UDP-glucuronosyltransferase, the role of the amino terminus in dimerization

UGT2B13 and UGT2C1 are involved in lambda-cyhalothrin resistance in Rhopalosiphum padi

Uridine diphosphate-glucuronosyltransferases (UGTs) are major multifunctional detoxification phase II enzymes involved in the metabolic detoxification of xenobiotics. However, their roles in insecticides resistance are still unclear. In this study, we identified two UGTs genes (UGT2B13 and UGT2C1) in Rhopalosiphum padi, a serious insect pest of wheat worldwide. Bioassays results showed that the resistance ratio of R. padi resistance strain (LC-R) to lambda-cyhalothrin (LC) was 2963.8 fold. The roles of UGT2B13 and UGT2C1 in lambda-cyhalothrin resistance were evaluated. Results indicated that the UGTs contents were significantly increased in the LC resistant strain of R. padi. UGT2B13 and UGT2C1 were significantly overexpressed in the LC-R strain. Transcription levels of UGT2B13 and UGT2C1 were relatively higher in the gut of LC-R strain. RNA interference (RNAi) of UGT2B13 or UGT2C1 significantly decreased the UGTs contents of the LC-R aphids and increased mortality of R. padi exposure to the LC50 concentration of LC. This study provides a new view that UGTs are involved in LC resistance of R. padi. The findings will promote further work to detailed the functions of UGTs in the metabolism resistance of insects to insecticides.

Adenine-related compounds modulate UDP-glucuronosyltransferase (UGT) activity in mouse liver microsomes

Adenine-related compounds are allosteric inhibitors of UDP-glucuronosyltransferase (UGT) in rat liver microsomes (RLM) and human UGT isoforms treated with detergent or pore-forming peptide, alamethicin.To clarify whether the same is true beyond species, the effects of adenine-related compounds on 4-methylumbelliferone (4-MU) glucuronidation were examined using detergent-treated mouse liver microsomes (MLM).Brij-58 treatment of MLM increased the V_max and the Michaelis constant, K_m, of 4-MU. This study was performed using Brij-58-treated MLM as an enzyme source. ATP- and ADP-inhibited 4-MU glucuronidation. In contrast, AMP caused a 1.5-fold increase in glucuronidation. Oxidised forms, NAD⁺ and NADP⁺, potently inhibited 4-MU glucuronidation, whereas the reduced forms, NADH and NADPH, did not. Furthermore, the IC₅₀ values of ATP, ADP, NAD⁺, and NADP⁺ were approximately 15 μM.In our previous study, ATP was the strongest inhibitor of UGT activity in RLM. However, in this study, the above-mentioned compounds inhibited 4-MU UGT in a comparable and non-competitive manner. Furthermore, AMP antagonised the inhibitory effects of ATP and ADP.These results suggest that ATP, ADP, NAD⁺, and NADP⁺ are common endogenous inhibitors of UGT beyond species.

Functional Interaction between Cytochrome P450 and UDP-Glucuronosyltransferase on the Endoplasmic Reticulum Membrane: One of Post-translational Factors Which Possibly Contributes to Their Inter-Individual Differences

Cytochrome P450 (P450) and uridine 5'-diphosphate (UDP)-glucuronosyltransferase (UGT) catalyze oxidation and glucuronidation in drug metabolism, respectively. It is believed that P450 and UGT work separately because they perform distinct reactions and exhibit opposite membrane topologies on the endoplasmic reticulum (ER). However, given that some chemicals are sequentially metabolized by P450 and UGT, it is reasonable to consider that the enzymes may interact and work cooperatively. Previous research by our team detected protein-protein interactions between P450 and UGT by analyzing solubilized rat liver microsomes with P450-immobilized affinity column chromatography. Although P450 and UGT have been known to form homo- and hetero-oligomers, this is the first report indicating a P450-UGT association. Based on our previous study, we focused on the P450-UGT interaction and reported lines of evidence that the P450-UGT association is a functional protein-protein interaction that can alter the enzymatic capabilities, including enhancement or suppression of the activities of P450 and UGT, helping UGT to acquire novel regioselectivity, and inhibiting substrate binding to P450. Biochemical and molecular bioscientific approaches suggested that P450 and UGT interact with each other at their internal hydrophobic domains in the ER membrane. Furthermore, several in vivo studies have reported the presence of a functional P450-UGT association under physiological conditions. The P450-UGT interaction is expected to function as a novel post-translational factor for inter-individual differences in the drug-metabolizing enzymes.

Enzyme Kinetics of Uridine Diphosphate Glucuronosyltransferases (UGTs)

Glucuronidation, catalyzed by uridine diphosphate glucuronosyltransferases (UGTs), is an important process for the metabolism and clearance of many lipophilic chemicals, including drugs, environmental chemicals, and endogenous compounds. Glucuronidation is a bisubstrate reaction that requires the aglycone and the cofactor, UDP-GlcUA. Accumulating evidence suggests that the bisubstrate reaction follows a compulsory-order ternary mechanism. To simplify the kinetic modeling of glucuronidation reactions in vitro, UDP-GlcUA is usually added to incubations in large excess. Many factors have been shown to influence UGT activity and kinetics in vitro, and these must be accounted for during experimental design and data interpretation. While the assessment of drug-drug interactions resulting from UGT inhibition has been challenging in the past, the increasing availability of UGT enzyme-selective substrate and inhibitor "probes" provides the prospect for more reliable reaction phenotyping and assessment of drug-drug interaction potential. Although extrapolation of the in vitro intrinsic clearance of a glucuronidated drug often underpredicts in vivo clearance, careful selection of in vitro experimental conditions and inclusion of extrahepatic glucuronidation may improve the predictivity of in vitro-in vivo extrapolation. Physiologically based pharmacokinetic (PBPK) modeling has also shown to be of value for predicting PK of drugs eliminated by glucuronidation.

The carboxyl-terminal di-lysine motif is essential for catalytic activity of UDP-glucuronosyltransferase 1A9

UDP-Glucuronosyltransferase (UGT) is a type I membrane protein localized to the endoplasmic reticulum (ER). UGT has a di-lysine motif (KKXX/KXKXX) in its cytoplasmic domain, which is defined as an ER retention signal. However, our previous study has revealed that UGT2B7, one of the major UGT isoform in human, localizes to the ER in a manner that is independent of this motif. In this study, we focused on another UGT isoform, UGT1A9, and investigated the role of the di-lysine motif in its ER localization, glucuronidation activity, and homo-oligomer formation. Immunofluorescence microscopy indicated that the cytoplasmic domain of UGT1A9 functioned as an ER retention signal in a chimeric protein with CD4, but UGT1A9 itself could localize to the ER in a di-lysine motif-independent manner. In addition, UGT1A9 formed homo-oligomers in the absence of the motif. However, deletion of the di-lysine motif or substitution of lysines in the motif for alanines, severely impaired glucuronidation activity of UGT1A9. This is the first study that re-defines the cytoplasmic di-lysine motif of UGT as an essential peptide for retaining glucuronidation capacity.

Homology Modeling of Human Uridine-5'-diphosphate-glucuronosyltransferase 1A6 Reveals Insights into Factors Influencing Substrate and Cosubstrate Binding

The elimination of numerous endogenous compounds and xenobiotics via glucuronidation by uridine-5'-diphosphate glycosyltransferase enzymes (UGTs) is an essential process of the body's chemical defense system. UGTs have distinct but overlapping substrate preferences, but the molecular basis for their substrate specificity remains poorly understood. Three-dimensional protein structures can greatly enhance our understanding of the interactions between enzymes and their substrates, but because of the inherent difficulties in purifying and crystallizing integral endoplasmic reticulum membrane proteins, no complete mammalian UGT structure has yet been produced. To address this problem, we have created a homology model of UGT1A6 using I-TASSER to explore, in detail, the interactions of human UGT1A6 with its substrates. Ligands were docked into our model in the presence of the cosubstrate uridine-5'-diphosphate-glucuronic acid, interacting residues were examined, and poses were compared to those cocrystallized with various plant and bacterial glycosyltransferases (GTs). Our model structurally resembles other GTs, and docking experiments replicated many of the expected UGT-substrate interactions. Some bias toward the template structures' protein-substrate interactions and binding preferences was evident.

Hetero-oligomer formation of mouse UDP-glucuronosyltransferase (UGT) 2b1 and 1a1 results in the gain of glucuronidation activity towards morphine, an activity which is absent in homo-oligomers of either UGT

UDP-Glucuronosyltransferase (UGT, Ugt) is a major drug metabolizing enzyme family involved in the glucuronidation and subsequent elimination of drugs and small lipophilic molecules. UGT forms homo- and hetero-oligomers that enhance or suppress UGT activity. In our previous study, we characterized mouse Ugt1a1 and all the Ugt isoform belonging to the Ugt2b subfamily and revealed that mouse Ugt2b1 and Ugt1a1 cannot metabolize morphine. Mouse Ugt2b1 had been believed to function similarly to rat UGT2B1, which plays a major role in morphine glucuronidation in rat liver. Thus, in this study, we hypothesized that hetero-oligomerization with another Ugt isoform may affect Ugt2b1 catalytic ability. We co-expressed Ugt1a1 and Ugt2b1 in a baculovirus-insect cell system, and confirmed hetero-oligomer formation by co-immunoprecipitation. As reported previously, microsomes singly expressing Ugt1a1 or Ugt2b1 were inactive towards the glucuronidation of morphine. Interestingly, in contrast, morphine-3-glucuronide, a major metabolite of morphine was formed, when Ugt2b1 and Ugt1a1 were co-expressed. This effect of hetero-oligomerization of Ugt1a1 and Ugt2b1 was also observed for 17β-estradiol glucuronidation. This is the first report demonstrating that UGT acquires a novel catalytic ability by forming oligomers. Protein-protein interaction of Ugts may contribute to robust detoxification of xenobiotics by altering the substrate diversity of the enzymes.

The Human UGT2B7 Nanodisc

The 20 uridine diphosphate glycosyl-transferases (UGTs) encoded in the human genome form an essential homeostatic network of overlapping catalytic functions that surveil and regulate the activity and clearance of scores of small molecule metabolites. Biochemical and biophysical UGT studies have been hampered by the inability to purify these membrane-bound proteins. Here, using cell-free expression and nanodisc technology, we assemble and purify to homogeneity the first UGT nanodisc-the human UGT2B7•nanodisc. The complex is readily isolated in milligram quantities. It is stable and its initial-rate parameters are identical within error to those associated with UGT2B7 in microsomal preparations (i.e., Supersomes). The high purity of the nanodisc preparation simplifies UGT assays, which allows complexities traditionally associated with microsomal assays (latency and the albumin effect) to be circumvented. Each nanodisc is shown to harbor a single UGT2B7 monomer. The methods described herein should be widely applicable to UGTs, and these findings are expected to set the stage for experimentalists to more freely explore the structure, function, and biology of this important area of phase II metabolism. SIGNIFICANCE STATEMENT: Lack of access to pure, catalytically competent human uridine diphosphate glucuronosyl-transferases (UGTs) has long been an impediment to biochemical and biophysical studies of this disease-relevant enzyme family. Here, we demonstrate this barrier can be removed using nanodisc technology-a human UGT2B7•nanodisc is assembled, purified to homogeneity, and shown to have activity comparable to microsomal UGT2B7.

The UGTome: The expanding diversity of UDP glycosyltransferases and its impact on small molecule metabolism

The UDP glycosyltransferase (UGT) superfamily of enzymes is responsible for the metabolism and clearance of thousands of lipophilic chemicals including drugs, toxins and endogenous signaling molecules. They provide a protective interface between the organism and its chemical-rich environment, as well as controlling critical signaling pathways to maintain healthy tissue function. UGTs are associated with drug responses and interactions, as well as a wide range of diseases including cancer. The human genome contains 22 UGT genes; however as befitting their exceptionally diverse substrate ranges and biological activities, the output of these UGT genes is functionally diversified by multiple processes including alternative splicing, post-translational modification, homo- and hetero-oligomerization, and interactions with other proteins. All UGT genes are subject to extensive alternative splicing generating variant/truncated UGT proteins with altered functions including the capacity to dominantly modulate/inhibit cognate full-length forms. Heterotypic oligomerization of different UGTs can alter kinetic properties relative to monotypic complexes, and potentially produce novel substrate specificities. Moreover, the recently profiled interactions of UGTs with non-UGT proteins may facilitate coordination between different metabolic processes, as well as providing opportunities for UGTs to engage in novel 'moonlighting' functions. Herein we provide a detailed and comprehensive review of all known modes of UGT functional diversification and propose a UGTome model to describe the resulting expansion of metabolic capacity and its potential to modulate drug/xenobiotic responses and cell behaviours in normal and disease contexts.

Characterization of UDP-Glucuronosyltransferases and the Potential Contribution to Nicotine Tolerance in Myzus persicae

Uridine diphosphate (UDP)-glycosyltransferases (UGTs) are major phase II detoxification enzymes involved in glycosylation of lipophilic endobiotics and xenobiotics, including phytoalexins. Nicotine, one of the most abundant secondary plant metabolites in tobacco, is highly toxic to herbivorous insects. Plant-herbivore competition is the major impetus for the evolution of large superfamilies of UGTs and other detoxification enzymes. However, UGT functions in green peach aphid (Myzus persicae) adaptation are unknown. In this study, we show that UGT inhibitors (sulfinpyrazone and 5-nitrouracil) significantly increased nicotine toxicity in M. persicae nicotianae, suggesting that UGTs may be involved in nicotine tolerance. In total, 101 UGT transcripts identified in the M. persicae genome/transcriptome were renamed according to the UGT Nomenclature Committee guidelines and grouped into 11 families, UGT329, UGT330, UGT339, UGT341-UGT345, and UGT348-UGT350, with UGT344 containing the most (57). Ten UGTs (UGT330A3, UGT339A2, UGT341A6, UGT342B3, UGT343C3, UGT344D5, UGT344D8, UGT348A3, UGT349A3, and UGT350A3) were highly expressed in M. persicae nicotianae compared to M. persicae sensu stricto. Knockdown of four UGTs (UGT330A3, UGT344D5, UGT348A3, and UGT349A3) significantly increased M. persicae nicotianae sensitivity to nicotine, suggesting that UGT expression in this subspecies may be associated with nicotine tolerance and thus host adaptation. This study reveals possible UGTs relevant to nicotine adaptation in tobacco-consuming M. persicae nicotianae, and the findings will facilitate further validation of the roles of these UGTs in nicotine tolerance.

The UDP-Glycosyltransferase (UGT) Superfamily: New Members, New Functions, and Novel Paradigms

UDP-glycosyltransferases (UGTs) catalyze the covalent addition of sugars to a broad range of lipophilic molecules. This biotransformation plays a critical role in elimination of a broad range of exogenous chemicals and by-products of endogenous metabolism, and also controls the levels and distribution of many endogenous signaling molecules. In mammals, the superfamily comprises four families: UGT1, UGT2, UGT3, and UGT8. UGT1 and UGT2 enzymes have important roles in pharmacology and toxicology including contributing to interindividual differences in drug disposition as well as to cancer risk. These UGTs are highly expressed in organs of detoxification (e.g., liver, kidney, intestine) and can be induced by pathways that sense demand for detoxification and for modulation of endobiotic signaling molecules. The functions of the UGT3 and UGT8 family enzymes have only been characterized relatively recently; these enzymes show different UDP-sugar preferences to that of UGT1 and UGT2 enzymes, and to date, their contributions to drug metabolism appear to be relatively minor. This review summarizes and provides critical analysis of the current state of research into all four families of UGT enzymes. Key areas discussed include the roles of UGTs in drug metabolism, cancer risk, and regulation of signaling, as well as the transcriptional and posttranscriptional control of UGT expression and function. The latter part of this review provides an in-depth analysis of the known and predicted functions of UGT3 and UGT8 enzymes, focused on their likely roles in modulation of levels of endogenous signaling pathways.

Thiamethoxam Resistance in Aphis gossypii Glover Relies on Multiple UDP-Glucuronosyltransferases

Uridine diphosphate (UDP)-glycosyltransferases (UGTs) are major phase II enzymes that conjugate a variety of small lipophilic molecules with UDP sugars and alter them into more water-soluble metabolites. Therefore, glucosidation plays a major role in the inactivation and excretion of a great variety of both endogenous and exogenous compounds. In this study, two inhibitors of UGT enzymes, sulfinpyrazone and 5-nitrouracil, significantly increased the toxicity of thiamethoxam against the resistant strain of Aphis gossypii, which indicates that UGTs are involved in thiamethoxam resistance in the cotton aphid. Based on transcriptome data, 31 A. gossypii UGTs belonging to 11 families (UGT329, UGT330, UGT341, UGT342, UGT343, UGT344, UGT345, UGT348, UGT349, UGT350, and UGT351) were identified. Compared with the thiamethoxam-susceptible strain, the transcripts of 23 UGTs were elevated, and the transcripts of 13 UGTs (UGT344J2, UGT348A2, UGT344D4, UGT341A4, UGT343B2, UGT342B2, UGT350C3, UGT344N2, UGT344A14, UGT344B4, UGT351A4, UGT344A11, and UGT349A2) were increased by approximately 2.0-fold in the resistant cotton aphid. The suppression of selected UGTs significantly increased the insensitivity of resistant aphids to thiamethoxam, suggesting that the up-regulated UGTs might be associated with thiamethoxam tolerance. This study provides an overall view of the possible metabolic factor UGTs that are relevant to the development of insecticide resistance. The results might facilitate further work to validate the roles of these UGTs in thiamethoxam resistance.

Advances in the Understanding of Protein-Protein Interactions in Drug Metabolizing Enzymes through the Use of Biophysical Techniques

In recent years, a growing appreciation has developed for the importance of protein-protein interactions to modulate the function of drug metabolizing enzymes. Accompanied with this appreciation, new methods and technologies have been designed for analyzing protein-protein interactions both in vitro and in vivo. These technologies have been applied to several classes of drug metabolizing enzymes, including: cytochrome P450's (CYPs), monoamine oxidases (MAOs), UDP-glucuronosyltransferases (UGTs), glutathione S-transferases (GSTs), and sulfotransferases (SULTs). In this review, we offer a brief description and assessment of the impact of many of these technologies to the study of protein-protein interactions in drug disposition. The still expanding list of these techniques and assays has the potential to revolutionize our understanding of how these enzymes carry out their important functions in vivo.

Inter-isoform Hetero-dimerization of Human UDP-Glucuronosyltransferases (UGTs) 1A1, 1A9, and 2B7 and Impacts on Glucuronidation Activity

Human UDP-glucuronosyltransferases (UGTs) play a pivotal role in phase II metabolism by catalyzing the glucuronidation of endobiotics and xenobiotics. The catalytic activities of UGTs are highly impacted by both genetic polymorphisms and oligomerization. The present study aimed to assess the inter-isoform hetero-dimerization of UGT1A1, 1A9, and 2B7, including the wild type (1A1*1, 1A9*1, and 2B7*1) and the naturally occurring (1A1*1b, 1A9*2/*3/*5, and 2B7*71S/*2/*5) variants. The related enzymes were double expressed in Bac-to-Bac systems. The fluorescence resonance energy transfer (FRET) technique and co-immunoprecipitation (Co-IP) revealed stable hetero-dimerization of UGT1A1, 1A9, and 2B7 allozymes. Variable FRET efficiencies and donor-acceptor distances suggested that genetic polymorphisms resulted in altered affinities to the target protein. In addition, the metabolic activities of UGTs were differentially altered upon hetero-dimerization via double expression systems. Moreover, protein interactions also changed the regioselectivity of UGT1A9 for querectin glucuronidation. These findings provide in-depth understanding of human UGT dimerization as well as clues for complicated UGT dependent metabolism in humans.

Structure and Protein-Protein Interactions of Human UDP-Glucuronosyltransferases

Mammalian UDP-glucuronosyltransferases (UGTs) catalyze the transfer of glucuronic acid from UDP-glucuronic acid to various xenobiotics and endobiotics. Since UGTs comprise rate-limiting enzymes for metabolism of various compounds, co-administration of UGT-inhibiting drugs and genetic deficiency of UGT genes can cause an increased blood concentration of these compounds. During the last few decades, extensive efforts have been made to advance the understanding of gene structure, function, substrate specificity, and inhibition/induction properties of UGTs. However, molecular mechanisms and physiological importance of the oligomerization and protein–protein interactions of UGTs are still largely unknown. While three-dimensional structures of human UGTs can be useful to reveal the details of oligomerization and protein–protein interactions of UGTs, little is known about the protein structures of human UGTs due to the difficulty in solving crystal structures of membrane-bound proteins. Meanwhile, soluble forms of plant and bacterial UGTs as well as a partial domain of human UGT2B7 have been crystallized and enabled us to predict three-dimensional structures of human UGTs using a homology-modeling technique. The homology-modeled structures of human UGTs do not only provide the detailed information about substrate binding or substrate specificity in human UGTs, but also contribute with unique knowledge on oligomerization and protein–protein interactions of UGTs. Furthermore, various in vitro approaches indicate that UGT-mediated glucuronidation is involved in cell death, apoptosis, and oxidative stress as well. In the present review article, recent understandings of UGT protein structures as well as physiological importance of the oligomerization and protein–protein interactions of human UGTs are discussed.

Enzyme kinetics of uridine diphosphate glucuronosyltransferases (UGTs)

Glucuronidation, catalyzed by uridine diphosphate glucuronosyltransferases (UGTs), is an important process for the metabolism and clearance of many lipophilic chemicals, including drugs, environmental chemicals, and endogenous compounds. Glucuronidation is a bi-substrate reaction that requires the aglycone and a cofactor, UDPGA. Accumulating evidence suggests that the bi-substrate reaction follows a compulsory-order ternary mechanism. To simplify the kinetic modelling of glucuronidation reactions in vitro, UDPGA is usually added to incubations in large excess. Many factors have been shown to influence UGT activity and kinetics in vitro, and these must be accounted for in experimental design and data interpretation. Assessing drug-drug interactions (DDIs) involving UGT inhibition remains challenging. However, the increasing availability of UGT enzyme-specific substrate and inhibitor "probes" provides the prospect for more reliable reaction phenotyping and assessment of DDI potential. Although extrapolation of the in vitro intrinsic clearance of a glucuronidated drug often under-predicts in vivo clearance, careful selection of in vitro experimental conditions and inclusion of extrahepatic glucuronidation may improve the predictivity of in vitro-in vivo extrapolation (IVIVE).

UDP-glucuronosyltransferase 1a enzymes are present and active in the mouse blastocyst

The UDP-glucuronosyltransferase (UGT) enzymes are critical for regulating nutrients, hormones, and endobiotics, as well as for detoxifying xenobiotics. Human and murine fetuses are known to express glucuronidation enzymes, but there are currently no data prior to implantation. Here we addressed this gap in knowledge and tested whether Ugt enzymes are already present in preimplantation-stage embryos. Blastocysts were obtained after in vitro fertilization with gametes from B6D2F1 hybrid mice and from embryo culture. Protein expression and localization were determined using pan-specific UGT1A and UGT2B, as well as anti-human isoform-specific antibodies. Immunofluorescence analysis showed that blastocysts expressed Ugt1a globally, in the cytoplasm and nuclei of all of the cells. Western blots demonstrated the presence of Ugt1a6 but not Ugt1a1, Ugt1a3, Ugt1a4, or Ugt1a9. The Ugt2b proteins were not detected by either assay. The level of Ugt activity in murine blastocysts was comparable with that of the adult human liver (per milligram of protein), but the activity of β-glucuronidase, an Ugt-partnering enzyme responsible for substrate regeneration, was lower. Altogether, these data confirm that Ugt1a proteins are present and active in preimplantation murine embryos and point to a potential role for these proteins in implantation and early embryonic and fetal development.

Absolute quantification of UGT1A1 in various tissues and cell lines using isotope label-free UPLC-MS/MS method determines its turnover number and correlates with its glucuronidation activities

Uridine 5'-diphosphate-glucuronosyltransferase (UGT)1A1 is a major phase II metabolism enzyme responsible for glucuronidation of drugs and endogenous compounds. The purpose of this study was to determine the expression level of UGT1A1 in human liver microsomes and human cell lines by using an isotope label-free LC-MS/MS method. A Waters Ultra performance liquid chromatography (UPLC) system coupled with an API 5500Qtrap mass spectrometer was used for the analysis. Two signature peptides (Pep-1, and Pep-2) were employed to quantify UGT1A1 by multiple reaction monitoring (MRM) approach. Standard addition method was used to validate the assay to account for the matrix effect. 17β-Estradiol was used as the marker substrate to determine UGT1A1 activities. The validated method has a linear range of 200-0.0195nM for both signature peptides. The precision, accuracy, and matrix effect were in acceptable ranges. UGT1A1 expression levels were then determined using 8 individual human liver microsomes, a pooled human liver microsomes, three UGT1A1 genotyped human liver microsomes, and four cell lines (Caco-2, MCF-7, Hela, and HepG2). The correlations study showed that the UGT1A1 protein levels were strongly correlated with its glucuronidation activities in human liver microsomes (R(2)=0.85) and in microsomes prepared from cell lines (R(2)=0.95). Isotope-labeled peptides were not necessary for LC-MS/MS quantitation of proteins. The isotope label-free absolute quantification method used here had good accuracy, sensitivity, linear range, and reproducibility, and were used successfully for the accurate determination of UGT1A1 from tissues and cell lines.

Evaluation of UGT protein interactions in human hepatocytes: effect of siRNA down regulation of UGT1A9 and UGT2B7 on propofol glucuronidation in human hepatocytes

Previous experiments performed in recombinant systems have suggested that protein-protein interactions occur between the UGTs and may play a significant role in modulating enzyme activity. However, evidence of UGT protein-protein interactions either in vivo or in more physiologically relevant in vitro systems has yet to be demonstrated. In this study, we examined oligomerization and its ability to affect glucuronidation in plated human hepatocytes. siRNA down regulation experiments and activity studies were used to examine changes in metabolite formation of one UGT isoform due to down regulation of a second UGT isoform. Selective siRNA directed towards UGT1A9 or UGT2B7 resulted in significant and selective decreases in their respective mRNA levels. As expected, the metabolism of the UGT1A9 substrate propofol decreased with UGT1A9 down regulation. Interestingly, UGT1A9 activity, but not UGT1A9 mRNA expression, was also diminished when UGT2B7 expression was selectively inhibited, implying potential interactions between the two isoforms. Minor changes to UGT1A4, UGT2B4 and UGT2B7 activity were also observed when UGT1A9 expression was selectively down regulated. To our knowledge, this represents the first piece of evidence that UGT protein-protein interactions occur in human hepatocytes and suggests that expression levels of UGT2B7 may directly impact the glucuronidation activity of selective UGT1A9 substrates.

Protein-protein interactions between the bilirubin-conjugating UDP-glucuronosyltransferase UGT1A1 and its shorter isoform 2 regulatory partner derived from alternative splicing

The oligomerization of UGTs [UDP (uridine diphosphate)-glucuronosyltransferases] modulates their enzyme activities. Recent findings also indicate that glucuronidation is negatively regulated by the formation of inactive oligomeric complexes between UGT1A enzymes [i1 (isoform 1)] and an enzymatically inactive alternatively spliced i2 (isoform 2). In the present paper, we assessed whether deletion of the UGT-interacting domains previously reported to be critical for enzyme function might be involved in i1-i2 interactions. The bilirubin-conjugating UGT1A1 was used as a prototype. We also explored whether intermolecular disulfide bonds are involved in i1-i2 interactions and the potential role of selected cysteine residues. Co-immunoprecipitation assays showed that UGT1A1 lacking the SP (signal peptide) alone or also lacking the transmembrane domain (absent from i2) did not self-interact, but still interacted with i2. The deletion of other N- or C-terminal domains did not compromise i1-i2 complex formation. Under non-reducing conditions, we also observed formation of HMWCs (high-molecular-mass complexes) for cells overexpressing i1 and i2. The presence of UGTs in these complexes was confirmed by MS. Mutation of individual cysteine residues throughout UGT1A1 did not compromise i1-i1 or i1-i2 complex formation. These findings are compatible with the hypothesis that the interaction between i1 and i2 proteins (either transient or stable) involves binding of more than one domain that probably differs from those involved in i1-i1 interactions.

Substrate selectivity of human intestinal UDP-glucuronosyltransferases (UGTs): in silico and in vitro insights

The current drug development process aims to produce safe, effective drugs within a reasonable time and at a reasonable cost. Phase II metabolism (glucuronidation) can affect drug action and pharmacokinetics to a considerable extent and so its studies and prediction at initial stages of drug development are very imperative. Extensive glucuronidation is an obstacle to oral bioavailability because the first-pass glucuronidation [or premature clearance by UDP-glucuronosyltransferases (UGTs)] of orally administered agents frequently results in poor oral bioavailability and lack of efficacy. Modeling of new chemical entities/drugs for UGTs and their kinetic data can be useful in understanding the binding patterns to be used in the design of better molecules. This review concentrates on first-pass glucuronidation by intestinal UGTs, including their topology, expression profile, and pharmacogenomics. In addition, recent advances are discussed with respect to substrate selectivity at the binding pocket, structural requirements, and mechanism of enzyme actions.

Modulation of UDP-glucuronosyltransferase activity by endogenous compounds

Glucuronidation is one of the major pathways of metabolism of endo- and xenobiotics. UDP-Glucuronosyltransferase (UGT)-catalyzed glucuronidation accounts for up to 35% of phase II reactions. The expression and function of UGT is modulated by gene regulation, post-translational modifications and protein-protein association. Many studies have focused on drug-drug interactions involving UGT, and there are a number of reports describing the inhibition of UGT by xenobiotics. However, studies about the role of endogenous compounds as an inhibitor or activator of UGT are limited, and it is important to understand any change in the function and regulation of UGT by endogenous compounds. Recent studies in our laboratory have shown that fatty acyl-CoAs are endogenous activators of UGT, although fatty acyl-CoAs had been considered as inhibitors of UGT. Further, we have also suggested that adenine and related compounds are endogenous allosteric inhibitors of UGT. In this review, we summarize the endogenous modulators of UGT and discuss their relevance to UGT function.

Modulation of UDP-glucuronosyltransferase activity by protein-protein association

Drug oxidation and conjugation mediated by cytochrome P450 (P450) and UDP-glucuronosyltransferase (UGT) have long been considered to take place separately. However, our recent studies have suggested that CYP3A4 specifically associates with UGT2B7 and alters the regioselectivity of morphine glucuronidation. This observation strongly supports the view that there is functional cooperation between P450 and UGT to facilitate multistep drug metabolism. In recent years, accumulating evidence has suggested an interaction between UGT isoforms or between P450 and UGTs and a change in UGT function by protein-protein association. In this review, we summarize these interactions and discuss their relevance to UGT function.

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.

Modulation of the human glucuronosyltransferase UGT1A pathway by splice isoform polypeptides is mediated through protein-protein interactions

This study investigated the molecular mechanisms underlying the regulatory effect of the newly discovered 45-kDa enzymatically inactive UGT1A spliced polypeptides, named isoform i2, upon UGT1A-mediated glucuronidation. Initially, using an inducible system that mimics the relative abundance of isoforms 1 and 2 of UGT1A1 in human tissues, the rates of formation of glucuronides were significantly reduced. We then used a heterologous system constitutively expressing both isoforms i1 and i2 for an in-depth investigation of the presence of spliced i2 on glucuronidation kinetics. UGT1A1, UGT1A7, and UGT1A8 were selected as candidates for these studies. In all cases, co-expression of i1 and i2 in HEK293 cells leads to a significant reduction of the velocity of the glucuronidation reaction without affecting the affinity (K(m) (app)) for all substrates tested and the K(m) for the co-substrate, UDP-glucuronic acid. The data are consistent with a dominant-negative model of inhibition but do not sustain with an UGT1A_i2-mediated inhibition by competitive binding for substrate or the co-substrate. In contrast, the data from the co-immunoprecipitation experiments are indicative of the existence of a mixture homo-oligomeric (i1-i1 or i2-i2) and hetero-oligomeric (i1-i2) complexes in which the i2-i2 and i1-i2 subunits would be inactive. Thus, protein-protein interactions are likely responsible for the inhibition of active UGT1A_i1 by i2 spliced polypeptides. This new regulatory mechanism may alternatively modulate cellular response to endo/xeno stimulus.

A systems biology perspective on Nrf2-mediated antioxidant response

Cells in vivo are constantly exposed to reactive oxygen species (ROS) generated endogenously and exogenously. To defend against the deleterious consequences of ROS, cells contain multiple antioxidant enzymes expressed in various cellular compartments to scavenge these toxic species. Under oxidative stresses, these antioxidant enzymes are upregulated to restore redox homeostasis. Such an adaptive response results from the activation of a redox-sensitive gene regulatory network mediated by nuclear factor E2-related factor 2. To more completely understand how the redox control system is designed by nature to meet homeostatic goals, we have examined the network from a systems perspective using engineering approaches. As with man-made control devices, the redox control system can be decomposed into distinct functional modules, including transducer, controller, actuator, and plant. Cells achieve specific performance objectives by utilizing nested feedback loops, feedforward control, and ultrasensitive signaling motifs, etc. Given that endogenously generated ROS are also used as signaling molecules, our analysis suggests a novel mode of action to explain oxidative stress-induced pathological conditions and diseases. Specifically, by adaptively upregulating antioxidant enzymes, oxidative stress may inadvertently attenuate ROS signals that mediate physiological processes, resulting in aberrations of cellular functions and adverse consequences. Lastly, by simultaneously considering the two competing cellular tasks-adaptive antioxidant defense and ROS signaling-we re-examine the premise that dietary antioxidant supplements is generally beneficial to human health. Our analysis highlights some possible adverse effects of these widely consumed antioxidants.

Single nucleotide polymorphism discovery and functional assessment of variation in the UDP-glucuronosyltransferase 2B7 gene

OBJECTIVE

UDP-glucuronosyltransferase 2B7 (UGT2B7) plays a central role in the liver-mediated biotransformation of endogenous and exogenous compounds. The genetic basis of interindividual variability in UGT2B7 function is unknown. This study aimed to discover novel gene variants of functional significance.

METHODS

Caucasian human livers (n=54) were used. UGT2B7 was resequenced in 12 samples [(six highest and six lowest for the formation of morphine-3-glucuronide (M3G)]. Haplotype-tagging single nucleotide polymorphisms were genotyped in the entire sample set. Samples were phenotyped for mRNA expression.

RESULTS

10 haplotype-tagging single nucleotide polymorphisms were identified and their haplotypes were inferred. Haplotype 4 (-45597G; -6682_-6683A; 372A; IVS1+9_IVS1+10A; IVS1+829T; IVS1+985G; IVS1+999C; IVS1+1250G; 801T; IVS4+185C) (frequency of 0.12) was associated with an increase in enzyme activity and gene expression. The 1/4 and 4/6 diplotypes had higher M3G formation compared with 1/1 (P<0.05) and 2/3 (P<0.01) diplotypes. Diplotypes containing haplotype 4 resulted in a significant 45% average increase in the formation of M3G compared with diplotypes without haplotype 4 (P=0.002). There was also an association between haplotype 4 and increased mRNA expression. IVS1+985A>G, 735A>G, and 1062C>T are the putative functional variants of haplotype 4. We also identified two mRNA splicing variants (UGT2B7_v2 and UGT2B7_v3) splicing out exon 1, 4, 5, and 6 but sharing exons 2 and 3 with the involvement of additional 5' exons. UGT2B7_v2 was detected in all livers tested, but UGT2B7_v3 was present at much lower levels compared with UGT2B7_v2. The UGT2B7 reference sequence mRNA is now named UGT2B7_v1.

CONCLUSION

UGT2B7 haplotype 4 is functional and its effects on the biotransformation of UGT2B7 substrates should be tested in controlled clinical trials. Biochemical studies should investigate the functional role of the newly discovered mRNA splicing variants.

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.

Genetic diversity at the UGT1 locus is amplified by a novel 3' alternative splicing mechanism leading to nine additional UGT1A proteins that act as regulators of glucuronidation activity

BACKGROUND

The gene UGT1 encodes phase II detoxification proteins involved in the elimination of small hydrophobic substances of both endogenous and exogenous origin. To date, nine functional UGT1A proteins are known to be produced from a single gene composed of alternative first exons shared with four common exons. Recently, a novel exon (referred to as exon 5b) was identified in the common shared region.

RESULTS

We now reveal a novel alternative splicing mechanism and demonstrate that the exon 5a and the new exon 5b are alternatively spliced, generating several variant mRNAs and up to nine previously unknown variant UGT1A proteins, referred to as isoforms 2 or i2. Isoform-specific RT-PCR analyses reveal that the alternatively spliced mRNAs are widely distributed in human tissues. Immunoreactive proteins at the predicted molecular weight of approximately 45 kDa were confirmed in microsomes of human tissues using antibodies against UGT1A1 and anti-UGT1A7/8/9/10. Functional enzyme assays demonstrate that i2 proteins containing exon 5b are enzymatically inactive. On the other hand, co-expression experiments of i2 of UGT1A1, UGT1A7, UGT1A8 and UGT1A9 with their classical isoform 1 homologs results in a significant repression (15 to 79%) of UGT1A_i1-mediated drug metabolism.

CONCLUSION

The UGT1A isoforms 2 act as negative modulators of their isoform 1 homologs in microsome preparations, revealing a new regulatory mechanism of the glucuronidation pathway. Findings further provide the first direct evidence of a novel alternative splicing mechanism at the 3' end of the UGT1 locus that further increases the number of proteins derived from this single gene.

Protein-protein interactions between rat hepatic cytochromes P450 (P450s) and UDP-glucuronosyltransferases (UGTs): evidence for the functionally active UGT in P450-UGT complex

The interaction between cytochrome P450s (CYP, P450) and UDP-glucuronosyltransferases (UGTs) was studied by co-immunoprecipitation. P450 isoform-selective antibody was used as a probe to co-precipitate UGTs with the P450s from solubilized rat liver microsomes. Antibodies toward CYP3A2, CYP2B2, CYP2C11/13 and CYP1A2 co-precipitated UGTs with corresponding P450s. However, calnexin, a type-I membrane protein, in the endoplasmic reticulum was not co-precipitated by anti-P450 antibodies. UGT activity toward 4-methylumbelliferone was detected in all co-precipitates, suggesting that UGT in the complex with P450s is functionally active. Repeated washing of co-immunoprecipitates revealed differences among P450 isoforms with regard to the affinity for UGT. Larger amounts of UGT1A1 and UGT1A6, compared with UGT2B1, were washed out from UGTs-CYP2C11/13 co-precipitates, whereas UGT-CYP3A2 and UGT-CYP2Bs complexes were resistant to thorough washing. Thus, CYP2C11/13 could associate with UGTs, but the affinity is assumed to be weaker than that of CYP2B/3As. These results suggest that there is isoform specificity in the interaction between P450s and UGTs.

Inhibitory effects of adenine nucleotides and related substances on UDP-glucuronosyltransferase: structure-effect relationships and evidence for an allosteric mechanism

The inhibitory effects of nucleotides and related substances on rat hepatic UDP-glucuronosyltransferase (UGT) were studied. ATP and NADP+ markedly reduced 4-methylumbelliferone (4-MU) UGT activity only when detergent-treated rat liver microsomes were used as the enzyme source. The IC50 values of adenine, ATP, NAD+ and NADP+ were estimated to be below 20 microM, whereas AMP had no inhibitory effect. From the kinetic behavior observed, these adenine-related compounds were assumed to inhibit UGT activity non-competitively without competing with either 4-MU or UDP-glucuronic acid. Among guanine, cytosine and their related nucleotides, only triphosphate nucleotides (CTP and GTP) exhibited potent UGT inhibition, although the effect of GTP was weak. Estradiol 3- and 17-glucuronidation were also inhibited by the inhibitors of 4-MU UGT. The only exception was that estradiol 17-glucuronidation activity was inhibited by AMP (IC50=31 microM). In addition, AMP antagonized the inhibitory effects of adenine, ATP, and NADP+ on 4-MU and estradiol 3- glucuronidation activities. These results suggest that (1) a number of cellular nucleotides present within the endoplasmic reticulum regulate UGT function; and (2) these substances bind to a common allosteric site on UGT to reduce catalytic function.

Molecular mechanism of phase I and phase II drug-metabolizing enzymes: implications for detoxification

Enzymes that catalyze the biotransformation of drugs and xenobiotics are generally referred to as drug-metabolizing enzymes (DMEs). DMEs can be classified into two main groups: oxidative or conjugative. The NADPH-cytochrome P450 reductase (P450R)/cytochrome P450 (P450) electron transfer systems are oxidative enzymes that mediate phase I reactions, whereas the UDP-glucuronosyltransferases (UGTs) are conjugative enzymes that mediate phase II enzymes. Both enzyme systems are localized to the endoplasmic reticulum (ER) where a number of drugs are sequentially metabolized. DMEs, including P450s and UGTs, generally have a highly plastic active site that can accommodate a wide variety of substrates. The P450 and UGT genes constitute a supergene family, in which UGT proteins are encoded by distinct genes and a complex gene. Both the P450 and UGT genes have evolved to diversify their functions. This chapter reviews advances in understanding the structure and function of the P450R/P450 and UGT enzyme systems. In particular, the coordinate biotransformation of xenobiotics by phase I and II enzymes in the ER membrane is examined.

Interactions between Human UGT1A1, UGT1A4, and UGT1A6 Affect Their Enzymatic Activities

Protein-protein interactions between human UDP-glucuronosyltransferase (UGT) 1A1, UGT1A4, and UGT1A6 were investigated using double expression systems in HEK293 cells (UGT1A1/UGT1A4, UGT1A1/UGT1A6, and UGT1A4/UGT1A6). The substrates specific for UGT1A1 (estradiol and bilirubin), UGT1A4 (imipramine and trifluoperazine), and UGT1A6 (serotonin and diclofenac) were used to determine the effects of the coexpression of the other UGT1A isoforms on the enzymatic activity. The coexpression of UGT1A4 and UGT1A6 decreased the S(50) and V(max) values of UGT1A1-catalyzed estradiol 3-O-glucuronide formation and increased the V(max) value of UGT1A1-catalyzed bilirubin O-glucuronide formation. The coexpression of UGT1A1 decreased the V(max) value of UGT1A4-catalyzed imipramine N-glucuronide formation but had no effect on UGT1A4-catalyzed trifluoperazine N-glucuronide formation. The coexpression of UGT1A6 had no effect on UGT1A4-catalyzed imipramine N-glucuronide formation but increased the K(m) and V(max) of UGT1A4-catalyzed trifluoperazine N-glucuronide formation. The coexpression of both UGT1A1 and UGT1A4 increased the V(max) values of UGT1A6-catalyzed serotonin and diclofenac O-glucuronide formation. Thus, the effects of the coexpression of other UGT1A isoforms on the kinetics of specific activities were different depending on the UGT1A isoforms and substrates. Native polyacrylamide gel electrophoresis analysis of the double expression systems showed multiple bands at approximately 110 kDa, indicating the existence of heterodimers as well as homodimers of UGTs. In conclusion, we found that human UGT1A1, UGT1A4, and UGT1A6 interact with each other, possibly by heterodimerization, and that their effects on the enzymatic activities are complex depending on the isoforms and substrates.

Stereoselective Glucuronidation of 5-(4′-Hydroxyphenyl)-5-phenylhydantoin by Human UDP-Glucuronosyltransferase (UGT) 1A1, UGT1A9, and UGT2B15: Effects of UGT-UGT Interactions

5-(4'-Hydroxyphenyl)-5-phenylhydantoin (4'-HPPH), a major metabolite of phenytoin in human, is exclusively metabolized to a glucuronide. 4'-HPPH has a chiral center. (S)-4'-HPPH is a predominant form produced from phenytoin in humans, and (R)-4'-HPPH is an extremely toxic form with respect to gingival hyperplasia. In the present study, we investigated stereoselective 4'-HPPH O-glucuronide formation in human liver microsomes. Human liver microsomes predominantly formed (S)-4'-HPPH O-glucuronide rather than (R)-4'-HPPH O-glucuronide from racemic 4'-HPPH. Among human UDP-glucuronosyltransferase (UGT) enzymes, UGT1A1, UGT1A9, and UGT2B15 showed 4'-HPPH O-glucuronide formation. Interestingly, UGT1A1 stereoselectively formed (R)-4'-HPPH O-glucuronide, whereas UGT1A9 and UGT2B15 stereoselectively formed (S)-4'-HPPH O-glucuronide from racemic 4'-HPPH. By using UGT1A double-expression systems in HEK293 cells that we previously established, the effects of UGT-UGT interactions on 4'-HPPH O-glucuronide formation were investigated. It was demonstrated that coexpression of UGT1A4 increased the V(max) values of (S)- and (R)-4'-HPPH O-glucuronide formation catalyzed by UGT1A1 but decreased the V(max) values of (S)- and (R)-4'-HPPH O-glucuronide formation catalyzed by UGT1A9. Coexpression of UGT1A6 increased the S(50) values and decreased the V(max) values of (S)- and (R)-4'-HPPH glucuronide formation catalyzed by UGT1A1 and UGT1A9. However, the interaction did not alter the stereoselectivity. In conclusion, we found that 4'-HPPH O-glucuronide formation in human liver microsomes is catalyzed by UGT1A1, UGT1A9, and UGT2B15 in a stereoselective manner, being modulated by interaction with other UGT1A isoforms.

Comparative modelling of the human UDP-glucuronosyltransferases: insights into structure and mechanism

UDP-glucuronosyltranferases (UGTs) affect the disposition of drugs and other xenobiotics by catalysing the conjugation of glucuronic acid to available oxygen, nitrogen, and sulfur atoms. Several related mammalian isoforms of UGT are expressed that have different binding affinities and turnover rates for the substrates they encounter in the liver and other tissues. Because no high-resolution structural information is available to dissect the enzyme-substrate interactions that give rise to different specificities, a search was conducted to find the best available templates to use for comparative protein modelling. Sequence identity analysis was used to identify some recently crystallized plant UGTs as homologues of microsomal UGTs. Because UGTs contain a Rossman fold motif predicted to bind the UDP-containing sugar donor substrate, this consensus sequence was used to aid sequence alignment, as were other conserved residues thought to be involved in catalysis or substrate binding, and the predicted secondary structure. Docking of UDP-glucuronic acid to a model of UGT1A1 resulted in a root mean square deviation of only 0.37 angstroms vs. UDP co-crystallized with the plant UGT71G1 template. The significance of a comparative model generated for UGT1A1 with respect to both the sugar donor and aglycone binding sites, and mechanism, is discussed.

Amino terminal domains of human UDP-glucuronosyltransferases (UGT) 2B7 and 2B15 associated with substrate selectivity and autoactivation

Despite the important role of UDP-glucuronosyltransferases (UGT) in the metabolism of drugs, environmental chemicals and endogenous compounds, the structural features of these enzymes responsible for substrate binding and selectivity remain poorly understood. Since UGT2B7 and UGT2B15 exhibit distinct, but overlapping, substrate selectivities, UGT2B7-UGT2B15 chimeras were constructed here to identify substrate binding domains. A UGT2B7-15-7 chimera that incorporated amino acids 61-194 of UGT2B15 glucuronidated the UGT2B15 substrates testosterone and phenolphthalein, but not the UGT2B7 substrates zidovudine and 11alpha-hydroxyprogesterone. Derived apparent K(m) values for testosterone and phenolphthalein glucuronidation by UGT2B7-15((61-194))-7 were similar in magnitude to those determined for UGT2B15. Moreover, glucuronidation of the non-selective substrate 4-methylumbelliferone (4MU) by UGT2B7-15((61-194))-7 and UGT2B15 followed Michaelis-Menten and weak substrate inhibition kinetics, respectively, whereas 4MU glucuronidation by UGT2B7 exhibited sigmoidal kinetics characteristic of autoactivation. Six UGT2B7-15-7 chimeras that incorporated smaller domains of UGT2B15 were subsequently generated. Of these, UGT2B7-15((61-157))-7, UGT2B7-15((91-157))-7 and UGT2B7-15((61-91))-7 glucuronidated 4MU, but activity towards the other substrates investigated here was not detected. Like UGT2B7, the UGT2B7-15((61-157))-7, UGT2B7-15((91-157))-7 and UGT2B7-15((61-91))-7 chimeras exhibited sigmoidal 4MU glucuronidation kinetics. The sigmoidal 4MU kinetic data were well modelled using both the Hill equation and the expression for a two-site model that assumes the simultaneous binding of two substrate molecules at equivalent sites. It may be concluded that residues 61-194 of UGT2B15 are responsible for substrate binding and for conferring the unique substrate selectivity of UGT2B15, while residues 158-194 of UGT2B7 appear to facilitate the binding of multiple 4MU molecules within the active site.

Crystal structure of the cofactor-binding domain of the human phase II drug-metabolism enzyme UDP-glucuronosyltransferase 2B7

Human UDP-glucuronosyltransferases (UGT) are the dominant phase II conjugative drug metabolism enzymes that also play a central role in processing a range of endobiotic compounds. UGTs catalyze the covalent addition of glucuronic acid sugar moieties to a host of therapeutics and environmental toxins, as well as to a variety of endogenous steroids and other signaling molecules. We report the 1.8-A resolution apo crystal structure of the UDP-glucuronic acid binding domain of human UGT isoform 2B7 (UGT2B7), which catalyzes the conjugative elimination of opioid, antiviral, and anticancer drugs. This is the first crystal structure of any region of a mammalian UGT drug metabolism enzyme. Designated UGT2B7 mutants at residues predicted to interact with the UDP-glucuronic acid cofactor exhibited significantly impaired catalytic activity, with maximum effects observed for amino acids closest to the glucuronic acid sugar transferred to the acceptor molecule. Homology modeling of UGT2B7 with related plant flavonoid glucosyltransferases indicates human UGTs share a common catalytic mechanism. Point mutations at predicted catalytic residues in UGT2B7 abrogated activity, strongly suggesting human UGTs also utilize a serine hydrolase-like catalytic mechanism to facilitate glucuronic acid transfer.

Interactions with other human UDP-glucuronosyltransferases attenuate the consequences of the Y485D mutation on the activity and substrate affinity of UGT1A6

OBJECTIVES

To explore the possible role of hetero-oligomerization among the human UDP-glucuronosyltransferases in attenuating the consequences of the pathological Y486D mutation (UGT1A1 numbering) that often causes hyperbilirubinaemia. Owing to exon sharing in the human UGT1A gene, the equivalent mutation is present in all other UGT1As of the affected individuals. It is unknown, however, if this mutation results in clinical conditions, other than impaired bilirubin conjugation by UGT1A1.

METHODS

The main experimental approach in this study was to try and form hetero-oligomers of selected UDP-glucuronosyltransferases by coinfecting insect cells with recombinant baculoviruses that encode different human UDP-glucuronosyltransferases and mutants thereof. The infected cells were analysed for both relative expression levels and catalytic activity in each case, the combination of which yielded normalized activity. Kinetic analyses and copurification by affinity chromatography were also performed.

RESULTS

Coinfections with UGT1A4 increased the normalized scopoletin glucuronidation of 6YD (the Y485D mutant of UGT1A6) much more than it affected 1YD (the Y486D mutant of UGT1A1). Serotonin glucuronidation analyses revealed that coexpression of 6YD with most other human UDP-glucuronosyltransferases significantly increased the normalized activity of this mutant. Using 1-naphthol as the aglycone substrate, the Km of 6YD for the cosubstrate UDP-glucuronic acid was about 50 times higher than in UGT1A6. Yet, coexpression of 6YD with UGT1A4 lowered the Km for UDP-glucuronic acid to the level of UGT1A6. Coexpression also influenced wild-type UGT1A6 and UGT2B7, increasing the normalized activity of UGT1A6, but decreasing it for UGT2B7.

CONCLUSION

Hetero-oligomerization may play an important role in UDP-glucuronosyltransferases activity.

Regulation of the UGT1A1 bilirubin-conjugating pathway: role of a new splicing event at the UGT1A locus

UDP-glucuronosyltransferase 1A1 (UGT1A1) is involved in a wide range of biological and pharmacological processes because of its critical role in the conjugation of a diverse array of endogenous and exogenous compounds. We now describe a new UGT1A1 isoform, referred to as isoform 2 (UGT1A1_i2), encoded by a 1495-bp complementary DNA isolated from human liver and generated by an alternative splicing event involving an additional exon found at the 3' end of the UGT1A locus. The N-terminal portion of the 45-kd UGT1A1_i2 protein is identical to UGT1A1 (55 kd, UGT1A1_i1); however, UGT1A1_i2 contains a unique 10-residue sequence instead of the 99-amino acid C-terminal domain of UGT1A1_i1. RT-PCR and Western blot analyses with a specific antibody against UGT1A1 indicate that isoform 2 is differentially expressed in liver, kidney, colon, and small intestine at levels that reach or exceed, for some tissues, those of isoform 1. Western blots of different cell fractions and immunofluorescence experiments indicate that UGT1A1_i1 and UGT1A1_i2 colocalize in microsomes. Functional enzymatic data indicate that UGT1A1_i2, which lacks transferase activity when stably expressed alone in HEK293 cells, acts as a negative modulator of UGT1A1_i1, decreasing its activity by up to 78%. Coimmunoprecipitation of UGT1A1_i1 and UGT1A1_i2 suggests that this repression may occur via direct protein-protein interactions.

CONCLUSION

Our results indicate that this newly discovered alternative splicing mechanism at the UGT1A locus amplifies the structural diversity of human UGT proteins and describes the identification of an additional posttranscriptional regulatory mechanism of the glucuronidation pathway.

Oligomerization of the UDP-glucuronosyltransferase 1A proteins: homo- and heterodimerization analysis by fluorescence resonance energy transfer and co-immunoprecipitation

UDP-glucuronosyltransferases (UGTs) are membrane-bound proteins localized to the endoplasmic reticulum and catalyze the formation of beta-d-glucopyranosiduronic acids (glucuronides) using UDP-glucuronic acid and acceptor substrates such as drugs, steroids, bile acids, xenobiotics, and dietary nutrients. Recent biochemical evidence indicates that the UGT proteins may oligomerize in the membrane, but conclusive evidence is still lacking. In the present study, we have used fluorescence resonance energy transfer (FRET) to study UGT1A oligomerization in live cells. This technique demonstrated that UGT1A1, UGT1A3, UGT1A4, UGT1A6, UGT1A7, UGT1A8, UGT1A9, and UGT1A10 self-oligomerize (homodimerize). Heterodimer interactions were also explored, and it was determined that UGT1A1 was capable of binding with UGT1A3, UGT1A4, UGT1A6, UGT1A7, UGT1A8, UGT1A9, and UGT1A10. In addition to the in vivo FRET analysis, UGT1A protein-protein interactions were demonstrated through co-immunoprecipitation experiments. Co-expression of hemagglutinin-tagged and cyan fluorescent protein-tagged UGT1A proteins, followed by immunoprecipitation with anti-hemagglutinin beads, illustrated the potential of each UGT1A protein to homodimerize. Co-immunoprecipitation results also confirmed that UGT1A1 was capable of forming heterodimer complexes with all of the UGT1A proteins, corroborating the FRET results in live cells. These preliminary studies suggest that the UGT1A family of proteins form oligomerized complexes in the membrane, a property that may influence function and substrate selectivity.

Molecular and functional characterization of microsomal UDP-glucuronic acid uptake by members of the nucleotide sugar transporter (NST) family

Transport of the co-substrate UDPGA (UDP-glucuronic acid) into the lumen of the endoplasmic reticulum is an essential step in glucuronidation reactions due to the intraluminal location of the catalytic site of the enzyme UGT (UDP-glucuronosyltransferase). In the present study, we have characterized the function of several NSTs (nucleotide sugar transporters) and UGTs as potential carriers of UDPGA for glucuronidation reactions. UDPGlcNAc (UDP-N-acetylglucosamine)-dependent UDPGA uptake was found both in rat liver microsomes and in microsomes prepared from the rat hepatoma cell line H4IIE. The latency of UGT activity in microsomes derived from rat liver and V79 cells expressing UGT1A6 correlated well with mannose-6-phosphatase latency, confirming the UGT in the recombinant cells retained a physiology similar to rat liver microsomes. In the present study, four cDNAs coding for NSTs were obtained; two were previously reported (UGTrel1 and UGTrel7) and two newly identified (huYEA4 and huYEA4S). Localization of NSTs within the human genome sequence revealed that huYEA4S is an alternatively spliced form of huYEA4. All the cloned NSTs were stably expressed in V79 (Chinese hamster fibroblast) cells, and were able to transport UDPGA after preloading of isolated microsomal vesicles with UDPGlcNAc. The highest uptake was seen with UGTrel7, which displayed a V(max) approx. 1% of rat liver microsomes. Treatment of H4IIE cells with beta-naphthoflavone induced UGT protein expression but did not affect the rate of UDPGA uptake. Furthermore, microsomes from UGT1-deficient Gunn rat liver showed UDPGA uptake similar to those from control rats. These data show that NSTs can act as UDPGA transporters for glucuronidation reactions, and indicate that UGTs of the 1A family do not function as UDPGA carriers in microsomes. The cell line H4IIE is a useful model for the study of UDPGA transporters for glucuronidation reactions.

Epirubicin Glucuronidation and UGT2B7 Developmental Expression

The usefulness of epirubicin in the treatment of adult and childhood malignant diseases is related in part to the potential reduction in cardiac toxicity compared with that of other anthracyclines given at equivalent doses. An important pathway for epirubicin detoxification is UGT2B7-dependent glucuronidation. This study was implemented to provide a preclinical evaluation of the metabolism of epirubicin with respect to age-related changes in epirubicin glucuronidation in pediatric liver microsomes. Rates of epirubicin glucuronidation and levels of UGT2B7 were determined for liver microsomes from four pediatric age categories (n = 32) and one adult age category (n = 8). Both sets of data showed an increase in UGT2B7 activity and content with increasing age. Epirubicin glucuronidation activity in the adult group was statistically higher compared with all pediatric age groups (p < or = 0.01). UGT2B7 expression also was statistically higher in adults compared with children below 11 years of age, with evidence of significant differences in protein levels among the pediatric age categories. A positive correlation (r = 0.68) between UGT2B7 levels and postnatal age was observed, suggesting a progressive increase in UGT2B7 protein expression with increasing age. However, allometric scaling using the (3/4) power rule suggested no difference in activity between any of the pediatric age categories and the adult, although only a single neonatal sample was included in the analysis. In summary, these in vitro data show differences in epirubicin glucuronidation and UGT2B7 content within pediatric age groups and support the use of epirubicin in pediatric patients at least 6 months of age.