Reaction of bilirubin glucuronides with serum albumin

Differentiation of Deprotonated Acyl-, N-, and O-Glucuronide Drug Metabolites by Using Tandem Mass Spectrometry Based on Gas-Phase Ion-Molecule Reactions Followed by Collision-Activated Dissociation

Glucuronidation, a common phase II biotransformation reaction, is one of the major in vitro and in vivo metabolism pathways of xenobiotics. In this process, glucuronic acid is conjugated to a drug or a drug metabolite via a carboxylic acid, a hydroxy, or an amino group to form acyl-, O-, and/or N-glucuronide metabolites, respectively. This process is traditionally thought to be a detoxification pathway. However, some acyl-glucuronides react with biomolecules in vivo, which may result in immune-mediated idiosyncratic drug toxicity (IDT). In order to avoid this, one may attempt in early drug discovery to modify the lead compounds in such a manner that they then have a lower probability of forming reactive acyl-glucuronide metabolites. Because most drugs or drug candidates bear multiple functionalities, e.g., hydroxy, amino, and carboxylic acid groups, glucuronidation can occur at any of those. However, differentiation of isomeric acyl-, N-, and O-glucuronide derivatives of drugs is challenging. In this study, gas-phase ion-molecule reactions between deprotonated glucuronide metabolites and BF₃ followed by collision-activated dissociation (CAD) in a linear quadrupole ion trap mass spectrometer were demonstrated to enable the differentiation of acyl-, N-, and O-glucuronides. Only deprotonated N-glucuronides and deprotonated, migrated acyl-glucuronides form the two diagnostic product ions: a BF₃ adduct that has lost two HF molecules, [M - H + BF₃ - 2HF]^-, and an adduct formed with two BF₃ molecules that has lost three HF molecules, [M - H + 2BF₃ - 3HF]^-. These product ions were not observed for deprotonated O-glucuronides and unmigrated, deprotonated acyl-glucuronides. Upon CAD of the [M - H + 2BF₃ - 3HF]^- product ion, a diagnostic fragment ion is formed via the loss of 2-fluoro-1,3,2-dioxaborale (MW of 88 Da) only in the case of deprotonated, migrated acyl-glucuronides. Therefore, this method can be used to unambiguously differentiate acyl-, N-, and O-glucuronides. Further, coupling this methodology with HPLC enables the differentiation of unmigrated 1-β-acyl-glucuronides from the isomeric acyl-glucuronides formed upon acyl migration. Quantum chemical calculations at the M06-2X/6-311++G(d,p) level of theory were employed to probe the mechanisms of the reactions of interest.

Detecting and characterizing reactive metabolites by liquid chromatography/tandem mass spectrometry

Metabolic activation of a drug leading to reactive metabolite(s) that can covalently modify proteins is considered an initial step that may lead to drug-induced organ toxicities. Characterization of reactive metabolites is critical to designing new drug candidates with an improved toxicological profile. High performance liquid chromatography (HPLC) coupled with mass spectrometry (MS) predominates over all analytical tools used for screening and characterization of reactive metabolites. In this review, a brief description of experimental approaches employed for assessing reactive metabolites is followed by a discussion on the reactivity of acyl glucuronides and acyl coenzyme A thioesters. Techniques for high-throughput screening and quantitation of reactive metabolite formation are also described, along with proteomic approaches used to identify protein targets and modification sites by reactive metabolites. Strategies for dealing with reactive metabolites are reviewed. In conclusion, we discuss the challenges and future needs in this field of research.

Rapid LC-TOFMS method for identification of binding sites of covalent acylglucuronide-albumin complexes

A method for rapid identification of binding sites of covalent adducts was developed using delta bilirubin as a model compound. Delta bilirubin, containing intact human serum albumin (HSA), was digested with trypsin and the peptide fragments were monitored at 436 nm, but no predominant peaks were detected indicating the instability of the digested peptides containing bilirubin-related compounds. Therefore, the high-performance liquid chromatography time-of-flight mass spectrometer (LC-TOFMS) data of digested fragments of delta bilirubin were compared with those of control digests of HSA, revealing a characteristic peptide in the digest mixture of delta bilirubin. This peptide was sequenced by high-performance liquid chromatography time-of-flight tandem mass spectrometry (LC-TOFMS/MS) and identified as LDELRDEGKASSAK (Leu182 to Lys195) with a modification of a 178 Da increase at Lys190. This indicated the Lys190 to be a predominant covalent binding site of BGs on HSA via the imine mechanism and the binding between the bilirubin moiety and the glucuronic acid moiety to be unstable to digestion with trypsin. The method of comparing LC-TOFMS data requires no specific detection such as fluorescence or radioactivity for every compound. This should accelerate the structure elucidation of covalent adducts and be helpful for studying the relationship between the structure of ligands and specific binding sites.

Significance of serum delta-bilirubin in patients with obstructive jaundice

BACKGROUND

Delta-bilirubin is a bilirubin covalently bound with albumin, which is nontoxic and excreted neither in urine nor in bile. We previously reported that the percentage of delta-bilirubin increased after biliary drainage and that the rapidly excretable bilirubin fraction (total minus delta-bilirubin) was a better parameter to predict the effectiveness of biliary decompression in the dog model. The aim of the present study was to elucidate whether it is applicable to humans.

MATERIALS AND METHODS

The serum bilirubin concentration was measured and its fractions were analyzed by high-performance liquid chromatography in 22 patients with obstructive jaundice before and after biliary drainage. In addition, the patients were subgrouped into good and poor drainage groups according to the decline index of serum bilirubin to examine the significance of delta-bilirubin.

RESULTS

The concentration of total bilirubin decreased from 14.1 mg/dl before biliary drainage to 5.4 mg/dl 28 days after drainage. During this period, the percentage of conjugated bilirubin steeply declined from 47.1 to 8.8% and that of excretable bilirubin from 63.4 to 28.6%. In contrast, the proportion of serum delta-bilirubin increased from 36.6 to 71.4%. There was an inverse correlation between percentage of delta-bilirubin and total bilirubin concentration (r = -0.69, P < 0.01). In the good drainage group, the percentage of delta-bilirubin increased above 60% within 7 days after biliary drainage, but it did not reach 60% by 28 days in the poor drainage group. A decreasing rate of total bilirubin minus delta-bilirubin, the excretable bilirubin fraction, was a better index than that of total bilirubin to assess the efficacy of biliary drainage (P< 0.01).

CONCLUSIONS

The increase in the percentage of serum delta-bilirubin indicates an effectiveness of biliary drainage in man. An analysis of serum delta-bilirubin for 7 days can distinguish the good drainage patients from the poor drainage patients.

Drug-protein binding sites. New trends in analytical and experimental methodology

In the last few years, continuous progress in instrumental analytical methodology has been achieved with a substantial increase in the number of new, more specific and more flexible methods for ligand-protein assays. In general, the methods used for drug-protein binding studies can be divided into two main groups: separation methods (enabling the calculation of binding parameters, i.e. the number of binding sites and their respective affinity constants) and non-separation methods (describing predominantly qualitative parameters of the ligand-protein complex). This review will be focussed particularly on recent trends in the development of drug-protein binding methods including stereoselective and non-stereoselective aspects using chromatography, capillary electrophoresis and microdialysis as compared to the "conventional approach" using equilibrium dialysis, ultrafiltration or size exclusion chromatography. The advantages and limitations of various methods will be discussed including a focus on "optimal" experimental strategies taking into account in vitro, ex vivo and/or in vivo studies. Furthermore, the importance of some particular aspects concerning the drug binding to proteins (covalent binding of drugs and metabolites, stereoselective interactions and evaluation of binding data) will be outlined in more detail.

Dynamic aspects of glutathione metabolism in obstructive jaundice

The dynamic aspects of glutathione metabolism during obstructive jaundice were analyzed in rats. Plasma bilirubin levels increased after ligation of the bile duct, with a concomitant increase in hepatorenal glutathione levels. When the bile duct was recanalized, plasma bilirubin levels rapidly decreased, with a concomitant decrease in hepatorenal glutathione levels. The half-life of hepatic glutathione turnover increased markedly after bile duct obstruction, returning to normal after recanalization of the bile duct. Intravenous administration of a loading dose of bilirubin inhibited the biliary secretion of glutathione in a dose-dependent manner. On the other hand, renal glutathione efflux increased markedly after bile duct obstruction. These observations suggest that glutathione status is significantly affected in obstructive jaundice, predominantly due to the inhibition of hepatic secretion by increased bilirubin.

Studies on the in vitro reactivity of clofibryl and fenofibryl glucuronides. Evidence for protein binding via a Schiff's base mechanism

Clofibryl and fenofibryl acyl (ester) glucuronides (CAG and FAG) are major metabolites in humans of the hypolipidaemic drugs clofibrate and fenofibrate, respectively. We have investigated three inter-related aspects of the reactivity of CAG and FAG in human serum albumin (HSA) solution, human plasma and in buffer at pH 7.0: namely (a) rearrangement via acyl migration to glucuronic acid esters of clofibric acid (CA) and fenofibric acid (FA), (b) hydrolysis of the parent glucuronide and rearrangement products to yield CA and FA and (c) the formation of covalent adducts with albumin and plasma protein. CAG was more reactive than FAG in all media, especially the protein solutions. The reactivity of both glucuronides was accelerated in protein solution compared with buffer and this was more marked in plasma than in HSA solution. The predominant reaction during the initial stages of the incubation was formation of isomeric rearrangement products. In the protein solutions, CA and FA were the major reaction products after 24 hr, compared to the rearranged isomers in buffer. Protein binding of 14C to HSA was markedly higher after incubation of CAG and FAG labelled on the glucuronyl moiety compared with the label on the aglycone. This is consistent with the covalent binding of CAG and FAG to protein proceeding via the formation of a Schiff's base rather than by transacylation.

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.

Studies on the reactivity of acyl glucuronides--III. Glucuronide-derived adducts of valproic acid and plasma protein and anti-adduct antibodies in humans

The major metabolite of the anti-epileptic agent valproic acid (VPA) is its acyl glucuronide conjugate (VPA-G), which undergoes non-enzymic, pH-dependent rearrangement via acyl migration to a mixture of beta-glucuronidase-resistant forms (collectively VPA-G-R). We have compared the reactivity of VPA-G and VPA-G-R towards covalent VPA-protein adduct formation by incubation in buffer, human serum albumin (HSA) and fresh human plasma at pH 7.4 and 37 degrees. In all three media, the predominant reaction of VPA-G over 30 hr was rearrangement to VPA-G-R (ca. 24%). Hydrolysis was quite minor (ca. 2%) and covalent adduct formation negligible (when protein was present). On the other hand, both hydrolysis (ca. 27%) and adduct formation (ca. 7%) were extensive when VPA-G-R was incubated with HSA or plasma. These data do not support a transacylation mechanism for VPA-protein adduct formation, since this pathway should be much more highly favoured by VPA-G (an acyl-substituted acetal) than VPA-G-R (simple esters). VPA-protein adducts were found in the plasma of epileptic patients taking VPA chronically (mean 0.77 +/- SD 0.63 microgram VPA equivalents/mL, N = 17). An enzyme linked immunosorbent assay was developed, using HSA modified by incubation with VPA-G-R, to test the immunoreactivity of the patients' plasma. Of 57 patients tested, nine showed measurable levels of antibodies to these adducts, but the titres were very low, with no difference in response to modified and unmodified protein detectable at plasma dilutions of 1:16 or greater. These results suggest that the VPA-protein adducts have little immunogenicity, and are in agreement with clinical observations that drug hypersensitivity responses have not been associated with VPA therapy. Thus, although the in vitro data show that VPA-G is an example of a relatively unreactive acyl glucuronide, covalent VPA-plasma protein adducts and anti-adduct antibodies are nonetheless formed in vivo, at least in some patients on chronic therapy with the drug.

Studies on the reactivity of acyl glucuronides--II. Interaction of diflunisal acyl glucuronide and its isomers with human serum albumin in vitro

A major metabolite of diflunisal (DF) is its reactive acyl glucuronide conjugate (DAG) which can undergo hydrolysis (regeneration of DF), intramolecular rearrangement (isomerization via acyl migration) and intermolecular reactions with nucleophiles. We have compared the fate of DAG and its individual 2-, 3- and 4-O-acyl positional isomers (at ca. 55 micrograms DF equivalents/mL) after incubation with human serum albumin (HSA, 40 mg/mL) at pH 7.4 and 37 degrees. Initial half-lives (T1/2) for DAG and its 2-, 3- and 4-isomers were 53, 75, 61 and 26 min, respectively. DAG was more labile to hydrolysis than any of its isomers but the latter, in particular the 4-isomer, were much better substrates for formation of covalent DF-HSA adducts. After a 2-hr incubation, 2.4, 8.2, 13.7 and 36.6% of substrate DAG and its 2-, 3- and 4-isomers (respectively) were present as DF-HSA adducts. With long term incubation, the concentrations of adducts so generated in situ declined in a biphasic manner, with apparent terminal T1/2 values of ca. 28 days. DAG was much more labile to transacylation with methanol (i.e. formation of DF methyl ester) than an equimolar mixture of its isomers after incubation in a 1:1 methanol:pH 7.4 buffer solution at 37 degrees (T1/2 values of 5 and 70 min, respectively). The data do not support direct transacylation with nucleophilic groups on protein as the predominant mechanism of formation of covalent DF-HSA adducts in vitro.

Bilirubin diglucuronide as the main source for in vitro formation of delta bilirubin

To clarify which of the bilirubin moieties is responsible for the formation of bilirubin bonded to albumin (delta bilirubin) in icteric serum, the in vitro formation of delta bilirubin from bile acid-free bilirubin glucuronides and unconjugated bilirubin was examined by high-performance liquid chromatography. Bovine serum albumin (150 mumol/liter) was mixed with equimolar bilirubin diglucuronide (BDG), bilirubin monoglucuronide (BMG), or unconjugated bilirubin (UCB) and incubated in the dark at 37 degrees C under argon gas saturation. Although no delta bilirubin was formed immediately, formation eventually occurred and increased with time. A similar amount of delta bilirubin was formed when human serum albumin was used instead of bovine serum albumin. Of the three types of bilirubin, BDG was found to be the greatest source of delta bilirubin, whereas UCB produced the least. On the other hand, photoirradiation of a mixture of bovine serum albumin and UCB at a molar ratio of 1:1 resulted 6 hr later in the formation of three times as much delta bilirubin as in nonirradiated specimens. This photoinduced delta bilirubin formation increased further when the UCB/albumin molar ratio was increased to 2:1.

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.

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.

Irreversible binding of tolmetin glucuronic acid esters to albumin in vitro

Tolmetin glucuronide (TG), extracted and purified from human urine, was incubated with albumin in vitro. The degradation profile and irreversible binding to protein were investigated and kinetic parameters calculated. Standard conditions were as follows: TG, 30 micrograms/ml; human serum albumin (HSA), 3%; pH 7.45; 37 degrees C. Lower pH enhanced TG stability and reduced both the extent and the rate of irreversible binding. HSA also increased TG stability, compared to protein-free buffer, but the opposite was observed with bovine serum albumin (BSA). With BSA, irreversible binding was much less, but the rate of adduct formation was the same as with HSA. Essentially fatty acid free HSA behaved similarly to HSA. Preincubation of HSA with warfarin, or diazepam, or an excess of tolmetin, did not influence irreversible binding significantly. In buffer, acyl migration led predominantly to one isomer. This isomer bound irreversibly to HSA, although more slowly and to a lesser extent than the beta 1-isomer. Incubation of TG with poly-L-lysine also resulted in irreversible binding but to a lesser extent than with HSA. Our results suggest that there is more than one binding mechanism, with the preferential pathway a function of the isomers present and the experimental conditions.

Serum bilirubin fractions in analbuminemic rats after bile duct ligation. Albumin requirement in the formation of covalently protein-bound bilirubin

To clarify the role of albumin in the production of covalently protein-bound bilirubin (delta bilirubin, B delta) in the blood and the pathophysiological relevance of B delta, changes in the serum bilirubin level and histological findings in the liver, kidneys and myocardium were studied in Sprague-Dawley (SD) rats and Nagase analbuminemic rats (NAR) after bile duct ligation (BDL). In SD rats, the serum bilirubin level increased until 3 days after BDL, and was followed by the appearance of B delta. The increase in serum bilirubin was smaller in NAR, and no serum B delta was noted. Albumin administration to NAR 1 day after BDL increased serum bilirubin with the appearance of B delta. Serum bilirubin decreased in both SD rats and NAR 7 days after BDL. Marked deposition of bile pigments was noted in NAR in the renal tubular epithelium. The renal bilirubin content was decreased after albumin administration. From these results it is concluded that albumin is necessary for the production of B delta, and that renal deposition of bile pigments progresses in the absence of albumin.

Human serum bilirubin fractionation in various hepatobiliary diseases by the newly developed high performance liquid chromatography

Serum Bilirubin was fractionated by newly developed reversed phase high performance liquid chromatography (HPLC) into 5 fractions: delta (delta-Bilirubin, B delta), gamma (bilirubin diglucuronide, BDG), beta (Bilirubin monoglucuronide, BMG), beta' ((Z, E,)- and/or (E, Z)-bilirubin IX alpha) and alpha ((Z, Z)-bilirubin IX alpha). Sera of healthy subjects and of patients with unconjugated hyperbilirubinemia showed predominantly alpha fraction with a small amount of beta' fraction. Trace amounts of delta fraction were detected in a few cases. The results of fractionation of serum bilirubin in 159 patients with various hepatobiliary diseases suggested that the ratios B delta/(B delta + BDG + BMG) and BMG/B delta can be useful parameters to follow patients with jaundice, compared with the reported B delta/total bilirubin which did not always reflect the jaundice stage, especially in cases with low serum bilirubin levels.

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.