Separation and characterization of bilirubin conjugates by high-performance liquid chromatography

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.

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.

High-performance liquid chromatography of bilirubin conjugates in bile: effect of beta-glucuronidase on the bile pigments

A simple, specific, and technically easy high-performance liquid chromatography (HPLC) method for the separation and quantification of unconjugated bilirubin, bilirubin monoglucoside-monoglucuronide, bilirubin diglucuronide, and bilirubin monoglucuronide has been developed. The method was used to determine the bilirubin compounds of bile obtained endoscopically from the common bile duct in 43 patients with gallstone disease and in 6 subjects without gallstones or liver disease. The bile samples were also assessed for the presence of beta-glucuronidase-producing bacteria. The amount of unconjugated bilirubin was significantly higher (p less than 0.01) in bile containing bacteria producing beta-glucuronidase than in bile without such bacterial strains. In six 'normal' bile samples the following quantities of bilirubin conjugates and unconjugated bilirubin were found (median and range): bilirubin monoglucoside-monoglucuronide (mixed conjugate), 61 (27-80) mumol/l; bilirubin diglucuronide, 632 (512-861) mumol/l; bilirubin monoglucuronide, 113 (70-175) mumol/l; and unconjugated bilirubin, 3 (1-7) mumol/l. These results are in good agreement with those obtained with other HPLC methods. The concentrations of unconjugated bilirubin were lower than those found when using conventional diazo methods and thin-layer chromatography. HPLC proved to be a useful tool in gallstone pathogenesis studies. Our results support bacterial glucuronidase as a possible pathogenic factor in pigment gallstone disease.

Reversed-phase high-performance liquid chromatography of conjugated and unconjugated bilirubins in body fluids

A novel high-performance liquid chromatography (HPLC) system is described for the separation of bilirubin and its conjugates in body fluids. Biliary bilirubin mono- and diglucuronide can be analysed directly on a C18 reversed-phase column with acetonitrile-dimethyl sulphoxide-0.1 M ammonium acetate (pH 5.16) (50:50:85, v/v/v) as mobile phase. However, the simultaneous determination of conjugated and unconjugated bilirubins in plasma required conversion of the conjugates into their methyl esters by alkaline methanolysis before HPLC separation of the C18 column eluted with acetonitrile-dimethyl sulphoxide-0.50 M ammonium acetate (pH 4.6) (50:50:40, v/v/v). The method is superior to, and more flexible than, previously described reversed-phase systems by allowing precise control of retention times by adjustment of pH, buffer concentration and the relative proportion of organic modifier in the mobile phase.

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

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

Hereditary chronic conjugated hyperbilirubinemia in mutant rats caused by defective hepatic anion transport

A mutant rat strain is described with autosomal recessive conjugated hyperbilirubinemia. Transport of conjugated bilirubin and tetrabromosulfophthalein from liver to bile is severely impaired whereas uptake of organic anions from plasma to liver is normal. During the first 10 days of life, serum bilirubin levels are 147 +/- 11 mumoles per liter with 68.7% diconjugates and 27.9% monoconjugates. In adult rats, serum bilirubin is 33 +/- 8 mumoles per liter with 81.8% diconjugates and 12.1% monoconjugates vs. 0.3 +/- 0.1 mumole per liter unconjugated bilirubin in normal adult rats. Bile acid metabolism is only mildly affected. In young rats, serum bile acid levels are normal. In adult rats, bile acid levels are elevated to 49 +/- 11 mumoles per liter vs. 10 +/- 6 mumoles per liter in normal rats. The bile flow in mutant rats is reduced to about 50%. This might be caused by a reduction of the bile acid-independent bile fraction. Liver marker enzyme activities in mutant rat serum are normal. Liver morphology is also normal. Total urinary coproporphyrin excretion is not elevated but urinary coproporphyrin isomer I excretion is increased, a pattern like that in Dubin-Johnson syndrome in humans. However, unlike Dubin-Johnson syndrome, the mutant rats do not have the characteristic black hepatic pigment. These rats provide a unique model to study mechanisms of bile formation and cholestasis.

Quantitation of bilirubin conjugates with high-performance liquid chromatography in patients with low total serum bilirubin levels

Bilirubin mono- and diconjugates were determined by alkaline methanolysis and high-performance liquid chromatography (HPLC) in serum from patients with metastatic liver disease and liver cirrhosis. Conjugates could be detected and quantitated at normal or low total bilirubin levels. Comparison with serum alkaline phosphatase activity revealed that in cirrhosis bilirubin conjugates were sometimes detectable at normal or slightly elevated alkaline phosphatase activities. In patients with metastatic liver disease alkaline phosphatase activity was a more sensitive indicator. In normal controls and in patients with Gilbert's syndrome no bilirubin conjugates were detected whereas serum of patients with haemolysis contained conjugated bilirubin. Therefore HPLC appears to be an excellent method to diagnose Gilbert's syndrome. In liver cirrhosis HPLC is a useful liver function test.

Properties and determination of serum and bilirubin

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

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.

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

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

beta-Glucuronidase-resistant bilirubin glucuronide isomers in cholestatic liver disease--determination of bilirubin metabolites in serum by means of high-pressure liquid chromatography

"Direct reacting bilirubin" in serum of patients with cholestatic liver disease and in serum of bile duct-ligated rats consists of a complex mixture of bilirubin metabolites. These metabolites were studied by means of high-pressure liquid chromatography. Bilirubin glucuronides in normal bile are beta-glycosidic 1-O-acyl conjugates which are completely hydrolyzed on incubation with beta-glucuronidase. Cholestatic serum contains glucuronide and non-glucuronide bilirubin metabolites. The glucuronides were only partially hydrolyzable with beta-glucuronidase. Compernolle et al. [11] showed that the 1-O-acyl bond of bilirubin glucuronides is labile and prone to migrate from the C1 position at the glucuronosyl residue to positions C2, C3 and C4. The isomerisation products are non-beta-glycosidic, beta-glucuronidase-resistant conjugates. The main beta-glucuronidase-resistant conjugates in cholestatic serum were characterized as: non-beta-glycosidic bilirubin monoglucuronide, non-beta-glycosidic diglucuronide and a diglucuronide isomer with beta-glycosidic and non-beta-glycosidic glucuronosyl groups. Moreover, a substantial amount of bilirubin monoglucoside monoglucuronide was detected in cholestatic human serum.