UDP-glucuronosyltransferases: gene structures of UGT1 and UGT2 families

Properties of Dietary Flavone Glycosides, Aglycones, and Metabolites on the Catalysis of Human Endoplasmic Reticulum Uridine Diphosphate Glucuronosyltransferase 2B7 (UGT2B7)

Flavone glycosides, their aglycones, and metabolites are the major phytochemicals in dietary intake. However, there are still many unknowns about the cellular utilization and active sites of these natural products. Uridine diphosphate glucuronosyltransferases (UGTs) in the endoplasmic reticulum have gene polymorphism distribution in the population and widely mediate the absorption and metabolism of endogenous and exogenous compounds by catalyzing the covalent addition of glucuronic acid and various lipophilic chemicals. Firstly, we found that rutin, a typical flavone O-glycoside, has a stronger UGT2B7 binding effect than its metabolites. After testing a larger number of flavonoids with different aglycones, their aglycones, and metabolites, we demonstrated that typical dietary flavone O-glycosides generally have high binding affinities towards UGT2B7 protein, but the flavone C-glycosides and the phenolic acid metabolites of flavones had no significant effect on this. With the disposition of 4-methylumbelliferone examined by HPLC assay, we determined that 10 μM rutin and nicotifiorin could significantly inhibit the activity of recombinant UGT2B7 protein, which is stronger than isovitexin, vitexin, 3-hydroxyphenylacetic acid and 3,4-dihydroxyphenylacetic acid. In addition, in vitro experiments showed that in normal and doxorubicin-induced lipid composition, both flavone O-glycosides rutin and flavone C-glycosides isovitexin at 10 μM had no significant effect on the expression of UGT1A1, UGT2B4, UGT2B7, and UGT2B15 genes for 24 h exposure. The obtained results enrich the regulatory properties of dietary flavone glycosides, aglycones, and metabolites towards the catalysis of UGTs and will contribute to the establishment of a precise nutritional intervention system based on lipid bilayers and theories of nutrients on endoplasmic reticulum and mitochondria communication.

Duplication, Loss, and Evolutionary Features of Specific UDP-Glucuronosyltransferase Genes in Carnivora (Mammalia, Laurasiatheria)

UDP-glucuronosyltransferases (UGTs) are one of the most important enzymes for xenobiotic metabolism or detoxification. Through duplication and loss of genes, mammals evolved the species-specific variety of UGT isoforms. Among mammals, Carnivora is one of the orders that includes various carnivorous species, yet there is huge variation of food habitat. Recently, lower activity of UGT1A and 2B were shown in Felidae and pinnipeds, suggesting evolutional loss of these isoforms. However, comprehensive analysis for genetic or evolutional features are still missing. This study was conducted to reveal evolutional history of UGTs in Carnivoran species. We found specific gene expansion of UGT1As in Canidae, brown bear and black bear. We also found similar genetic duplication in UGT2Bs in Canidae, and some Mustelidae and Ursidae. In addition, we discovered contraction or complete loss of UGT1A7–12 in phocids, some otariids, felids, and some Mustelids. These studies indicate that even closely related species have completely different evolution of UGTs and further imply the difficulty of extrapolation of the pharmacokinetics and toxicokinetic result of experimental animals into wildlife carnivorans.

Bilirubin-Induced Neurological Damage: Current and Emerging iPSC-Derived Brain Organoid Models

Bilirubin-induced neurological damage (BIND) has been a subject of studies for decades, yet the molecular mechanisms at the core of this damage remain largely unknown. Throughout the years, many in vivo chronic bilirubin encephalopathy models, such as the Gunn rat and transgenic mice, have further elucidated the molecular basis of bilirubin neurotoxicity as well as the correlations between high levels of unconjugated bilirubin (UCB) and brain damage. Regardless of being invaluable, these models cannot accurately recapitulate the human brain and liver system; therefore, establishing a physiologically recapitulating in vitro model has become a prerequisite to unveil the breadth of complexities that accompany the detrimental effects of UCB on the liver and developing human brain. Stem-cell-derived 3D brain organoid models offer a promising platform as they bear more resemblance to the human brain system compared to existing models. This review provides an explicit picture of the current state of the art, advancements, and challenges faced by the various models as well as the possibilities of using stem-cell-derived 3D organoids as an efficient tool to be included in research, drug screening, and therapeutic strategies for future clinical applications.

Study single nucleotide polymorphism in Promoter region of UGT1A1 Gene in Iraqi Patients with Gilbert's syndrome

This study aimed to detect genetic variants of the UGT1A1 gene in patients with Gilbert's syndrome. To detect this, primers were designed; PCR and direct sequencing were done for the promoter area of the gene as a diagnostic tool for the detection of any polymorphism. Variation and polymorphism were detected within the promoter mutants of the UDP glycosyltransferase _UGT1A1 gene that causes hyperbilirubinemia in a group of Iraqi patients compared with a group of the normal healthy individual as controls. The patients with hyperbilirubinemia in this study were 30 in which the total bilirubin level was more than 12 mg/dl serum; they included 25 males and 5 females, while the control group consisted of 20 healthy individuals. This study was carried out from September 2019 till April 2021. The result displayed high occurrence of Gilbert syndrome within male patients than in females, and regarding the analyses of mutation of bilirubin UDP glycosyltransferase _UGT1A1 gene, it is clear that the genotypic distribution of variation among the hyperbilirubinemia patients included all 30 patients, while SNP was detected in 18 patients out of 30 which indicate that the UGT1A1 gene mutation was a likely risk factor for the development of hyperbilirubinemia related Gilbert syndrome in Iraq. The homozygous and heterozygous polymorphisms A/G inside the promoter region of the UGT1A1 gene were effectively identified by sequencing. Our finding suggests that TA repeats and allele of UGT1A1 polymorphism A/G are associated with Gilbert's syndrome and act as genetic markers of this disease in Iraqi patients. To analyze data and sequence variation in gene, generous software was used after amplifying the gene. All processes include DNA extraction, PCR amplification, sequencing, and assembly.

Characterization of the Ontogeny of Hepatic UDP-Glucuronosyltransferase Enzymes Based on Glucuronidation Activity Measured in Human Liver Microsomes

An understanding of the postnatal development of hepatic UDP-glucuronosyltransferase (UGT) enzymes is required for accurate prediction of the age-dependent changes in pharmacokinetics of many drugs used in children. However, the maturation rate of hepatic UGT isoforms remains a major knowledge gap. This study aimed to establish the age-associated changes in glucuronidation activity of 10 major hepatic UGT isoforms in humans, namely, UGT1A1, UGT1A3, UGT1A4, UGT1A6, UGT1A9, UGT2B4, UGT2B7, UGT2B10, UGT2B15, and UGT2B17. Human liver microsomes from pediatric and adult donors were incubated under optimized incubation conditions to assess the activity rates of hepatic UGT isoforms using a panel of 19 in vitro UGT probe substrates and clinically used drugs. Statistically strong correlations of glucuronidation activities allowed the ontogeny of UGT1A1, UGT1A4, UGT2B7, UGT2B10, and UGT2B15 to be established using multiple selective UGT substrates and matched human liver microsome samples. The postnatal development of hepatic UGTs is isoform-dependent using either individual or cross-correlated selective isoform substrates. Maximal adult activity was reached at different times ranging from within a month (UGT1A1, UGT2B4, UGT2B7, UGT2B10, and UGT2B15), during infancy (UGT1A3, UGT1A4, and UGT1A9), to adolescence (UGT1A6 and UGT2B17). This study provides an extensive characterization of the postnatal ontogeny profiles of hepatic UGT enzymes that are instrumental for predicting drug disposition via in vitro-in vivo extrapolation algorithms and verifying pharmacokinetic predictions against in vivo observations via pediatric physiologically based pharmacokinetic modeling in pediatric patients.

Characterization of function and genetic feature of UDP-glucuronosyltransferase in avian species

Birds are exposed to many xenobiotics during their lifetime. For accurate prediction of xenobiotic-induced toxic effects on avian species, it is necessary to understand metabolic capacities in a comprehensive range of bird species. However, there is a lack of information about avian xenobiotic metabolizing enzymes (XMEs), particularly in wild birds. Uridine diphosphate glucuronosyltransferase (UGT) is an XME that plays an important role in phase II metabolism in the livers of mammals and birds. This study was performed to determine the characteristics of UGT1E isoform in avian species, those are related to mammals UGT 1A. To understand the characteristics of avian UGT1E isoforms, in vitro metabolic activity and genetic characteristics were investigated. Furthermore, mRNA expression levels of all chicken UGT1E isoforms were measured. On in vitro enzymatic analysis, the white-tailed eagle, great horned owl, and Humboldt penguin showed lower UGT-dependent activity than domestic birds. In synteny analysis, carnivorous birds were shown to have fewer UGT1E isoforms than herbivorous and omnivorous birds, which may explain why they have lower in vitro UGT activity. These observations suggested that raptors and seabirds, in which UGT activity is low, may be at high risk if exposed to elevated levels of xenobiotics in the environment. Phylogenetic analysis suggested that avian UGT1Es have evolved independently from mammalian UGT1As. We identified the important UGT isoforms, such as UGT1E13, and suspected their substrate specificities in avian xenobiotic metabolism by phylogenetic and quantitative real-time PCR analysis. This is the first report regarding the genetic characteristics and interspecies differences of UGT1Es in avian species.

Functional metabolomics discover pentose and glucuronate interconversion pathways as promising targets for Yang Huang syndrome treatment with Yinchenhao Tang

Yinchenhao Tang (YCHT), a classic traditional Chinese medicine (TCM) formulae, plays an important role in the treatment of Yang Huang syndrome (YHS). With the emergence of new biomarkers of YHS uncovered via metabonomics, the underlying functional mechanisms are still not clear. Functional metabolomics aims at converting biomarkers derived from metabonomics into disease mechanisms. Here, an integrated non-target metabolomics and IPA strategy were used to investigate the YCHT intervention on YHS. Our metabolomics study has shown that the potential protective effect of YCHT on YHS mice leads to significant changes in the metabolic profile by modulating the biomarkers and regulating the metabolic disorders. Twenty two differential metabolite biomarkers and fifteen involved metabolic pathways were correlated with the regulation of YCHT treatment on YHS. Functional metabolomics identified a core biomarker, d-glucuronic acid in pentose and glucuronate interconversion pathways, which was directly related to the target prediction of UDP-glucuronosyltransferase 1A1 and eventually leaded to a series of disturbances. In conclusion, this study shows that functional metabolomics can discover metabolic pathways as promising targets.

Yinchenhao Tang (YCHT), a classic traditional Chinese medicine (TCM) formulae, plays an important role in the treatment of Yang Huang syndrome (YHS).

Species differences in drug glucuronidation: Humanized UDP-glucuronosyltransferase 1 mice and their application for predicting drug glucuronidation and drug-induced toxicity in humans

More than 20% of clinically used drugs are glucuronidated by a microsomal enzyme UDP-glucuronosyltransferase (UGT). Inhibition or induction of UGT can result in an increase or decrease in blood drug concentration. To avoid drug-drug interactions and adverse drug reactions in individuals, therefore, it is important to understand whether UGTs are involved in metabolism of drugs and drug candidates. While most of glucuronides are inactive metabolites, acyl-glucuronides that are formed from compounds with a carboxylic acid group can be highly toxic. Animals such as mice and rats are widely used to predict drug metabolism and drug-induced toxicity in humans. However, there are marked species differences in the expression and function of drug-metabolizing enzymes including UGTs. To overcome the species differences, mice in which certain drug-metabolizing enzymes are humanized have been recently developed. Humanized UGT1 (hUGT1) mice were created in 2010 by crossing Ugt1-null mice with human UGT1 transgenic mice in a C57BL/6 background. hUGT1 mice can be promising tools to predict human drug glucuronidation and acyl-glucuronide-associated toxicity. In this review article, studies of drug metabolism and toxicity in the hUGT1 mice are summarized. We further discuss research and strategic directions to advance the understanding of drug glucuronidation in humans.

Glucuronidation: driving factors and their impact on glucuronide disposition

Glucuronidation is a well-recognized phase II metabolic pathway for a variety of chemicals including drugs and endogenous substances. Although it is usually the secondary metabolic pathway for a compound preceded by phase I hydroxylation, glucuronidation alone could serve as the dominant metabolic pathway for many compounds, including some with high aqueous solubility. Glucuronidation involves the metabolism of parent compound by UDP-glucuronosyltransferases (UGTs) into hydrophilic and negatively charged glucuronides that cannot exit the cell without the aid of efflux transporters. Therefore, elimination of parent compound via glucuronidation in a metabolic active cell is controlled by two driving forces: the formation of glucuronides by UGT enzymes and the (polarized) excretion of these glucuronides by efflux transporters located on the cell surfaces in various drug disposition organs. Contrary to the common assumption that the glucuronides reaching the systemic circulation were destined for urinary excretion, recent evidences suggest that hepatocytes are capable of highly efficient biliary clearance of the gut-generated glucuronides. Furthermore, the biliary- and enteric-eliminated glucuronides participate into recycling schemes involving intestinal microbes, which often prolong their local and systemic exposure, albeit at low systemic concentrations. Taken together, these recent research advances indicate that although UGT determines the rate and extent of glucuronide generation, the efflux and uptake transporters determine the distribution of these glucuronides into blood and then to various organs for elimination. Recycling schemes impact the apparent plasma half-life of parent compounds and their glucuronides that reach intestinal lumen, in addition to prolonging their gut and colon exposure.

UDP-glucuronosyltransferases (UGTs) and their related metabolic cross-talk with internal homeostasis: A systematic review of UGT isoforms for precision medicine

UDP-glucuronosyltransferases (UGTs) are the primary phase II enzymes catalyzing the conjugation of glucuronic acid to the xenobiotics with polar groups for facilitating their clearance. The UGTs belong to a superfamily that consists of diverse isoforms possessing distinct but overlapping metabolic activity. The abnormality or deficiency of UGTs in vivo is highly associated with some diseases, efficacy and toxicity of drugs, and precisely therapeutic personality. Despite the great effects and fruitful results achieved, to date, the expression and functions of individual UGTs have not been well clarified, the inconsistency of UGTs is often observed in human and experimental animals, and the complex regulation factors affecting UGTs have not been systematically summarized. This article gives an overview of updated reports on UGTs involving the various regulatory factors in terms of the genetic, environmental, pathological, and physiological effects on the functioning of individual UGTs, in turn, the dysfunction of UGTs induced disease risk and endo- or xenobiotic metabolism-related toxicity. The complex cross-talk effect of UGTs with internal homeostasis is systematically summarized and discussed in detail, which would be of great importance for personalized precision medicine.

Inherited disorders of bilirubin clearance

Inherited disorders of hyperbilirubinemia may be caused by increased bilirubin production or decreased bilirubin clearance. Reduced hepatic bilirubin clearance can be due to defective (i) unconjugated bilirubin uptake and intrahepatic storage, (ii) conjugation of glucuronic acid to bilirubin (e.g., Gilbert syndrome, Crigler-Najjar syndrome, Lucey-Driscoll syndrome, breast milk jaundice), (iii) bilirubin excretion into bile (Dubin-Johnson syndrome), or (iv) conjugated bilirubin re-uptake (Rotor syndrome). In this review, the molecular mechanisms and clinical manifestations of these conditions are described, as well as current approaches to diagnosis and therapy.

Phytoremediation potential of Arabidopsis with reference to acrylamide and microarray analysis of acrylamide-response genes

Acrylamide (ACR) is a widely used industrial chemical. However, it is a dangerous compound because it showed neurotoxic effects in humans and act as reproductive toxicant and carcinogen in many animal species. In the environment, acrylamide has high soil mobility and may travel via groundwater. Phytoremediation is an effective method to remove the environmental pollutants, but the mechanism of plant response to acrylamide remains unknown. With the purpose of assessing remediation potentials of plants for acrylamide, we have examined acrylamide uptake by the model plant Arabidopsis grown on contaminated substrates with high performance liquid chromatography (HPLC) analysis. The result revealed that acrylamide could be absorbed and degraded by Arabidopsis. Further microarray analysis showed that 527 transcripts were up-regulated within 2-days under acrylamide exposure condition. We have found many potential acrylamide-induced genes playing a major role in plant metabolism and phytoremediation.

Xenobiotic Metabolism in Mice Lacking the UDP-Glucuronosyltransferase 2 Family

UDP-Glucuronosyltransferases (UGTs) conjugate a glucuronyl group from glucuronic acid to a wide range of lipophilic substrates to form a hydrophilic glucuronide conjugate. The glucuronide generally has decreased bioactivity and increased water solubility to facilitate excretion. Glucuronidation represents an important detoxification pathway for both endogenous waste products and xenobiotics, including drugs and harmful industrial chemicals. Two clinically significant families of UGT enzymes are present in mammals: UGT1s and UGT2s. Although the two families are distinct in gene structure, studies using recombinant enzymes have shown considerable overlap in their ability to glucuronidate many substrates, often obscuring the relative importance of the two families in the clearance of particular substrates in vivo. To address this limitation, we have generated a mouse line, termed ΔUgt2, in which the entire Ugt2 gene family, extending over 609 kilobase pairs, is excised. This mouse line provides a means to determine the contributions of the two UGT families in vivo. We demonstrate the utility of these animals by defining for the first time the in vivo contributions of the UGT1 and UGT2 families to glucuronidation of the environmental estrogenic agent bisphenol A (BPA). The highest activity toward this chemical is reported for human and rodent UGT2 enzymes. Surprisingly, our studies using the ΔUgt2 mice demonstrate that, while both UGT1 and UGT2 isoforms can conjugate BPA, clearance is largely dependent on UGT1s.

Posttranscriptional regulation of uridine diphosphate glucuronosyltransferases

INTRODUCTION

The uridine diphosphate (UDP)-glucuronosyltransferase (UGT) superfamily of enzymes (EC 2.4.1.17) conjugates glucuronic acid to an aglycone substrate to make them more polar and readily excreted. In general, this reaction terminates the activities of chemicals, drugs and toxins, although occasionally a more active or toxic species is produced.

AREAS COVERED

In addition to their well-known transcriptional responsiveness, UGTs are also regulated by posttranscriptional mechanisms. Here, the authors review these mechanisms, including latency, modulation of co-substrate accessibility and binding, dimerization and oligomerization, protein-protein interactions, allosteric inhibition and activation, posttranslational structural and functional modifications and developmental switching for UGTs.

EXPERT OPINION

Posttranscriptional regulation of UGTs has traditionally received less attention than nuclear regulation, in part because mechanisms involving ribosomes and endoplasmic reticula are challenging to investigate. Most promising of the posttranscriptional mechanisms reviewed are likely to be effects on co-substrate (UDP-glucuronic acid) transport and availability and structure-function changes to UGT proteins through, for example, glycosylation and phosphorylation. Although classical biochemistry continues to illuminate many aspects of UGT function, advances in proteomics and structural biology are beginning to assist in the determination of posttranscriptional regulation mechanisms for UGTs.

Crigler-Najjar syndrome type II in a Chinese boy resulting from three mutations in the bilirubin uridine 5'-diphosphate-glucuronosyltransferase (UGT1A1) gene and a family genetic analysis

Background

The UGT1A1 gene encodes a responsible enzyme, UDP-glucuronosyltransferase1A1 (UGT1A1), for bilirubin metabolism. Many mutations have already been identified in patients with inherited disorders with unconjugated hyperbilirubinemia, such as Crigler-Najjar syndromes and Gilbert’s syndrome.

Case presentation

In this report, we presented a boy with intermittent unconjugated hyperbilirubinemia, whose genetic analysis showed a new compound heterozygote determined by three mutations, c.211G > A (p.G71R), c.508_510delTTC (p.F170-) and c.1456 T > G (p.Y486D) in the hotspot regions of the UGT1A1 gene (exons 1 and 5) in Asian populations, presenting a genotype compatible with clinical picture of CNS-II. The family genetic analysis confirmed the origin of these mutations.

Conclusion

UGT1A1 gene analysis should be performed in all cases with unexplained unconjugated hyperbilirubinemia. The description of patients with peculiar genotypes especially including family analysis could help explain the relationship between the genotype and phenotype,it is helpful for clinicians to predict the outcome of the patients.

Inherited disorders of bilirubin transport and conjugation: new insights into molecular mechanisms and consequences

Inherited disorders of bilirubin metabolism might reduce bilirubin uptake by hepatocytes, bilirubin conjugation, or secretion of bilirubin into bile. Reductions in uptake could increase levels of unconjugated or conjugated bilirubin (Rotor syndrome). Defects in bilirubin conjugation could increase levels of unconjugated bilirubin; the effects can be benign and frequent (Gilbert syndrome) or rare but severe, increasing the risk of bilirubin encephalopathy (Crigler-Najjar syndrome). Impairment of bilirubin secretion leads to accumulation of conjugated bilirubin (Dubin-Johnson syndrome). We review the genetic causes and pathophysiology of disorders of bilirubin transport and conjugation as well as clinical and therapeutic aspects. We also discuss the possible mechanisms by which hyperbilirubinemia protects against cardiovascular disease and the metabolic syndrome and the effects of specific genetic variants on drug metabolism and cancer development.

Characterization of the zebrafish Ugt repertoire reveals a new class of drug-metabolizing UDP glucuronosyltransferases

The zebrafish genome contains a gene superfamily of 40 Ugt genes that can be divided into Ugt1, Ugt2, and Ugt5 families. Because the encoded zebrafish UDP glucuronosyltransferase (UGT) proteins do not display orthologous relationships to any of the mammalian and avian UGT enzymes based on molecular phylogeny, it is difficult to predict their substrate specificity. Here, we mapped their tissue-specific expression patterns. We showed that the zebrafish UGT enzymes can be glycosylated. We determined their substrate specificity and catalytic activity toward diverse aglycone substrates. Specifically, we measured mRNA levels of each of the 40 zebrafish Ugt genes in 11 adult tissues and found that they are expressed in a tissue-specific manner. Moreover, functional analyses with the donor of UDP glucuronic acid (UDPGA) for each of the 40 zebrafish UGT proteins revealed their substrate specificity toward 10 important aglycones. In particular, UGT1A1, UGT1A7, and UGT1B1 displayed good glucuronidation activities toward most phenolic aglycones (4-methylumbelliferone, 4-nitrophenol, 1-naphthol, bisphenol A, and mycophenolic acid) and the two carboxylic acids (bilirubin and diclofenac). Importantly, some members of the UGT5, a novel UGT family identified recently, are capable of glucuronidating multiple aglycones with the donor cofactor of UDPGA. In particular, UGT5A5, UGT5B2, and UGT5E1 glucuronidate phenols and steroids with high specificity toward steroid hormones of estradiol and testosterone and estrogenic alkylphenols 4-tert-octylphenol. These results shed new insights into the mechanisms by which fish species defend themselves against vast numbers of xenobiotics via glucuronidation conjugations and may facilitate the establishment of zebrafish as a model vertebrate in toxicological, developmental, and pathologic studies.

New insights in bilirubin metabolism and their clinical implications

Bilirubin, a major end product of heme breakdown, is an important constituent of bile, responsible for its characteristic colour. Over recent decades, our understanding of bilirubin metabolism has expanded along with the processes of elimination of other endogenous and exogenous anionic substrates, mediated by the action of multiple transport systems at the sinusoidal and canalicular membrane of hepatocytes. Several inherited disorders characterised by impaired bilirubin conjugation (Crigler-Najjar syndrome type I and type II, Gilbert syndrome) or transport (Dubin-Johnson and Rotor syndrome) result in various degrees of hyperbilirubinemia of either the predominantly unconjugated or predominantly conjugated type. Moreover, disrupted regulation of hepatobiliary transport systems can explain jaundice in many acquired liver disorders. In this review, we discuss the recent data on liver bilirubin handling based on the discovery of the molecular basis of Rotor syndrome. The data show that a substantial fraction of bilirubin conjugates is primarily secreted by MRP3 at the sinusoidal membrane into the blood, from where they are subsequently reuptaken by sinusoidal membrane-bound organic anion transporting polypeptides OATP1B1 and OATP1B3. OATP1B proteins are also responsible for liver clearance of bilirubin conjugated in splanchnic organs, such as the intestine and kidney, and for a number of endogenous compounds, xenobiotics and drugs. Absence of one or both OATP1B proteins thus may have serious impact on toxicity of commonly used drugs cleared by this system such as statins, sartans, methotrexate or rifampicin. The liver-blood cycling of conjugated bilirubin is impaired in cholestatic and parenchymal liver diseases and this impairment most likely contributes to jaundice accompanying these disorders.

Nuclear receptors and endobiotics glucuronidation: the good, the bad, and the UGT

The recent progresses in molecular biology and pharmacology approaches allowed the characterization of a series of nuclear receptors (NRs) as efficient regulators of uridine diphosphate glucuronosyltransferase (UGT) genes activity. These regulatory processes ensure an optimized UGT expression in response to specific endo- and/or exogenous stimuli. Many of these NRs are activated by endobiotics that also are substrates for UGTs. Thus, by activating their receptors, these endogenous substances control their own conjugation, leading to the concept that glucuronidation is an important part of feed-forward/feedback mechanisms by which bioactive molecules control their own concentrations. On the other hand, numerous studies have established the pharmacological relevance of NR-UGT regulatory pathways in the response to therapeutic ligands. The present review article aims at providing a comprehensive view of the physiological and pharmacological importance of the NR regulation of the expression and activity of endobiotics-conjugating UGT enzymes. Selected examples will illustrate how the organism profits from the feed-forward/feedback mechanisms involving NR-UGT pathways, but also how such regulatory processes are involved in the initiation and/or progression of several pathological situations. Finally, we will discuss how the present pharmacopeia involves NR-dependent regulation of endobiotics glucuronidation, and whether the unexploited NR-UGT axes could serve as pharmacological targets for novel therapeutics to restore endobiotics homeostasis.

Gene expression variability in human hepatic drug metabolizing enzymes and transporters

Interindividual variability in the expression of drug-metabolizing enzymes and transporters (DMETs) in human liver may contribute to interindividual differences in drug efficacy and adverse reactions. Published studies that analyzed variability in the expression of DMET genes were limited by sample sizes and the number of genes profiled. We systematically analyzed the expression of 374 DMETs from a microarray data set consisting of gene expression profiles derived from 427 human liver samples. The standard deviation of interindividual expression for DMET genes was much higher than that for non-DMET genes. The 20 DMET genes with the largest variability in the expression provided examples of the interindividual variation. Gene expression data were also analyzed using network analysis methods, which delineates the similarities of biological functionalities and regulation mechanisms for these highly variable DMET genes. Expression variability of human hepatic DMET genes may affect drug-gene interactions and disease susceptibility, with concomitant clinical implications.

Human UDP-glucuronosyltransferase UGT2A2: cDNA construction, expression, and functional characterization in comparison with UGT2A1 and UGT2A3

OBJECTIVES

Characterize the expression and glucuronidation activities of the human uridine 5'-diphospho (UDP)-glucuronosyltransferase (UGT) 2A2.

METHOD

UGT2A1 was cloned from nasal mucosa mRNA. Synthetic cDNA for UGT2A2 was constructed assuming exon sharing between UGT2A1 and UGT2A2 (Mackenzie et al., Pharmacogenetics and Genomics 2005, 15:677-685). Exon 1 of UGT2A2 was amplified from genomic DNA and combined with exons 2-6 of UGT2A1. UGT2A3 was cloned from liver mRNA. Quantitative reverse-transcribed-PCR (RT-PCR) was used to evaluate the expression of all the three UGTs of subfamily 2A in different tissues. Recombinant UGT2A1, UGT2A2 and UGT2A3 were expressed in baculovirus-infected insect cells and analyzed for glucuronidation activity towards different substrates.

RESULTS

DNA sequencing of RT-PCR products from human nasal mucosa mRNA, confirmed exon sharing between UGT2A1 and UGT2A2. In addition, it indicated that the N-terminal signal peptide sequence of UGT2A2 is the longest among the human UGTs. Quantitative RT-PCR revealed that both UGT2A1 and UGT2A2 are mainly expressed in the nasal mucosa, and that their expression level in fetal samples was much higher than in adults. Activity assays with recombinant UGTs 2A1-2A3 showed broad substrate selectivity for UGT2A1 and UGT2A2. Although glucuronidation rates and substrate affinities were mostly higher in UGT2A1, the Km values for UDP-glucuronic acid were similar in both UGTs. In addition, there were regioselectivity differences between the two UGTs and, with a few substrates, particularly ethinylestradiol, the activity of UGT2A2 was higher.

CONCLUSION

UGT2A2 is mainly expressed in the nasal mucosa and it has glucuronidation activity towards several different endobiotic and xenobiotic substrates.

Phytoremediation and phytosensing of chemical contaminant, toluene: identification of the required target genes

As an industrial chemical produced worldwide in high volumes, toluene is commonly detected in ambient air and water. It can combine with oxygen and form compounds that are harmful to humans. In recent years, phytoremediation has been increasingly applied to repair the environmental damage caused by pollutants. However, insufficient knowledge is available regarding the response of plants to toluene. To detect the potential genes in plants that are related to the sensing mechanism and metabolism of toluene, a microarray analysis has been conducted on Arabidopsis thaliana seedlings grown on toluene-containing media. Following the validation of data and the application of appropriate selection criteria, the results show a coordinated induction and suppression of 202 and 67 toluene-responsive genes, respectively. Within the functional class "metabolism", the genes encoding detoxification proteins represent the most strongly up-regulated group. These include genes encoding cytochrome P450s, glucosyl transferases, and transporters. Subsequently, the toluene-induced genes of Arabidopsis are analyzed in detail.

Genetic variations and haplotype diversity of the UGT1 gene cluster in the Chinese population

Vertebrates require tremendous molecular diversity to defend against numerous small hydrophobic chemicals. UDP-glucuronosyltransferases (UGTs) are a large family of detoxification enzymes that glucuronidate xenobiotics and endobiotics, facilitating their excretion from the body. The UGT1 gene cluster contains a tandem array of variable first exons, each preceded by a specific promoter, and a common set of downstream constant exons, similar to the genomic organization of the protocadherin (Pcdh), immunoglobulin, and T-cell receptor gene clusters. To assist pharmacogenomics studies in Chinese, we sequenced nine first exons, promoter and intronic regions, and five common exons of the UGT1 gene cluster in a population sample of 253 unrelated Chinese individuals. We identified 101 polymorphisms and found 15 novel SNPs. We then computed allele frequencies for each polymorphism and reconstructed their linkage disequilibrium (LD) map. The UGT1 cluster can be divided into five linkage blocks: Block 9 (UGT1A9), Block 9/7/6 (UGT1A9, UGT1A7, and UGT1A6), Block 5 (UGT1A5), Block 4/3 (UGT1A4 and UGT1A3), and Block 3′ UTR. Furthermore, we inferred haplotypes and selected their tagSNPs. Finally, comparing our data with those of three other populations of the HapMap project revealed ethnic specificity of the UGT1 genetic diversity in Chinese. These findings have important implications for future molecular genetic studies of the UGT1 gene cluster as well as for personalized medical therapies in Chinese.

Stress responses to phenol in Arabidopsis and transcriptional changes revealed by microarray analysis

Phenols are toxic, environmentally persistent products of the chemical industry that are capable of bioaccumulation and biomagnifications in the food chain. Little is known of how plants respond to this compound. To understand the transcriptional changes under phenol, microarray experiments on Arabidopsis thaliana were performed. Microarray results revealed numerous perturbations in signaling and metabolic pathways. The results indicated that the phenol response was related to reactive oxygen species (ROS) accumulation and oxidative conditions, including ROS generated for pathogen defense.

Red wine consumption is inversely associated with 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine-DNA adduct levels in prostate

In humans, genetic variation and dietary factors may alter the biological effects of exposure to 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP), one of the major heterocyclic amines generated from cooking meats at high temperatures that has carcinogenic potential through the formation of DNA adducts. Previously, we reported grilled red meat consumption associated with PhIP-DNA adduct levels in human prostate. In this study, we expanded our investigation to estimate the associations between beverage consumption and PhIP-DNA adduct levels in prostate for 391 prostate cancer cases. Of the 15 beverages analyzed, red wine consumption had the strongest association with PhIP-DNA adduct levels showing an inverse correlation in both tumor (P = 0.006) and nontumor (P = 0.002) prostate cells. Red wine consumption was significantly lower in African American compared with white cases, but PhIP-DNA adduct levels in prostate did not vary by race. In African Americans compared with whites, however, associations between red wine consumption and PhIP-DNA adduct levels were not as strong as associations with specific (e.g., SULT1A1 and UGT1A10 genotypes) and nonspecific (e.g., African ancestry) genetic variation. In a multivariable model, the covariate for red wine consumption explained a comparable percentage (13%-16%) of the variation in PhIP-DNA adduct levels in prostate across the two racial groups, but the aforementioned genetic factors explained 33% of the PhIP-DNA adduct variation in African American cases, whereas only 19% of the PhIP-DNA adduct variation in whites. We conclude that red wine consumption may counteract biological effects of PhIP exposure in human prostate, but genetic factors may play an even larger role, particularly in African Americans.

Hereditary hyperbilirubinemia and its molecular diagnosis

Hereditary hyperbilirubinemia is caused by genetic defects in the enzymes that control bilirubin metabolism. It includes Gilbert syndrome (GS), Crigler-Najjar syndrome (CNS), Lucey-Driscoll syndrome (LDS), Dubin-Johnson syndrome (DJS), Rotor syndrome (RS) and progressive familial intrahepatic cholestasis (PFIC). This literature review covers the molecular basis of and laboratory detection methods for hereditary hyperbilirubinemia.

Snapback primer genotyping of the Gilbert syndrome UGT1A1 (TA)(n) promoter polymorphism by high-resolution melting

BACKGROUND

Gilbert syndrome, a chronic nonhemolytic unconjugated hyperbilirubinemia, is associated with thymine-adenine (TA) insertions in the UGT1A1 (UDP glucuronosyltransferase 1 family, polypeptide A1) promoter. The UGT1A1 promoter genotype also correlates with toxicity induced by the chemotherapeutic drug irinotecan. Current closed-tube assays for genotyping the UGT1A1 (TA)(n) promoter polymorphism require multiple labeled probes and/or have difficulty classifying the (TA)(5) and (TA)(8) alleles.

METHODS

An unlabeled 5' extension on one primer that creates a hairpin after asymmetric PCR was used to develop a snapback primer high-resolution melting assay for the (TA)(n) polymorphism. A new method that plots the local deviation from exponential decay to improve genotype clustering was used to remove background fluorescence and to analyze the data. The snapback assay was compared with small-amplicon melting and fragment length analyses in a blinded study of DNA samples from 100 African Americans.

RESULTS

Genotyping results obtained by small-amplicon melting and snapback primer melting were 83% and 99% concordant, respectively, with results obtained by fragment analysis. Reanalysis of the single discordant sample in the results of the snapback genotyping assay and the fragment analysis revealed an error in the fragment analysis. High-resolution melting was required for accurate snapback genotyping of the UGT1A1 (TA)(n) polymorphism. The 100% accuracy obtained with a capillary-based instrument fell to ≤81% with plate-based instruments.

CONCLUSIONS

In contrast to small-amplicon genotyping, snapback primer genotyping can distinguish all UGT1A1 promoter genotypes. Rapid-cycle PCR combined with snapback primer analysis with only 2 unlabeled PCR primers (one with a 5' extension) and a saturating DNA dye can genotype loci with several alleles in <30 min.

Microarray analysis of the phytoremediation and phytosensing of occupational toxicant naphthalene

Naphthalene is of global environmental concern because it is assumed to contribute considerably to human cancer risk. Plants are important in removing naphthalene from the atmosphere and soil. However, there remains insufficient knowledge on plant response to this compound. To determine the mechanism of naphthalene uptake and transduction in plants, as well as plant response to this compound, a microarray system was used to analyze gene expression patterns in Arabidopsis thaliana after irrigation with 2.0mM naphthalene. A total of 247 differentially expressed genes were identified as upregulated by naphthalene. These genes might specifically contribute to naphthalene uptake, transformation, conjugation, and compartmentalization in the plant. The potential role of upregulated genes in plant defense to naphthalene and the use of phytosensing for naphthalene detection were also discussed.

Intestinal transport of scutellarein and scutellarin and first-pass metabolism by UDP-glucuronosyltransferase-mediated glucuronidation of scutellarein and hydrolysis of scutellarin

Scutellarin (SG) is a bioactive flavonoid used to treat cardiovascular disease. Scutellarein (S) is the aglycone form of SG. This study aimed to characterize their intestinal transport and first-pass metabolism by UDP-glucuronosyltransferase-mediated glucuronidation and β-glucuronidase-mediated hydrolysis. Results showed that S is more readily passed through Caco-2 cell monolayers by passive diffusion than SG. SG was the predominant metabolite of S, which was formed during the transportation of S across Caco-2 cell monolayers or following incubation of S with human microsomes. SG was extensively generated in human liver microsomes (HLMs), which was demonstrated by its higher catalyzing efficiency (C(lint)) in liver microsomes than in human intestinal microsomes (HIMs). Enzymatic kinetic analysis indicated that the catalyzing efficiency of UGT1A9 was the highest among the tested UGTs under the present experimental conditions, followed by UGT1A1 and UGT1A3. No significant P450-mediated hydroxylation of S was found. SG may be hydrolyzed into S in both HLMs and HIMs.

First-pass metabolism via UDP-glucuronosyltransferase: a barrier to oral bioavailability of phenolics

Glucuronidation mediated by UDP-glucuronosyltransferases (UGTs) is a significant metabolic pathway that facilitates efficient elimination of numerous endobiotics and xenobiotics, including phenolics. UGT genetic deficiency and polymorphisms or inhibition of glucuronidation by concomitant use of drugs are associated with inherited physiological disorders or drug-induced toxicities. Moreover, extensive glucuronidation can be a barrier to oral bioavailability as the first-pass glucuronidation (or premature clearance by UGTs) of orally administered agents usually results in the poor oral bioavailability and lack of efficacies. This review focused on the first-pass glucuronidation of phenolics including natural polyphenols and pharmaceuticals. The complexity of UGT-mediated metabolism of phenolics is highlighted with species-, gender-, organ- and isoform-dependent specificity, as well as functional compensation between UGT1A and 2B subfamily. In addition, recent advances are discussed with respect to the mechanisms of enzymatic actions, including the important properties such as binding pocket size and phosphorylation requirements.

Validation of a candidate deoxynivalenol-inactivating UDP-glucosyltransferase from barley by heterologous expression in yeast

Resistance to the virulence factor deoxynivalenol (DON) due to formation of DON-3-O-glucoside (D3G) is considered to be an important component of resistance against Fusarium spp. which produce this toxin. Multiple candidate UDP-glycosyltransferase (UGT) genes from different crop plants that are either induced by Fusarium spp. or differentially expressed in cultivars varying in Fusarium disease resistance have been described. However, UGT are encoded by a very large gene family in plants. The study of candidate plant UGT is highly warranted because of the potential relevance for developing Fusarium-spp.-resistant crops. We tested Arabidopsis thaliana genes closely related to a previously identified DON-glucosyltransferase gene by heterologous expression in yeast and showed that gene products with very high sequence similarity can have pronounced differences in detoxification capabilities. We also tested four candidate barley glucosyltransferases, which are highly DON inducible. Upon heterologous expression of full-length cDNAs, only one gene, HvUGT13248, conferred DON resistance. The conjugate D3G accumulated in the supernatant of DON-treated yeast transformants. We also present evidence that the product of the TaUGT3 gene recently proposed to encode a DON-detoxification enzyme of wheat does not protect yeast against DON.

Gene polymorphisms and their role in epilepsy treatment and prognosis

The human genome carries an enormous number of genetic variants, many of them of functional consequence. In epilepsy, they are likely to be involved in drug-specific treatment efficacy, unwanted or even toxic drug reactions, teratogenic risks in pregnancy as well as in the long-term prognosis of patients with epilepsy. As in many other disorders with a complex genetic background, the associated genetic variants that could be verified successfully in replication studies are still only a few. However, new techniques and improved research strategies are likely to increase their number in the foreseeable future, although at a much slower pace as initially expected.

Functions and transcriptional regulation of adult human hepatic UDP-glucuronosyl-transferases (UGTs): mechanisms responsible for interindividual variation of UGT levels

Ten out of 19 UDP-glucuronosyltransferases (UGTs) are substantially expressed in adult human liver (>1% of total UGTs); 5 UGT1 isoforms (UGT1A1, 1A3, 1A4, 1A6 and 1A9) and 5 UGT2 family members (UGT2B4, 2B7, 2B10, 2B15 and 2B17) (Izukawa et al. [11]). Surprisingly, UGT2B4 and UGT2B10 mRNA were found to be abundant in human liver suggesting an underestimated role of the liver in detoxification of their major substrates, bile acids and eicosanoids. Among factors responsible for high interindividual variation of hepatic UGT levels (genetic diversity including polymorphisms and splice variants, regulation by liver-enriched transcription factors such as HNF1 and HNF4, and ligand-activated transcription factors) nuclear receptors (PXR, CAR, PPARalpha, etc.), and the Ah receptor are discussed. Unraveling the mechanisms responsible for interindividual variation of UGT expression will be beneficial for drug therapy but still remains a major challenge.

Ligand orientation governs conjugation capacity of UDP-glucuronosyltransferase 1A1

UDP-glucuronosyltransferase 1A1 (UGT1A1) is an endoplasmic reticulum membrane protein that catalyses glucuronidation. Mutant UGT1A1 possesses a different conjugation capacity, and the molecular mechanisms regulating these conjugation reactions are as yet unclear. To elucidate these molecular mechanisms, we simulated and analysed the glucuronidation of wild-type UGT1A1 and six UGT1A1 mutants, with bilirubin as the substrate. We found that only the orientation of the substrates correlated with the conjugation capacity in in vitro experiments. Inasmuch as glucuronidation is an intermolecular rearrangement reaction, we find that the conjugation reaction proceeds only when the hydroxyl group of the substrate is oriented towards the coenzyme, which allows the proton transfer to occur.

Cloning and comparative analyses of the zebrafish Ugt repertoire reveal its evolutionary diversity

UDP-glucuronosyltransferases (Ugts) are a supergene family of phase II drug-metabolizing enzymes that catalyze the conjugation of numerous hydrophobic small molecules with the UDP-glucuronic acid, converting them into hydrophilic molecules. Here, we report the identification and cloning of the complete zebrafish Ugt gene repertoire. We found that the zebrafish genome contains 45 Ugt genes that can be divided into three families: Ugt1, Ugt2, and Ugt5. Both Ugt1 and Ugt2 have two unlinked clusters: a and b. The Ugt1a, Ugt1b, Ugt2a, and Ugt2b clusters each contain variable and constant regions, similar to that of the protocadherin (Pcdh), immunoglobulin (Ig), and T-cell receptor (Tcr) clusters. Cloning the full-length coding sequences confirmed that each of the variable exons is separately spliced to the set of constant exons within each zebrafish Ugt cluster. Comparative analyses showed that both a and b clusters of the zebrafish Ugt1 and Ugt2 genes have orthologs in other teleosts, suggesting that they may be resulted from the "fish-specific" whole-genome duplication event. The Ugt5 genes are a novel family of Ugt genes that exist in teleosts and amphibians. Their entire open reading frames are encoded by single large exons. The zebrafish Ugt1, Ugt2, and Ugt5 genes can generate additional transcript diversity through alternative splicing. Based on phylogenetic analyses, we propose that the ancestral tetrapod and teleost Ugt1 clusters contained multiple Ugt1 paralogs. After speciation, these ancestral Ugt1 clusters underwent lineage-specific gene loss and duplication. The ancestral vertebrate Ugt2 cluster also underwent lineage-specific duplication. The intronless Ugt5 open reading frames may be derived from retrotransposition followed by gene duplication. They have been expanded dramatically in teleosts and have become the most abundant Ugt family in these lineages. These findings have interesting implications regarding the molecular evolution of genes with diversified variable exons in vertebrates.

Regulation of sulfotransferase and UDP-glucuronosyltransferase gene expression by the PPARs

During phase II metabolism, a substrate is rendered more hydrophilic through the covalent attachment of an endogenous molecule. The cytosolic sulfotransferase (SULT) and UDP-glucuronosyltransferase (UGT) families of enzymes account for the majority of phase II metabolism in humans and animals. In general, phase II metabolism is considered to be a detoxication process, as sulfate and glucuronide conjugates are more amenable to excretion and elimination than are the parent substrates. However, certain products of phase II metabolism (e.g., unstable sulfate conjugates) are genotoxic. Members of the nuclear receptor superfamily are particularly important regulators of SULT and UGT gene transcription. In metabolically active tissues, increasing evidence supports a major role for lipid-sensing transcription factors, such as peroxisome proliferator-activated receptors (PPARs), in the regulation of rodent and human SULT and UGT gene expression. This review summarizes current information regarding the regulation of these two major classes of phase II metabolizing enzyme by PPARs.

Amino acid positions 69-132 of UGT1A9 are involved in the C-glucuronidation of phenylbutazone

Phenylbutazone (PB) is known to be biotransformed to its O- and C-glucuronide. Recently, we reported that PB C-glucuronide formation is catalyzed by UGT1A9. Interestingly, despite UGT1A8 sharing high amino acid sequence identity with UGT1A9, UGT1A8 had no PB C-glucuronidating activity. In the present study, we constructed eight UGT1A9/UGT1A8 chimeras and evaluated which region is important for PB C-glucuronide formation. All of the chimeras and UGT1A8 and UGT1A9 had 7-hydroxy-(4-trifluoromethyl)coumarin (HFC) O-glucuronidating activity. The K(m) values for HFC glucuronidation of UGT1A8, UGT1A9 and their chimeras were divided into two types, UGT1A8 type (high K(m)) and UGT1A9 type (low K(m)), and these types were determined according to whether their amino acids at positions 69-132 were those of UGT1A8 or UGT1A9. Likewise, PB O-glucuronidating activity was also detected by all of the chimeras, and their K(m) values were divided into two types. On the contrary, PB C-glucuronidating activity was detected by UGT1A9((1-132))/1A8((133-286)), UGT1A9((1-212))/1A8((213-286)), UGT1A8((1-68))/1A9((69-286)), and UGT1A8((1-68))/1A9((69-132))/1A8((133-286)) chimeras. The region 1A9((69-132)) was common among chimeras having PB C-glucuronidating activity. Of interest is that UGT1A9((1-68))/1A8((69-132))/1A9((133-286)) had lost PB C-glucuronidation activity, but retained activities of PB and HFC O-glucuronidation. These results strongly suggested that amino acid positions 69-132 of UGT1A9 are responsible for chemoselectivity for PB and affinity to substrates such as PB and HFC.

Phytochemical regulation of UDP-glucuronosyltransferases: implications for cancer prevention

Uridine 5'-diphospho-glucuronosyltransferases (UGTs) are Phase II biotransformation enzymes that metabolize endogenous and exogenous compounds, some of which have been associated with cancer risk. Many phytochemicals have been shown to induce UGTs in humans, rodents, and cell culture systems. Because UGTs maintain hormone balance and facilitate excretion of potentially carcinogenic compounds, regulation of their expression and activity may affect cancer risk. Phytochemicals regulate transcription factors such as the nuclear factor-erythroid 2-related factor 2 (Nrf2), aryl hydrocarbon, and pregnane X receptors as well as proteins in several signal transduction cascades that converge on Nrf2 to stimulate UGT expression. This induction can be modified by several factors, including phytochemical dose and bioavailability and interindividual variation in enzyme expression. In this review, we summarize the knowledge of dietary modulation of UGTs, particularly by phytochemicals, and discuss the potential mechanisms by which phytochemicals regulate UGT transcription.

Piscine UDP-glucuronosyltransferase 1B

Glucuronidation is an important detoxification pathway for organic pollutants in fish. We report here the isolation and characterisation of UDP-glucuronosyltransferases (UGT) genes from the closely related marine flatfish, plaice (Pleuronectes platessa) and flounder (Platichthys flesus). The deduced amino acid sequences share greater similarity with mammalian UGT1 family genes than UGT2 genes (44-47% and 39-40% amino acid identity, respectively) and have been designated UGT1B. Both plaice and flounder UGT1B mRNAs are expressed in all tissues and are most highly expressed in liver, with high levels in intestine, gill, kidney and adipose tissue and much lower levels in muscle, heart and brain. Plaice UGT1B mRNA is undetectable in gametes or fertilised eggs and there is a large increase in expression between gastrulation and myotome formation after which levels decline some 5-10-fold. Flounder UGT1B mRNA was increased in liver after intraperitoneal injection of Arochlor 1254 or lindane (gamma-hexachlorocyclohexane), but not after perflourooctanoic acid or 3-methylcholanthrene treatment. In isolated flounder hepatocytes UGT1B mRNA was increased after exposure to benzo(a)pyrene but not by 17alpha-ethynylestradiol. Expression of a cDNA for plaice UGT1B in cos7 cells resulted in higher 1-naphthol conjugation in cell homogenates compared to steroid conjugation, whilst bilirubin and bile acid conjugation were undetectable. This indicates that the plaice gene codes for the phenol-conjugating UGT previously purified in our laboratory from this species and that it is likely to play a major role in the detoxification of polyaromatic hydrocarbons in flatfish. Its role in development is unknown. UGT1B genes are also present in pufferfish (Tetraodon nigroviridis) and zebrafish (Danio rerio) genomes, but they differ in their genic organisation. Pufferfish possess multiple (repeated) complete UGT1 genes and Southern blots indicate that the homologous plaice UGT1B gene may also be organised in this way. In contrast, zebrafish appear to have two UGT1 loci whose sequences and intron/exon structures are closely related to that of plaice, however, the organisation of these genes is similar to the mammalian UGT1 family since each has multiple repeated exon 1's which are alternatively spliced to a common set of exons encoding the aglycone binding domain. Taken together with evidence from phylogenetic comparison of fish sequences with UGT1 and UGT2 families in mammals, we suggest these homologous fish UGTs should all be included within the vertebrate UGT1 family and designated as UGT1B.

Eudismic analysis of tricyclic sesquiterpenoid alcohols: lead structures for the design of potent inhibitors of the human UDP-glucuronosyltransferase 2B7

The epimeric tricyclic sesquiterpenoid alcohols globulol, epiglobulol, cedrol, epicedrol, longifolol, and isolongifolol were investigated in their ability to inhibit the recombinant human UDP-glucuronosyltransferase (UGT) 2B7. The stereoisomers displayed rapidly reversible competitive inhibition, which was substrate-independent. Longifolol and its stereoisomer isolongifolol displayed the lowest competitive inhibition constants (K(ic)) of 23 and 26 nM, respectively. The K(ic) values of cedrol and its epimer epicedrol were 0.15 and 0.21 microM, those of globulol and epiglobulol were 5.4 and 4.0 microM, respectively. The diastereomeric alcohols exhibited nearly identical affinities toward UGT2B7 indicating that the spatial arrangement of the hydroxy group had no influence on the dissociation of the enzyme-terpenoid complex. The high affinities stemmed presumably from mere hydrophobic interactions between the hydrocarbon scaffold of the terpenoid alcohol and the binding site of the enzyme. Glucuronidation assays revealed that there were large differences in the rates at which the epimeric alcohols were conjugated. Therefore, the spatial arrangement of the hydroxy group controlled the rate of the UGT2B7-catalyzed reaction. The introduction of a methyl group into the side chain of isolongifolol and longifolol increased the steric hindrance. As a result, the rate of the UGT2B7-catalyzed reaction was decreased by more than 88%. The findings indicated that the rate of the UGT2B7-catalyzed glucuronidation is significantly controlled by stereochemical and steric factors. Considering the high inhibition levels exerted by the tricyclic sesquiterpenoid alcohols, these compounds might serve as valuable lead structures for the design of potent inhibitors for UGT2B7.