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

The origins and kinetics of bilirubin in dogs with hepatobiliary and haemolytic diseases

In 35 dogs with spontaneous hepatobiliary liver disease the kinetics and the sources of bilirubin were quantified. The disorders were extrahepatic bile duct obstruction (n = 4), fulminant hepatitis (n = 2), (sub)acute hepatitis (n = 5), chronic active hepatitis (CAH) with cirrhosis (n = 6), hepatic lymphosarcoma (n = 5), centrizonal necrosis secondary to haemolytic anaemia (n = 6) and other (n = 2). The plasma disappearance of [3H]bilirubin was analyzed with a two-compartment model in all dogs. The ratio early labeled/late labeled bilirubin was determined by measuring the incorporation of [14C]glycine into erythrocyte haem and faecal stercobilin. By introducing this relation in the model analysis the bilirubin production rates from erythrocyte destruction (PE), ineffective erythropoiesis (PI) and hepatic haemoprotein (PL) could be quantified. Total bilirubin turnover was increased in both primary haemolytic disease and most cases of hepatobiliary disease. Erythrocyte survival was reduced in all cases but one. The bilirubin clearance was impaired to 30-50% of the normal value in most cases of hepatobiliary disease and also in primary haemolysis. In dogs with fulminant hepatitis, and cirrhosis with or without CAH, the clearance rates were reduced to values below 15% of normal. In these dogs both an impaired clearance and an increased production were important determinants of hyperbilirubinaemia. In other cases plasma bilirubin was primarily determined by increased production. These clearances and production rates were similar in haemolysis and in many cases of primary hepatobiliary disease. The hepatic haemoprotein turnover was quite variable in all subgroups, ranging from 1-74% of the total bilirubin turnover.(ABSTRACT TRUNCATED AT 250 WORDS)

Subfractionation of serum bilirubins by alkaline methanolysis and thin-layer chromatography. An aid in the differential diagnosis of icteric diseases

The determination of direct and indirect-reacting bilirubin fractions by diazo procedures does not allow a definite diagnosis of icteric diseases. Therefore, the clinical relevance of serum bilirubin subfractionation by alkaline methanolysis and subsequent thin-layer chromatography (AM-TLC) was evaluated. Esterified bilirubins could be detected and quantitated in all serum samples investigated. The ratio of serum esterified to total bilirubin was 10-28% in 60 healthy adults (mean 17 +/- 5% S.D.), 1-11% in 77 patients with Gilbert's syndrome (mean 6 +/- 2%), and 2 and 3%, respectively, in two patients with Crigler-Najjar disease type II. The difference was highly significant (p less than 0.001) and the overlap was restricted to three of 139 individuals. The ratio of esterified to total bilirubin was similar to that obtained with HPLC when corrected for with a blank run. The absolute concentration of bilirubin esters in serum from Gilbert's syndrome patients was similar to that from healthy controls, but the unconjugated pigment was increased. In patients with chronic haemolysis (n = 9) and chronic persistent hepatitis (n = 12), the hyperbilirubinaemia consisted of a proportional increase of both unconjugated and esterified bilirubin. As such, the ratio of conjugated to total bilirubin was not significantly different from control values. Patients with acute hepatitis during the first (n = 18) and third ('remission') week of the disease (n = 15), liver cirrhosis (n = 34), and extrahepatic cholestasis (n = 20) predominantly showed an increase in bilirubin conjugates.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Changes in serum and bile bilirubin induced by acute hemolysis

The association between chronic hemolysis and pigment gallstones is well known but not adequately understood. To define the source and quantity of insoluble pigment resulting from hemolysis, hepatic bile in miniature swine was studied during the intravenous infusion of distilled water. Concentrations of both total bilirubin and insoluble unconjugated bilirubin rose rapidly during the first 2 hr to levels more than seven times above control values. In the third hour, unconjugated pigment continued to increase while total bilirubin plateaued, suggesting that glucuronidation or transport maxima had been reached. Elevations in unconjugated bilirubin could not be accounted for by the intrabiliary hydrolysis of conjugated bilirubin. It is concluded that hemolysis results in the secretion of large quantities of insoluble bilirubin directly into bile. This process may represent an important step in the development of pigment polymers or precipitates.

Analysis of bilirubins in biological fluids by extraction and thin-layer chromatography of the intact tetrapyrroles: application to bile of patients with Gilbert's syndrome, hemolysis, or cholelithiasis

A method was developed to extract quantitatively the bilirubins from bile, urine, serum, stool, and preparations from liver with a chloroform-ethanol mixture at pH 1.8 in the presence of ascorbic acid and NaCl. Extracted pigment was submitted to thin-layer chromatography, and the separated bilirubins were either immediately eluted and determined spectrophotometrically or individually converted to ethyl anthranilate azo derivatives for thin-layer chromatographic analysis of each isolated pigment band. Bilirubins in duodenal bile of eight healthy adults comprised 1.5 +/- 1.3% unconjugated bilirubin-IX alpha, 69 +/- 6% bilirubin diglucuronide, and 16 +/- 4% bilirubin monoglucuronides. Mixed diconjugates containing one glucuronosyl moiety and either one xylosyl or one glucosyl group amounted to 10 +/- 3%. Most samples (6 of 8) contained trace amounts (0.6 +/- 0.6%) of unconjugated bilirubin-IX beta, in agreement with nearly exclusive cleavage of heme at the alpha-meso position. The composition of the bilirubins in bile was normal in 6 patients with cholesterol gallstones, 4 with chronic hepatitis, and 3 with hemolysis. In duodenal bile of individuals with Gilbert's syndrome (n = 10), the concentration of bilirubin conjugates was comparable to that in healthy adults, but the proportion of bilirubin diglucuronides (52 +/- 8%) was decreased. The concentration of unconjugated bilirubin-IX alpha showed a fair positive correlation with that of bilirubin monoglucuronide and was increased in half of the patients with Gilbert's syndrome.

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

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

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

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

High-performance liquid chromatography of bile pigments: application to the determination of unconjugated bilirubin in serum

A rapid, sensitive and specific method for the determination of unconjugated bilirubin in serum is described. Reversed-phase chromatography on short-chain alkylsilica (C2) or octadecylsilica (C18) with acetonitrile--dimethylsulphoxide--water as eluent is used for the separation of bilirubin. The method is applied to the analysis of bilirubin in patients with disorders associated with jaundice.

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

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

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

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

The formation and distribution of bilirubin monoglucuronide and diglucuronide in rat liver slices

1. Bilirubin conjugation in rat liver slices was reassessed by using analysis of ethyl anthranilate azopigments to estimate separately the formation of bilirubin mono- and di-glucuronides. 2. Conjugation in slices resembles the situation in vivo more closely than does microsomal conjugation, in that diglucuronide is formed in appreciable quantity. 3. Both bilirubin mono- and di-glucuronides were present in slices in approximately equal amounts, but the monoglucuronide was the major product found in the incubation medium. 4. These results are discussed in relation to recent theories on the relationship between bilirubin mono- and di-glucuronide formation in vivo.