Excretion in dog bile of glucose and xylose conjugates of bilirubin

Bile pigments in fishes: a review

Gallbladder (GB) bile of most cyclostomes, elasmobranchs, and teleosts contains appreciable amounts of biliverdin (BV) and bilirubin (BR) conjugates with lesser amounts of unconjugated BR in certain species. Certain elasmobranch and teleost species have been reported to have primarily BV or BR in bile. The appearance of the enzyme BV reductase, which converts BV to BR in mammals, evolved quite early in the evolution of vertebrate species; however, exceptions exist in certain fishes, amphibians, reptiles, and mammals whose bile contains primarily BV. Nearly all analytical studies on bile pigment composition in fishes to date have utilized only GB bile, which may or may not always be representative of the pigments excreted in freshly collected hepatic duct bile. The concentration of BV and BR in GB bile of fishes increases markedly during prolonged fasts. From the limited data currently available, there appears to be no systematic development from primitive to advanced forms in the appearance of certain bile pigments in fishes. While bile of most aquatic species contains appreciable amounts of both BV and BR, it is interesting that the bile of most terrestrial avian and reptilian forms contains primarily BV. The serum of fishes, except for certain bony species such as eels (Anguilleformes) and cottids (Scorpaeniformes), is a light yellow color due to the presence of BR and is similar to that observed in higher vertebrates. Serum from certain eels and cottids is bluish green in color due to the presence of a variety of chromoproteins that contain BV firmly bound as the prosthetic group.

Metabolism and lack of DNA reactivity of the mycotoxin ochratoxin a in cultured rat and human primary hepatocytes

It is still unclear whether the carcinogenic mycotoxin ochratoxin A (OTA) is bioactivated to DNA-binding metabolites in rodents and humans. Therefore, we have incubated cultured rat and human primary hepatocytes with noncytotoxic concentrations of (3)H-OTA ranging from 10(-7) to 10(-5) M for 8 h and determined its metabolism and covalent DNA binding. In rat hepatocytes, OTA was metabolized to small amounts of three products, which were further studied by electrospray ionization (ESI)-MS/MS techniques. In addition to 4-hydroxy-OTA, which is a known product of OTA biotransformation, two novel metabolites were detected and tentatively identified as hexose and pentose conjugates of OTA. The in vitro induction with 3-methylcholanthrene (3MC) increased the formation of 4-hydroxy-OTA but did not alter the formation of the conjugated metabolites. No covalent binding of (3)H-OTA or its metabolites to DNA was observed in rat hepatocytes with or without 3MC induction with a limit of detection of 2 adducts per 10(9) nucleotides. However, the cellular ratio of reduced glutathione to oxidized glutathione was significantly decreased by treatment with OTA. In cultured human hepatocytes, (3)H-OTA was only very poorly metabolized, and no covalent DNA binding was observed. In conclusion, the results of this in vitro study do not support the notion that OTA has the potential to undergo metabolic activation and form covalent DNA adducts in rodents and humans.

Structural and functional studies of UDP-glucuronosyltransferases

UDP-Glucuronosyltransferases (UGTs) are glycoproteins localized in the endoplasmic reticulum (ER) which catalyze the conjugation of a broad variety of lipophilic aglycon substrates with glucuronic acid using UDP-glucuronic acid (UDP-GIcUA) as the sugar donor. Glucuronidation is a major factor in the elimination of lipophilic compounds from the body. In this review, current information on the substrate specificities of UGT1A and 2B family isoforms is discussed. Recent findings with regard to UGT structure and topology are presented, including a dynamic topological model of UGTs in the ER. Evidence from experiments on UGT interactions with inhibitors directed at specific amino acids, photoaffinity labeling, and analysis of amino acid alignments suggest that UDP-GIcUA interacts with residues in both the N- and C-terminal domains, whereas aglycon binding sites are localized in the N-terminal domain. The amino acids identified so far as crucial for substrate binding and catalysis are arginine, lysine, histidine, proline, and residues containing carboxylic acid. Site-directed mutagenesis experiments are critical for unambiguous identification of the active-site architecture.

Microtubular disruption prolongs the expression of human bilirubin-uridinediphosphoglucuronate-glucuronosyltransferase-1 gene transferred into Gunn rat livers

DNA delivered to the liver by asialoglycoprotein receptor-mediated endocytosis is degraded in lysosomes within 48 h. To test the hypothesis that microtubular disruption should promote transgene persistence by interrupting endosomal translocation to lysosomes, plasmids containing bacterial chloramphenicol acetyltransferase (pSV2-CAT) or human bilirubin-UDP-glucuronosyltransferase-1 (pSVK3-hBUGT1) genes were complexed with asialoglycoprotein-polylysine conjugates, and 1 mg of the complexed DNA was injected intravenously into bilirubin-UDP-glucuronosyltransferase-deficient Gunn rats. 30 min before DNA injection, one group received 0.75 mg of colchicine/kg of body weight intraperitoneally, which was shown by immunofluorescent confocal microscopy to disrupt the microtubular network. Control rats received normal saline. In colchicine-pretreated rats receiving pSV2-CAT, hepatic chloramphenicol acetyltransferase activity persisted for 9-14 weeks, whereas in the saline-pretreated group the activity was detectable for 48 h only. In colchicine-pretreated Gunn rats receiving pSVK3-hBUGT1, the DNA persisted in liver for 10 weeks, bilirubin glucuronides were excreted in bile, and serum bilirubin levels declined by 25-35% in 2-4 weeks and remained reduced for 8 weeks. Without colchicine pretreatment, the DNA was detectable in liver for 2 days only, and serum bilirubin levels were not reduced. Thus, microtubular disruption provides a noninvasive method for prolonging the effect of liver-targeted gene therapy.

Photoaffinity labeling for evaluation of uridinyl analogs as specific inhibitors of rat liver microsomal UDP-glucuronosyltransferases

The UDP-glucuronosyltransferases (UGT) involved in glucuronidation of endogenous and exogenous toxic compounds transfer the glucuronic acid residue from UDP-glucuronic acid (UDP-GlcUA), to various acceptor groups. A series of compounds that contain N-acyl phenylaminoalcohol derivatives linked to uridine or isopropylideneuridine were tested as UGT inhibitors. The potency of these inhibitors was determined by studying their effect on the photoaffinity labeling of rat liver microsomal UGTs by two photoaffinity probes, [beta-32P]5-azido-UDP-glucuronic acid (5N3UDP-GlcUA) and [beta-32P]5-azido-UDP-glucose (5N3UDP-Glc) and on the enzymatic formation of the two glucuronide conjugates (3-O- and carboxyl-specific) of lithocholic acid. All but one of the compounds tested proved to have an inhibitory effect on UGTs, both in the photoaffinity labeling system and in the enzymatic glucuronidation assay. In the photoaffinity labeling system, the inhibitors containing the isopropylidene moiety were less effective than their unprotected derivatives; however, the protected forms were, with one exception, more potent inhibitors of enzymatic activity. The photoaffinity labeling of UGTs with [beta-32P]5N3UDP-Glc was more susceptible to inhibition by all derivatives than that with [beta-32P]5N3UDP-GlcUA. The effect of one inhibitor, PP50B, on the two enzymatic activities involved in LA glucuronidation was extensively tested. A double-reciprocal plot suggested a competitive inhibition for UDP-GlcUA with an apparent Ki of 35 microM for LA 3-O-glucuronide formation and 94 microM for the carboxyl-linked glucuronide of the same substrate.

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.

Novel method for high-performance liquid chromatography of azo derivatives of conjugated and unconjugated bilirubin

A method for the separation and quantitation of ethyl anthranilate or p-iodoaniline azo derivatives of bile pigments was developed using reversed-phase high-performance liquid chromatography. A convenient separation was achieved in 15 min, permitting the quantitation of the unconjugated azo-dipyrrole (alpha o) and its glucuronide (delta), xyloside (alpha 2) and glucoside (alpha 3) conjugates. The pathological beta- and gamma-azo pigments, derived from bilirubin glucuronide isomers that occur in cholestatic bile or plasma, are also detected in this system. The results of this method as applied to bile from 25 healthy dogs were in excellent agreement with the values obtained by reversed-phase chromatography of bilirubin and its mono- and dimethyl esters produced from the corresponding conjugates by alkaline methanolysis. This system permits the sensitive and convenient determination of bilirubin and its conjugation pattern in biological fluids.

Effect of the tridecamer of compound 48/80, a Ca2+-dependent histamine releaser, on phospholipid metabolism during the early stage of histamine release from rat mast cells

When the tridecamer component of compound 48/80 (Fraction D, Fr.D), a Ca2+-dependent histamine releaser, was incubated with rat mast cells that had been prelabeled with [32P]phosphate, [3H]inositol or [3H]glycerol, it induced a rapid decrease in [32P]phosphatidylinositol-4,5-bisphosphate (PIP2) followed by increases of [3H]inositol-1,4,5-trisphosphate (Ins P3) and [3H]diacylglycerol during the 10 sec prior to detectable histamine release. Fr.D-induced changes of the metabolism of these compounds occurred even in the absence of Ca2+, but to a lesser extent than in the presence of Ca2+. In contrast, the accumulation of [3H]arachidonic acid into phosphatidylcholine (PC), phosphatidylinositol (PI) and phosphatidic acid (PA) in [3H]arachidonic acid-prelabeled mast cells was Ca2+-dependently stimulated by Fr.D with a concomitant decrease in [3H]phosphatidylethanolamine (PE). These Ca2+-dependent changes in PC and PE were not observed in mast cells preloaded with [32P]phosphate, while [32P]PI and [32P]PA increased Ca2+ independently. Fr.D also increased 45Ca2+ uptake by mast cells within 5 sec after the stimulation. These results indicate that Fr.D binding to mast cell Ca2+ independently induces rapid changes of PI cycle-related metabolism of plasma membrane components, while it also induces Ca2+-dependent accumulation of arachidonic acid into PC, PI and PA in association with the decrease of PE, which may be important during the latent period prior to the Ca2+-dependent release of histamine from Fr.D-stimulated mast cells.

Methods for determination of conjugated bilirubin in rat faeces

Conjugated bilirubin was prepared from the faeces of germ-free (GF) rats by three different preparative methods. The bilirubin conjugate preparations were coupled with diazotized ethyl anthranilate and the formed ethyl anthranilate azopigments were quantified spectrophotometrically and separated by thin-layer chromatography (tlc). The most polar azopigment was purified by tlc and subjected to ammonolysis followed by tlc of the released saccaride. As a result of this procedure, only glucuronic acid was detected as the conjugating saccaride thus indicating that the most polar azopigment prepared from GF rat faeces was the delta ethyl anthranilate azopigment. Reference azopigments were prepared from GF rat small intestinal contents and subjected to separation by tlc. The azopigment pattern was very similar to the pattern obtained with the faecal azopigment preparations and a maximum of ten separated azopigment spots were detected. The findings indicated that, in addition to bilirubin glucuronides, other bilirubin conjugates with unknown structure are excreted with the faeces of GF rats. One of the preparative methods used for the preparation of conjugated bilirubin from GF rat faeces was tested on faeces from conventional (CONV) rats. From these preparations, no ethyl anthranilate azopigments were formed, thus indicating that faeces from CONV rats is devoid of conjugated bilirubin.

Plasma bile pigment conjugation modalities in icterus syndromes of various origin

Plasma azopigments derived from conjugated bilirubin were analyzed by thin-layer chromatography according to HEIRWEGH et al. in 14 cases of obstructive jaundice and in 11 of acute hepatitis. The chromatographic patterns were compared with those obtained from azopigments derived from 8 normal bile samples. The plasma pigment patterns did not differ from those of the bile in number and chromatographic mobility of the spots. However, the quantitative percentages of the plasma azopigments were significantly modified: the alpha 0 fraction (free azodipyrrolic pigment) increased in both icteric syndromes, while the delta fraction (mainly glucuronide azopigment) decreased. Moreover, the behavior of two closed components of the delta group showed significant differences in both icteric syndromes. It can be postulated that the synthesis of bilirubin diconjugates decreases both in hepatocellular and cholestatic jaundice, while monoglucuronidated as well as saccharide and glucoside conjugates increase. In cholestatic jaundice the conjugation with glucuronic acid mainly takes place in the normal way, whereas compounds with different features are formed in hepatitis.

Rearranged glucuronic acid conjugates of bilirubin-IX alpha in post-obstructive bile. Structure elucidation of azopigments beta and gamma as ethyl anthranilate N-glycosides derived from 2-, 3- and 4-o-acyl glucuronides

Azopigment analysis was performed on conjugates of bilirubin-IXalpha in bile of man and rats obtained after obstruction of the bile duct or in bile incubated under N2. The azopigments beta and gamma, formed by applying a pH 2.7 diazonium reagent containing an excess of ethyl anthranilate, correspond to rearranged ethyl athranilate N-glucuronides having the azodipyrrole acyl group on positions 2, 3 and 4 of the sugar. These assignments were verified, first by conversion of the structurally known 2-, 3- and 4-O-acyl glucuronide azopigments, unsubstituted at C-1, into ethyl anthranilate N-glucuronide reference compounds, and second, by mass spectrometry of trimethylsilyl ether methyl ester derivatives of unknown and reference compounds. The C-1 ethyl anthranilate group of the N-glucuronides triggers characteristics fragmentation reactions of the carbohydrate moiety revealing the position of the azodipyrrole O-acyl group.

Inaccuracies in measurement of conjugated and unconjugated bilirubin in bile with ethyl anthranilate diazo and solvent-partition methods

A criticial evaluation was made of the ethyl anthranilate diazo and two solvent-partition methods for the determination of conjugated and unconjugated bilirubin in human and rat bile. The ethyl anthranilate diazo reagent, which reacts only with conjugated bilirubin in serum, also diazotized a variable proportion of unconjugated bilirubin in bile and thus overestimated the concentration of monoconjugates. With the Weber-Schalm and modified Folsch solvent-partition methods applied to human or rat bile, 4--9% of added 14C-labelled unconjugated bilirubin partitioned with the conjugated bilirubin in the upper phase, and 4--9% of added 14C-labelled conjugated bilirubin partitioned into the lower phase. With dog bile, the spill-over of 14C-labelled bilirubin into the lower phase was 9--11%. Analysis of azopigments from the Weber-Schalm partition confirmed that over two-thirds of the bilirubin in the lower phase represents monoconjugates, principally the less-polar monoxylosides and monoglucosides. These solvent-partition methods thus overestimate the concentration of unconjugated bilirubin in bile.

Twenty-five years of progress in bilirubin metabolism (1952-77)

This review deals with the development of our understanding of the chemistry of bilirubin and its glucuronide derivatives during the years 1952-1977. It examines the relation between haem metabolism and bilirubin formation and our present knowledge of hepatic transport of bilirubin. The heterogeneity of familial hyperbilirubinaemia is discussed.

The fate of bilirubin-IXalpha glucuronide in cholestasis and during storage in vitro. Intramolecular rearrangement to positional isomers of glucuronic acid

1. In aqueous solution above pH7 bilirubin-IXalpha 1-O-acylglucuronide rapidly isomerizes to the non-C-1 glucuronides by sequential migration of the bilirubin acyl group from position 1 to positions 2, 3 and 4 of the sugar moiety. The transformations are enhanced by increasing the pH. Compared with the rates at 37 degrees C the transformations are rather slow at 0 degrees C. Virtually complete inhibition is observed at values below pH6. The isomerization at 25 degrees C and pH 7.4 is not affected by the presence in the solutions of a molar excess of human serum albumin. 2. Isomerization in bile kept at 37 degrees C at pH7.7-7.8 is probably non-enzymic, as the rates of change are similar to those observed under comparable conditions for aqueous solutions of glucuronides of bilirubin-1Xalpha and of azodipyrrole. 3. Analysis without delay of normal biles of man and rats collected at 0 degrees C over a maximum period of 10 min shows that the bilirubin-IXalpha mono- and di-glucuronides consist exclusively of the 1-O-acyl isomers. 4. The mixtures of the four positional isomers of bilirubin-IXalpha glucuronide found in freshly collected biles of man and rats with cholestasis probably originate from initially synthesized 1-O-acylglucuronide by the same mechanism of sequential migration as has been observed in aqueous solutions of conjugated bilirubin-IXalpha.

Conjugated and unconjugated bilirubins in bile of humans and rhesus monkeys. Structure of adult human and rhesus-monkey bilirubins compared with dog bilirubins

1. Bilirubin-IXalpha, -IXalpha diglucuronide, -IXalpha monoglucuronide, -IXalpha monoglucoside -IXalpha monoxyloside, a bilirubin-IXalpha diconjugate containing glucose and another unknown compound, and bilirubin-IXbeta are present in gall-bladder bile of adult human, rhesus monkey and dog. Dog bile normally also contains other bilirubin-IXalpha diconjugates, i.e. compounds containing two conjugating sugars such as glucuronic acid and glucose, glucuronic acid and xylose and glucose xylose. 2. Azopigments alphaF, alphaO, alpha2, alpha3, betax and delta derived from human and rhesus-monkey bilirubins are identical in their chemical composition with those obtained from the dog. 3. Azopigments alphaF and betax found in diazotized biles of adult humans, rhesus monkeys and dogs are products of unconjugated bilirubin-IXbeta. 4. Technical modifications of previously published procedures [Heirwegh, Fevery, Michiels, Van Hees & Compernolle, (1975) Biochem. J. 145, 185-199] were introduced which make it possible to separate the bilirubins, diazotize the separated bilirubins, extract the azopigments and chromatograph them in one working day (6-8h).

Unconjugated bilirubin and an increased proportion of bilirubin monoconjugates in the bile of patients with Gilbert's syndrome and Crigler-Najjar disease

Bilirubin pigments were studied in the bile of 20 normal adults, 25 patients with Gilbert's syndrome, 9 children with Crigler-Najjar disease, and 6 patients with hemolysis, to determine how a deficiency of hepatic bilirubin UDP-glucuronosyltransferase would affect the end products of bilirubin biotransformation. In the bile from patients with Gilbert's syndrome, a striking increase was found in the proportion of bilirubin monoconjugates (48.6+/-9.8% of total conjugates) relative to that in normal bile (27.2+/-7.8%). This increase was even more pronounced in children with Crigler-Najjar disease, in whom, even in the most severe cases, glucuronide could always be demonstrated in the bile. Furthermore, unconjugated bilirubin-IXalpha was unquestionably present in the bile of these children and amounted to 30-57% of their total bilirubin pigments (<1% in the controls). It was not possible to predict from the biliary bilirubin composition whether a child would respond to phenobarbital therapy or not. Bile composition was normal in patients with hemolysis, except when there was associated deficiency of hepatic glucuronosyltransferase. Therefore, the observed alterations were not a simple consequence of unconjugated hyperbilirubinemia. The present findings suggest that Crigler-Najjar disease represents a more pronounced expression than Gilbert's syndrome of a common biochemical defect. Hepatic bilirubin UDP-glucuronosyltransferase deficiency leads to decreased formation of diconjugates with an ensuing increase in the proportion of bilirubin monoconjugates in bile; in the most severe cases, an elevated content of biliary unconjugated bilirubin is also found.

The isolation and further characterization of the bilirubin tetrapyrroles in bile-containing human duodenal juice and dog gall-bladder bile

Bilirubin and its conjugates were extracted from either dog gall-bladder bile or bile-containing human duodenal juice into chloroform containing 10mm-tetraheptylammonium chloride. The intact bilirubin tetrapyrroles were then separated by t.l.c. Structural elucidation was made after coupling of the individual pigments with diazonium salts. Four azopigments were detected: azopigment alpha(o) or dipyrrolic azobilirubin; azopigment delta or dipyrrolic azobilirubin monoglucuronide; azopigment alpha(3) or dipyrrolic azobilirubin monoglucoside; and, from dog gall-bladder bile, azopigment alpha(2). The last conjugate required further verification of its structure. After methanolysis, it was shown by combined g.l.c.-mass spectrometry to contain xylose in a 1:1 molar ratio with the azopigments of bilirubin. Human bile contained 86% bilirubin diglucuronide, 7% bilirubin monoglucuronide monoglucoside diester, 4% bilirubin monoglucuronide and 3% bilirubin. Dog gall-bladder bile had a considerably different composition; it contained 47% bilirubin diglucuronide, 40% bilirubin monoglucuronide monoglucoside diester, 8% bilirubin monoglucuronide, 4% bilirubin diglucoside, 1-2% bilirubin and traces of conjugates containing xylose. The total bilirubin content and proportions of the conjugates did not change in bile that was frozen and stored at -20 degrees C under N(2), whereas in the chloroform/tetraheptylammonium chloride extract, similarly stored, total pigment was slowly lost and the diglucuronide conjugate converted into the monoglucuronide.

Comparison in different species of biliary bilirubin-IX alpha conjugates with the activities of hepatic and renal bilirubin-IX alpha-uridine diphosphate glycosyltransferases

The bilrubin-IXalpha conjugates in bile and the activities of bilirubin-IX alpha--UDP-glycosyltransferases in liver and kidney were determined for ten species of mammals and for the chicken. 1. In the mammalian species, bilirubin-IX alpha glucuronide was the predominant bile pigment. Excretion of neutral glycosides was unimportant, except in the cat, the mouse, the rabbit and the dog, where glucose and xylose represented 12--41% of total conjugating groups bound to bilirubin-IX alpha. In chicken bile, glucoside and glucuronide conjugates were of equal importance. They probably represent only a small fraction of the total bile pigment. 2. The transferase activities in liver showed pronounced species variation. This was also apparent with regard to activation by digitonin, pH optimum and relative activities of transferases acting on either UDP-glucuronic acid or neutral UDP-sugars. 3. Man, the dog, the cat and the rat excrete bilirubin-IX alpha largely as diconjugated derivatives. In general, diconjugated bilirubin-IX alpha could also be synthesized in vitro with liver homogenate, bilirubin-IX alpha and UDP-sugar. In contrast, for the other species examined, bilirubin pigments consisted predominantly of monoconjugated bilirubin-IX alpha. Synthesis in vitro with UDP-glucuronic acid, UDP-glucose or UDP-xylose as the sugar donor led exclusively to the formation of monoconjugated bilirubin-IX alpha. 4. The transferase activities in the kidney were restricted to the cortex and were important only for the rat and the dog. No activity at all could be detected for several species, including man. 5. Comparison of the transferase activities in liver with reported values of the maximal rate of excretion in bile suggests a close linkage between conjugation and biliary secretion of bilirubin-IX alpha.

The isolation and characterization of bilirubin diglucuronide, the major bilirubin conjugate in dog and human bile

The chemical structure of the major conjugate of bilirubin was unequivocally elucidated by structural analysis. The conjugated bilirubins were first separated from the lipid components of human duodenal aspirates or dog gall-bladder bile, and then resolved by t.l.c. into a series of tetrapyrroles. The major tetrapyrrole was then converted into its more stable dipyrrolic azo derivative for further analysis. The conjugated moiety of the azopigment was characterized after methanolysis with sodium methoxide. This reaction yields two types of product, those soluble in water and those soluble in organic solvents. The organic-soluble fraction was shown by t.l.c. and mass spectrometry to contain the methyl esters of the dipyrrolic azo derivatives of bilirubin. The water-soluble materials were analysed by enzymic procedures, t.l.c., n.m.r. spectrometry and combined g.l.c. and mass spectrometry. This analysis showed that the only water-soluble product resulting from the methanolysis was glucuronic acid. The structure was identical with that of pure standards, on both mass spectrometry and n.m.r. spectroscopy. No contaminating moieties were found. Quantitative measurement indicated that the glucuronic acid had been released in a 1:1 molar ratio with the resulting methyl esters of the dipyrrolic azo derivatives of bilirubin. This unequivocally establishes bilirubin diglucuronide as the major pigment present in bile. Past problems with identification of bilirubin diglucuronide were shown to originate from procedures which resulted in incomplete separation and isolation of the azopigments of the conjugated bilirubins, owing to contamination by biliary lipids.

Bilirubin conjugates in bile of man, rat and dog. Semi-quantitative analysis of bile composition by thin-layer chromatography

1. Conjugated bile pigments, separated in two fractions by semi-quantitative t.l.c. performed on silicic acid with phenol/water as the developing solvent, were treated with diazotized ethyl anthranilate. Resulting dipyrrylazo derivatives were analysed by quantitative t.l.c. 2. The tentative structure elucidation of tetrapyrrolic bilirubin conjugates and semi-quantitative evaluation of rat bile, post-obstructive human bile and dog bile composition is presented. 3. Homogeneous and mixed hexuronic acid diesters of bilirubin containing glucuronic acid constitute 51% of the total conjugates in normal rat bile, 45% of those in human post-obstructive bile and 38% of those in obstructed rat biles. 4. Monoconjugated bilirubin amounts to 33% of total conjugated bile pigments in normal rat bile, and 17 and 14% in post-obstructive hepatic human bile and gall-bladder bile of dog respectively. After loading with unconjugated bilirubin a greater amount of monoconjugates (56%) occur in the rat bile, whereas bilirubin diglucuronide excretion is decreased (34%). 5. In gall-bladder bile of normal dog, 40% of glucose-containing diconjugates, 32% of homogeneous and/or mixed hexuronic acid (mainly glucuronic acid) diesters of bilirubin and 14% of xylose-containing diconjugates are estimated. 6. Increased amounts of bilirubin conjugates, including some with unidentified uronic acid groups, were observed in cholestatic rat biles and quantities of conjugates with glucuronic acid were decreased.

Heterogeneity of bile pigments formed by the breakdown of haemoglobin in the isolated male-dog kidney

Normothermic perfused isolated male-dog kidneys formed radioactive bile pigments by the breakdown of radioactive haemoglobin prepared from [2-14C]glycin. After column chromatographic separation and preparation of dipyrrolic azopigments, 86.3 +/- 2.2% of the bile pigments seemed to be conjugated bilirubin. Thin-layer chromatographic separation of the azopigments of ethyl anthranilate revealed a good correlation between photometric scanning, radiochromatographic scanning and the radioactivity of the azopigments scraped off the thin-layer glass plates and counted in a liquid scintillation counter. Although the same heterogenity of the azopigments was observed as in dog bile, the isolated male-dog kidney formed significantly less alpha2- and significantly more gamma-fractions.

A polypeptide conjugate of bilirubin from human bile

An alkali-stable bilirubin conjugate has been obtained from human T-tube bile as its phenylazo derivative. The conjugate consists of a polypeptide probably of molecular weight 7000 to which the azo pigment of bilirubin is linked covalently through its carboxyl group. It thus constitutes the first biliprotein found in mammals. It is not known whether both carboxyl groups of native bilirubin participate in the binding of the conjugating protein, nor has it been possible to determine the number of pigment moieties occurring on a single polypeptide chain. The isolation makes use of the tendency of the conjugate to form large aggregates and involves the following steps: azo coupling of the native bile, (NH4)2S04 precipitation of macromolecules and aggregates, removal of low molecular weight contaminants by dialysis and gel filtration (first on Sepharose 6B IN 6 M guanidine, then on Sephadex LH-20 in 50% acqueous 2-chloroethanol) and a concluding purification by chromatography on p-aminobenzyl cellulose using a PH gradient. The final preparation appeared to be homogeneous on polyacrylamide electrophoresis.

Separation by thin-layer chromatography and structure elucidation of bilirubin conjugates isolated from dog bile

1. A system for separation of bile pigments by t.l.c. and for their structure elucidation is presented. Separated bile pigments are characterized by t.l.c. of derived dipyrrolic azopigments. 2. At the tetrapyrrolic stage hydrolysis in strongly alkaline medium followed by t.l.c. demonstrates the presence of bilirubin-IIIalpha, -IXalpha and -XIIIalpha and allows assessment of their relative amounts. 3. Most structural information is derived from analysis of dipyrrolic azopigments. Such derivatives, obtained by treatment of separated bile pigments with diazotized ethyl anthranilate, were separated and purified by t.l.c. Micro methods showed (a) the nature of the dipyrrolic aglycone, (b) the nature of the bonds connecting aglycone to a conjugating group, (c) the ratio of vinyl/isovinyl isomers present in the aglycone and, (d) the nature of the conjugating groups (by suitable derivative formation and t.l.c. with reference to known compounds). 4. In bile of normal dogs at least 20 tetrapyrrolic, diazo-positive bile pigments could be recognized. Except for two pigments the tetrapyrrolic nucleus corresponded predominantly to bilirubin-IXalpha. All conjugated pigments had their conjugating groups connected in ester linkage to the tetrapyrrolic aglycone, Apart from bilirubin-IXalpha, monoconjugates and homogeneous and mixed diconjugates of bilirubin were demonstrated; conjugating groups of major importance were xylose, glucose and glucuronic acid. 5. Bilirubin isomer determination on native bile and isolated bile pigments, and dipyrrole-exchange assays with [14C8]bilirubin indicated (a) that the conjugates pre-exist in bile, and (b) that no significant dipyrrole exchange occurs during isolation of the pigments.

The isolation of an azobilirubin beta-D-monoglucoside from dog gall-bladder bile

An ethyl anthranilate azopigment of bilirubin conjugated to beta-d-monoglucoside was isolated from dog gall-bladder bile. Glucose was cleaved from the azopigment by treatment with beta-glucosidase and beta-glucuronidase. Mild alkaline hydrolysis of the compound by sodium methoxide yielded two kinds of compounds, water-soluble and organic-soluble. The former were shown, by enzymic analysis, t.l.c., nuclear magnetic resonance, and combined g.l.c. and mass spectrometry, to contain glucose. No evidence was obtained from these data that a disaccharide was present in this fraction. The organic-soluble compounds formed during this methanolysis were shown, by t.l.c. and mass spectrometry, to be the isomeric dipyrrole azopigments of bilirubin. These findings contribute further evidence to the controversy surrounding the nature of conjugated bilirubin.

The synthesis of esters of bilirubin

1-Alkyl-3-p-tolytriazenes were used to synthesize the methyl, ethyl, isopropyl and benzyl esters of bilirubin. Treatment of a chloroform solution of bilirubin with the triazene at room temperature gave high yields of the corresponding esters. These were identified by n.m.r. and mass spectroscopy together with elemental analysis. N.m.r. studies also suggest that bilirubin dimethyl ester is in the lactam rather than the lactim form.

Structure and conformation of bilirubin. Opposing views that invoke tautomeric equilibria, hydrogen bonding and a betaine may be reconciled by a single resonance hybrid

A novel conformational structure of bilirubin is presented which obtains maximum stabilization through a system of four intramolecular hydrogen bonds. Two hydrogen bonds link oxygen and nitrogen atoms of each end ring to the contralateral carboxyl group. The proposed structure can explain a variety of uncommon features of bilirubin, and reconciles many seemingly contradictory hypotheses by accommodating them in individual structures which are mesomeric forms of one resonance hybrid. In the light of this newly conceived structure the following characteristics of bilirubin are re-evaluated: the stability of the compound, its reaction with diazomethane, the conformational behaviour of its dimethyl ester, its spectral properties, the chirality of the compound when complexed to serum albumin, and the structure of its metal chelates.

Studies of mammalian glucoside conjugation

The mammalian glucoside-conjugation pathway was studied by using p-nitrophenol as the model substrate and mouse liver microsomal preparations as the source of enzyme. The microsomal preparations supplemented with UDP-glucose glucosylated p-nitrophenol; p-nitrophenyl glucoside was identified by chromatography in six solvent systems. The unsolubilized glucosyltransferase of fresh microsomal preparations did not follow the usual Michaelis-Menten kinetics and was easily inhibited by many steroids. All the steroids tested inhibited glucosylation of p-nitrophenol to a greater degree than glucuronidation of p-nitrophenol when assayed in the same microsomal preparations. The steroids inhibited glucosylation with the following decreasing effectiveness: pregnan-3alpha-ol-20beta-one (3alpha-hydroxypregnan-20-beta-one)>oestradiol-17beta 3-methyl ether>oestradiol-17beta>oestriol>pregnane-3alpha,20beta-diol>oestrone. Pregnan-3alpha-ol-20beta-one, pregnane-3alpha,20beta-diol and oestrone had negligible effect on glucuronidation.

Assay and properties of dititonin-activated bilirubin uridine diphosphate glucuronyltransferase from rat liver

1. The bilirubin UDP-glucuronyltransferase assay described by Van Roy & Heirwegh (1968) has been improved. 2. Extraction of final azo-derivatives is rendered more simple and efficient by thorough emulsification and by cooling. 3. Pretreatment of homogenates and cell fractions with digitonin increases the sensitivity of the assays and gives less variable results than those with untreated preparations. The activation procedure is flexible. 4. Blank values (obtained from incubation mixtures from which activating bivalent metal ion and UDP-glucuronic acid were omitted) are low. No endogenous conjugate formation could be detected except with untreated, fresh liver homogenates. Control incubation mixtures containing the latter preparations are preferably kept at 0 degrees C. 5. With activated microsomal preparations, rates of breakdown of UDP-glucuronic acid (as monitored by release of P(i)) were low. Little if any increase in enzyme activity was found when UDP-N-acetylglucosamine was included in the incubation mixtures. 6. Slight deviation from Michaelis-Menten kinetics with respect to bilirubin observed at low substrate concentrations is probably related to the use of binding protein in the assay mixtures. Michaelis-Menten kinetics were followed with respect to UDP-glucuronic acid. Part of the enzyme in microsomal preparations from rat liver functioned independently of added bivalent metal ions. Mn(2+) was slightly more, and Ca(2+) somewhat less, stimulatory than Mg(2+). The Mg(2+)-dependent fraction showed Michaelis-Menten kinetics with respect to the added Mg(2+). 7. The enzyme activities found were higher than values reported in the literature for untreated or purified preparations from rat liver. They were above reported values of the maximal biliary excretion rate of bilirubin.

Enzymic transfer of glucose and xylose from uridine diphosphate glucose and uridine diphosphate xylose to bilirubin by untreated and digitonin-activated preparations from rat liver

1. Digitonin-treated and untreated homogenates, cell extracts and washed microsomal preparations from liver of Wistar R rats are capable of transferring sugar from UDP-glucose or UDP-xylose to bilirubin. No formation of bilirubin glycosides occurred with UDP-galactose or d-glucose, d-xylose or d-glucuronic acid as the sources of sugar. 2. Procedures to assay digitonin-activated and unactivated bilirubin UDP-glucosyltransferase and bilirubin UDP-xylosyltransferase were developed. 3. In digitonin-activated microsomal preparations the transferring enzymes had the following properties. Both enzyme activities were increased 2.5-fold by pretreatment with digitonin. They were optimum at pH6.6-7.2. Michaelis-Menten kinetics were followed with respect to UDP-glucose. In contrast, double-reciprocal plots of enzyme activity against the concentration of UDP-xylose showed two intersecting straight-line sections corresponding to concentration ranges where either bilirubin monoxyloside was formed (at low UDP-xylose concentrations) or where mixtures of both the mono- and di-xyloside were synthesized (at high UDP-xylose concentrations). Both enzyme activities were stimulated by Mg(2+); Ca(2+) was slightly less, and Mn(2+) slightly more, stimulatory than Mg(2+). Of the activities found in standard assay systems containing Mg(2+), 58-78% (substrate UDP-glucose) and 0-38% (substrate UDP-xylose) were independent of added bivalent metal ion. Double-reciprocal plots of the Mg(2+)-dependent activities against the concentration of added Mg(2+) were linear. 4. In comparative experiments the relative activities of liver homogenates obtained with UDP-glucuronic acid, UDP-glucose and UDP-xylose were 1:1.5:2.7 for untreated preparations and 1:0.29:0.44 after activation with digitonin. 5. Bilirubin UDP-glucuronyltransferase was protected against denaturation by human serum albumin, whereas bilirubin UDP-xylosyltransferase was not. 6. Digitonin-treated and untreated liver homogenates from Gunn rats were inactive in transferring sugar to bilirubin from UDP-glucuronic acid (in agreement with the work of others), UDP-glucose or UDP-xylose.

Structures of bilirubin conjugates synthesized in vitro from bilirubin and uridine diphosphate glucuronic acid, uridine diphosphate glucose or uridine diphosphate xylose by preparations from rat liver

1. In incubation mixtures containing digitonin-activated or untreated preparations from rat liver, albumin-solubilized bilirubin as the acceptor substrate and (a) UDP-glucuronic acid, (b) UDP-glucose or (c) UDP-xylose as the sugar donor, formation of the following ester glycosides was demonstrated: with (a), bilirubin beta-d-monoglucuronoside, with (b), bilirubin beta-d-monoglucoside and with (c), bilirubin monoxyloside or mixtures of the mono-and di-xyloside. 2. With UDP-glucuronic acid prolonged incubation and variation of the composition of the incubation mixtures yielded equimolar amounts of azodipyrrole (I) and azodipyrrole beta-d-monoglucuronoside (II) after treatment of the incubation mixtures with the diazonium salt of ethyl anthranilate. The azo-derivatives were identified by t.l.c. by reference to known compounds and by the following chemical tests. After ammonolysis the conjugated azo-derivative (II) yielded d-glucuronic acid and the carboxylic acid amide of azodipyrrole, indicating transfer of a glucuronic acid residue to the carboxylic acid groups of bilirubin. The beta-d-configuration of the sugar moiety and binding at C-1 were demonstrated by enzymic hydrolysis tests. 3. Analogous evidence established the structure of the reaction product obtained with UDP-glucose as the sugar donor, as bilirubin beta-d-monoglucoside. 4. With UDP-xylose as the sugar donor xylosyl transfer to the carboxylic acid groups of bilirubin with attachment at C-1 was demonstrated in an analogous way. A beta-d-configuration is considered very likely, but requires confirmation. 5. Monoxyloside formation was predominant at pH7.4, whereas at decreasing pH values increasing fractions of the substrate were converted into the dixyloside. Prolonged incubation, low concentrations of bilirubin and high concentrations of UDP-xylose favoured diconjugate formation. The available evidence supports the synthesis sequence: bilirubin --> bilirubin monoxyloside --> bilirubin dixyloside.

Bilirubin conjugates in bile of man and rat in the normal state and in liver disease

Conjugates of bilirubin were studied in normal bile of man and rat, and in bile of liver patients. In general human bile was obtained by duodenal intubation. In addition T-tube bile was examined in patients operated on for mechanical obstruction. The bile pigment compositions of duodenal and T-tube bile were similar in two patients where comparison was possible. Obstruction of the bile duct in rats was used as an animal model for obstructive jaundice. Diazotized ethyl anthranilate was used for determination of total conjugated bile pigment and for thin-layer chromatography (t.l.c.) analysis of the derived azopigments. The available t.l.c. procedures are versatile and allow rapid and quantitative analysis. A variety of conjugated azopigments can be distinguished. With chloroform, negligible amounts of unconjugated bilirubin are extracted from bile of man. Therefore, the percentage of monoconjugated bile pigments present in the initial bile sample can be calculated from the percentage of azodipyrrole found after diazotization. Normal bile from man and rat yields similar azopigment patterns. The dominant component is azopigment-delta (azodipyrrole beta-D-monoglucuronoside). Small amounts of azopigments with complex conjugating structures (gamma-azopigments) are present in both cases. Human bile further yields small amounts of azopigments containing xylose or glucose (called azopigments-alpha(2) and -alpha(3), respectively). Monoconjugated bilirubin (estimated from the percentage of azodipyrrole) amounts of 22% of total bile pigments in human bile and to 39% in murine bile. In both, the bulk of bile pigment is bilirubin diglucuronoside. From bile of patients with acquired liver diseases a new azopigment group (beta-azopigment) was derived. The gamma-azopigment group was increased; the delta-azopigment group (containing azodipyrrole beta-D-monoglucuronoside) was decreased. No differentiation was possible between intra- and extrahepatic cholestasis. The percentage of beta-azopigment showed a positive correlation with serum bilirubin concentration (r = 0.6). Recovery of the diseases was accompanied by normalization of the azopigment patterns. In rats, hydrostatic or mechanical obstruction induced increases in beta- and gamma-azopigments and a decrease in delta-azopigment similar to the changes observed in bile of liver patients. Complete normalization was obtained 6 hr after relieving the hydrostatic obstruction (duration 15-21 hr). In contrast, with man after surgery for extrahepatic obstruction, T-tube bile was not normalized when the T-tube was withdrawn (10 days after operation). Hydrostatic obstruction in rats provides an easy model when postobstructive bile pigment composition and parameters have to be investigated. The present investigations stress the importance of the physiopathological state when studying bilirubin conjugation. Hindrance to bile secretion induced heterogeneity of bilirubin conjugates and stimulated the formation of complex structures.

The formation of glucosides of isoflavones and of some other phenols by rabbit liver microsomal fractions

1. Rabbit liver microsomal fractions in vitro effected the transfer of glucuronic acid from UDP-glucuronic acid to biochanin A, formononetin, daidzein, genistein and equol. Only monoglucuronides were formed. 2. The same isoflavones were converted into monoglucosides when UDP-[6-(3)H]glucose was substituted for UDP-glucuronic acid in the incubation medium in vitro. The glucosides were formed in much lesser yield than were the glucuronides. 3. The glucoside of genistein was identified as genistin (genistein 7-glucoside) by Sephadex chromatography and reverse isotope dilution. 4. The specificity of the glucuronyl- and glucosyl-transfer mechanisms was compared for a series of steroids and other phenols in addition to the isoflavones. It was concluded that separate transferases were responsible for the formation of the two types of glycosides.

Mass-spectrometric structure elucidation of dog bile azopigments as the acyl glycosides of glucopyranose and xylopyranose

1. The structures of the alpha(2)- and alpha(3)-azopigments, prepared by diazotization of dog bile with ethyl anthranilate, were shown by mass spectrometry and g.l.c. to correspond to azobilirubin beta-d-xylopyranoside and azobilirubin beta-d-glucopyranoside respectively. 2. Both azopigments consist of a mixture of two methyl vinyl isomers having structures (IIIa) and (IIIb) for the alpha(2)-azopigment and structures (IVa) and (IVb) for the alpha(3)-azopigment. Separation of methyl vinyl isomers was obtained by t.l.c. or column chromatography performed on the acetylated azopigments. Hydrolysis of the less polar acetates derived from components (IIIa) and (IVa) gave rise to the azopigment (Ia), whereas hydrolysis of the more polar acetates derived from components (IIIb) and (IVb) gave rise to the azopigment acid (Ib). The positions of methyl and vinyl substituents in compounds (Ia) and (Ib) were assigned on the basis of their n.m.r. spectra. 3. Molecular ions in the mass spectra of the trimethylsilyl and acetyl derivatives of the azopigments indicated the presence of a pentose and a hexose conjugating sugar. 4. The ester functions linking the sugars to the propionic acid side chain of azobilirubin were demonstrated by ammonolysis and identification of the amide of azobilirubin as the aglycone derivative. 5. The sugar moieties were shown to occur as xylopyranose (alpha(2)) and glucopyranose (alpha(3)), bound at C-1, by application of a sequence of reactions performed on a micro-scale. The sugar hydroxyl groups were acetylated and the 1-acyl aglycone removed selectively by treatment with hydrogen bromide in acetic acid. Hydrolysis of the 1-bromo sugar acetates followed by acetylation afforded the alpha- and beta-xylopyranose tetra-acetates and alpha- and beta-glucopyranose penta-acetates, identified by a combination of g.l.c. and mass spectrometry. 6. The validity of this degradation scheme was confirmed (a) by g.l.c.-mass spectrometry identification of the alpha- and beta-1-propionyl derivatives of glucopyranose tetra-acetate, obtained from the alpha(3)-azopigment after final reaction with propionic anhydride; (b) by subjecting the acetates of alphabeta-glucopyranose, alphabeta-xylofuranose and alphabeta-glucofuranose to the same sequence of reactions.