New developments in glucuronidation research: report of a workshop on "glucuronidation, its role in health and disease".
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Animals↗
Biomedical subjects
B Burchell
Publications and source records attributed to B Burchell.
Differential expression and induction of UDP-glucuronosyltransferase isoforms in hepatic and extrahepatic tissues of a fish, Pleuronectes platessa: immunochemical and functional characterization.
Glucuronidation of three substrates prototypical for different UDP-glucuronosyltransferase (UDPGT) isoforms in hepatic, renal, intestinal, and branchial microsomes of corn oil, 3-methylcholanthrene, Aroclor 1254, and clofibrate-pretreated plaice was investigated. The differential expression of UDPGT in the four tissues clearly demonstrated for the first time that multiple isoforms with differing substrate specificities were present in fish. The liver was quantitatively the most important site for the glucuronidation of all three compounds studied. Phenol UDPGT activity was ubiquitous to all tissues and was induced by 3-methylcholanthrene and Aroclor 1254 in hepatic tissue and by Aroclor 1254 in renal tissue. The glucuronidation of testosterone was restricted to liver and intestinal tissue, while conjugation of bilirubin was expressed solely in hepatic tissue. The biotransformation of the endogenous compounds was not induced in the xenobiotic-treated animals. The presence of immunoreactive UDPGTs in the four tissues was demonstrated by immunoblot analysis using sheep anti-plaice UDPGT antibodies. Hepatic tissue displayed a range of immunoreactive polypeptides of 52 to 57 kDa, while a 55-kDa polypeptide was detected in extrahepatic tissues. An increased intensity of the latter polypeptide species was demonstrated in liver and kidney microsomes in which there was a concomitant induction of phenol UDPGT activity in xenobiotic-treated fish. The results indicate that the 55-kDa polypeptide was the major polyaromatic hydrocarbon-inducible UDPGT isoenzyme in hepatic and renal microsomes.
Animals↗
Functional and immunochemical comparison of hepatic UDP-glucuronosyltransferases in a piscine and a mammalian species.
1. The aglycone specificity of hepatic microsomal glucuronidation was compared under uniform conditions in a fish, Pleuronectes platessa and a mammal, Rattus norvegicus, representative of the most primitive and advanced vertebrate classes. 2. Both species exhibited comparable UDP-glucuronosyltransferase (UDPGT) activity towards planar phenolic substrates (1-naphthol, 4-nitrophenol); however, plaice activity towards bulky non-planar substrates such as (-)-morphine was either 200-fold lower, or for an arylacetic acid (RS-2-phenylpropionic acid) and an aryloxyacetic acid (clofibric acid) non-detectable. 3. Conjugation of the endogenous substrates, bilirubin and steroids were 4- to 40-fold lower in the plaice than in the rat. Whilst both species formed diglucuronides of the asymmetrical bilirubin IX alpha, they displayed a reciprocal preference for the initial esterification, conjugation of the C-8 side chain predominating in the rat and of C-12 in the fish. 4. Immunoblot analysis using two polyclonal antisera preparations raised against rat UDPGTs demonstrated the presence of multiple weakly cross-reacting polypeptides in fish microsomes indicative of multiple isoforms and conservation of common structural motifs over more than 350 million years since evolutionary divergence of the mammals.
Animals↗
Localization of a bile acid UDP-glucuronosyltransferase gene (UGT2B) to chromosome 4 using the polymerase chain reaction.
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Bile↗
Chromosomal assignment of human phenol and bilirubin UDP-glucuronosyltransferase genes (UGT1A-subfamily).
DNA probes were prepared from the 5'-terminal portion of four cDNA clones encoding human phenol and bilirubin UDP-glucuronosyltransferases (UGTs). An additional sequence common to all four clones was isolated from the 3'-terminal portion of one of the clones (UGT1A1). The four specific and the one common DNA sequences were used as probes on a panel of 16 human--rodent somatic cell hybrid DNAs by Southern-blot analysis. The results obtained indicate that all four cDNA clones are encoded by gene(s) located on human chromosome 2.
Animals↗
Mono- and diglucuronide formation from chrysene and benzo(a)pyrene phenols by 3-methylcholanthrene-inducible phenol UDP-glucuronosyltransferase (UGT1A1).
Mono- and diphenols of chrysene and benzo(a)pyrene are suspected substrates of a 3-methylcholanthrene (MC)-inducible phenol UDP-glucuronosyltransferase (UGT1A1). Mono- and diglucuronide formation from these compounds was studied in two systems, (a) livers of MC-treated rats (homologous expression) and (b) a Chinese hamster lung fibroblast cell line (V79) containing rat UGT1A1 cDNA and stably expressing this isozyme (heterologous expression). In liver microsomes of MC-treated rats, glucuronidation of 6-hydroxychrysene was stimulated 11-fold by MC treatment. With 3,6-dihydroxychrysene, formation of its 3-hydroxy-6-monoglucuronide and of the diglucuronide was increased 24- and 310-fold, respectively. Induction factors obtained for monoglucuronide formation (but not for diglucuronide formation) were in line with published data on the increase of immunodetectable UGT1A1 protein and of its mRNA. It is suggested that the high induction factors for diglucuronide formation are the result of a combination of the induction of UGT1A1 and facilitated interaction of neighboring UGT1A1 molecules in endoplasmic reticulum membranes. Glucuronidation of 6-hydroxychrysene, 3-hydroxybenzo(a)pyrene, and 3,6-dihydroxybenzo(a)pyrene to their mono- and diglucuronides was clearly detectable in V79 cells expressing UGT1A1. However, conjugation of 3,6-dihydroxychrysene to its monoglucuronides was low and diglucuronide formation was not detectable, suggesting that UGT isozymes other than UGT1A1 are responsible for these reactions.
Animals↗
Cloning and stable expression of a new member of the human liver phenol/bilirubin: UDP-glucuronosyltransferase cDNA family.
A new human liver UDP-glucuronosyltransferase (HlugP4) has been cloned and expressed in cell culture. The expressed enzyme has a molecular mass of 56 kDa and preferentially catalysed the glucuronidation of halogenated and bulky alkyl phenols. The C-terminal half of the sequence (246 amino acids) is 96% identical with the same portion of HlugP1, whereas the N-terminal half of the deduced protein sequences are only 38% identical. These results suggest that the two isoenzymes may be derived from the same gene by differential splicing of the gene product.
Amino Acid Sequence↗
Turning on and turning off the sense of smell.
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Animals↗
Investigation of the molecular basis of the genetic deficiency of UDP-glucuronosyltransferase in Crigler-Najjar syndrome.
Liver biopsy samples were obtained from eight Crigler-Najjar patients. Bilirubin UDPGT activity, assayed by a microassay with HPLC analysis, was not detectable in type I livers, and low levels (9-26% of controls) of monoglucuronide conjugates only were observed in type II livers. 1-Naphthol UDPGT activity was normal in most patients, where membrane integrity was maintained by correct sample procurement and preparation. Our data on type II livers suggest that a defect in UDPGA transport is an unlikely cause of the hyperbilirubinaemia, but reduced affinity for UDPGA was observed in one sample. Analysis of four patient liver samples by immunoblot analysis revealed the heterogeneous nature of this inherited disease within the patient population, and one sample where 1-naphthol UDPGT activity was considerably reduced appeared to correlate with the non-detection of a phenol UDPGT protein. Progress towards a molecular genetic diagnosis of Crigler-Najjar syndromes is discussed.
Aged↗
Selective induction of bilirubin UDP-glucuronosyl-transferase by perfluorodecanoic acid.
Differential effects of perfluorodecanoic acid (PFDA) on rat liver UDP-glucuronosyltransferase isoenzymes have been observed after a single i.p. administration of the compound to young male Sprague-Dawley rats. (1) Bilirubin glucuronidation was induced 2-fold. The induced state was stable for at least 3 weeks. (2) Glucuronidation of 1-naphthol, morphine and testosterone was decreased to half of the control values. These decreases were maximal after 12 days but all three activities returned to normal levels after 3 weeks. (3) Immunoblotting experiments indicated that the differential effects of PFDA on UDP-glucuronosyltransferase activities were due to modulation of enzyme protein concentrations rather than activation/inactivation mechanisms. With respect to its influence on UDP-glucuronosyltransferase isoenzymes, PFDA may be classified as a clofibrate-type inducer. The persistence of the induction after a single application however is unique among peroxisome proliferators and therefore PFDA may be a useful, elective inducer of bilirubin glucuronidation.
Animals↗
Mapping genes encoding drug-metabolizing enzymes in recombinant inbred mice.
Probes for cytochrome P450IVA (P450IVA), alpha- and pi-class glutathione S-transferases (GST), and phenol-metabolizing UDP-glucuronyltransferase (UDPGT-K39) detected restriction fragment length variants (RFLVs) between C57BL/6J and DBA/2J mice. These variants were used to map the P450IVA genes (Cyp4 alpha) to chromosome 4, close to Mtv-13 and Pmv-19, midway between brown (b) and Gpd-1; GST alpha genes were mapped to chromosome 9, with a cross-hybridizing sequence mapping to another chromosome; the GST pi genes were mapped to the distal end of chromosome 1 near Pmv-21; one UDPGT-K39 variant to chromosome 1, between Acrg and Emv-17, and another showed linkage to Odc-10 on an unidentified chromosome. No RFLVs were detected with probes for P450IID, P450 reductase, androsterone-metabolizing UDPGT, GST mu, or microsomal GST.
Animals↗
The UDP glucuronosyltransferase gene superfamily: suggested nomenclature based on evolutionary divergence.
A nomenclature system for the UDP glucuronosyltransferase superfamily is proposed, based on divergent evolution of the genes. A total of 26 distinct cDNAs in five mammalian species have been sequenced to date. Comparison of the deduced amino acid sequences leads to the definition of two families and a total of three subfamilies. For naming each gene, we propose that the root symbol UGT for human (Ugt for mouse), representing "UDP glucuronosyltransferase," be followed by an Arabic number denoting the family, a letter designating the subfamily, and an Arabic numeral representing the individual gene within the family or subfamily (hyphen before the Arabic number for mouse), e.g., human UGT2B1 and murine Ugt2b-1. Whereas the gene and cDNA should be italicized, the corresponding transcript, protein, and enzyme activity should not be written with lowercase letters or in italics, e.g., human or murine UGT2B1. Recent experimental evidence suggests that several exons of the UGT1 gene might be shared, indicating that distinct UGT1 transcripts and proteins may arise via alternative splicing; the gene and gene product of alternative splicing will be designated with an asterisk, e.g., UGT1*6 and UGT1*6, respectively. When an orthologous gene between species cannot be identified with certainty, as occurs in the UGT2B subfamily, we recommend sequential naming of the genes chronologically as they become characterized. We suggest that the human nomenclature system be used for species other than the mouse. We anticipate that this UGT gene nomenclature system will require updating on a regular basis.
Animals↗
Stable expression of two human UDP-glucuronosyltransferase cDNAs in V79 cell cultures.
Two human liver UDP-glucuronosyltransferase cDNA clones (HLUGP1 and HLUG25) were individually inserted into the eukaryotic expression vector pKCRH2. Each recombinant plasmid was cotransfected with a SFVneo vector, thereby allowing establishment of several V79 cell lines retaining the exogenous UDP-glucuronosyltransferase cDNA after selection with G418 (Geneticin). Southern blot analysis suggested that the cDNAs were integrated into the host cell genome. Northern blot and immunoblot analyses indicated that the cDNAs were correctly transcribed and translated for the production of functional enzymes. The established recombinant V79 cell lines stably expressed the UDP-glucuronosyltransferase activities towards 1-naphthol (HLUGP1) and hyodeoxycholic acid (HLUG25) at levels 10-20-fold higher than with transient expression, and in the range found in human liver. These high levels of expression of UDP-glucuronosyltransferase activity allowed the determination of apparent kinetic constants and substrate specificities of glucuronidation in the genetically engineered cell lines. HLUG25 cDNA encoded an isoform with restricted specificity towards the 6-OH group of the bile acid hyodeoxycholic acid. The other steroids, bile acids, endobiotics, and xenobiotics tested as substrates were glucuronidated in various samples of human liver microsomes, but not by this isoenzyme. This study, allowing the expression of individual UDP-glucuronosyltransferases in heterologous cells with no endogenous transferases, offered a unique solution for the characterization of UDP-glucuronosyltransferase functional heterogeneity.
Animals↗
17 Beta-hydroxysteroid UDP-glucuronosyl transferase is expressed in bile ductular epithelial cells under physiological conditions.
Using an improved procedure to isolate pure bile ductular epithelial cells from rat liver, we were able to define UDP-glucuronosyl transferase (UDPGT) isozymes expressed in these cells under physiological conditions. The cells contained mRNA for a 17 beta-hydroxysteroid, a 3 alpha-hydroxysteroid and a phenol UDPGT. Concomitantly, a distinct pattern of UDPGT-immunoreactive proteins was expressed, including a putative 17 beta-hydroxysteroid UDPGT. The presence of this UDPGT isozyme was confirmed by a high level of testosterone glucuronidation activity. This is the first demonstration of a metabolic pathway in bile ductular epithelial cells that is primarily dedicated to the processing of endogenous compounds.
Animals↗
Membrane topology of epoxide hydrolase.
The amino acid sequences of epoxide hydrolase from rat, rabbit and human have been subjected to hydropathy analysis and a novel model for the membrane topology of this enzyme is presented. The enzyme would appear to be retained in microsomal membranes by a single transmembrane segment located at the N-terminus and the majority (96%) of the protein is exposed at the cytosolic membrane surface. This model is significantly different from a scheme suggested by analysis of the rat enzyme alone which proposed six transmembrane domains (Porter et al. (1988) Arch. Biochem. Biophys. 248, 121-129). Experiments with rat microsomal membranes were conducted to distinguish between the two models and used proteolytic enzymes and non-permeant chemical probes. Epoxide hydrolase of intact and permeabilised membranes was resistant to digestion by a number of proteinases. However, this is likely to be related to a compact fold of the protein rather than membrane association since purified, delipidated enzyme preparations were also resistant to proteolysis. While the use of proteinases did not provide useful membrane topological information, experiments with the fluorescent probe, 3-azido-2,7-naphthalenedisulphonate strongly support the view that the majority of the protein is indeed exposed at the cytosolic surface of the membranes. The analysis illustrates the caution which must be employed in the formulation of topological models based on hydropathy plots alone and the value of considering homologous proteins.
Animals↗
Characterization and primary sequence of a human hepatic microsomal estriol UDPglucuronosyltransferase.
A human liver microsomal UDP glucuronosyltransferase (UDPGT) that demonstrates reactivity with estriol (pI 7.4 UDPGT) has been purified to homogeneity and characterized further. No activity toward morphine, 4-hydroxybiphenyl, bilirubin, or tripelennamine was observed. The estriol UDPGT shows immunoreactivity with antibodies raised against rat hepatic microsomal 3 alpha- and 17 beta-hydroxysteroid UDPGTs but not with antibodies raised against rat hepatic microsomal p-nitrophenol UDPGT. The NH2-terminal sequence of the purified protein was determined and found to correspond to an identical sequence in the deduced amino acid sequence of a cDNA obtained from a human liver library in lambda gt11 (HLUG4). Sequence analysis revealed that HLUG4 is 2094 bp in length and encodes a protein of 523 amino acids which has a 16 amino acid leader sequence, followed by an untranslated 3' region of 525 bp. Three potential N-glycosylation sites were identified in the predicted sequence. The deduced amino acid sequence of estriol UDPGT showed 82% identity with the deduced amino acid sequence of another human hepatic cDNA (HLUG25), which has been expressed as a UDPGT capable of 6 alpha-hydroxyglucuronidation of hyodeoxycholic acid, strongly suggesting that these proteins are members of the same gene subfamily.
Amino Acid Sequence↗
Inhibition of UDP-glucuronosyltransferase activity by possible transition-state analogues in rat-liver microsomes.
A series of possible transition state analogues of the glucuronidation reaction catalyzed by UDP-glucuronosyltransferase were tested for their inhibitory effect on glucuronidation of various substrates in a rat liver microsomal fraction. In general 4-nitrophenol glucuronidation was more effectively inhibited than that of 1-naphthol, bilirubin or testosterone. 2-(1-Naphthyl)ethyl-UDP and 2,2,2-(triphenyl)ethyl-UDP were the most effective inhibitors. Their inhibitory effect was competitive towards both UDP-glucuronic acid and 4-nitrophenol. These compounds were much more effective inhibitors than UDP; therefore addition of a lipophilic group enhances the inhibitory potency of UDP. The various UDP derivatives showed differences in their abilities to inhibit the glucuronidation of the four acceptor substrates, supporting the concept that the different forms of UDP-glucuronosyl transferase have different active sites.
Animals↗
UDP-glucuronosyltransferases: a family of detoxifying enzymes.
Glucuronidation is an important process in the metabolism of xenobiotic and endogenous substances leading to enhancement of excretion of these compounds from the body. A multigene family encodes a number of UDP-glucuronosyltransferase enzymes which catalyse this route of metabolism. Recent advances in biochemical and molecular biological approaches, reviewed here by Thomas Tephly and Brian Burchell, have given new insight into the function and structure of UDP-glucuronosyltransferases. These proteins have surprising similarities and yet appear to be capable of conjugating a remarkable number of different chemicals.
Glucuronosyltransferase↗