Glucuronic acid conjugates of bilirubin-IXalpha in normal bile compared with post-obstructive bile. Transformation of the 1-O-acylglucuronide into 2-, 3-, and 4-O-acylglucuronides

Methyl tetra-O-acetyl-α-D-glucopyranuronate: crystal structure and influence on the crystallisation of the β anomer

Methyl tetra-O-acetyl-β-D-glucopyranuronate (1) and methyl tetra-O-acetyl-α-D-glucopyranuronate (3) were isolated as crystalline solids and their crystal structures were obtained. That of the β anomer (1) was the same as that reported by Root et al., while anomer (3) was found to crystallise in the orthorhombic space group P212121 with two independent molecules in the asymmetric unit. No other crystal forms were found for either compound upon recrystallisation from a range of solvents. The α anomer (3) was found to be an impurity in initially precipitated batches of β-anomer (1) in quantities <3%; however, it was possible to remove the α impurity either by recrystallisation or by efficient washing, i.e. the α anomer is not incorporated inside the β anomer crystals. The β anomer (1) was found to grow as prisms or needles elongated in the a crystallographic direction in the absence of the α impurity, while the presence of the α anomer (3) enhanced this elongation.

A primer for best practices in tissue preparation for bioanalysis

Analysis of drugs, biomarkers and their metabolites in tissue samples has always been an important aspect of the drug-development process. In the last decade, significant improvements in equipment and processes have made handling such samples far more efficient, with higher precision, accuracy and ruggedness. The purpose of this paper is to provide a primer for best practices of tissue analysis, including brief but specific tutorials on basic principles and laboratory operation. Included will be a discussion of what to consider when designing a study, tools available to make appropriate pre-study decisions, approaches for tissue acquisition and extraction, sample processing methods, and tips on creation of standards and QCs. We will offer some practical advice to help scientists who have good analytical skills, but are not experienced in tissue analysis to quickly start their own analyses with the minimum amount of time, labor and cost.

Unconjugated bilirubin induces apoptosis in colon cancer cells by triggering mitochondrial depolarization

Bilirubin is the principal end product of heme degradation. Prompted by epidemiologic analyses demonstrating an inverse correlation between serum bilirubin levels and cancer mortality, we examined the effect(s) of bilirubin on the growth and survival of colon adenocarcinoma cells. Adenocarcinoma cell monolayers were treated with bilirubin over a range of bilirubin:BSA molar ratios (0-0.6), and viability was assessed colorimetrically. Apoptosis was characterized by TUNEL assay, annexin V staining and caspase-3 activation. The mechanism(s) by which bilirubin induces apoptosis was investigated by Western blotting for cytochrome c release, assaying for caspase-8 and caspase-9 activation and for mitochondrial depolarization by JC-1 staining. The direct effect of bilirubin on the membrane potential of isolated mitochondria was evaluated using light-scattering and fluorescence techniques. Bilirubin decreased the viability of all colon cancer cell lines tested in a dose-dependent manner. Cells exhibited substantial apoptosis when exposed to bilirubin concentrations ranging 0-50 microM, as demonstrated by an 8- to 10-fold increase in TUNEL and annexin V staining and in caspase-3 activity. Bilirubin treatment evokes specific activation of caspase-9, enhances cytochrome c release into the cytoplasm and triggers the mitochondrial permeability transition in colon cancer monolayers. Additionally, bilirubin directly induces the depolarization of isolated rat liver mitochondria, an effect that is not inhibited by cyclosporin A. Bilirubin stimulates apoptosis of colon adenocarcinoma cells in vitro through activation of the mitochondrial pathway, apparently by directly dissipating mitochondrial membrane potential. As this effect is triggered at concentrations normally present in the intestinal lumen, we postulate a physiologic role for bilirubin in modulating colon tumorigenesis.

Acyl glucuronide reactivity in perspective: biological consequences

The metabolic conjugation of exogenous and endogenous carboxylic acid substrates with endogenous glucuronic acid, mediated by the uridine diphosphoglucuronosyl transferase (UGT) superfamily of enzymes, leads to the formation of acyl glucuronide metabolites. Since the late 1970s, acyl glucuronides have been increasingly identified as reactive electrophilic metabolites, capable of undergoing three reactions: intramolecular rearrangement, hydrolysis, and intermolecular reactions with proteins leading to covalent drug-protein adducts. This essential dogma has been accepted for over a decade. The key question proposed by researchers, and now the pharmaceutical industry, is: does or can the covalent modification of endogenous proteins, mediated by reactive acyl glucuronide metabolites, lead to adverse drug reactions, perhaps idiosyncratic in nature? This review evaluates the evidence for acyl glucuronide-derived perturbation of homeostasis, particularly that which might result from the covalent modification of endogenous proteins and other macromolecules. Because of the availability of acyl glucuronides for test tube/in vitro experiments, there is now a substantial literature documenting their rearrangement, hydrolysis and covalent modification of proteins in vitro. It is certain from in vitro experiments that serum albumin, dipeptidyl peptidase IV, tubulin and UGTs are covalently modified by acyl glucuronides. However, these in vitro experiments have been specifically designed to amplify any interference with a biological process in order to find biological effects. The in vivo situation is not at all clear. Certainly it must be concluded that all humans taking carboxylate drugs that form reactive acyl glucuronides will form covalent drug-protein adducts, and it must also be concluded that this in itself is normally benign. However, there is enough in vivo evidence implicating acyl glucuronides, which, when backed up by in vivo circumstantial and documented in vitro evidence, supports the view that reactive acyl glucuronides may initiate toxicity/immune responses. In summary, though acyl glucuronide-derived covalent modification of endogenous macromolecules is well-defined, the work ahead needs to provide detailed links between such modification and its possible biological consequences.

Isolation and characterization of protoporphyrin glycoconjugates from rat harderian gland by HPLC, capillary electrophoresis and HPLC/electrospray ionization MS

It has been widely reported that the Harderian gland, present in most vertebrates, accumulates high levels of porphyrins, particularly protoporphyrin. The present study describes the extraction, identification and characterization of a group of hitherto unreported protoporphyrin glycoconjugates in the rat Harderian gland using HPLC, capillary electrophoresis, on-line HPLC/electrospray ionization MS and tandem MS. The major glycoconjugate was identified as protoporphyrin-1-O-acyl beta-xyloside with a smaller amount of protoporphyrin-1-O-acyl beta-glucoside also detected. In the Harderian glands studied, 50-70% of the porphyrins present were in the form of protoporphyrin glycoconjugates. This is the first reported occurrence of glycoconjugates of porphyrins in Nature and suggests that previous studies have wrongly identified the major porphyrin in the Harderian gland as the unconjugated protoporphyrin.

Direct characterization of drug glucuronide isomers in human urine by HPLC-NMR spectroscopy: application to the positional isomers of 6,11-dihydro-11-oxodibenz[b,e]oxepin-2-acetic acid glucuronide

In this work, 400 and 600 MHz 1H HPLC-NMR spectroscopic methods were developed and applied to separate and identify the positional glucuronide isomers and anomers of the model nonsteroidal antiinflammatory drug, 6,11-dihydro-11-oxodibenz[b,e]oxepin-2-acetic acid, in whole human urine. The HPLC methods utilized either an isocratic system, comprising 30% acetonitrile in water at pH 2.5, or a gradient elution system increasing from 30% to 60% acetonitrile, in order to achieve improved separation of the 2-, 3-, and 4-O-acylglucuronide isomers from the faster eluting endogenous urinary metabolites. Directly coupled stop-flow 1H HPLC-NMR spectroscopic measurements were made at the retention times indicated by the UV-monitored chromatographic peaks. The glucuronide isomers were identified from the 1H NMR spectra on the basis of their chemical shifts and spin-spin coupling patterns. The elution order was 4-O-acyl-, 3-O-acyl-, and finally 2-O-acylglucuronide, with tR values of 10.04, 11.68, and 12.64 min, respectively. Although the alpha- and beta-anomers of each of the positional isomers could not be separated in these solvent systems, they could be identified in the individual 1H NMR spectra. This work shows for the first time that directly coupled HPLC-NMR spectroscopy can be used directly to isolate and characterize acyl-migrated isomers of drug glucuronides in whole urine. This approach will be of value in the study of glucuronide acyl migration reactions of nonsteroidal antiinflammatory drugs and other xenobiotic ester glucuronides in whole biofluids.

Chromatographic fractionation and analysis by mass spectrometry of conjugated metabolites of bis(2-ethylhexyl)phthalate in urine

Mono(2-ethylhexyl)phthalate (MEHP), the primary metabolite of the plasticizer bis(2-ethylhexyl)phthalate (DEHP), was given to guinea pigs and mice and the methods for the isolation, separation and analysis of its metabolites in urine were developed. Following solid-phase extraction with octadecylsilane-bonded silica, individual metabolites were purified and separated using a combination of ion-exchange chromatography on lipophilic gels and reversed-phase high-performance liquid chromatography. Analysis of intact conjugates, as well as nonconjugated metabolites, was performed by fast atom bombardment mass spectrometry (FAB-MS) and, after derivatization, by gas chromatography-mass spectrometry. Enzymatic methods were used for further characterization. The study confirms glucuronidation as the major conjugation pathway for MEHP in the investigated species. Although less important quantitatively, glucosidation is shown to be an alternative conjugation pathway in mice. The methods developed were applied to a sample of urine from a hyperbilirubinemic newborn infant subjected to DEHP-exposure in conjunction with an exchange transfusion. It was demonstrated that metabolites of DEHP were excreted in amounts which could be analyzed by FAB-MS.

Evidence for covalent binding of acyl glucuronides to serum albumin via an imine mechanism as revealed by tandem mass spectrometry

Acyl glucuronide metabolites of bilirubin and many drugs can react with serum albumin in vivo to form covalent adducts. Such adducts may be responsible for some toxic effects of carboxylic nonsteroidal antiinflammatory agents. The mechanism of formation of the adducts and their chemical structures are unknown. In this paper we describe the use of tandem mass spectrometry to locate binding sites and elucidate the binding mechanism involved in the formation of covalent adducts from tolmetin glucuronide and albumin in vitro. Human serum albumin and excess tolmetin glucuronide were coincubated in the presence of sodium cyanoborohydride to trap imine intermediates. The total protein product was reduced, carboxymethylated, and digested with trypsin. Six tolmetin-containing peptides (indicated by absorbance at 313 nm) were isolated by high-pressure liquid chromatography and analyzed by liquid secondary-ion mass spectrometry and collision-induced dissociation, using a four-sector tandem mass spectrometer. All six peptides contained tolmetin linked covalently via a glucuronic acid to protein lysine groups. Major attachment sites on the protein were Lys-195, -199, and -525; minor sites were identified as Lys-137, -351, and -541. Our results show unambiguously that the glucuronic acid moiety of acyl glucuronides can be retained within the structure when these reactive metabolites bind covalently to proteins, and they suggest that acyl migration followed by Schiff base (imine) formation is a credible mechanism for the generation of covalent adducts in vivo.

Biosynthesis and chemical synthesis of carboxyl-linked glucuronide of lithocholic acid

The glucuronidation of lithocholic acid (LA) by phenobarbital-induced male Fischer 344 rat liver microsomes supplemented with UDP-glucuronic acid was studied. A single radioactive metabolite was formed and its structure was determined by high pressure liquid chromatography/particle beam/mass spectrometry (HPLC/PB/MS), both with and without prior methylation and acetylation of the sample. The reaction product was rigorously identified as the 1-O-acyl-beta-D-glucuronide of LA by comparison with a chemically synthesized standard. The chemical synthesis of the acyl glucuronide of LA was accomplished via a condensation reaction using benzyl 2,3,4-tri-O-benzyl-D-glucopyranuronate. The latter compound was prepared in two steps from benzyl 2,3,4-tri-O-benzyl-1-O-methyl-alpha-D-glucopyranuronate via the 1-O-acetyl derivative. The stereoselective beta coupling of LA with 2,3,4-tri-O-benzyl-D-glucopyranuronate was achieved by the Mitsunobu reaction, in the presence of the free hydroxyl function of LA, using triphenylphosphine and diisopropyl azodicarboxylate in THF followed by preparative TLC. The benzylic ester and ether groups were cleaved by hydrogenation with Pd on charcoal as the catalyst. Positive identification of the glucuronide was established by HPLC/PB/MS and 1H-NMR spectra. No side products formed by acyl migration were detected, but the free acyl glucuronide underwent rapid transesterification in methanol.

Metabolism of (-)-6(S)-hydroxy-4(R)-(1-hydroxy-1-methylethyl)-1- cyclohexene-1-ethanol in rat and dog

1. The metabolism of a new mucoactive drug, chemically (-)-6(S)-hydroxy-4(R)- (1-hydroxy-1-methylethyl)-1-cyclohexene-1-ethanol (CO/1408), has been studied in rat and dog after a single oral dose; eight metabolites were identified. 2. Oxidation of the primary and secondary alcohol groups, hydroxylation in allylic positions and conjugation with glucuronic acid occurred in both species. Products of oxidation on the double bond have not been identified. 3. Using reversed-phase h.p.l.c. and beta-cyclodextrin in the eluent it was found that the glucuronide metabolites varied with species and with the biological fluid examined.

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.

Novel direct high-performance liquid chromatographic method for determination of salicylate glucuronide conjugates in human urine

A novel direct high-performance liquid chromatographic (HPLC) assay for the simultaneous determination of three salicylate glucuronide conjugates and other salicylate metabolites in human urine has been developed. Salicylate glucuronide conjugates were purified by HPLC from the urine of a volunteer after oral administration of aspirin and identified by selective hydrolysis with beta-glucuronidase and with sodium hydroxide. This method gave high reproducibility with coefficients of variation less than 10%. The total urinary recovery of salicylic acid after a single 1.2-g dose of soluble aspirin was greater than 90%. This assay has been successfully used to re-evaluate the capacity-limited pharmacokinetics of salicylic acid in humans.

The measurement of bilirubin fractions in serum

Bilirubin fractions are measured by (1) the direct diazo reaction, (2) high-performance liquid chromatography (HPLC), (3) direct spectrophotometry, and (4) enzymatic methods. HPLC, which effects separation and quantitation of the four bilirubin fractions, is the method of choice, but impractical for routine use. A special application of direct spectrophotometry allows the measurement of unconjugated bilirubin and the sum of bilirubin conjugates. This approach, which provides essentially the same information as HPLC, unfortunately is available only in one clinical analyzer. The direct diazo reaction measures bilirubin conjugates plus delta-bilirubin, albeit not very accurately. Direct diazo methods that measure unconjugated bilirubin as direct could obscure the clinical diagnosis. At acid pH, enzymatic methods measure all direct reacting bilirubins, while at pH 10 only conjugated bilirubins are measured. Because the measurement of conjugated bilirubins is clearly more helpful than that of direct bilirubin in the differential diagnosis of jaundice, direct diazo methods should be replaced by methods specific for bilirubin conjugates.

Rapid high-performance liquid chromatographic assay for the simultaneous determination of probenecid and its glucuronide in urine. Irreversible binding of probenecid to serum albumin

A reversed-phase high-performance liquid chromatographic (HPLC) assay for the simultaneous determination of probenecid and its glucuronide in urine has been developed. The genuine glucuronide conjugate was isolated from urine by the use of solid-phase extraction on Amberlite XAD-2 and finally purified by the use of preparative HPLC on a Sepharon Hema 1000 RP-18 column. The purity of the product obtained was 88.9%. The isolated glucuronide was used as a standard sample. Of a p.o. dose of 500 mg to two volunteers, 26 and 29% were excreted as the ester glucuronide, while 1.0 and 2.7% were excreted unmetabolized. The stability of the ester glucuronide was investigated in aqueous buffers, buffered urine and human serum albumin solutions. The glucuronide was unstable in neutral and mildly alkaline solutions, and special precautions have to be taken during sampling and sample treatment in order to preserve the genuine glucuronide. The presence of human serum albumin in the solution stabilized the glucuronide against isomerization/rearrangements but catalysed the hydrolysis of the glucuronide. When incubating human serum albumin with the ester glucuronide, probenecid was shown to be covalently bound to the protein probably via a transacylation reaction.

HPLC separation and quantification of bilirubin and its glucuronide conjugates in faeces and intestinal contents of germ-free rats

We describe an accurate reverse-phase high-performance liquid chromatography (HPLC) method for the separation and quantification of unconjugated bilirubin (UCB) and its monoglucuronide (BMG) and diglucuronide (BDG) conjugates in faeces and intestinal contents from germ-free (GF) rats. We demonstrated that female GF rats excreted predominantly BMG and that the percentage of this conjugate was at most 71.7% of the total bilirubin excreted with the faeces. The highest percentages for BDG and the UCB were 27.9% and 6.0%, respectively. The bile pigment composition in duodenal contents was 59.8% BDG and 40.2% BMG (median percentage) and was 47.7% BDG, 50.1% BMG and 2.2% UCB in ileal contents. Deconjugation of BDG to BMG was profound in caecal contents with 26.0% BDG, 67.4% BMG and 6.6% UCB. Endogenous (mammalian) beta-glucuronidase activity was present in intestinal contents throughout the entire length of the intestine and in faeces of the GF rats. The results indicated that it is very likely that endogenous beta-glucuronidase plays a role in the deconjugation of bilirubin glucuronides as well as of other glucuronides in the intestine of the GF rat.

High-performance liquid chromatographic analysis of bile pigments as their native tetrapyrroles and as their dipyrrolic azosulfanilate derivatives

A reversed-phase high-performance liquid chromatographic (HPLC) analysis of bile pigments is described that provides baseline separation of the major bilirubin conjugates found in bile. The advantage of the technique is that the bile pigments can be analyzed directly as their native tetrapyrroles without prior solvent extractions or derivatization. The use of ammonium acetate in place of sodium salts permits preparative isolation and lyophilization of the pigments for mass spectroscopy. The derivatization of the pigments as their dipyrrolic azosulfanilates with subsequent HPLC analysis demonstrates baseline separation of the endo- and exovinyl azodipyrroles and allows identification of that half of the tetrapyrrole which contains the conjugate in the instances of monoglycosides.

Reactivity of diflunisal acyl glucuronide in human and rat plasma and albumin solutions

Diflunisal acyl glucuronide (DAG) is a major metabolite of diflunisal (DF) in rats and humans. We have investigated the reactivity of DAG, in purified albumin solutions and plasma from both rat and human sources, along three interrelated pathways: rearrangement via acyl migration to yield positional isomers of DAG, hydrolysis of DAG and/or its isomers to liberate DF, and formation of covalent adducts of DF (via DAG and/or its isomers) with plasma protein. Two initial concentrations of DAG (ca. 50 and 10 micrograms DF equivalents/mL) were used throughout. In all incubations, the order of quantitative importance of the reactions was: rearrangement greater than hydrolysis greater than covalent binding. At pH 7.4 and 37 degrees, degradation of DAG in albumin solutions (e.g. half-life ca. 95 min in fatty acid-free human serum albumin) was retarded in comparison to that found in buffer alone (half-life ca. 35 min). Degradation in unbuffered rat and human plasma containing heparin was comparable to that found in buffer. Maximal covalent binding to protein was achieved after 4-8 hr incubation, and was greatest for fatty acid-free human serum albumin (165 ng DF/mg albumin). Thereafter, slow degradation of the adducts was observed. Formation of DF-plasma protein adducts in vivo was also found in rats and humans dosed with DF.

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.

Pharmacodynamics and causes of dose-dependent pharmacokinetics of flavone-8-acetic acid (LM-975; NSC-347512) in mice

Flavone acetic acid (FAA) is a novel antitumor agent with broad solid-tumor activity. However, this drug has shown a steep dose-response curve in preclinical trials, with a narrow sublethal window of efficacy. To investigate this threshold behavior, we studied various aspects of FAA pharmacology in mice after i.v. administration. Mice bearing advanced-stage s.c. colon 38 adenocarcinoma were treated at four dose levels (39, 65, 108 and 180 mg/kg), and only the highest dose produced significant antitumor activity, showing a steep dose-response curve. Using an HPLC assay, FAA pharmacokinetics in both plasma and tumors were found to be dose-dependent. As the dose increased, there was a decrease in both total body clearance and volume of distribution at steady state. The increase in tumor area under the curve (AUC) was more pronounced than the corresponding increase in plasma AUC, showing a better tumor exposure to FAA at high doses. The distribution of FAA in normal tissues showed a short-term retention in the liver and kidneys; low concentrations were observed in the heart, spleen, and brain, with some retention in the latter. The highest FAA concentrations were found in the gastrointestinal (GI) tract, mainly in the duodenum, suggesting an important biliary excretion of the drug. Various possible causes of FAA nonlinear pharmacokinetics were investigated. Serum protein binding was high (79%) and remained constant up to 100 micrograms/ml, but decreased thereafter at higher FAA concentrations, e.g., 76% at 500 micrograms/ml and 64% at 1,000 micrograms/ml. Urinary and biliary clearances were dose-dependent and decreased 5- and 9-fold, from the 39- to the 180-mg/kg dose levels, respectively. A direct assessment of FAA enterohepatic circulation using intercannulated mice showed that 27% of the plasma AUC was accounted for by enterohepatic circulation. FAA acyl glucuronide was identified as the major metabolite in mice and was found to contribute to the nonlinear pharmacokinetics due to its facile hydrolysis under physiological conditions, regenerating FAA. In conclusion, the steep FAA dose-response curve was found to be caused by dose-dependent pharmacokinetics in mice. The nonlinear pharmacokinetics of this drug was attributed to a dose-dependent decrease in both urinary and biliary clearances, concentration-dependent serum protein binding, enterohepatic circulation, and the instability of FAA acyl glucuronide under physiological conditions forming a futile cycle.(ABSTRACT TRUNCATED AT 400 WORDS)

Characterization of acyl-linked glucuronides by electron impact and fast atom bombardment mass spectrometry

Thirty-one electron impact (EI) mass spectra and 22 fast atom bombardment (FAB) mass spectra were evaluated with regard to providing molecular weights and information about the structures of 1-O-acyl glucuronides. Molecular ion species were obtained by both techniques. Fragmentation of the glycosidic and carboxylate bonds produced ions characteristic of glucuronides as a class, and also ions unique to acyl glucuronides. In EI mass spectra of pertrimethylsilylated derivatives, pairs of [M - 481]+ and [M - 509]+ ions characterized the acyl linkage. Relative abundances within these pairs correlated with the benzylic, benzoic or aliphatic nature of the carboxylate group. Positive ion FAB spectra contained three sets of ions, with intervals of 28 and 46 mass units, which characterized the linkage. In negative ion FAB spectra, a characteristic pair of fragment ions 44 mass units apart were accompanied by anions of mass 193, which appeared to distinguish acyl from phenol and hydroxyl glucuronides.

Isolation and identification of the rearrangement products of diflunisal 1-O-acyl glucuronide

A preparative reversed-phase high-performance liquid chromatographic method is described for the simultaneous separation of eight different isomers formed from the 1-O-acyl glucuronide of diflunisal. All isomers were formed when the acyl glucuronide was incubated under mildly alkaline conditions in aqueous solution. Various forms of two-dimensional NMR studies were performed in order to identify each isomer. Seven of the isomers were identified as alpha- and beta-forms of esters in which diflunisal forms an ester with one of the four alcohol groups in the glucupyranuronic acid. One isomer was identified as the ether glucuronide of diflunisal. To establish the exact chemical shift of the different protons, simulation of the one-dimensional NMR spectra and iterative analyses were performed.

Reversed-phase high-performance liquid chromatographic assay for the simultaneous determination of diflunisal and its glucuronides in serum and urine. Rearrangement of the 1-O-acylglucuronide

A reversed-phase ion-pair high-performance liquid chromatographic assay for the simultaneous determination of diflunisal and its ester and ether glucuronide in urine and serum has been developed. The determination of the ester glucuronide in serum has not been previously reported. The genuine glucuronide conjugates isolated from urine were used as standards. The ester glucuronide is found to be unstable, especially in neutral and basic solutions, and special precautions therefore have to be taken during sampling and sample treatment. Nine rearrangement/degradation products of the ester glucuronide were detected.

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.

Isomers of bilirubin glucuronide in serum and bile before and after relief of common duct obstruction

Isomers of bilirubin glucuronide with the bilirubin acyl group attached to the C1-, C2-, C3- and C4-positions of the glucuronyl residue are present in bile of patients with extrahepatic cholestasis, whereas in normal bile only C1-isomers are found. In the present study, these bilirubin glucuronide isomers, and the fractions of unconjugated bilirubin, and bilirubin mono- and diconjugates were determined in serum and bile of 8 patients before and after relief of common duct obstruction by endoscopic papillotomy. Before papillotomy we found 39.6% C1-isomers (median value), 22.2% C2-isomers, 19.3% C3-isomers and 11.4% C4-isomers in the bile. The values in serum before papillotomy were comparable. Twenty-four hours after papillotomy, the level of C1-isomers in bile increased significantly to 56.3% (P less than 0.05) with a concomitant decrease of the non-C1-isomers. In contrast, in serum the isomers of bilirubin glucuronide did not change significantly at 24 h after papillotomy. Before papillotomy, the fraction of unconjugated bilirubin in bile was 3.6% of the total, with 15.8% bilirubin monoconjugates and 75.5% bilirubin disconjugates. After papillotomy, unconjugated bilirubin decreased to 1.6% (n.s.) and bilirubin monoconjugates to 11.9% (n.s.), while bilirubin diconjugates increased to 86.1% (P less than 0.05). In serum, the elevated fractions of bilirubin diconjugates and monoconjugates decreased from 38.4 to 32.2% (P less than 0.05) and from 29.6 to 23.4% (n.s.), respectively. In parallel, the fraction of unconjugated bilirubin in serum increased from 24.1 to 37.0% (P less than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

Rearrangement of valproate glucuronide in a patient with drug-associated hepatobiliary and renal dysfunction

Formation of beta-glucuronidase-resistant "glucuronides" of valproic acid (VPA) by intramolecular rearrangement of biosynthetic valproate glucuronide in vivo was investigated in a patient diagnosed with VPA-associated hepatobiliary and renal dysfunction. Plasma elimination half-life of VPA following cessation of the drug was 13.9 h. At the time of the toxicity, the concentration of conjugated VPA in plasma was very high (36-54% of nonconjugated VPA levels) relative to that in normal patients (2.9%). The fraction of conjugated VPA resistant to beta-glucuronidase hydrolysis was 0.28-0.47 in plasma and 0.15-0.42 in urine. The corresponding fraction in urine from normal patients receiving VPA therapy was 0.044. The evidence was consistent with retarded elimination of biosynthetic VPA glucuronide caused by renal and hepatobiliary dysfunction. Consequent prolongation of circulation of VPA glucuronide at the slightly alkaline pH of blood would permit extensive intramolecular rearrangement which is known to be pH-, temperature-, and time-dependent. The biological consequences of the presence of such beta-glucuronidase-resistant conjugated VPA in vivo are largely unknown.

Isolation and characterization of the four antipyrine glucuronides and determination of their urinary excretion pattern in man by a reversed-phase h.p.l.c. assay

A large-scale procedure for the isolation of four urinary glucuronides in antipyrine metabolism is described; the isolated compounds are used as standards in a direct h.p.l.c. assay. The four glucuronides were characterized by u.v. and 1H-n.m.r. spectroscopy, and after hydrolysis by a t.l.c. assay of the corresponding aglycones. A reversed-phase h.p.l.c. assay procedure has been developed for the direct quantification of the four antipyrine glucuronides; this separates 3-hydroxymethyl-antipyrine glucuronide, 4,4'-dihydroxy-antipyrine glucuronide, norantipyrine glucuronide and 4-hydroxy-antipyrine glucuronide in a single run. Urinary elimination patterns of these glucuronides have been determined in five female and five male volunteers after antipyrine (1200 mg) administration. The direct assay of urinary glucuronides enables the simultaneous determination of glucuronidation activities and four different phase-I metabolites of antipyrine in vivo.

Liver microsomal bilirubin UDP-glucuronyltransferase disturbances in bile duct ligated rats

The activity of bilirubin UDP-Glucuronyltransferase was determined in microsomes from normal and bile duct ligated rats. It was measured after 2 and 8 days following bile duct ligation and compared with normal rats. A decrease of 33% in the total enzyme activity was observed on day 2; a fall of 70% was founded on day 8. Bilirubin diglucuronide represented approximately 20% of total conjugates in both groups of cholestatic rats, as compared with 65% found in normals. It was concluded that bilirubin microsomal conjugating capacity is markedly altered during cholestasis. This can be attributed to microsomal membrane damage produced by stagnant bile.

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.

Origin of mammalian biliprotein and rearrangement of bilirubin glucuronides in vivo in the rat

In hepatobiliary disease bilirubin becomes bound covalently to serum albumin, producing a nondissociable bile pigment-protein complex (biliprotein). To elucidate the mechanism of biliprotein formation we studied the bile pigment composition of blood from animals with experimental cholestasis and carried out comparative studies on the rate of biliprotein formation in vivo and in vitro during incubation of bilirubin glucuronides with albumin. Bile duct ligation in the rat and guinea pig led to rapid accumulation in the circulation of bilirubin, heterogeneous bilirubin esters of glucuronic acid, and a biliprotein that migrated along with albumin on high performance liquid chromatography. When the obstruction was removed, biliprotein remained longer in the circulation than did the other bile pigment species. Biliprotein and heterogeneous bilirubin esters of glucuronic acid were not formed in bile duct-ligated homozygous Gunn rats but they were formed when bilirubin glucuronides were incubated with Sprague-Dawley rat serum or human serum albumin at 37 degrees C in vitro. Bilirubin glucuronide rearrangement in vitro was accompanied by nonenzymic hydrolysis. We conclude that the formation of biliprotein in vivo is probably nonenzymic and suggest that mammalian biliprotein is formed by acyl migration of bilirubin from a bilirubin-glucuronic acid ester to a nucleophilic site on albumin.

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.

Isovalerylglucuronide, a new urinary metabolite in isovaleric acidemia. Identification problems due to rearrangement reactions

Isovaleryl-beta-D-glucuronide, a new metabolite in the urine of patients with isovaleric acidemia, is described. Its gas chromatographic and mass spectrometric parameters are presented. In alkaline solution this glucuronide exhibited intramolecular rearrangements, resulting in isomers bearing the acyl moiety on C-2, C-3 and C-4. The isomers showed similar mass spectra but different positions on the gas chromatogram. In the index patient isovalerylglucuronide was a main metabolite, but the excretion was a transient phenomenon. Only traces of isovalerylglucuronide could also be detected in the urine of three other patients with isovaleric acidemia. The significance of this metabolite for the detoxication of isovalerate in isovaleric acidemia is discussed.

Microsomal conjugation and oxidation of bilirubin

Bilirubin diglucuronide and bilirubin monoglucuronide are formed on incubation of microsomal preparations from rat liver with bilirubin and UDPglucuronate. Microsomal diglucuronide formation is a two-step reaction: first monoglucuronide is formed and this is subsequently converted to diglucuronide. Both steps require UDPglucuronate and have a similar pH optimum at pH 7.8. Albumin inhibits the conversion of monoto diglucuronide. Factors favouring diglucuronide formation are: (a) low bilirubin concentration; (b) relatively high UDPglucuronate concentration; (c) complete removal of UDPglucuronyltransferase latency. For the latter, trypsin-treatment appeared superior over digitonin or UDP-N-acetylglucosamine. Trypsin-treatment had to be done under strictly anaerobic conditions. If trypsin treatment was done under aerobic conditions, reactive molecules were formed which initiated the rapid oxidation of bilirubin and its glucuronides. Microsomal oxidation of bilirubin and glucuronides also occurred in untreated and digitonin-treated microsomes and was stimulated by NADPH and by the cytochrome P-450 inhibitor, metyrapone. This suggests that lipid peroxides act as initiators of bilirubin oxidation. Indirect evidence was found that trypsin inactivates nucleotide pyrophosphatase. This is an active UDPglucuronate-consuming enzyme in microsomal preparations which must be inactivated before meaningful kinetic studies can be done. With trypsin-treated microsomal preparations the Vmax for bilirubin monoglucuronide formation was 1.7 X 10(-9) mol . mg protein-1 . min-1 and KUDPglucuronatem 43 X 10(-6) M. For bilirubin diglucoronide formation the apparent Vmax was 0.7 X 10(-9) mol . mg protein-1 . min-1 and the apparent KUDPglucuronate m 1.0 X 10(-3) M.

UDP-glucuronosyltransferase activity and bilirubin conjugation in the bullfrog

Bile pigments of bile and serum of Rana catesbeiana were investigated by means of high-pressure liquid chromatography. The major pigment in both bile and serum was bilirubin IX alpha. Bilirubin UDP-glucuronosyltransferase activity was found in the livers of all animals examined, but no conjugated bilirubin was detectable in the bile. Frog bile was found to contain large amounts of beta-glucuronidase. When the beta-glucuronidase inhibitor saccharo-1,4-lactone was introduced into the gall bladder followed by an exogenous bilirubin load, bilirubin glucuronide appeared in the bile.