the diconjugate increased by only 20 to 50%, depending on the dose of thyroxine administered; this resulted in a decreased di- to monoconjugate ratio in bile. A linear positive relationship was found between bilirubin UDP-glucuronosyltransferase activity and the ratio of bilirubin di- to monoconjugates present in bile or formed by in vitro incubation of liver homogenates at low concentration of bilirubin (10 to 15 microM), indicating that bile pigment composition is mainly determined by the conjugation activity in the liver. The inverse relationship observed between hepatic beta-glucuronidase activity and the ratio of di- to monoconjugates in bile warrants further investigation to analyze whether this enzyme activity also plays a possible role in the changes in bile pigment composition in hypo- and hyperthyroid rats.

Glycoside conjugation in microsomes from hepatic and renal carcinoma of man

Human hepatoma which had been xenografted into nude mice have been estimated for their ability to catalyze glucuronic acid and glucose conjugation of endogenous compounds and p-nitrophenol. The xenobiotic p-nitrophenol was glucuronidated with a comparable rate in microsomes from human hepatoma, human liver and host liver. With regard to glucuronic acid or glucose conjugation of the endogenous compounds of bile acids, bilirubin and steroid hormones, glucosidation of bile acids was the only conjugation mechanism that was not decreased or deficient in microsomes from hepatoma, but showed about a 2-fold increase in reaction rate compared to normal human liver. Human hepatoma and host liver were shown to respond to phenobarbital treatment which led to about a 2-fold increase in UDP-glucuronosyltransferase activity toward bilirubin in hepatoma and in host liver. Compared to normal tissues, alterations in the pattern of glycoside conjugating enzymes were not only observed in microsomes from human hepatoma, but also in microsomes from human adenocarcinoma of the kidney, exhibiting negligible UDP-glucuronosyltransferase activities toward bile acids and steroid hormones. Bile acid glucoside formation was measurable in kidney adenocarcinoma with an activity which was similar to the activity observed in hepatoma. In comparison to normal renal tissue, glucose-conjugating activity toward bile acids decreased about 2-fold in kidney adenocarcinoma.

Membrane-membrane interactions associated with rapid transfer of liposomal bilirubin to microsomal UDP-glucuronyltransferase. Relevance for hepatocellular transport and biotransformation of hydrophobic substrates

Bilirubin may be transported within intracellular membranes of the hepatocyte and may undergo membrane-membrane transfer to gain access to the conjugating enzyme UDP-glucuronyltransferase in the endoplasmic reticulum. We have demonstrated previously that the lipid composition of liposomal membranes incorporating bilirubin substrate influences the rate of transfer and glucuronidation of bilirubin by hepatic microsomes. To examine the mechanism(s) of substrate transfer, we incorporated radiolabelled bilirubin into small unilamellar model membranes of egg phosphatidylcholine or natural phospholipids in the proportions present in native hepatic microsomes. The rate at which bilirubin was transferred to rat liver microsomes and glucuronidated was then examined in the presence of various endogenous compounds that promote membrane fusion. For bilirubin substrate in membranes of egg phosphatidylcholine, the addition of Ca2+ (2 mM) increased the microsomal glucuronidation rate, whereas retinol enhanced microsomal conjugation rates for bilirubin in membranes of both lipid compositions. When the transfer of [3H]bilirubin from dual-labelled liposomes to microsomes was enhanced by Ca2+ or retinol, there was no associated increase in [14C]phospholipid transfer. Thus it appears likely that bilirubin is transferred to the endoplasmic reticulum by rapid cytosolic diffusion or membrane-membrane collisions, rather than by membrane fusion; this process may be modulated by changes in the lipid microenvironment of the substrate or the effective intracellular concentrations of Ca2+ or retinol. The observation that polymyxin B induced concomitant membrane-membrane transfer of [3H]bilirubin and [14C]phospholipid suggests that under certain circumstances membrane fusion or aggregation may promote the movement of lipophilic substrates in hepatocytes.

Hepatic disposition and biliary excretion of bilirubin and bilirubin glucuronides in intact rats. Differential processing of pigments derived from intra- and extrahepatic sources

Mechanisms for transport of bilirubin and its conjugates in hepatocytes have not been defined. We investigated the hepatic processing of bilirubin glucuronides and their precursors, and characterized the disposition of bile pigments arising from intraversus extrahepatic sources. Tracer doses of purified radiolabeled biliverdin, bilirubin, bilirubin monoglucuronide (BMG) or diglucuronide (BDG) were administered intravenously to intact normal or jaundiced homozygous Gunn rats. Rapid sequential analysis of radiolabeled BMG and BDG in bile revealed comparable excretion patterns following biliverdin and bilirubin injection, with BDG as the major pigment. Biliary excretion of radiolabeled conjugates from injected BMG was more rapid, with BMG predominating. Excretion of injected BDG in normal rats and BMG or BDG in Gunn rats was virtually identical to that of unaltered BMG in normal rats. Model independent analysis by deconvolution provided objective comparison of the disposition of radiolabeled pigments from the different sources. These findings indicate that bilirubin glucuronides formed in the liver from endogenous (hepatic) and exogenous (extrahepatic) sources of bilirubin follow a similar excretory pathway. BMG formed endogenously is converted preferentially to BDG, whereas circulating BMG is excreted predominantly unchanged. Exogenous conjugated bilirubins are excreted more rapidly than those generated intrahepatically, by a transcellular pathway that is largely independent of the conjugation system.

Enzyme-catalyzed detoxication reactions: mechanisms and stereochemistry

Enzyme catalyzed detoxication reactions are one of the primary defenses organisms have against chemical insult. This article reviews current chemical approaches to understanding the cooperative role of enzymes in the metabolism of foreign compounds. Emphasis is placed on chemical and stereochemical studies which help elucidate the mechanism of action and active-site topologies of the detoxication enzymes. The stereoselectivity of the cytochromes P-450 and flavin containing monooxygenases as well as the role of hemoglobin and lipid peroxidation in the primary metabolism of xenobiotics is discussed. Current knowledge of the mechanism and stereoselectivity of epoxide hydrolase is also presented. Three enzymes involved in secondary metabolism of xenobiotics, UDP-glucuronosyltransferase, sulfotransferase and glutathione S-transferase are discussed with particular emphasis on active site topology and cooperative participation with the enzymes of primary metabolism.

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.

Effects of ioglycamide on the hepatic transport of bilirubin and its mono- and diconjugates in the rat

Bilirubin seems to share the biliary excretion pathway with other organic anions, but not with bile acids. We studied the effects of the organic anion ioglycamide, an iodinated contrast agent, on bilirubin metabolism in Wistar rats. This compound does not undergo conjugation and is characterized by a maximal biliary secretory rate (Tm). The results show that in spite of producing a 3-fold increase in bile flow, ioglycamide excretion under Tm conditions decreased the output of unconjugated bilirubin and its monoconjugate by approximately 90%. Diconjugated bilirubin decreased by only 50% and became by far the predominant pigment in bile (86.5 +/- 6.0% of total pigment vs. 61.0 +/- 4.0% in basal conditions, n = 12). Unconjugated and monoconjugated bilirubins changed in parallel suggesting that the former arises from the monoconjugates. In serum, diconjugated bilirubin augmented from trace amounts to 1.15 +/- 0.17 mumole per liter. Total conjugated pigments in serum increased from 5 to 85% of total bilirubin. Bile acid output remained unchanged. Pretreatment of rats with ioglycamide altered neither the activity of bilirubin UDP-glucuronyltransferase nor the ratio of diconjugate to monoconjugate formed at both low (25 microM) and high (164 microM) bilirubin concentrations. The observed biological effects of ioglycamide were dose-dependent and fully reversible. We suggest that ioglycamide interferes with the excretion of conjugated bilirubins ("bilirubinostasis"). The monoconjugates retained in the hepatocyte might then undergo more efficient transformation to diconjugates, the latter thus becoming the most important bile pigments in serum and bile.

Effects of anesthetic agents on bile pigment excretion in the rat

Anesthesia-induced alterations in bilirubin conjugation were studied. Rats were fitted with bile duct and jugular vein catheters while anesthetized with diethyl ether, ketamine or pentobarbital. As anesthesia abated, bile was collected for the next 5 hr and analyzed for flow rate, total bilirubin excretion and bilirubin glucuronide composition. The high-performance liquid chromatography method used allowed direct analysis of bile without derivatization or extraction. Ether anesthesia was associated with a reversible suppression of diglucuronide formation and total bilirubin excretion, with reciprocal monoglucuronide changes. Bile flow and pigment excretion were variable with ketamine. Pentobarbital provided the most uniform excretion data, although the ratio of C-8:C-12 monoglucuronide varied with all drugs. These data are consistent with recently reported drug-induced alterations in hepatic uridine diphosphoglucuronic acid concentration and support the hypothesis that alterations in this substrate concentration are capable of influencing rates of hepatic glucuronide formation.

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.

Radioassay of UDP-glucuronyltransferase-catalysed formation of bilirubin monoglucuronides and bilirubin diglucuronide in liver microsomes

A radioassay for specific determination of the rates of UDP-glucuronic acid-dependent conversion of bilirubin into the two isomeric (C-8, C-12) bilirubin monoglucuronides and bilirubin diglucuronide is described and illustrated by its application to rat liver microsomes. The method is based on measurement of the relative amounts of radiolabel in unesterified bilirubin and its mono- and di-esterified reaction products after incubation with [14C]bilirubin as substrate. This analysis is performed by the alkaline-methanolysis procedure, combined with one of two t.l.c. systems developed in order to enhance the sensitivity, accuracy and precision of the radioassay. Results for rates of total bilirubin glucuronide formation obtained with the new assay and the standard enzyme assay based on the ethyl anthranilate diazo-method were identical. However, the sensitivity of the latter technique is approx. 10-fold lower than that of the radioassay.

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.

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.

Properties and determination of serum and bilirubin

The accurate determination of the types and amounts of bilirubin species in serum is important for diagnostic purposes as well as for therapeutic monitoring. However, of the determinations routinely performed in the clinical laboratory, those for bilirubins are not among the more accurate and exhibit significant method variability. In this review, the structural, stability, solubility, and albumin-binding properties of serum bilirubins are discussed with respect to their impact on analytical methods. Following a consideration of analytical standards, methods for the determination of unconjugated and conjugated bilirubins are reviewed and recent developments are evaluated. Finally, the present capabilities and future potential of the methods for producing information applicable to the development of new or improved methods of determination are summarized.

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.

Metabolism of heme and bilirubin in rat and human small intestinal mucosa

Formation of heme, bilirubin, and bilirubin conjugates has been examined in mucosal cells isolated from the rat upper small intestine. Intact, viable cells were prepared by enzymatic dissociation using a combined vascular and luminal perfusion and incubated with an isotopically labeled precursor, delta-amino-[2,3-3H]levulinic acid. Labeled heme and bile pigment were formed with kinetics similar to those exhibited by hepatocytes. Moreover, the newly formed bilirubin was converted rapidly to both mono- and diglucuronide conjugates. In addition, cell-free extracts of small intestinal mucosa from rats or humans exhibited a bilirubin-UDP-glucuronyl transferase activity that was qualitatively similar to that present in liver. The data suggest that the small intestinal mucosa normally contributes to bilirubin metabolism.

Bilirubin diglucuronide formation in intact rats and in isolated Gunn rat liver

Bilirubin diglucuronide (BDG) may be formed in vitro by microsomal UDP glucuronosyl transferase (EC 2.4.1.17)-mediated transfer of a second mole of glucuronic acid from UDP-glucuronic acid, or by dismutation of bilirubin monoglucuronide (BMG) to BDG and unconjugated bilirubin, catalyzed by an enzyme (EC 2.4.1.95) that is concentrated in plasma membrane-enriched fractions of rat liver. To evaluate the role of these two enzymatic mechanisms in vivo, [(3)H]bilirubin mono-[(14)C]glucuronide was biosynthesized, purified by thin-layer chromatography, and tracer doses were infused intravenously in homozygous Gunn (UDP glucuronyl transferase-deficient) rats or Wistar rats. Bilirubin conjugates in bile were separated by high-pressure liquid chromatography and (3)H and (14)C were quantitated. In Gunn rats, the (14)C:(3)H ratio in BDG excreted in bile was twice the ratio in injected BMG. In Wistar rats the (14)C:(3)H ratio in biliary BDG was 1.25 +/- 0.06 (mean +/- SEM) times the ratio in injected BMG. When double labeled BMG was injected in Wistar rats after injection of excess unlabeled unconjugated bilirubin (1.7 mumol), the (14)C:(3)H ratio in BDG excreted in bile was identical to the ratio in injected BMG. Analysis of isomeric composition of bilirubin conjugates after alkaline hydrolysis or alkaline methanolysis indicated that the bile pigments retained the IX(alpha) configuration during these experiments. The results indicate that both enzymatic dismutation and UDP glucuronyl transferase function in vivo in BDG formation, and that dismutation is inhibited by a high intrahepatic concentration of unconjugated bilirubin. This hypothesis was supported by infusion of [(3)H]bilirubin-monoglucuronide in isolated perfused homozygous Gunn rat liver after depletion of intrahepatic bilirubin by perfusion with bovine serum albumin (2.5%), and after bilirubin repletion following perfusion with 0.34 mM bilirubin. From 20 to 25% of injected radioactivity was recovered in BDG in bile in the bilirubin-depleted state; only 8-10% of radioactivity was in BDG in bile after bilirubin repletion. After infusion of [(3)H]bilirubin di-[(14)C]glucuronide in homozygous Gunn rats, 5-7% of the injected pigment was excreted in bile as BMG. The (14)C:(3)H ratio in the injected BDG was 10% greater than the (14)C:(3)H ratio in BMG excreted in bile. These results indicate that in vivo, dismutation rather than partial hydrolysis, is responsible for BMG formation. Incubation of [(3)H]bilirubin, BDG and a rat liver plasma membrane preparation resulted in formation of BMG (3.3 nmol/min per mg protein) indicating that dismutation is also reversible in vitro.

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

TWO ROUTES HAVE BEEN PROPOSED FOR CONVERSION OF BILIRUBIN MONOGLUCURONIDE TO THE DIGLUCURONIDE: glucuronyl transfer (a) from UDP-glucuronic acid to bilirubin monoglucuronide, catalyzed by a microsomal UDP-glucuronyltransferase, and (b) from one molecule of bilirubin monoglucuronide to another (transglucuronidation), catalyzed by an enzyme present in liver plasma membranes. The evidence regarding the role of the latter enzyme for in vivo formation of bilirubin diglucuronide is conflicting. We therefore decided to reexamine the transglucuronidation reaction in plasma membranes and to study the conversion of bilirubin monoglucuronide to diglucuronide in vivo. Purified bilirubin monoglucuronide was incubated with homogenates and plasma membrane-enriched fractions from liver of Wistar and Gunn rats. Stoichiometric formation of bilirubin and bilirubin diglucuronide out of 2 mol of bilirubin monoglucuronide was paralleled by an increase of the IIIalpha- and XIIIalpha-isomers of the bilirubin aglycone, thus showing that dipyrrole exchange, not transglucuronidation, is the underlying mechanism. Complete inhibition by ascorbic acid probably reflects intermediate formation of free radicals of dipyrrolic moieties. The reaction was nonenzymic because it proceeded independently of the protein concentration and heat denaturation of the plasma membranes did not result in decreased conversion rates. Collectively, these findings show spontaneous, nonenzymic dipyrrole exchange when bilirubin monoglucuronide is incubated in the presence of rat liver plasma membranes. Because bilirubin glucuronides present in biological fluids contain exclusively the bilirubin-IXalpha aglycone, formation of the diglucuronide from the monoglucuronide by dipyrrole exchange does not occur in vivo. Rapid excretion of unchanged bilirubin monoglucuronide in Gunn rat bile after injection of the pigment provides confirmatory evidence for the absence of a UDP-glucuronic acid-independent process.

Bilirubin mono- and diglucuronide formation by human liver in vitro: assay by high-pressure liquid chromatography

Bilirubin diglucuronide, the major pigment in human bile is formed in two steps. Bilirubin is converted to bilirubin monoglucuronide by transfer of the glucuronosyl moiety of uridine diphosphoglucuronic acid catalyzed by the microsomal enzyme, uridine diphosphoglucuronate glucuraonosyl transferase (UDP glucuronyl transferase, EC 2.4.1.17). Bilirubin monoglucuaronide is converted to bilirubin diglucuronide in vitro by two enzymatic mechanisms: (a) UDP glucuronyl transferase-mediated transfer of a second mole of glucuronic acid form UDP-glucuronic acid to bilirubin monoglucuronide; (b) dismutation of 2 moles of bilirubin monoglucuronide to 1 mole of bilirubin diglucuronide and 1 mole of unconjugated bilirubin, catalyzed by bilirubin monoglucuronide dismutase (bilirubin glucuronoside glucuronosyl transferase EC 2.4.1.95). Assay methods for the three enzymatic mechanisms in human liver homogenate by high-pressure liquid chromatography analysis of underivatized bilirubin tetrapyrroles have been developed. UDP glucuronyl transferase was activated in five human liver homogenates with digitonin, Triton X-100, or UDP-N-acetylglucosamine. Greatest activation was observed with Triton X-100. The pH optimum for conversion of bilirubin to bilirubin monoglucuronide was 7.4, and UDP glucuronyl transferase activity was 625 +/- 51 nmoles per 20 min per gm liver. At high initial bilirubin concentrations (342 microM), the product of UDP glucuronyl transferase assay with bilirubin as substrate was predominantly bilirubin monoglucuronide. At lower initial bilirubin concentrations (6.5 to 34 microM), up to 15% bilirubin diglucuronide was formed. Glucuronyl transferase-mediated UDP glucuronic acid-dependent conversion of bilirubin monoglucuronide to diglucuronide was assayed using UDP-14-C-glucuronic acid. The pH optimum was 7.4, and the rate was 21 +/- 7 nmoles per gm liver per 20 min. The rate of bilirubin diglucuronide formation by enzymatic dismutation of bilirubin monoglucuronide was 470 +/- 112 nmoles per gm liver per min. The pH optimum was 6.6. The products of enzymatic dismutation were of the IX alpha configuration.

The presence of a microsomal UDP-glucuronyl transferase for bilirubin in homozygous jaundiced Gunn rats and in the Crigler-Najjar syndrome

The infusion of a closely related derivative of bilirubin, its dimethyl diester (DME), into jaundiced (jj) Gunn rats were associated with biliary excretion of mono- and diglucuronides of bilirubin. In vitro incubation of DME with liver microsomes from jj rats demonstrated sequential demethylation and glucuronidation of DME. Liver microsomes from a patient with the Crigler-Najjar syndrome were unable to form glucuronides of bilirubin in vitro unless DME was used as substrate. The results suggest that the deficiency in Gunn rats and in the Crigler-Najjar syndrome may be due to a structural defect in the microsomal matrix which contains glucuronyl transferase. This interpretation envisions a microenvironment of the transferase enzyme which is either impermeable to bilirubin or induces conformational changes which interfere with glucuronidation.

Bilirubin kinetics in intact rats and isolated perfused liver. Evidence for hepatic deconjugation of bilirubin glucuronides

Most previous compartmental models describing bilirubin transport and metabolism in the liver have been validated solely by analysis of the plasma disappearance of radiolabeled bilirubin in human subjects. We now have determined the transport kinetics of a bilirubin tracer pulse by analysis of plasma, liver, and bile radioactivity data from 30 intact rats. Plasma [3H]bilirubin disappearance was best described by the sum of three exponentials, and a six-compartment model, derived by simulation analysis, was necessary and adequate to describe all experimental data. Examination of the injected radiolabeled bilirubin by extraction with hexadecyltrimethylammonium bromide and thin-layer chromatography revealed that 6.6% (mean) of the original pigment had been degraded to labeled nonbilirubin derivatives during preparation of the tracer dose. This material exhibited a significantly longer half-life (mean 50.6 min) of the plasma terminal exponential than that of authentic radiobilirubin (20.6 min). In isolated perfused rat liver, the kinetics of [3H]bilirubin in perfusate and bile readily fitted the proposed model. Compatibility of the model with the data obtained, both in the isolated liver and in vivo, required that a fraction of bilirubin conjugated in the liver be deconjugated and returned to the plasma. Deconjugation of bilirubin glucuronides was evaluated directly by infusion of bilirubin monoglucuronides, containing 14C in the glucuronosyl group, into rats with an external bile fistula. Since metabolic degradation of hydrolyzed 14C-labeled glucuronic acid yields 14CO2, this was measured in expired air. Whereas 86% of the administered labeled pigment was recovered in bile, 7% of the label appeared in 14CO2. These findings directly validate a portion of the proposed kinetic model and suggest that hepatic deconjugation of a small fraction of bilirubin glucuronides is a physiological event. Deconjugation may also account, at least in part, for the presence of increased concentrations of unconjugated bilirubin in the plasma of patients with cholestasis.

The photoinduced isomerization of bilirubin in cationic detergent solutions

When bilirubin IX alpha in solution in a buffered aqueous cationic detergent near neutral pH is irradiated with visible light, a rapid equilibrium with bilirubin III alpha and XIII alpha is set up. Little isomerization can be detected under comparable conditions in anionic or neutral detergents. The rapid disproportionation of bilirubin monoglucuronide into unconjugated bilirubin and bilirubin diglucuronide also takes place on irradiation in a solution of a cationic detergent.

Mechanism of bilirubin diglucuronide formation in intact rats: bilirubin diglucuronide formation in vivo

Although it is well established that bilirubin monoglucuronide is formed in the liver from bilirubin by a microsomal bilirubin uridine diphosphate (UDP)-glucuronosyltransferase, the subcellular site of conversion of monoglucuronide to diglucuronide and the molecular mechanism involved in diglucuronide synthesis have not been identified. Based on in vitro studies, it has been proposed that two fundamentally different enzyme systems may be involved in diglucuronide synthesis in rat liver: (a) a microsomal UDP-glucuronosyltransferase system requiring UDP-glucuronic acid as sugar donor or (b) a transglucuronidation mechanism that involves transfer of a glucuronosyl residue from one monoglucuronide molecule to another, catalyzed by a liver plasma membrane enzyme. To clarify the mechanism by which bilirubin monoglucuronide is converted in vivo to diglucuronide, three different experimental approaches were used. First, normal rats were injected with either equal amounts of bilirubin-IIIalpha [(14)C]monoglucuronide and unlabeled bilirubin-XIIIalpha monoglucuronide, or bilirubin-XIIIalpha [(14)C]monoglucuronide and unlabeled bilirubin-IIIalpha monoglucuronide. Analysis of radiolabeled diglucuronide excreted in bile showed that [(14)C]glucuronosyl residues were not transferred between monoglucuronide molecules. Second, in normal rats infused intravenously with dual-labeled [(3)H]bilirubin [(14)C]monoglucuronide, no transfer or exchange of the [(14)C]glucuronosyl group between injected and endogenously produced bilirubin monoglucuronide could be detected in the excreted bilirubin diglucuronide. Third, in homozygous Gunn rats, injected (14)C-labeled or unlabeled bilirubin mono- or diglucuronides were excreted in bile unchanged (except that diglucuronide was hydrolyzed to a minor degree). This indicates that Gunn rats, which lack bilirubin UDP-glucuronosyltransferase activity, are unable to convert injected monoglucuronide to diglucuronide. Collectively, these findings establish that a transglucuronidation mechanism is not operational in vivo and support the concept that bilirubin diglucuronide is formed by a microsomal UDP-glucuronosyltransferase system.

Analysis of bilirubin and bilirubin mono- and di-conjugates. Determination of their relative amounts in biological samples

1. A novel method for determination of the relative amounts of unconjugated bilirubin and sugar mono- and di-conjugates of bilirubin in biological samples, including serum, is described and illustrated by its application to the analysis of bilinoids in rat bile. 2. The method is based on specific conversion of the carbohydrate conjugates of bilirubin into the corresponding mono- or di-methyl esters by base-catalysed transesterification in methanol. Under the selected reaction conditions, unconjugated biliru-in remains intact and no dipyrrole exchange in the bilinoids is detectable; transesterification of bilirubin mono- or di-glucuronide is virtually complete (approx. 99%), and sponification is negligible (less than 1%); recovery of the pigments is approx. 95%. 3. The reaction products bilirubin and its methyl esters are separated by t.l.c. and determined spectrophotometrically; the two isomeric bilirubin-IX alpha monomethyl esters are separated and therefore can be determined individually. 4. Reference bilirubin mono- and di-methyl esters have been synthesized and characterized, and the two isomers of bilirubin-IX alpha monomethyl ester and bilirubin dimethyl ester were obtained individually, in crystalline form. 5. With this new method, virtually all bilinoids (over 99%) in normal rat bile have been found to be conjugated, with diconjugates (71%) predominating. A significantly increased proportion of monoconjugates is present in bile collected from heterozygous Gunn rats or from normal rats that were refused with large amounts of bilirubin.