Microsomal conjugation and oxidation of bilirubin

Heme Oxygenase-1 and Its Role in Colorectal Cancer

Heme oxygenase-1 (HO-1) is an enzyme located at the endoplasmic reticulum, which is responsible for the degradation of cellular heme into ferrous iron, carbon monoxide and biliverdin-IXa. In addition to this main function, the enzyme is involved in many other homeostatic, toxic and cancer-related mechanisms. In this review, we first summarize the importance of HO-1 in physiology and pathophysiology with a focus on the digestive system. We then detail its structure and function, followed by a section on the regulatory mechanisms that control HO-1 expression and activity. Moreover, HO-2 as important further HO isoform is discussed, highlighting the similarities and differences with regard to HO-1. Subsequently, we describe the direct and indirect cytoprotective functions of HO-1 and its breakdown products carbon monoxide and biliverdin-IXa, but also highlight possible pro-inflammatory effects. Finally, we address the role of HO-1 in cancer with a particular focus on colorectal cancer. Here, relevant pathways and mechanisms are presented, through which HO-1 impacts tumor induction and tumor progression. These include oxidative stress and DNA damage, ferroptosis, cell cycle progression and apoptosis as well as migration, proliferation, and epithelial-mesenchymal transition.

Bilirubin degradation by uncoupled cytochrome P450. Comparison with a chemical oxidation system and characterization of the products by high-performance liquid chromatography/electrospray ionization mass spectrometry

Bilirubin is a protective antioxidant; however, when its conjugation and excretion are impaired, as in neonatal and hereditary jaundice, bilirubin accumulates and may cause severe neurotoxicity. Degradation of bilirubin takes place (a) on interaction with oxidative free radicals and (b) when cytochrome P450 (CYP) enzymes are uncoupled by polyhalogenated substrate analogues. The products of pathways (a) and (b) above have now been characterized by high-performance liquid chromatography/electrospray ionization mass spectrometry (HPLC/ESI-MS) and the mechanisms of fragmentation in part clarified. Oxidation of bilirubin by uncoupled CYP1A5 and by a Fe-EDTA/H2O2 system produced both biliverdin and an identical profile of dipyrrolic fragments, as detected by positive ESI-MS. A similar profile of oxidation products was found from mesobilirubin, all showing the expected increase in mass, thus providing direct evidence for fragmentation at the central methene bridge of the tetrapyrroles. Two degradation products, also detected by negative ESI-MS, were characterized as dipyrroles retaining the central bridge carbon, with one or two oxygen atom(s) bound (probably as the aldehyde and hydroperoxide derivatives). Ions compatible with propentdyopents and bilifuscins were also detected, but here the assignment was less certain. It is concluded that the first step in the oxidation of bilirubin may be hydrogen abstraction at the central methene bridge. This is followed either by loss of another hydrogen to give biliverdin, or by oxygen binding and fragmentation. Fe-EDTA/H2O2 and uncoupled CYP(Fe=O) may both initiate the reaction, the latter in an attempt to reduce the ferryl oxygen to water. These studies shed light on the CYP uncoupling mechanism and are of potential significance for the therapy of severe jaundice.

Antioxidant activities of bile pigments

Biliverdin and bilirubin are reducing species and hence potential antioxidants formed by the action of heme oxygenase and biliverdin reductase. Indeed, there is increasing evidence for the suggestion that a beneficial role of the potentially toxic bilirubin may be to act as a powerful chain-breaking antioxidant in biological systems, and that bilirubin may contribute to the cellular and tissue protection seen with increased heme oxygenase. This article reviews the in vitro antioxidant activities of the two bile pigments with emphasis on the different physiological forms of bilirubin and types of oxidants, and discusses these properties in light of the presence and reactivity other nonproteinaceous antioxidants.

Combined polymorphisms in UDP-glucuronosyltransferases 1A1 and 1A6: implications for patients with Gilbert's syndrome

BACKGROUND/AIMS

UDP-glucuronosyltransferases (UGTs) are important enzymes involved in glucuronidation of various exogenous and endogenous compounds. Studies were undertaken on the variability of three UGT enzyme activities in human livers. Enzyme activities were associated with genetic polymorphisms in UGT1A1 (UGT1A1*28) and UGT1A6 (UGT1A6*2). UGT1A1*28 is associated with Gilbert's syndrome, a deficiency in glucuronidation of bilirubin leading to mild hyperbilirubinemia, whereas UGT1A6*2 may result in low glucuronidation rates of several drugs.

METHODS

Enzyme activities and genetic polymorphisms were assessed in 39 human liver samples, and polymorphisms were also assessed in blood of 253 healthy controls.

RESULTS

Associations were found between UGT enzyme activities of bilirubin (B) and 4-nitrophenol (NP; r=0.47, P=0.0024), B and 4-methylumbelliferone (MUB; r=0.54, P=0.0003), and NP and MUB (r=0.89, P<0.0001). In addition to the association between B-UGT enzyme activity and UGT1A1*28 (r=0.45, P=0.0034) as reported earlier, an association between B-UGT and UGT1A6*2 (r=0.43, P=0.007) was found. In 253 Dutch Caucasian controls, co-occurrence of UGT1A1*28 and UGT1A6*2 was found (r=0.9, P<0.0001).

CONCLUSIONS

Most patients with Gilbert's syndrome, in addition to their reduced B-UGT enzyme activity, may have abnormalities in the glucuronidation of aspirin or coumarin- and dopamine-derivatives, due to this combination of UGT1A1*28 and UGT1A6*2 genotypes.

Association of human liver bilirubin UDP-glucuronyltransferase activity with a polymorphism in the promoter region of the UGT1A1 gene

BACKGROUND/AIMS

Gilbert's syndrome is a benign form of a deficiency in bilirubin glucuronidation. It is associated with a homozygous polymorphism, A(TA)7TAA instead of A(TA)6TAA, in the TATA-box of the promoter region of the bilirubin UDP-glucuronyltransferase gene. In this study the correlation between this promoter region polymorphism and in vitro human liver bilirubin UDP-glucuronyltransferase enzyme activity was investigated.

METHODS

Liver samples from organ transplant donors n=39) and two known Gilbert's syndrome patients were used for measuring bilirubin UDP-glucuronyltransferase enzyme activity and for isolation of DNA followed by detection of the promoter region polymorphism by polymerase chain reaction. Genotypes were assigned as follows; 6/6: homozygous for the A(TA)6TAA-allele, 7/7: homozygous for the A(TA)7TAA-allele, and 6/7: heterozygous with one of each alleles.

RESULTS

Seventeen out of 39 subjects (44%) had the homozygous 6/6 genotype, 18 subjects (46%) had the heterozygous 6/7 genotype, whereas four individuals (10%) and the two individuals with Gilbert's syndrome had the 7/7 genotype correlated with Gilbert's syndrome. This resulted in an allele frequency of 0.33 for the A(TA)7TAA-allele. The median bilirubin UDP-glucuronyltransferase enzyme activity of the 17 subjects with the 6/6 genotype (1565 nmol/g liver/h) was significantly higher than the activity of the 18 subjects with the 6/7 genotype (985 nmol/g liver/h; p<0.05) and the six individuals with the 7/7 genotype (749 nmol/g liver/h; p<0.005). No significant differences in enzyme activity were found between the 6/7 and the 7/7 genotype groups.

CONCLUSIONS

The results indicate a close association between the promoter region genotype and the expression of hepatic bilirubin UDP-glucuronyltransferase enzyme activity. Subjects who have a 7/7 genotype have the lowest enzyme activity, whereas subjects with the heterozygous 6/7 genotype have an intermediate enzyme activity.

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.

Investigation of the role of reactive oxygen species in bilirubin metabolism in the Gunn rat

It has been previously established that the attenuation of hepatic lipid peroxidation by a fat-free diet is accompanied by a marked rise in plasma bilirubin in Gunn rats. Present in vitro studies confirmed that microsomal lipid peroxidation caused the concurrent degradation of added bilirubin but failed to show that microsomal superoxide, hydroxyl radical or hydrogen peroxide would degrade bilirubin. Moreover, although injection of vitamin E completely inhibited microsomal lipid peroxidation and bilirubin degradation it had no effect on plasma bilirubin. No evidence has therefore been obtained that in Gunn rats, in the absence of bilirubin glucuronidation, that reactive oxygen species provide a significant physiological pathway of bilirubin disposal.

Degradation of heme IX in rats pretreated with cobaltous chloride. A study in isolated perfused liver

The effect of the administration of cobaltous chloride on the degradation of heme IX was investigated using perfusions of isolated rat livers. The presence of biliverdin IX beta (2%) in the bile fluid of cobaltous chloride treated rats support the hypothesis of the presence of a chemical oxidation of heme IX induced by the cobalt salt. In control rats the absence of biliverdin IX beta in the bile fluid was observed. When biliverdin IX beta was added to the perfusate it was excreted in the bile fluid as bilirubin IX beta without conjugation. An increase from approximately twofold to three-fold of the total bilirubins in the bile fluids of cobaltous chloride pretreated rats in relation with control rats was observed due to heme oxygenase enhanced activity. Bilirubin IX alpha diconjugates increased ca. 15% in the bile fluids of cobaltous chloride pretreated rats, after the addition of hemin IX. The increase could reflect the presence of a new molecular form of UDP-glucuronosyltransferase, which favours the formation of bilirubin IX alpha diglucuronide and therefore its preferential excretion into the bile. In the bile fluid of the rats pretreated with cobaltous chloride, other diconjugates of bilirubin IX alpha were also detected 120 min after the addition of hemin IX to the perfusate, consisting of glucose and glucuronate (5%) and xylose and glucuronate (5%).

Two pathways of iron-catalyzed oxidation of bilirubin: effect of desferrioxamine and trolox, and comparison with microsomal oxidation

The bilirubin-degrading activity of liver microsomes from rats induced with 3-methylcholanthrene has been shown to be markedly stimulated by addition of 3,3',4,4',5,5'-hexabromobiphenyl, a polyhalogenated chemical which resembles in size and shape the most effective inducers of cytochrome P450IA1, but lacks the structural features necessary for it to be metabolised. The degradation of bilirubin by this microsomal system has been compared to oxidation by a chemical model system involving H2O2 and Fe-EDTA (ethylenediaminetetraacetic acid). In both systems bilirubin disappearance was accompanied by bleaching. However, when either desferrioxamine or Trolox were present in the chemical model system, the rate of bilirubin oxidation was greatly enhanced and, at the same time, bilirubin was largely or entirely converted to biliverdin, a pathway of oxidation which proceeds by dehydrogenation. In the presence of desferrioxamine, biliverdin was also further oxidised to an unidentified red pigment.

Serum bilirubin fractions in healthy subjects and patients with unconjugated hyperbilirubinemia

Serum bilirubin fractions were determined by a newly developed, high performance liquid chromatography method in 23 healthy subjects, and in 15 patients with Gilbert's syndrome, five with type 2 Crigler-Najjar syndrome, seven with hemolytic disorders, and 26 with neonatal jaundice. In the healthy subjects, 92.6% of the bilirubin was unconjugated (UCB), 6.2% was bilirubin monoglucoronide (BMG), and 0.5% was bilirubin diglucuronide (BDG). Delta bilirubin (B delta) was not detected. In the patients, the percentage of UCB was significantly higher and that of BMG was significantly lower than in the healthy subjects. The proportion of BDG tended to decrease in Gilbert's and type 2 Crigler-Najjar syndromes; the proportion of B delta tended to increase in hyperbilirubinemia, except in Gilbert's syndrome. In particular, B delta was frequently detected in serum which also contained BDG (mainly in hemolytic disorders) or which presented with high concentrations (above 100 mumol/L) of UCB (mainly in type 2 Crigler-Najjar syndrome and neonatal jaundice). Trace amounts of (Z,E)- and/or (E,Z)-UCB were detected in approximately one fourth of the serum samples analyzed.

Potential role of conjugated bilirubin and copper in the metabolism of lipid peroxides in bile

Conjugated bilirubin and copper ions at their physiological concentrations in bile may play an important role in hydroperoxide and other detoxification. Conjugated bilirubin may also be an important chain-breaking antioxidant preventing lipid peroxidation. Bilirubin ditaurine (BR-DT), a water-soluble model compound of conjugated bilirubin, completely prevents the peroxyl radical-induced oxidation of phosphatidylcholine in either multilamellar liposomes or micelles. This antioxidant activity is associated with the bilirubin moiety of BR-DT, since taurine alone is inefficient in scavenging peroxyl radicals. The number of peroxyl radicals trapped per molecule of BR-DT is 1.9, compared to 4.7 trapped per molecule of biliverdin, the water-soluble physiological precursor of bilirubin. Peroxyl radical-induced oxidation of BR-DT results in a spectral shift in maximal absorbance toward shorter wavelengths; biliverdin is not formed as a major oxidation product. BR-DT, but neither taurine nor biliverdin, greatly accelerates the cupric ion-catalyzed decomposition of linoleic acid hydroperoxide. In the presence of ferric ion, BR-DT shows no lipid hydroperoxide-degrading activity. Addition of cupric ion to BR-DT results in formation of a complex with spectral features similar to that of a biliverdin-cupric ion complex, indicating that BR-DT and cupric ion undergo redox reactions.

Relationships between serum bilirubins and production and conjugation of bilirubin. Studies in Gilbert's syndrome, Crigler-Najjar disease, hemolytic disorders, and rat models

The pattern of serum bilirubins was determined in serum of humans and rats with unconjugated hyperbilirubinemia due to increased pigment load or defective hepatic conjugation. Bilirubin ester conjugates were present in all serum samples tested and were identified as bilirubin 1-O-acyl glucuronides. In Gilbert's syndrome, the concentration of total conjugates was comparable to the values in healthy control subjects. Because the concentration of unconjugated pigment was increased, the fraction of conjugated relative to total bilirubins was markedly decreased. Sera from patients with Crigler-Najjar disease differed from those with Gilbert's syndrome by the higher unconjugated bilirubin levels and the undetectability of diconjugated bilirubins. A striking finding was that in hemolytic disease, the concentration of both monoconjugates and diconjugates was enhanced in parallel with the increase of unconjugated pigment. Therefore, the fraction of conjugated relative to total bilirubins remained within the normal range. As in Gilbert's syndrome, heterozygote R/APfd-j/+ rats with impaired hepatic bilirubin conjugation exhibit an increased unconjugated bilirubin level in serum, whereas the concentration of total conjugates was comparable to the values in normal rats. In serum of normal rats loaded intraperitoneally with unconjugated bilirubin, both unconjugated and mono- and diconjugated bilirubins were increased in parallel so that the ratio of unconjugated to esterified pigment remained unaffected. Decreased hepatic conjugation or increased bilirubin load was associated with a lower percentage of diconjugates relative to total conjugates both in human and rat serum. The present results are consistent with a compartmental model in which there is bidirectional transfer across the sinusoidal membrane for unconjugated bilirubin as well as for the bilirubin glucuronides. Because typical patterns of serum bilirubins are found in Gilbert's syndrome and patients with hemolytic hyperbilirubinemia, determination of esterified bilirubins in serum is of value to study the pathophysiology and the differential diagnosis of unconjugated hyperbilirubinemia.

Deconjugation of glucuronides catalysed by UDPglucuronyltransferase

Evidence was found for UDPglucuronyltransferase-catalysed deconjugation of p-nitrophenol-, 4-methylumbelliferone- and phenolphthalein-glucuronides. The evidence is based on the following observations: 1, deconjugation is UDP-dependent and the reactions show Michaels-Menten kinetics with respect to UDP and glucuronide saturability; 2, UDP-glucuronic acid was identified as reaction product; 3, all studies were done in the presence of a beta-glucuronidase inhibitor; 4, induction profiles, using 3-methylcholanthrene and phenobarbital as inducing agents, were identical for conjugation and deconjugation reactions. Optimal deconjugation rates for p-nitrophenol- and 4-methylumbelliferone-glucuronides were at pH 5.1 and for phenolphthalein-glucuronide at pH 6.5. Only conjugation reactions showed latency; the corresponding deconjugation reactions were not latent. UDPglucuronyltransferase is a group of oligomeric isoenzymes with different molecular masses. The molecular masses of the isoenzyme species catalysing the forward and reverse reactions were determined by radiation-inactivation analysis. The molecular masses of the isoenzyme species mediating the catalyses of deconjugation reactions were significantly smaller than those mediating catalyses of conjugation reactions: 66 +/- 4 kDa vs. 109 +/- 7 kDa for p-nitrophenol; 82 +/- 8 kDa vs. 105 +/- 6 kDa for 4-methylumbelliferone; and 74 +/- 8 kDa vs. 159 +/- 14 kDa for phenolphthalein. This suggests that for catalyses of deconjugation reactions only part of a UDPglucuronyltransferase isoenzyme is needed, whereas for forward reactions the complete isoenzymes are required.

Regulation of bilirubin glucuronide synthesis in primate (Macaca fascicularis) liver. Kinetic analysis of microsomal bilirubin uridine diphosphate glucuronyltransferase

Hepatic bilirubin uridine diphosphate glucuronyl-transferase (UDP-glucuronyltransferase) catalyzes the formation of bilirubin monoglucuronides (BMG, C-8 and C-12 isomers) and bilirubin diglucuronide (BDG) from bilirubin and the cosubstrate, UDP-glucuronic acid. Distinctive patterns of bile pigment excretion occur in different species and in pathologic disorders (e.g., Gilbert's syndrome). In normal human and monkey (Macaca fascicularis) bile, the proportion of BDG exceeds that of BMG and the C-8/C-12 BMG isomer ratio approaches unity. To investigate the mechanisms responsible for the patterns of BDG and BMG isomers in bile, we used a radiochemical assay to analyze the kinetics and regulation of bilirubin UDP-glucuronyltransferase in microsomes prepared from monkey liver. The synthesis of BMG from bilirubin was a higher capacity, lower affinity step (Vmax = 295 pmol/mg protein X min, Km = 24 microM) than BDG synthesis from endogenously formed BMG (Vmax = 170 pmol/mg protein X min, Km = 14 microM). This observation was confirmed when biosynthetically prepared BMG was used as substrate. The rate of formation of BDG relative to BMG was modulated by both bilirubin and UDP-glucuronic acid concentration, whereas the C-8/C-12 BMG isomer ratio was influenced exclusively by UDP-glucuronic acid concentration. The data obtained with increasing UDP-glucuronic acid concentration did not obey conventional single-site kinetics, suggesting the presence of more than one binding site on the enzyme or a membrane transporter for this nucleotide sugar. These findings demonstrate that microsomal bilirubin UDP-glucuronyltransferase has a greater capacity for BMG than BDG synthesis, and thus support the concept that the decreased BDG/BMG ratio in the bile of patients with reduced hepatic enzyme activity (i.e., Gilbert's syndrome and type II Crigler-Najjar disease) reflects the diminished capacity of the enzyme to synthesize BDG from BMG. Bilirubin and UDP-glucuronic acid concentrations both appear to be pivotal in regulating the synthesis of individual bilirubin glucuronides and hence the patterns of bilirubin conjugates excreted in bile.

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.

Peroxidative oxidation of bilirubin during prostaglandin biosynthesis

The peroxidative oxidation of bilirubin has been characterized in the ram seminal vesicle microsomal system. The oxidation was monitored by following the loss in absorbance of bilirubin at 440 nm. Bilirubin behaves as a peroxidase substrate for prostaglandin H synthase. The oxidation may be initiated by the addition of arachidonic acid or peroxides to incubations containing ram seminal vesicle microsomes and bilirubin, and is sensitive to inhibition by reduced glutathione. The arachidonate-dependent oxidation, but not the peroxide-initiated case, is inhibited by indomethacin. Similar results were obtained using microsomal preparations from mouse, rat, and pig lungs. Spectral and chromatographic examination of the products of bilirubin oxidation in the ram seminal vesicle system demonstrate that biliverdin is produced in this system by the dehydrogenation of bilirubin, but that this product accounts for only about 15% of the bilirubin consumed. Biliverdin itself is not oxidized in this system. At least three highly polar, fluorescent products also are formed from bilirubin. Though not identified, these polar products differ markedly in chromatographic behavior from the major fluorescent products obtained following the singlet oxygen oxidation or the autoxidation of bilirubin.

UDP-glucuronyltransferase-catalyzed deconjugation of bilirubin monoglucuronide

Bilirubin monoglucuronide is rapidly deconjugated when incubated with UDP and rat liver microsomal preparations at pH 5.1. The following evidence was found that this reaction is catalyzed by UDP-glucuronyltransferase: (i) unconjugated bilirubin and UDP-glucuronic acid were identified as the reaction products; (ii) Gunn rat microsomal preparations lack bilirubin UDP-glucuronyltransferase deficiency and do not catalyze the deconjugation reaction, and (iii) neither saccharo-1,4-lactone, a beta-glucuronidase inhibitor, nor butylated hydroxytoluene, an inhibitor of spontaneous isomerisation, affect the rate of the deconjugation reaction. Deconjugation appears to be the reverse of UDP-glucuronyltransferase-catalyzed glucuronidation. The conditions for the reverse reaction differ in the following aspects from those of the forward reaction: (i) nucleotide triphosphates stimulate the reverse reaction probably allosterically; (ii) UDP-N-acetylglucosamine stimulates the forward reaction but has no effect on the reverse reaction; (iii) the optimal pH for the reverse reaction is pH 5.1 and for the forward reaction is pH 7.8, and (iv) Mg++ ion is not required for the reverse reaction but stimulates the forward reaction. Detergents stimulate both reactions. Stimulation of the reverse reaction by nucleotide triphosphates and detergents is mutually independent and additive which suggests different mechanisms of action. Deconjugation reactions may become important during parenchymatous liver disease when, as a result of anaerobic glycolysis, intracellular pH decreases. Elevated levels of unconjugated bilirubin in the serum of patients with parenchymatous liver disease may be a sign of sick liver cells rather than decreased UDP-glucuronyltransferase activity.