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

Hematological changes, oxidative stress assessment, and dysregulation of aquaporin-3 channel, prolactin, and oxytocin receptors in kidneys of lactating Wistar rats treated with monosodium glutamate

High consumption of locally produced delicacies could expose nursing mothers to high monosodium glutamate (MSG) levels, frequently used as a necessary condiment in low-income countries. Thus, this study evaluated some novel preliminary changes in renal hormonal receptors, the aquaporin-3 channel, oxidative stress markers, and hematological indices induced by monosodium glutamate in lactating rats. Post-parturition, twenty-four (24) lactating Wistar rats were divided into four (4) groups of six rats each (n = 6). Oral administration of distilled water and MSG started three (3) days postpartum as follows: group 1: distilled water (1 ml/kg BW), group 2: MSG (925 mg/kg BW), group 3: MSG (1850 mg/kg BW), and group 4: MSG (3700 mg/kg BW). At the end of the experiment, which lasted fourteen (14) days, animals were sacrificed and samples of blood and tissues were obtained for biochemical analysis. MSG administration significantly (p < 0.05) increased ROS and MDA, with a significant (p < 0.05) decrease in kidney antioxidants. Serum creatinine, total, conjugated, and unconjugated bilirubin significantly (p < 0.05) increased with MSG administration. The prolactin receptor was significantly reduced (p < 0.05), while the oxytocin receptor and aquaporin-3 channel were significantly (p < 0.05) increased in the MSG-administered groups. There were significant (p < 0.05) changes in the hematological indices of the MSG-administered animals. Thus, the findings of this study suggest that high MSG consumption causes hematological alterations and may alter renal function via increased ROS production and dysregulation of the AQP-3 channel, prolactin, and oxytocin receptors in the kidneys of lactating Wistar rats.

Evaluation of a Point-of-Care Test for Bilirubin in Malawi

OBJECTIVES

BiliSpec is a low-cost spectrophotometric reader and disposable paper-based strip to quantify total serum bilirubin from several blood drops. This study was a prospective evaluation of BiliSpec in 2 neonatal wards in Malawi compared with a reference standard bilirubinometer over a large range of bilirubin and hematocrit levels.

METHODS

The accuracy of BiliSpec and a transcutaneous bilirubinometer were compared with the reference standard of spectrophotometry for 475 blood samples collected from 375 subjects across a range of total serum bilirubin concentrations from 0.0 to 33.7 mg/dL. The development of error grids to assess the clinical effects of measurement differences is reported.

RESULTS

BiliSpec was found to have a mean bias of -0.48 mg/dL and 95% limits of agreement of -5.09 mg/dL to +4.12 mg/dL. Results show 90.7% of BiliSpec measurements would have resulted in the same clinical decision as the reference standard, whereas 55.0% of transcutaneous bilirubin measurements would have resulted in the same clinical decision as the reference standard.

CONCLUSIONS

This evaluation supports use of BiliSpec to provide accurate, low-cost, point-of-care bilirubin measurements in low-resource hospitals. Future work is needed to evaluate BiliSpec among a larger number of users.

Heme oxygenase-1 induction by hemin prevents oxidative stress-induced acute cholestasis in the rat

We previously demonstrated in in vitro and ex vivo models that physiological concentrations of unconjugated bilirubin (BR) prevent oxidative stress (OS)-induced hepatocanalicular dysfunction and cholestasis. Here, we aimed to ascertain, in the whole rat, whether a similar cholestatic OS injury can be counteracted by heme oxygenase-1 (HO-1) induction that consequently elevates endogenous BR levels. This was achieved through the administration of hemin, an inducer of HO-1, the rate-limiting step in BR generation. We found that BR peaked between 6 and 8 h after hemin administration. During this time period, HO-1 induction fully prevented the pro-oxidant tert-butylhydroperoxide (tBuOOH)-induced drop in bile flow, and in the biliary excretion of bile salts and glutathione, the two main driving forces of bile flow; this was associated with preservation of the membrane localization of their respective canalicular transporters, bile salt export pump (Bsep) and multidrug resistance-associated protein 2 (Mrp2), which are otherwise endocytosed by OS. HO-1 induction counteracted the oxidation of intracellular proteins and membrane lipids induced by tBuOOH, and fully prevented the increase in the oxidized-to-total glutathione (GSHt) ratio, a sensitive parameter of hepatocellular OS. Compensatory elevations of the activity of the antioxidant enzymes catalase (CAT) and superoxide dismutase (SOD) were also prevented. We conclude that in vivo HO-1 induction protects the liver from acute oxidative injury, thus preventing consequent cholestasis. This reveals an important role for the induction of HO-1 and the consequently elevated levels of BR in preserving biliary secretory function under OS conditions, thus representing a novel therapeutic tool to limit the cholestatic injury that bears an oxidative background.

Biochemical diagnosis of liver disease

It is important that clinicians and laboratorians, including clinical chemists and pathologists, recognize and understand the clinical significance of abnormal liver function tests. The liver regulates many important metabolic functions. Hepatic injury is associated with distortion of these metabolic functions. Hepatic disease can be evaluated and diagnosed by determining serum concentrations of a number of serum analytes. Many serum analytes exist to assist in the biochemical diagnosis of liver disease. The focus of this paper is on the analytes which are associated with hepatic necrosis, cholestasis, defects in excretion and end stage hepatic disease which results in decreased synthetic function. The abnormalities of these serum analytes will be correlated with the important types of liver disease.

Bilirubin in clinical practice: a review

Bilirubin is an endogenous compound that can be toxic under certain conditions but, on the other hand, mild unconjugated hyperbilirubinaemia might protect against cardiovascular diseases and tumour development. Serum bilirubin levels are often enhanced under a variety of clinical conditions. These are discussed and the mechanisms are outlined.

Protective role of unconjugated bilirubin on complement-mediated hepatocytolysis

Hyperbilirubinemia and complement-mediated immune attack on hepatocyte membrane are common features of certain hepatic diseases. To assess whether unconjugated bilirubin (UB) counteracts complement-mediated hepatocytolysis, we first generated a rabbit polyclonal antibody (Ab) against rat hepatocyte plasma membrane (RHPM). An assay performed with isolated rat hepatocytes in the presence of the polyclonal Ab and rat serum as complement donor demonstrated that UB inhibits cell lysis, as lactate dehydrogenase release into the medium was inhibited by the pigment in a dose-dependent manner. Immunofluorescence microscopy studies showed that UB significantly attenuates the binding of C3 to the hepatocyte-Ab complex. Further enzyme immunoassay studies showed that UB interferes the binding of C1q to purified anti-RHPM IgG, also in a dose-dependent manner. A dot-blot assay showed that [14C]-UB binds to C1q and human serum albumin (HSA) to a similar extent. A differential spectrum analysis of UB in the presence of C1q further confirmed that the pigment interacts with this protein. In conclusion, we demonstrated an inhibitory action of UB on complement-mediated Ab-induced hepatocytolysis, this action being evidenced at pathophysiological pigment concentrations (171 microM and higher). A direct binding of the pigment to C1q is likely involved.

24-norUrsodeoxycholic acid is superior to ursodeoxycholic acid in the treatment of sclerosing cholangitis in Mdr2 (Abcb4) knockout mice

BACKGROUND & AIMS

Current therapy for primary sclerosing cholangitis is of limited efficacy. Multidrug resistance gene 2 knockout mice (Mdr2(-/-)) represent a well-characterized model for sclerosing cholangitis. Experiments were performed to test in such mice the therapeutic effects of 24-norUrsodeoxycholic acid, a C(23) homologue of ursodeoxycholic acid with 1 fewer methylene group in its side chain.

METHODS

Mdr2(-/-) mice were fed a diet containing 24-norUrsodeoxycholic acid (0.5% wt/wt) or ursodeoxycholic acid (0.5% wt/wt) as a clinical comparator for 4 weeks; controls received standard chow. Effects on serum liver tests, liver histology, markers of inflammation and fibrosis, and bile acid transport and metabolism were compared. 24-norUrsodeoxycholic acid metabolism was studied in serum, liver, bile, and urine.

RESULTS

24-norUrsodeoxycholic acid markedly improved liver tests and liver histology and significantly reduced hydroxyproline content and the number of infiltrating neutrophils and proliferating hepatocytes and cholangiocytes. 24-norUrsodeoxycholic acid underwent extensive phase I/II metabolism (hydroxylation, sulfation, and glucuronidation), thereby increasing the hydrophilicity of biliary bile acid secretion. There was a coordinated induction of bile acid detoxifying enzymes (Cyp2b10, Cyp3a11, and Sult2a1) and efflux pumps (Mrp3 and Mrp4). Ursodeoxycholic acid, in contrast, increased alanine transaminase and alkaline phosphatase levels, had no significant effects on hydroxyproline content, and induced biliary transporters and detoxification enzymes to a much smaller extent than 24-norUrsodeoxycholic acid.

CONCLUSIONS

24-norUrsodeoxycholic acid ameliorates sclerosing cholangitis in Mdr2(-/-) mice. Its therapeutic mechanisms involve (1) increasing the hydrophilicity of biliary bile acids, (2) stimulating bile flow with flushing of injured bile ducts, and (3) inducing detoxification and elimination routes for bile acids.

Excretion of fetal biliverdin by the rat placenta-maternal liver tandem

Fetal liver immaturity is accompanied by active heme catabolism. Thus fetal biliary pigments must be excreted toward the mother by the placenta. To investigate biliverdin handling by the placenta-maternal liver tandem, biliverdin-IXalpha was administered to 21-day pregnant rats through the jugular vein or the umbilical artery of an in situ perfused placenta. Jugular administration resulted in the secretion into maternal bile of both bilirubin and biliverdin (3:1). However, when biliverdin was administered to the placenta, most of it was transformed into bilirubin before being transferred to the maternal blood. Injecting Xenopus laevis oocytes with mRNA from rat liver or placenta enhanced their ability to take up biliverdin, which was inhibited by estradiol 17beta-d-glucuronide. The expression of three OATP isoforms in this system revealed that they have a varying degrees of ability to transport biliverdin (Oatp1/1a1 > Oatp2/1a4 > Oatp4/1b2). The abundance of their mRNA in rat trophoblast was Oatp1/1a1 >> Oatp4/1b2 > Oatp2/1a4. The expression of biliverdin-IXalpha reductase in rat placenta was detected by RT-PCR/sequencing and Western blot analysis. The relative abundance of biliverdin-IXalpha reductase mRNA (determined by real-time quantitative RT-PCR) was fetal liver > placenta > maternal liver. Common bile duct ligation in the last week of pregnancy induced an upregulation of biliverdin-IXalpha reductase in maternal liver but had no effect on fetal liver and placenta. In conclusion, several members of the OATP family may contribute to the uptake of fetal biliverdin by the rat placenta. Before being transferred to the mother, biliverdin is extensively converted into bilirubin by biliverdin-IXalpha reductase, whose expression is maintained even though bilirubin excretion into maternal bile is impaired.

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

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

Study of hepatic function matching between banna minipig inbred and humans

As a unique inbred pig Banna minipig inbred (BMI) is potentially suitable for pig-to-human xenotransplantation due to its clear genetic background and minor interindividual differences. Previous studies of BMI have focused on immunological barriers between BMI and humans. However, a comparison of liver function between donor animals and humans is an essential premise for successful xenotransplantation. In this study, we investigated routine hepatic functions, protein electrophoresis, and drug metabolism to compare capacity of liver synthesis, metabolism, and drainage between BMI and humans. The results showed no significant differences in the concentrations of albumin and globulin synthesized in the liver (alpha1, alpha2, and beta-globulin). Serum enzyme activities in BMI were higher than those in humans, and levels of total bilirubin and direct-reacting bilirubin of BMI were lower than those of humans. In BMI, the clearance of antipyrine, a widely used model drug to study hepatic drug metabolism, was 16 times greater than that by humans, with a mean residual time of antipyrine in BMI, one-tenth of that in human. These findings suggested that BMI livers are similar to humans in albumin and alpha, beta-globulin synthesis, but stronger in bilirubin elimination, enzyme activity, and drug metabolism. BMI livers may have stronger functions compared with those of humans. No incompatibility was identified in hepatic function between BMI and humans.

Reversible cholestasis induced by experimental partial obstruction of the bile duct

The aim of this investigation was to reproduce in rats a partial stenosis of the common bile duct to analyze early liver functional and morphometric changes. The hepatic transport kinetics of sulfobromophthalein (organic anion) and rhodamine B (organic cation) was also investigated, and compartmental analysis of both compounds was performed. The humoral parameters of liver function indicated a cholestasis after 2 days of surgery, which reverted to reach normal values on the seventh day. Tumor necrosis factor alpha serum levels showed a tendency to increase on the second day of stenosis (7 out of 14 rats) while white blood cells increased on the second day of stenosis, and turned to normal levels on the seventh day. Histological studies showed increased volume of portal areas and ductular proliferation, which did no revert during the time of the study (up to 7 days post-op). Conversely, a moderate fibrosis and leukocyte infiltrates in portal areas predominated on the second day of stenosis, but normalized on the seventh day. Bile flow was considerably diminished on the second day of partial obstruction as compared to controls. The mean recovery in bile of sulfobromophthalein after 1h of being injected was low on the second day of stenosis, but normalized on the seventh day. Conversely, that of rhodamine B was very low in all animals. Sulfobromophthalein kinetics showed that hepatic uptake and canalicular excretion were impaired during the second but normalized on the seventh day of stenosis. However, rhodamine B kinetics showed that this compound was poorly excreted in all groups although canalicular excretion increased on the second day. The results suggested a model of obstructive cholestasis induced by the experimental stenosis of the bile duct which was not only reversible but also implicates the role of hepatic inflammation.

Unconjugated bilirubin in human bile: the nucleating factor in cholesterol cholelithiasis?

AIMS

To investigate the concentrations of bilirubin, bilirubin conjugates, phospholipid, and cholesterol in the gall bladder bile obtained at surgery from patients with and without cholesterol gallstones.

METHODS

Gall bladder bile was collected during surgery, by puncture, from 20 patients with gallstones undergoing routine cholecystectomy and from eight patients with normal liver blood tests. Concentrations of bilirubin, bilirubin conjugates, phospholipid, and cholesterol were measured using standard procedures.

RESULTS

The proportion of total bilirubin that was unconjugated was significantly higher in the bile from patients with stones than in bile from control patients, whether or not the bile from either group was saturated with cholesterol or not. Indeed, the mean concentration of cholesterol was significantly higher in control bile samples.

CONCLUSION

The presence of stones was more closely related to the proportion of unconjugated bilirubin than to the degree of saturation of bile with cholesterol. Bilirubin and its metabolites probably play an important part in the formation of cholesterol gallstones.

Excretion of foetal bilirubin by the rat placenta-maternal liver tandem

Using plasma membrane vesicles from human trophoblast, carrier-mediated transport of unconjugated bilirubin (UCBR) has been reported. In the present work, using the in situ perfused rat placenta-maternal liver tandem, the relevance of this pathway in vivo was investigated. After single-pass perfusion of rat placenta through the umbilical artery with 0.25 micromol [(3)H]-UCBR, approximately 15 per cent of it was taken up by the placenta, detected in maternal serum (>96 per cent was unconjugated) and subsequently secreted into maternal bile (approximately 15 per cent of administered dose; >88 per cent was glucuronidated bilirubin). Co-administration through the umbilical artery of 0.25 micromol [(3)H]-UCBR and 2.5 micromol unlabelled UCBR, bromosulfophthalein, cholic acid or biliverdin IXalpha, reduced [(3)H]-UCBR placenta uptake, and the amount of radioactivity found in the maternal serum and bile. Co-administration into maternal jugular vein of 0.1 micromol [(3)H]-UCBR-a dose 3-fold higher than that reaching the maternal compartment in placenta perfusion experiments-and 1.0 micromol bromosulfophthalein, cholic acid or biliverdin IXalpha, resulted in no marked inhibition of the amount of radioactivity bile output. When antipyrine and [(3)H]-UCBR were continuously co-infused to the mother, similar antipyrine concentrations in maternal and foetal serum were reached in approximately 15 min, while progressive increase in [(3)H]-bilirubin concentrations in maternal serum above 70 microM was accompanied by a very low transfer of this compound into foetal compartment where [(3)H]-bilirubin concentrations were always <10 microM. These results suggest that the transfer of UCBR across the rat placenta occurs, without biotransformation, via a foetal-to-maternal mainly unidirectional pathway that can be cis-inhibited by UCBR and other cholephilic organic anions.

Acute sodium arsenite treatment induces Cyp2a5 but not Cyp1a1 in the C57Bl/6 mouse in a tissue (kidney) selective manner

Modulation of hepatic and extrahepatic detoxication enzymes Cyp1a1, Cyp2a5, glutathione S-transferse Ya (GSTYa) and NAD(P)H:quinone oxidoreductase (QOR) dependent catalytic activity and mRNA levels were investigated at 1, 2, or 4 days in liver, lung, or kidney of male, adult CD57 Bl/6 mice treated sc with a single dose (85 micromol/kg) of sodium arsenite (As3+). Maximum decreases of total hepatic cytochrome P450 (CYP) monooxygenase content and catalytic activities, occurring at 24 h, corresponded with maximum increases of heme oxygenase (HO-1) in all tissues, as well as maximum plasma total bilirubin. Extrahepatic increases in CYP were observed only in non-AHR dependent isozymes in the kidney, where both Cyp2a5 mRNA and catalytic activity increased maximally 24 h after treatment. In contrast, no significant changes in Cyp2b1/2-dependent PROD or mRNA activity and decreases in Cyp1a1-dependent-EROD activity were noted 1, 2, or 4 days after treatment. Increases in QOR catalytic activities were observed in all tissues examined with increased mRNA in kidney. On the other hand, GSTYa catalytic activity and mRNA increases were only detected in kidney. This study demonstrates the differential modulation of CYP, QOR, and GST-Ya, important drug metabolizing enzymes after acute As3+ administration. The induction of Cyp2a5, QOR, and GSTYa catalytic activity and gene expression occurred primarily in kidney during or shortly after conditions of oxidant stress.

Zinc sulfate inhibits the enterohepatic cycling of unconjugated bilirubin in subjects with Gilbert's syndrome

We have previously observed that UCB binds to ZnSO4 in vitro, and suppressed the biliary bilirubin secretion in the hamster. The aim of this study was designed to investigate whether Zn salts might inhibit the enterohepatic cycling of UCB in subjects with Gilbert s syndrome. Fifteen patients with Gilbert s syndrome and 5 normal healthy volunteers were included in this study according to the following criteria: fasting hyperbilirubinemia, no hemolysis, and free of any medication. Patients were randomly assigned to receive acute o chronic treatment. Subjects treated in acute form and normal healthy volunteers were treated with 40 mg of ZnSO4 in a single dose, where as patients treated in chronic form received 100 mg ZnSO4 in a single dose daily for 7 days. The serum UCB levels (mg/dL) decreased from 2.64 +/- 1.04 to 2.02 +/- 0.87 (p < 0.001) and 1.8 +/- 0.36 to 1.48 +/- 0.32 (p < 0.005) in subjects treated in acute an chronic form respectively, but not in the control group. Whereas, the serum Zn levels (mg/dL) increased from 96.3 +/- 16.8 to 118.8 +/- 19. 5, (p < 0.01) and from 117.6 +/- 8.5 to 130.7 +/- 6.6 (p < 0.03) in subjects treated in acute an chronic form and also in subjects in the control group (98.0 +/- 7.3 to 128.0 +/- 21.9) p < 0.03. This study showed that acute and chronic oral administration of ZnSO4 decreased serum UCB levels significantly in subjects with Gilbert s syndrome. Most likely by the inhibition of the "normal" enterohepatic cycling of UCB.

Serum hepatic biochemical activity in two populations of workers exposed to styrene

OBJECTIVE

To determine whether hepatic biochemical changes, as measured by routinely available tests indicative of hepatocellular necrosis, cholestasis, or altered hepatic clearance of bilirubin, occur in association with low to moderate exposure to styrene commonly experienced in industrial production.

METHODS

Two independent cross sectional studies were performed comparing serum hepatic transaminases (alanine aminotransferase (ALT) and aspartate aminotransferase (AST)), cholestatic enzymes (alkaline phosphatase (AP) and gamma glutamyl transpeptidase (GGT)), and bilirubin in (a) 47 workers of fibreglass reinforced plastics who were exposed to styrene and (b) 21 boat and tank fabricators, with separate referent groups of unexposed workers. Exposure to styrene was assessed in air by dosimetry, and in venous blood by headspace analysis. Hepatic biochemical variables were assessed across strata of exposure to styrene defined as 25 ppm in air, or 0.275 mg/l in blood, adjusting for age, sex, body mass index, and ethanol consumption.

RESULTS

A consistent and significant linear trend for increasing direct bilirubin and direct/total bilirubin ratio was found in association with increasing exposure to styrene, by both air and blood monitoring, in both studies. Mean direct bilirubin concentrations increased from 0.05-0.08 mg% in referents to 0.12-0.19 in workers exposed above 25 ppm, with a significant exposure-response trend (p<0.005). Significantly increased direct/total bilirubin ratios, ranging from 0.22 to 0.35 were associated with exposure to styrene (p<0.001), indicating diminished hepatic clearance of conjugated bilirubin. Also, a significant linear association between the hepatic transaminases ALT and AST and exposure to styrene was found in pooled regression analyses, with an increase in AP of about 10 IU/ml in workers exposed above 25 ppm air or 0.275 mg/l blood styrene in pooled analyses from both studies.

CONCLUSIONS

The consistent finding of increased direct bilirubin and AP concentrations in these two independent studies provides evidence for diminished hepatic clearance of conjugated bilirubin with associated cholestasis in workers exposed to styrene. The finding of a significant linear association between hepatic transaminase concentrations and exposure to styrene in pooled analyses is consistent with mild hepatic injury and associated metabolic dysfunction.

Intestinal colonization leading to fecal urobilinoid excretion may play a role in the pathogenesis of neonatal jaundice

BACKGROUND

Neonatal hyperbilirubinemia remains of concern because of the potential danger for the central nervous system. Because urobilinogen is a nontoxic derivative of bilirubin, the current study was conducted to examine the fecal excretion of urobilinoids and bilirubin in healthy newborns and infants, as well as their intestinal bacteria capable of reducing bilirubin, to assess a possible relation to serum bilirubin levels during the first weeks of life.

METHODS

Bilirubin pigments, urobilinoids, and porphyrins were measured in stools of infants during the first week (group A, n = 60) and between the second week and the first 6 months of life (group B, n = 64). Microbiologic analysis of stools was performed in selected cases and bilirubin-converting activity of isolated bacteria was determined in vitro.

RESULTS

Urobilinoids were detectable in stools of 57% of the neonates at day 5, but not before. However, fecal urobilinoid production on that day was only a fraction of that observed in adults (0.07 vs. 0.7-3.6 mg/kg per day), whereas at week 6 it increased significantly to an average of 0.9 mg/kg per day. Microbiologic analysis of neonatal stools revealed two novel bacterial strains of Clostridium perfringens and Clostridium difficile capable of reducing bilirubin to urobilinoids.

CONCLUSIONS

Urobilinoids can be detected in stools of 57% of newborns at day 5 after delivery. However, the urobilinoid production during the first week of life is quantitatively insufficient to contribute significantly to the removal of bilirubin. Enhancement of the microbial conversion of bilirubin could decrease the intestinal concentration of bilirubin and may decrease the degree or enhance the removal of neonatal hyperbilirubinemia.

Inhibitory effect of bilirubin on complement-mediated hemolysis

We investigated the in vitro action of the bile pigments, unconjugated bilirubin (UB) and bilirubin monoglucuronide (BMG) on complement (C) cascade reaction. Both UB and BMG inhibited hemolysis in the classical pathway (CP) in a dose-dependent manner at low micromolar concentrations, UB showing a stronger effect than BMG. The analysis of the action of UB on the hemolytic activity of the C1, C4, C2 and C-EDTA components of the C cascade revealed that the C1 step was the most inhibited. An enzyme immunoassay was developed to evaluate the effect of UB on the binding of C1q, one of the subcomponents of C1, to human IgM and IgG. The study demonstrated that the unconjugated pigment interferes both the C1q-IgM and -IgG interactions, thus tentatively explaining the inhibitory action of UB on hemolytic activity of C1. We conclude that the anti-complement effect of UB is mainly exerted on the C1 component, the recognition unit of CP. The potential clinical implication of the reported effects in hyperbilirubinemia is discussed.

Correction of the UDP-glucuronosyltransferase gene defect in the gunn rat model of crigler-najjar syndrome type I with a chimeric oligonucleotide

Crigler-Najjar syndrome type I is characterized by unconjugated hyperbilirubinemia resulting from an autosomal recessive inherited deficiency of hepatic UDP-glucuronosyltransferase (UGT) 1A1 activity. The enzyme is essential for glucuronidation and biliary excretion of bilirubin, and its absence can be fatal. The Gunn rat is an excellent animal model of this disease, exhibiting a single guanosine (G) base deletion within the UGT1A1 gene. The defect results in a frameshift and a premature stop codon, absence of enzyme activity, and hyperbilirubinemia. Here, we show permanent correction of the UGT1A1 genetic defect in Gunn rat liver with site-specific replacement of the absent G residue at nucleotide 1206 by using an RNA/DNA oligonucleotide designed to promote endogenous repair of genomic DNA. The chimeric oligonucleotide was either complexed with polyethylenimine or encapsulated in anionic liposomes, administered i.v., and targeted to the hepatocyte via the asialoglycoprotein receptor. G insertion was determined by PCR amplification, colony lift hybridizations, restriction endonuclease digestion, and DNA sequencing, and confirmed by genomic Southern blot analysis. DNA repair was specific, efficient, stable throughout the 6-month observation period, and associated with reduction of serum bilirubin levels. Our results indicate that correction of the UGT1A1 genetic lesion in the Gunn rat restores enzyme expression and bilirubin conjugating activity, with consequent improvement in the metabolic abnormality.

Asymptomatic jaundice after fasting: a diagnostic dilemma

An 18-year-old female with asymptomatic jaundice presented to the emergency department after fasting. She was referred to the regional medical center for evaluation and treatment. The diagnosis of Gilbert's syndrome was made by fractionation of serum unconjugated and conjugated bilirubin fraction by alkaline methanolysis, followed by thin-layer chromatography and analysis of fasting-state levels of cholyl conjugated bile acids. Methods for diagnosing this disorder are discussed.

ATP-dependent transport of bilirubin glucuronides by the multidrug resistance protein MRP1 and its hepatocyte canalicular isoform MRP2

Bilirubin is secreted from the liver into bile mainly as monoglucuronosyl and bisglucuronosyl conjugates. We demonstrate for the first time that ATP-dependent transport of both bilirubin glucuronides is mediated by the multidrug resistance protein (MRP1) as well as by the distinct canalicular (apical) isoform MRP2, also termed cMRP or cMOAT (canalicular multispecific organic anion transporter). In membrane vesicles from MRP1-transfected HeLa cells mono[3H]glucuronosylbilirubin and bis[3H]glucuronosylbilirubin (each at 0.5 microM) were transported with rates of 5.3 and 3.1 pmol/min per mg of protein respectively. Rat hepatocyte canalicular membrane vesicles, which contain Mrp2 (the rat equivalent of MRP2), transported mono[3H]glucuronosylbilirubin and bis[3H]glucuronosylbilirubin at rates of 8.9 and 8.5 pmol/min per mg of protein, whereas membrane vesicles from mutant liver lacking Mrp2 showed no transport of the conjugates. In membrane vesicles from human hepatoma Hep G2 cells, which predominantly expressed MRP2, transport rates were 8.3 and 4.4 pmol/min per mg of protein for monoglucuronosylbilirubin and bisglucuronosylbilirubin respectively. ATP-dependent transport of the glutathione S-conjugate -3H-leukotriene C4, an established high-affinity substrate for MRP1 and MRP2, was inhibited by both bilirubin glucuronides with IC50 values between 0.10 and 0.75 microM. The ratios of leukotriene C4 transport and bilirubin glucuronide transport, determined in the same membrane vesicle preparation, indicated substrate specificity differences between MRP1 and MRP2 with a preference of MRP2 for the glucuronides.

Transepithelial fate of bilirubin in the isolated rat kidney

The renal handling of bilirubin in the rat was studied using an isolated kidney preparation by means of the determination of total pigment concentration decay in the perfusion medium and its renal clearance. Unconjugated bilirubin was incorporated in the perfusate at a concentration of about 4 microg/ml. In order to establish the potential role of secretion in renal handling of the pigment, experiments were also performed incorporating in the perfusate different doses of nicotinic acid (NA) (0.1 and 1.0 mM final concentration), which is considered an alternative substrate for the organic anion transport system, or probenecid (Prob) (0.1 and 1.0 mM final concentration), the classical inhibitor of organic anion transport process. The magnitude of pigment uptake from the perfusion medium, estimated by a first order exponential decay constant, was decreased in a dose-dependent way by NA (40 and 76% decrease for 0.1 and 1.0 mM of NA, respectively) and Prob (57 and 88% decrease for 0.1 and 1.0 mM of Prob, respectively). NA and Prob also induced a diminution in the ratio of pigment renal clearance to glomerular filtration rate (24 and 48% decrease for 0.1 and 1.0 mM of NA and 52 and 55% decrease for 0.1 and 1.0 mM of Prob). Based on these findings, it can be proposed that tubular secretion through the proximal cells contributes significantly to renal pigment depuration. In order to establish the possible contribution of cellular metabolism to the secretory process, a different set of experiments was conducted. The content of bilirubin mono and diconjugates (BMC and BDC) were determined in urine, in arterial and venous samples and in renal cortex. Studies performed using either an open or a closed circulating system, revealed that after conjugation in the renal cell, pigment derivatives can be secreted into both the tubule and the venous compartments. Total bilirubin concentration as well as the relative content of BMC and BDC in urine increased over time, representing the sum of both conjugates about 50% of the total pigment excreted by the end of experiments. Consequently, our results support the existence of a tubular transepithelial transport of bilirubin, playing the metabolism of the pigment an important role in this process.

The regulation of heme turnover and carbon monoxide biosynthesis in cultured primary rat olfactory receptor neurons

Heme oxygenase (HO) converts heme to carbon monoxide (CO) and biliverdin, which is metabolized rapidly to bilirubin. CO is implicated as an intercellular messenger, whereas bilirubin could function as an antioxidant. These cellular functions differ significantly from those of HO in peripheral tissues, in which it degrades heme from senescent erythrocytes, suggesting that the regulation of HO may differ in neurons from that in other tissues. Among neurons, olfactory receptor neurons have the highest level of HO activity. Metabolic labeling with [2-14C]glycine or delta-[3H]aminolevulinic acid ([3H]ALA) was used to investigate heme metabolic turnover and CO biosynthesis in primary cultures of olfactory receptor neurons. The production rates of heme precursors and metabolites from [14C]glycine over 6 hr were (in pmol/mg protein): 100 for ALA, 8.2 for heme, and 2.9 for CO. Taking into account endogenous heme content, the amount of total CO production was determined to be 1.6 nmol/mg protein per 6 hr. Heme biosynthesis usually is subject to end-product negative feedback at the level of ALA synthase. However, metabolic control in these neurons is different. Both heme concentration (heme formation) and HO activity (heme degradation) were enhanced significantly during immature stage of neuronal differentiation in culture. Neuronal maturation, which is accelerated by transforming growth factor-beta 2 (TGF-beta 2), suppressed the activities of both heme biosynthesis and degradation. To explore the physiological importance of this endogenous production of CO, we examined the potency of CO as a soluble guanylyl cyclase activator. Exogenous CO (10-30 microM), comparable to endogenous CO production, significantly activated guanylyl cyclase, suggesting that HO activity may regulate cGMP levels in the nervous system.

The regulation of heme turnover and carbon monoxide biosynthesis in cultured primary rat olfactory receptor neurons

Heme oxygenase (HO) converts heme to carbon monoxide (CO) and biliverdin, which is metabolized rapidly to bilirubin. CO is implicated as an intercellular messenger, whereas bilirubin could function as an antioxidant. These cellular functions differ significantly from those of HO in peripheral tissues, in which it degrades heme from senescent erythrocytes, suggesting that the regulation of HO may differ in neurons from that in other tissues. Among neurons, olfactory receptor neurons have the highest level of HO activity. Metabolic labeling with [2-14C]glycine or delta-[3H]aminolevulinic acid ([3H]ALA) was used to investigate heme metabolic turnover and CO biosynthesis in primary cultures of olfactory receptor neurons. The production rates of heme precursors and metabolites from [14C]glycine over 6 hr were (in pmol/mg protein): 100 for ALA, 8.2 for heme, and 2.9 for CO. Taking into account endogenous heme content, the amount of total CO production was determined to be 1.6 nmol/mg protein per 6 hr. Heme biosynthesis usually is subject to end-product negative feedback at the level of ALA synthase. However, metabolic control in these neurons is different. Both heme concentration (heme formation) and HO activity (heme degradation) were enhanced significantly during immature stage of neuronal differentiation in culture. Neuronal maturation, which is accelerated by transforming growth factor-beta 2 (TGF-beta 2), suppressed the activities of both heme biosynthesis and degradation. To explore the physiological importance of this endogenous production of CO, we examined the potency of CO as a soluble guanylyl cyclase activator. Exogenous CO (10-30 microM), comparable to endogenous CO production, significantly activated guanylyl cyclase, suggesting that HO activity may regulate cGMP levels in the nervous system.

Identification of bilirubin UDP-GTs in the human alimentary tract in accordance with the gut as a putative metabolic organ

The initial identification of traditionally hepatic enzymes expressed in the gut has led to the hypothesis that the gut may function as a metabolic organ. The UDP glucuronosyltransferases (UDP-GTs) play an important role as phase II metabolizing enzymes. Previously members of this family have been identified in the gut by non-isoform specific immunoreactivity, and a small amount of bilirubin glucuronosyltransferase activity was detected in the colon. Recent reports of gut transplantation to reverse the metabolic defect in Gunn rats raised further interest in the expression and distribution of human bilirubin (UDP-GTs (HUG Br 1 and HUG Br 2) in the human alimentary tract. The availability of molecular genetic probes for HUG Br 1 and HUG Br 2 permits the screening of the alimentary tract for the presence of isoform specific message. RNA samples extracted from pinch biopsy specimens of buccal mucosa, esophagus, stomach body, antrum, duodenum, and colon were analyzed for expression of HUG Br 1 and HUG Br 2. HUG Br 1 hybridization was detected in duodenum > colon, whereas HUG Br2 hybridization was detected in duodenum > esophagus > colon. Immunoreactivity data confirmed the presence of HUG Br 1 protein at low levels in the duodenum, whereas the less abundant HUG Br 2 protein was below the limits of detection of isoform specific anti-peptide antibodies. Bilirubin specific reactivity was demonstrated in duodenal samples but not antrum samples, consistent with the molecular genetic data. The presence of functional bilirubin UDP-GT isoforms in human alimentary tract supports the notion that the gut may function as a metabolic organ and may have diagnostic and therapeutic implications for disorders of bilirubin metabolism.

Differential effect of ursodeoxycholate and its taurine conjugate on biliary transport maximum of bilirubin in the rat

The effects of ursodeoxycholate and its taurine conjugate on biliary Tm of bilirubin were evaluated in rats. Ursodeoxycholate was administered at four different doses (4, 8, 12 or 16 mumol per 100 g body wt i.v., followed by an i.v. infusion of 0.3, 0.6, 0.9 or 1.2 mumol/min per 100 g body wt, respectively), whereas tauroursodeoxycholate was administered only at the maximal dose. A dose-dependent diminution of bilirubin Tm was observed during ursodeoxycholate administration, which ranged from no effect at the lowest dose to a virtual excretory blockage at the highest dose. This was associated with an increase in bilirubin concentrations in both plasma and liver as well as in the fractional amount of conjugated pigment in both sites, suggesting an impairment of bilirubin transfer at the canalicular level. Incomplete taurine conjugation of ursodeoxycholate well correlated with these effects. Unlike ursodeoxycholate, tauroursodeoxycholate had no inhibitory effect on bilirubin Tm, although a slight inhibition of bilirubin uptake and bilirubin conjugation became apparent. Taken together, these results suggest that ursodeoxycholate interferes with the hepatobiliary transport of bilirubin by impairing its transfer at the canalicular level and that incomplete taurine conjugation appears to be a key factor determining this effect.

Correction of bilirubin conjugation in the Gunn rat using hepatocytes immobilized in alginate gel beads as an extracorporeal bioartificial liver

A new extracorporeal bioartificial liver using alginate-entrapped hepatocytes was developed and evaluated for its ability to correct the lack of bilirubin conjugation in the Gunn rat. Hepatocytes were harvested from Sprague-Dawley rats by the two-step collagenase perfusion method and then immobilized in Ca(++)-alginate beads. The ability of immobilized hepatocytes to conjugate bilirubin was investigated in vitro by comparison with hepatocyte monolayer cultures. The bioartificial liver consisted of a cylindric bioreactor containing either alginate beads with hepatocytes (test group) or alginate beads alone (control group). Gunn rats were connected to this bioreactor via an extracorporeal circulation and bile fractions were collected at hourly intervals. Both bilirubin monoconjugates and bilirubin diconjugates were measured in the bile by high pressure liquid chromatography. Hepatocyte viability in alginate beads was determined prior to and at the end of each experiment and found to be unchanged (75%). In the test group, the concentration of bilirubin conjugates increase rapidly, attaining median values of 72.26 microM and 92.59 microM for mono and diconjugated bilirubin respectively, during a 3 h period of extracorporeal circulation. In the control group, the levels of either conjugate did not exceed 0.87 microM throughout the experiments. Statistical analysis showed a significant difference between the two groups (p < 0.0023). These results suggest that the bioartificial liver used in this study represents an effective method for the temporary correction of the Gunn rat's genetic defect. Such a system might be of therapeutic interest in acute liver failure.

Separation of serum bilirubin species by micellar electrokinetic chromatography with direct sample injection

Four major bilirubin species in serum were separated by micellar electrokinetic chromatography with 25 mM sodium dodecyl sulfate (SDS) and 20 mM sodium tetraborate-boric acid buffer at pH 8.5. Due to the solubilization of the serum proteins by the SDS micelles, serum samples were injected directly into a 50 cm x 75 microns I.D. fused-silica capillary and complete separation of the four bilirubin species was accomplished within ca. 10 min without extensive sample pretreatment. Detection was performed by absorbance at 450 nm and average limit of detection was in the 6.0 microM concentration range. The usefulness of this method was demonstrated for the separation and detection of a number of bilirubin species present in pathological human serum samples.

Bilirubin production and conjugation from newly formed heme in isolated rat hepatocytes

1. Heme synthesis from delta-aminolevulinic acid (delta-ALA) in freshly isolated rat hepatocytes was maximal at 100 microM with a rate of approx. 7 nmol being synthesized per g wet weight cells. 2. Approximately 8% of synthesized heme was converted to bilirubin and 50% of the newly synthesized bilirubin was conjugated. 3. The ratio of di to monoconjugate was approx. 2.5. Incorporation of delta-ALA into bilirubin was increased by additional delta-ALA, heme and was also doubled in cells isolated from animals treated with CoCl2. 4. Bilirubin formation was inhibited approx. 90% by in vitro treatment with heme oxygenase inhibitors zinc and tin protoporphyrin.

The origins and kinetics of bilirubin in healthy dogs, in comparison with man

The origins and the kinetics of unconjugated bilirubin were assessed in 25 healthy dogs. Bilirubin kinetics were measured by the analysis of the plasma disappearance of [3H]bilirubin in a two-compartment model. The relative bilirubin productions from erythrocyte haem degradation and early labeled bilirubin were determined by measuring the incorporation of [14C]glycine in erythrocyte haem and in fecal stercobilin. The incorporation of this relation into the model permitted the quantitation of the bilirubin production from erythrocyte destruction, ineffective erythropoiesis and the catabolism of hepatic haemoproteins. The contribution of the three bilirubin sources to the plasma concentration was derived from the calculated fraction reflux into the plasma of bilirubin produced in the liver from hepatic haemoproteins. Other calculated model-dependent and -independent parameters were plasma bilirubin clearance, hepatic bilirubin extraction efficiency, pool sizes, and the fractional transfer rates which reflect the hepatic uptake process, reflux from liver to the plasma, and the conjugating enzyme activity. In plasma of healthy dogs only unconjugated bilirubin was detected. It averaged 0.68 mumol/l, which is far below levels in man. This is probably due to the 20-fold higher hepatic clearance rate in dogs (median 32.2; range 21.6-43.9 ml/kg per min). In addition, fasting hyperbilirubinaemia could not be documented in the dog. The total bilirubin turnover was 14.9 (12.6-17.1) mumol/kg (median and 95% range), with 67 (60-70)% derived from erythrocyte degradation, 5.3 (4.7-5.5)% from ineffective erythropoiesis and 27.7 (24.5-35.3)% from hepatic haemoproteins. The figures for the plasma bilirubin turnover were 12.3 (10.3-14.2) mumol/kg per day, 79 (75-84)%, 6.3 (6.0-6.6)% and 14.8 (9.2-18.9)%, respectively. The presented model permits the simultaneous quantitation of both the origins and the kinetics of bilirubin. The application of this approach in pathological conditions is expected to provide better insight in the pathophysiology of acquired hyperbilirubinaemia.

Bile pigments in gallbladder and freshly-secreted hepatic duct bile from fed and fasted rainbow trout, Oncorhynchus mykiss

1. Chromatographic analyses of bile pigments in rainbow trout reveal the presence of primarily unconjugated biliverdin (BV) and bilirubin (BR) glycosyl conjugates. Only trace amounts of unconjugated BR are present in hepatic duct (HD) bile: no beta-glucuronidase activity is detectable. 2. The per cent of BV and BR in HD and gallbladder biles is similar in fasted trout; however, the per cent of BV is significantly increased in HD bile from fed fish. 3. Fasting decreases the rate of choleresis but does not alter the excretory rate of endogenous BV or BR. 4. Erythrocyte life span is estimated to be approximately 500 days.

N-acetylation and debrisoquine hydroxylation polymorphisms in patients with Gilbert's syndrome

1. N-acetylation and debrisoquine hydroxylation phenotypes were determined in 54 patients with Gilbert's syndrome and in 247 (sulphamethazine) and 76 (debrisoquine) non-related healthy volunteers. 2. Forty (74.1%) of the patients and 135 (54.7%) of the healthy volunteers were slow acetylators (chi 2 = 6.87). In the patients, the cumulative urinary excretion of sulphamethazine up to 6 h (Ae(0,6)) was significantly lower. No differences in the frequency of debrisoquine poor metabolizers were observed: Gilbert's syndrome 5/54 (9.3%), healthy volunteers 5/76 (6.6%). The metabolic ratios were similar in both groups as well as the urinary recoveries of debrisoquine and its 4-hydroxy metabolite. 3. Gilbert's syndrome seems to be related in some way to N-acetylation but not to the debrisoquine hydroxylation polymorphism.

Hepatic transport of bilirubin in rats with streptozotocin-induced diabetes

This study was undertaken to evaluate the effects of streptozotocin-induced diabetes on the hepatic transport of bilirubin in male Wistar rats. Rats were pretreated with streptozotocin (60 mg/kg i.p.) to induce uncontrolled diabetes. Six days later endogenous biliary excretion and plasma bilirubin concentration were significantly enhanced compared to control animals (+36% and +46%, respectively), while the blood levels of free hemoglobin remained unchanged. Following a bilirubin load, the maximal biliary excretion of the pigment (Tm) in diabetic animals was significantly enhanced compared to control animals (+49%). Liver and plasma bilirubin concentrations at the end of bilirubin administration were significantly reduced (-28% and -30%, respectively). Bilirubin UDP-glucuronosyltransferase activity and UDP-glucose concentration in liver were significantly enhanced (+31% and +81%, respectively), as was the biliary excretion of unconjugated bilirubin (+37%) and bilirubin mono- (+38%) and diconjugates (+53%). When streptozotocin-diabetic rats were treated with insulin, the parameters of bilirubin transport and metabolism were significantly reduced compared to diabetic animals receiving no hormone replacement. In summary, our data indicate that in short-term streptozotocin-diabetic rats there is increased bilirubin production as well as enhanced hepatic conjugation and subsequent biliary excretion of the pigment. These effects appear to be a direct consequence of diabetes.

Synthesis, chromatographic purification, and analysis of isomers of biliverdin IX and bilirubin IX

Neutral solvent systems were developed to isolate the alpha, beta, gamma, and delta isomers of biliverdin IX dimethyl ester by TLC. The individual free acids of biliverdin IX were obtained by saponification of the corresponding dimethyl esters. The bilirubin IX isomers were prepared by reducing the corresponding biliverdin IX isomers with NaBH3CN. Starting from a pure biliverdin IX dimethyl ester, the corresponding free acid of biliverdin IX or bilirubin IX was available within 3-4 h. Preparation of spectrally pure bile pigment required final TLC on acid-cleaned neutral TLC plates. The absorption spectra of the free acids and dimethyl esters of biliverdin IX in methanol showed a broad band at about 650 nm and a sharp band at about 375 nm. The long-wave-length band was extremely sensitive to the presence of strong acid. A 10-fold molar excess of HCl caused a 35- to 50-nm shift of the absorption maximum to longer wavelengths and near doubling of the maximum absorption. The molar absorption coefficients of biliverdins were identical for each free acid and dimethyl ester pair. In each case, Beer's law was followed in both methanol and acidified methanol. Methanol also proved to be a suitable solvent for spectroscopic determination of the non-alpha isomers of bilirubin IX. The wavelength of maximum absorption and molar absorption coefficient of each dipyrrolic ethyl anthranilate azo pigment derived from the various bilirubin IX isomers are also reported.

Hepatic handling of photoirradiated bilirubin. A study in isolated perfused Wistar rat liver

Conjugation has been considered the rate-limiting step for bilirubin hepatic transport, and bypass of this metabolic step could explain why photobilirubins can be rapidly cleared by the liver. In this paper we assessed whether photoirradiation may enhance the bilirubin overall hepatic transport in the isolated perfused Wistar rat liver, a model possessing intact transport and conjugating systems. Bilirubin was administered as a bolus so as to reach a perfusate concentration of approximately 10 microM (bilirubin/albumin molar ratio 1:17). Perfusate light exposure (0.56.10(15) quanta s-1 cm-2) yielded 7-10% of configurational photoisomers, which were further identified as (4Z,15E/4E,15Z)-bilirubin IX alpha. Under such conditions, the perfusate removal rate was increased by 39% over that from dark conditions. Likewise, biliary excretion, estimated as total bilirubin recovery at 60 min, was also increased (+48%). This later improvement was mainly produced at the expense of unconjugated bilirubin, which most likely derived from its configurational photoisomers that, once excreted into bile, readily re-isomerized to the parent compound. In addition, this increment was partially due to a delayed improvement of monoglucuronide pigment excretion. The calculated hepatic pigment content was significantly higher under light conditions. A direct assessment of hepatic content of different bilirubin moieties at 20 min after bilirubin administration confirmed that such an increment was fully accounted for by unconjugated pigment. Our finding that hepatic pigment content rose (despite a higher biliary excretion) when the bilirubin was irradiated suggests a higher net uptake of photoisomers than native pigment. This observation, and the finding that bilirubin photoisomers were usually excreted without undergoing conjugation even if the metabolic system is active, contribute to explain the greater appearance of unconjugated bilirubin in Wistar rat bile under light exposure.

The influence of gestational age on bilirubin conjugation in newborns

Unconjugated, mono- and diconjugated bilirubin levels were determined in serum soon after birth, and followed up for several days. Fourteen preterm neonates were studied with a gestational age below 33 weeks (n = 7) or between 34 and 37 weeks (n = 7), respectively, as well as 19 full-term newborns either untreated (n = 9) or treated by phototherapy (n = 10). Bilirubin and its derivatives were analysed by alkaline methanolysis and spectrometry after separation by thin-layer chromatography. In normal full-term neonates total and unconjugated bilirubin reached peak levels at days 2-4. Thereafter, a decline of 11% per day was detectable. Monoconjugates in serum amounted to 3.1 +/- 1.1% of total pigment and remained at that level. The relative amount of diconjugates increased from 0.55 +/- 0.25% (2-4th postnatal day) to 1.62 +/- 0.99% (9-13th day of life). The rapid decline of unconjugated bilirubin paralleled by an increase of diconjugates are an expression of the maturation process for bilirubin conjugation. The premature neonates with less than 33 weeks gestation exhibited an increase of unconjugated serum bilirubin up to the 4-5th postnatal day, the decline thereafter amounted 2% per day. The fraction of 2.3 +/- 1.1% monoconjugates was small and exhibited only a moderate increase in the follow up. In contrast diconjugates were undetectable or very low and remained at this level. These results suggest the presence of a more severe immaturity as well as a slower maturation process of bilirubin conjugation in preterm newborns.(ABSTRACT TRUNCATED AT 250 WORDS)