Determinants of UDP glucuronosyltransferase membrane association and residency in the endoplasmic reticulum

Endoplasmic reticulum transporter OAT2 regulates drug metabolism and interaction

Xenobiotic metabolic reactions in the hepatocyte endoplasmic reticulum (ER) including UDP-glucuronosyltransferase and carboxylesterase play central roles in the detoxification of medical agents with small- and medium-sized molecules. Although the catalytic sites of these enzymes exist inside of ER, the molecular mechanism for membrane permeation in the ER remains enigmatic. Here, we investigated that organic anion transporter 2 (OAT2) regulates the detoxification reactions of xenobiotic agents including anti-cancer capecitabine and antiviral zidovudine, via the permeation process across the ER membrane in the liver. Pharmacokinetic studies in patients with colorectal cancer revealed that the half-lives of capecitabine in rs2270860 (1324C > T) variants was 1.4 times higher than that in the C/C variants. Moreover, the hydrolysis of capecitabine to 5'-deoxy-5-fluorocytidine in primary cultured human hepatocytes was reduced by OAT2 inhibitor ketoprofen, whereas capecitabine hydrolysis directly assessed in human liver microsomes were not affected. The immunostaining of OAT2 was merged with ER marker calnexin in human liver periportal zone. These results suggested that OAT2 is involved in distribution of capecitabine into ER. Furthermore, we clarified that OAT2 plays an essential role in drug-drug interactions between zidovudine and valproic acid, leading to the alteration in zidovudine exposure to the body. Our findings contribute to mechanistically understanding medical agent detoxification, shedding light on the ER membrane permeation process as xenobiotic metabolic machinery to improve chemical changes in hydrophilic compounds.

Heterodimerization of Human UDP-Glucuronosyltransferase 1A9 and UDP-Glucuronosyltransferase 2B7 Alters Their Glucuronidation Activities

Human UDP-glucuronosyltransferases (UGTs) play a pivotal role as prominent phase II metabolic enzymes, mediating the glucuronidation of both endobiotics and xenobiotics. Dimerization greatly modulates the enzymatic activities of UGTs. In this study, we examined the influence of three mutations (H35A, H268Y, and N68A/N315A) and four truncations (signal peptide, single transmembrane helix, cytosolic tail, and di-lysine motif) in UGT2B7 on its heterodimerization with wild-type UGT1A9, using a Bac-to-Bac expression system. We employed quantitative fluorescence resonance energy transfer (FRET) techniques and co-immunoprecipitation assays to evaluate the formation of heterodimers between UGT1A9 and UGT2B7 allozymes. Furthermore, we evaluated the glucuronidation activities of the heterodimers using zidovudine and propofol as substrates for UGT2B7 and UGT1A9, respectively. Our findings revealed that the histidine residue at codon 35 was involved in the dimeric interaction, as evidenced by the FRET efficiencies and catalytic activities. Interestingly, the signal peptide and single transmembrane helix domain of UGT2B7 had no impact on the protein-protein interaction. These results provide valuable insights for a comprehensive understanding of UGT1A9/UGT2B7 heterodimer formation and its association with glucuronidation activity. SIGNIFICANCE STATEMENT: Our findings revealed that the H35A mutation in UGT2B7 affected the affinity of protein-protein interaction, leading to discernable variations in fluorescence resonance energy transfer efficiencies and catalytic activity. Furthermore, the signal peptide and single transmembrane helix domain of UGT2B7 did not influence heterodimer formation. These results provide valuable insights into the combined effects of polymorphisms and protein-protein interactions on the catalytic activity of UGT1A9 and UGT2B7, enhancing our understanding of UGT dimerization and its impact on metabolite formation.

Integrate thermostabilized fusion protein apocytochrome b562RIL and N-glycosylation mutations: A novel approach to heterologous expression of human UDP-glucuronosyltransferase (UGT) 2B7

Human UDP-glucuronosyltransferase (UGT) 2B7 is a crucial phase II metabolic enzyme that transfers glucuronic acid from UDP-glucuronic acid (UDPGA) to endobiotic and xenobiotic substrates. Biophysical and biochemical investigations of UGT2B7 are hampered by the challenge of the integral membrane protein purification. This study focused on the expression and purification of recombinant UGT2B7 by optimizing the insertion sites for the thermostabilized fusion protein apocytochrome b₅₆₂RIL (BRIL) and various mutations to improve the protein yields and homogeneity. Preparation of the recombinant proteins with high purity accelerated the measurement of pharmacokinetic parameters of UGT2B7. The dissociation constants (K_D) of two classical substrates (zidovudine and androsterone) and two inhibitors (schisanhenol and hesperetin) of UGT2B7 were determined using the surface plasmon resonance spectroscopy (SPR) for the first time. Using negative-staining transmission electron microscopy (TEM), UGT2B7 protein particles were characterized, which could be useful for further exploring its three-dimensional structure. The methods described in this study could be broadly applied to other UGTs and are expected to provide the basis for the exploration of metabolic enzyme kinetics, the mechanisms of drug metabolisms and drug interactions, changes in pharmacokinetics, and pharmacodynamics studies in vitro.

Uptake and time-dependent subcellular localization of native and micellar curcumin in intestinal cells

Uptake into intestinal cells and intracellular distribution into metabolically competent organelles, such as the endoplasmic reticulum, are important processes potentially limiting the bioavailability of xenobiotics. The incorporation of curcumin into polysorbate 80 micelles improves its naturally low oral bioavailability in humans. Here, we investigated uptake and time-dependent localization of curcumin in intestinal cells when administered as native or micellar formulation. Differentiated Caco-2 cells were incubated with 200 μmol/L native or micellar curcumin for up to 180 min and cellular uptake was quantified. Intracellular curcumin was detected already after 30 min and did not differ significantly between formulations or over time. Subcellular localization of native and micellar curcumin in Caco-2 cells was studied by density gradient centrifugation. After 30 min, curcumin from both formulations was mainly associated with mitochondria and lysosomes, after 180 min native curcumin was associated with mitochondria and peroxisomes, micellar curcumin with peroxisomes only. Uptake and localization of native and micellar curcumin in intestinal cells do not differ significantly and consequently do not explain differences in bioavailability in humans. The temporary co-localization with lysosomes is in agreement with the previously proposed role of endocytosis in cellular uptake of curcumin and warrants further investigation.

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.

Molecular Mechanism of the UDP-Glucuronosyltransferase 2B20-like Gene (AccUGT2B20-like) in Pesticide Resistance of Apis cerana cerana

UDP-glucuronosyltransferases (UGTs), being multifunctional detoxification enzymes, play a major role in the process of resistance to various pesticides in insects. However, the mechanism underlying the molecular regulation of pesticide resistance remains unclear, especially in Apis cerana cerana. In this study, all of the UGTs in Apis cerana cerana (AccUGT) have been identified through the multiple alignment and phylogenetic analysis. Expression of AccUGT genes under different pesticides, and antioxidant genes after silencing of AccUGT2B20-like, were detected by qRT-PCR. The resistance of overexpressed AccUGT2B20-like to oxidative stress was investigated by an Escherichia coli overexpression system. Also, antioxidant-related enzyme activity was detected after silencing of the AccUGT2B20-like gene. Expression pattern analysis showed that almost all UGT genes were upregulated under different pesticide treatments. This result indicated that AccUGTs participate in the detoxification process of pesticides. AccUGT2B20-like was the major gene because it was more highly induced than the others. Overexpression of AccUGT2B20-like in E. coli could effectively improve oxidative stress resistance. Specifically, silencing the AccUGT2B20-like gene increased oxidative stress by repressing the expression of oxidation-related genes, decreasing antioxidant-related enzyme activity, and increasing malondialdehyde concentration. Taken together, our results indicate that AccUGTs are involved in pesticide resistance, among which, AccUGT2B20-like contributes to the detoxification of pesticides by eliminating oxidative stress in Apis cerana cerana. This study explains the molecular basis for the resistance of bees to pesticides and provides an important safeguard for maintaining ecological balance.

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.

Network Pharmacology-Based Prediction and Verification of Qingluo Tongbi Formula to Reduce Liver Toxicity of Tripterygium wilfordii via UGT2B7 in Endoplasmic Reticulum

BACKGROUND The hepatotoxicity of Tripterygium wilfordii Hook. f. (TWHF) limits its clinic utilization. Qingluo Tongbi formula (QTF) was formulated based on a basic Chinese medicine theory. Previous studies have confirmed the safety and efficacy of QTF in treating rheumatoid arthritis. Therefore, we considered that TWHF could be detoxified based on its reasonable compatibility with QTF. We investigated the detoxicity mechanism of QTF in reducing the liver toxicity of TWHF. MATERIAL AND METHODS We used network pharmacology to determine the relevant metabolism targets of TWHF, focusing on the phase II metabolic enzymes uridine diphosphate-glucuronosyltransferase 1A1 (UGT1A1), UGT1A6, and UGT2B7. Based on the molecular mechanisms of these predictions and the results of the network analysis, we designed experiments to verify our hypothesis in vivo. We used western blotting, real-time quantitative polymerase chain reaction (RT-qPCR), double immunofluorescence, and laser confocal microscopy to detect the expression of UGTs. Finally, we used transmission electron microscopy to observe the endoplasmic reticulum structure. RESULTS The results confirmed that QTF reversed the TWHF-induced reduction of UGT content in liver microsomes, upregulated UGT1A1 and UGT1A6 but not UGT2B7 in the liver tissue. UGT2B7 expression in the liver and liver microsomes was inconsistent. QTF upregulated the expression of UGT2B7 in the endoplasmic reticulum, and QTF upregulated UGT2B7 expression levels in the endoplasmic reticulum compared with TWHF, which reduced liver toxicity. Structural changes were observed in the endoplasmic reticulum. CONCLUSIONS The Chinese traditional medicine compound QTF can achieve the effect of detoxification by upregulating the expression of UGT2B7 in the endoplasmic reticulum.

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.

UDP-Glucuronosyltransferase (UGT)-mediated attenuations of cytochrome P450 3A4 activity: UGT isoform-dependent mechanism of suppression

BACKGROUND AND PURPOSE

Cytochrome P450 (CYP, P450) 3A4 is involved in the metabolism of 50% of drugs and its catalytic activity in vivo is not explained only by hepatic expression levels. We previously demonstrated that UDP-glucuronosyltransferase (UGT) 2B7 suppressed CYP3A4 activity through an interaction. In the present study, we target UGT1A9 as another candidate modulator of CYP3A4.

EXPERIMENTAL APPROACH

We prepared co-expressed enzymes using the baculovirus-insect cell expression system and compared CYP3A4 activity in the presence and absence of UGT1A9. Wistar rats were treated with dexamethasone and liver microsomes were used to elucidate the role of CYP3A-UGT1A interactions.

KEY RESULTS

UGT1A9 and UGT2B7 interacted with and suppressed CYP3A4. Kinetic analyses showed that both of the UGTs significantly reduced V_max of CYP3A4 activity. In addition, C-terminal truncated mutants of UGT1A9 and UGT2B7 still retained the suppressive capacity. Dexamethasone treatment induced hepatic CYP3As and UGT1As at different magnitudes. Turnover of CYP3A was enhanced about twofold by this treatment.

CONCLUSION AND IMPLICATIONS

The changes of kinetic parameters suggested that UGT1A9 suppressed CYP3A4 activity with almost the same mechanism as UGT2B7. The luminal domain of UGTs contains the suppressive interaction site(s), whereas the C-terminal domain may contribute to modulating suppression in a UGT isoform-specific manner. CYP3A-UGT1A interaction seemed to be disturbed by dexamethasone treatment and the suppression was partially cancelled. CYP3A4-UGT interactions would help to better understand the causes of inter/intra-individual differences in CYP3A4 activity.

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.

Prediction of Drug-Drug Interactions Related to Inhibition or Induction of Drug-Metabolizing Enzymes

Drug-drug interaction (DDI) is the phenomenon of alteration of the pharmacological activity of a drug(s) when another drug(s) is co-administered in cases of so-called polypharmacy. There are three types of DDIs: pharmacokinetic (PK), pharmacodynamic, and pharmaceutical. PK is the most frequent type of DDI, which often appears as a result of the inhibition or induction of drug-metabolising enzymes (DME). In this review, we summarise in silico methods that may be applied for the prediction of the inhibition or induction of DMEs and describe appropriate computational methods for DDI prediction, showing the current situation and perspectives of these approaches in medicinal and pharmaceutical chemistry. We review sources of information on DDI, which can be used in pharmaceutical investigations and medicinal practice and/or for the creation of computational models. The problem of the inaccuracy and redundancy of these data are discussed. We provide information on the state-of-the-art physiologically- based pharmacokinetic modelling (PBPK) approaches and DME-based in silico methods. In the section on ligand-based methods, we describe pharmacophore models, molecular field analysis, quantitative structure-activity relationships (QSAR), and similarity analysis applied to the prediction of DDI related to the inhibition or induction of DME. In conclusion, we discuss the problems of DDI severity assessment, mention factors that influence severity, and highlight the issues, perspectives and practical using of in silico methods.

FABP1 controls hepatic transport and biotransformation of Δ9-THC

The increasing use of medical marijuana highlights the importance of developing a better understanding of cannabinoid metabolism. Phytocannabinoids, including ∆⁹-tetrahydrocannabinol (THC), are metabolized and inactivated by cytochrome P450 enzymes primarily within the liver. The lipophilic nature of cannabinoids necessitates mechanism(s) to facilitate their intracellular transport to metabolic enzymes. Here, we test the central hypothesis that liver-type fatty acid binding protein (FABP1) mediates phytocannabinoid transport and subsequent inactivation. Using X-ray crystallography, molecular modeling, and in vitro binding approaches we demonstrate that FABP1 accommodates one molecule of THC within its ligand binding pocket. Consistent with its role as a THC carrier, biotransformation of THC was reduced in primary hepatocytes obtained from FABP1-knockout (FABP1-KO) mice. Compared to their wild-type littermates, administration of THC to male and female FABP1-KO mice potentiated the physiological and behavioral effects of THC. The stark pharmacodynamic differences were confirmed upon pharmacokinetic analyses which revealed that FABP1-KO mice exhibit reduced rates of THC biotransformation. Collectively, these data position FABP1 as a hepatic THC transport protein and a critical mediator of cannabinoid inactivation. Since commonly used medications bind to FABP1 with comparable affinities to THC, our results further suggest that FABP1 could serve a previously unrecognized site of drug-drug interactions.

Investigation of the Endoplasmic Reticulum Localization of UDP-Glucuronosyltransferase 2B7 with Systematic Deletion Mutants

UDP-Glucuronosyltransferase (UGT) plays an important role in the metabolism of endogenous and exogenous compounds. UGT is a type I membrane protein, and has a dilysine motif (KKXX/KXKXX) in its C-terminal cytoplasmic domain. Although a dilysine motif is defined as an endoplasmic reticulum (ER) retrieval signal, it remains a matter of debate whether this motif functions in the ER localization of UGT. To address this issue, we generated systematic deletion mutants of UGT2B7, a major human isoform, and compared their subcellular localizations with that of an ER marker protein calnexin (CNX), using subcellular fractionation and immunofluorescent microscopy. We found that although the dilysine motif functioned as the ER retention signal in a chimera that replaced the cytoplasmic domain of CD4 with that of UGT2B7, UGT2B7 truncated mutants lacking this motif extensively colocalized with CNX, indicating dilysine motif-independent ER retention of UGT2B7. Moreover, deletion of the C-terminal transmembrane and cytoplasmic domains did not affect ER localization of UGT2B7, suggesting that the signal necessary for ER retention of UGT2B7 is present in its luminal domain. Serial deletions of the luminal domain, however, did not affect the ER retention of the mutants. Further, a cytoplasmic and transmembrane domain-deleted mutant of UGT2B7 was localized to the ER without being secreted. These results suggest that UGT2B7 could localize to the ER without any retention signal, and lead to the conclusion that the static localization of UGT results from lack of a signal for export from the ER.

Suppression of Cytochrome P450 3A4 Function by UDP-Glucuronosyltransferase 2B7 through a Protein-Protein Interaction: Cooperative Roles of the Cytosolic Carboxyl-Terminal Domain and the Luminal Anchoring Region

There is a large discrepancy between the interindividual difference in the hepatic expression level of cytochrome P450 3A4 (CYP3A4) and that of drug clearance mediated by this enzyme. However, the reason for this discrepancy remains largely unknown. Because CYP3A4 interacts with UDP-glucuronosyltransferase 2B7 (UGT2B7) to alter its function, the reverse regulation is expected to modulate CYP3A4-catalyzed activity. To address this issue, we investigated whether protein-protein interaction between CYP3A4 and UGT2B7 modulates CYP3A4 function. For this purpose, we coexpressed CYP3A4, NADPH-cytochrome P450 reductase, and UGT2B7 using a baculovirus-insect cell system. The activity of CYP3A4 was significantly suppressed by coexpressing UGT2B7, and this suppressive effect was lost when UGT2B7 was replaced with calnexin (CNX). These results strongly suggest that UGT2B7 negatively regulates CYP3A4 activity through a protein-protein interaction. To identify the UGT2B7 domain associated with CYP3A4 suppression we generated 12 mutants including chimeras with CNX. Mutations introduced into the UGT2B7 carboxyl-terminal transmembrane helix caused a loss of the suppressive effect on CYP3A4. Thus, this hydrophobic region is necessary for the suppression of CYP3A4 activity. Replacement of the hydrophilic end of UGT2B7 with that of CNX produced a similar suppressive effect as the native enzyme. The data using chimeric protein demonstrated that the internal membrane-anchoring region of UGT2B7 is also needed for the association with CYP3A4. These data suggest that 1) UGT2B7 suppresses CYP3A4 function, and 2) both hydrophobic domains located near the C terminus and within UGT2B7 are needed for interaction with CYP3A4.

Importance of UDP-glucuronosyltransferases 2A2 and 2A3 in tobacco carcinogen metabolism

UDP-glucuronosyltransferase A1 (UGT2A1) is expressed in the lung and exhibits activity against polycyclic aromatic hydrocarbons (PAHs), suggesting UGT2A1 involvement in the local metabolism of PAH tobacco carcinogens. The goal of the present study was to investigate the importance of two additional UGT2A enzymes, UGT2A2 and UGT2A3, in tobacco carcinogen metabolism. Real-time polymerase chain reaction suggested that wild-type UGT2A2 had the highest expression in the breast, followed by trachea > larynx > kidney. A novel splice variant of UGT2A2 lacking exon 3 (termed UGT2A2Δexon3) was investigated, with UGT2A2Δexon3 expression determined to be 25-50% that of wild-type UGT2A2 in all tissues examined. UGT2A3 was determined to be well expressed in the liver and colon, followed by pancreas > kidney > lung > tonsil > trachea > larynx. Cell homogenates prepared from human embryonic kidney (HEK)293 cells overexpressing wild-type UGT2A2 (termed UGT2A2_i1) exhibited glucuronidation activity, as observed by reverse-phase ultra-pressure liquid chromatography, against 1-hydroxy-(OH)-pyrene, 1-naphthol, and hydroxylated benzo(a)pyrene metabolites, whereas homogenates prepared from HEK293 cells overexpressing UGT2A3 only showed activity against simple PAHs like 1-OH-pyrene and 1-naphthol. Activity assays showed the UGT2A2Δexon3 protein (termed UGT2A2_i2) exhibited no detectable glucuronidation activity against all substrates examined; however, coexpression studies suggested that UGT2A2_i2 negatively modulates UGT2A2_i1 activity. Both UGT2A2 and UGT2A3 exhibited no detectable activity against complex PAH proximate carcinogens, tobacco-specific nitrosamines, or heterocyclic amines. These data suggest that, although UGT2A1 is the only UGT2A enzyme active against PAH proximate carcinogens (including PAH diols), both UGTs 2A1 and 2A2 play an important role in the local detoxification of procarcinogenic monohydroxylated PAH metabolites.

ABCC2 is involved in the hepatocyte perinuclear barrier for small organic compounds

Small organic molecules are believed to freely diffuse across nuclear pores but this may not be so if this route is blocked during protein and nucleic acid transfer. Here we have investigated the existence of transport mechanisms across the nuclear envelope (NE) of hepatocytes. Using nuclei isolated from rat liver cells, evidence for the existence of ATP-dependent transporters of organic compounds was found. In rat hepatocyte NE, with negligible contamination by other membranes, the presence of mature and glycosylated ABCC2, but not other ABC export pumps, was detected. ABCC2 was localized in the same membranes as the conjugating enzyme UGT1A1. Human ABCC2 ORF was tagged with V5 and transfected to human hepatoma cells. ABCC2-V5 protein was detected at perinuclear ER vesicles and at the NE. Both compartments expressing ABCC2-V5 were able to exclude calcein. ABCC2 abundance at the NE of rat hepatocytes was modified by treatments able to increase or reduce the expression of canalicular ABCC2. The sensitivity to mitoxantrone was higher for hepatocytes obtained from TR- rats whose NE lacked ABCC2. Incubation with mitoxantrone after depletion of ATP resulted in a marked accumulation of mitoxantrone in the nucleus of wild-type, but not TR-, hepatocytes. In sum, ABCC2 is present at the NE and perinuclear ER where, in combination with the activity of conjugating enzymes, this pump may be involved in the perinuclear barrier for small organic molecules, playing a role in protecting DNA from genotoxic compounds and in the control of intranuclear concentrations of ligands for nuclear receptors.

Determinants of the enzymatic activity and the subcellular localization of aspartate N-acetyltransferase

Aspartate N-acetyltransferase (NAT8L, N-acetyltransferase 8-like), the enzyme that synthesizes N-acetylaspartate, is membrane-bound and is at least partially associated with the ER (endoplasmic reticulum). The aim of the present study was to determine which regions of the protein are important for its catalytic activity and its subcellular localization. Transfection of truncated forms of NAT8L into HEK (human embryonic kidney)-293T cells indicated that the 68 N-terminal residues (regions 1 and 2) have no importance for the catalytic activity and the subcellular localization of this enzyme, which was exclusively associated with the ER. Mutation of conserved residues that precede (Arg81 and Glu101, in region 3) or follow (Asp168 and Arg220, in region 5) the putative membrane region (region 4) markedly affected the kinetic properties, suggesting that regions 3 and 5 form the catalytic domain and that the membrane region has a loop structure. Evidence is provided for the membrane region comprising α-helices and the catalytic site being cytosolic. Transfection of chimaeric proteins in which GFP (green fluorescent protein) was fused to different regions of NAT8L indicated that the membrane region (region 4) is necessary and sufficient to target NAT8L to the ER. Thus NAT8L is targeted to the ER membrane by a hydrophobic loop that connects two regions of the catalytic domain.

Static retention of the lumenal monotopic membrane protein torsinA in the endoplasmic reticulum

TorsinA is a membrane-associated enzyme in the endoplasmic reticulum (ER) lumen that is mutated in DYT1 dystonia. How it remains in the ER has been unclear. We report that a hydrophobic N-terminal domain (NTD) directs static retention of torsinA within the ER by excluding it from ER exit sites, as has been previously reported for short transmembrane domains (TMDs). We show that despite the NTD's physicochemical similarity to TMDs, it does not traverse the membrane, defining torsinA as a lumenal monotopic membrane protein and requiring a new paradigm to explain retention. ER retention and membrane association are perturbed by a subset of nonconservative mutations to the NTD, suggesting that a helical structure with defined orientation in the membrane is required. TorsinA preferentially enriches in ER sheets, as might be expected for a lumenal monotopic membrane protein. We propose that the principle of membrane-based protein sorting extends to monotopic membrane proteins, and identify other proteins including the monotopic lumenal enzyme cyclooxygenase 1 (prostaglandin H synthase 1) that share this mechanism of retention with torsinA.

The crystal structure of human UDP-glucuronosyltransferase 2B7 C-terminal end is the first mammalian UGT target to be revealed: the significance for human UGTs from both the 1A and 2B families

Human UDP-glucuronosyltransferases (EC 2.4.1.17) (UGTs) are major phase II metabolism enzymes that detoxify a multitude of endo- and xenobiotics through the covalent addition of a glucuronic acid moiety. UGTs are promiscuous enzymes that regulate the levels of numerous important endobiotics in a range of tissues, and inactivate most therapeutic compounds in concert with phase I enzymes. In spite of the importance of these enzymes, we have only a limited understanding of the molecular mechanisms governing their substrate specificity and catalytic activity. Until recently, no three-dimensional structural information was available for any mammalian UGT. The 1.8-å resolution apo crystal structure of the UDP-glucuronic acid binding domain of human UGT2B7 (2B7CT) is the only structure of a mammalian UGT target determined to date. In this review, we summarize what has been learned about human UGT function from the analysis of this and other related glycosyltransferase (GT) crystal structures.

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.

Identification of structural elements in Nox1 and Nox4 controlling localization and activity

Nox NADPH oxidases differ in their mode of activation, subcellular localization, and physiological function. Nox1 releases superoxide anions (O(2)(-)) and depends on cytosolic activator proteins, whereas Nox4 extracellularly releases hydrogen peroxide (H(2)O(2)), and its activity does not require cotransfection of additional proteins. We constructed chimeric proteins consisting of Nox1 and Nox4 expressed in HEK293 cells. When the cytosolic tail of Nox4 was fused with the transmembrane part of Nox1, Nox1 became constitutively active. The reciprocal construct was inactive, suggesting that cytosolic subunit-dependent activation requires elements in the transmembrane loops. By TIRF-microscopy, Nox1 was observed in the plasma membrane, whereas Nox4 colocalized with proteins of the endoplasmic reticulum. Fusion proteins of Myc and Nox revealed that the N-terminal part of Nox1 but not Nox4 is cleaved. When the potential signal peptide of Nox4 was inserted into Nox1, plasma-membrane localization was lost, and the protein was retained in vesicle-like structures below the plasma membrane. The potential signal peptide of Nox1 failed to translocate Nox4 to the plasma membrane but switched the extracellularly detectable ROS from H(2)O(2) to O(2)(-). Thus, the very N-terminal part of Nox proteins determines subcellular localization and the ROS type released, whereas the cytosolic tail regulates activity.

Glucuronidation of bioflavonoids by human UGT1A10: structure–function relationships

The extrahepatic human UDP glucuronosyltransferase 1A10 is found throughout the gastrointestinal tract and is thought to participate in the removal of orally ingested lipophilic chemicals. However, its substrate specificity towards these chemicals has not been fully characterized. The structurally diverse bioflavonoids are present in considerable amounts in fruits, vegetables and plant-derived beverages and have been shown to have many biological functions, including antioxidant properties. This study proposes features of the bioflavonoid structure necessary to confer it as a substrate of UGT1A10. The preferred substrates of UGT1A10 contain the hydroxyl group to be glucuronidated at C6 or C7, but not C5 of the A-ring or on C4' of the B-ring. Up to two additional hydroxyl groups on the A-ring enhance activity, whereas the presence of other groups, notably sugar groups, decreases activity. The high glucuronidation efficiency towards many bioflavonoids observed suggests that the contribution of UGT1A10 in the metabolism of these dietary compounds in the gastrointestinal tract may be significant.

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.

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.

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.

Fatty acyl-CoA as an endogenous activator of UDP-glucuronosyltransferases

The acyl-CoA-dependent modulation of hepatic microsomal UDP-glucuronosyltransferase (UGT) function in rats was studied. Oleoyl- and palmitoyl-CoAs inhibited UGT activity toward 4-methylumbelliferone in the presence of Brij 58. However, acyl-CoAs enhanced UGT activity in untreated microsomes. A maximum activation of about 8-fold over the control was observed at 15 microM oleoyl-CoA, whereas 50 microM or more oleoyl-CoA had an inhibitory effect on UGT function. Medium- and long-chain acyl-CoAs also exhibited similar effects. On the basis of resistance to tryptic digestion of UGTs, oleoyl-CoA at 15 microM has no ability to change the permeability of the endoplasmic reticulum (ER) membrane, although perturbation of the membrane occurred with 50 microM oleoyl-CoA. N-Ethylmaleimide and 5,5'-dithiobis(2-nitrobenzoic acid) abolished the oleoyl-CoA (15 microM)-dependent activation of microsomal UGT. These results suggest that: (1) acyl-CoAs play a role as an endogenous activator of UGTs, and (2) a sulfhydryl group is required for the activation of UGT by physiological concentrations of acyl-CoAs.

Uridine diphosphoglucuronosyltransferase pharmacogenetics and cancer

The uridine diphosphoglucuronosyltransferases (UGTs) belong to a superfamily of enzymes that catalyse the glucuronidation of numerous endobiotics and xenobiotics. Several human hepatic and extrahepatic UGT isozymes have been characterized with respect to their substrate specificity, tissue expression and gene structure. Genetic polymorphisms have been identified for almost all the UGT family members. A wide variety of anticancer drugs, dietary chemopreventives and carcinogens are known to be conjugated by members of both UGT1A and UGT2B subfamilies. This review examines in detail each UGT isozyme known to be associated with cancer and carcinogenesis. The cancer-related substrates for several UGTs are summarized, and the functionally relevant genetic polymorphisms of UGTs are reviewed. A number of genotype-phenotype association studies have been carried out to characterize the role of UGT pharmacogenetics in several types of cancer, and these examples are discussed here. In summary, this review focuses on the role of the human UGT genetic polymorphisms in carcinogenesis, chemoprevention and cancer risk.

Phenylalanine 90 and 93 Are Localized within the Phenol Binding Site of Human UDP-Glucuronosyltransferase 1A10 as Determined by Photoaffinity Labeling, Mass Spectrometry, and Site-Directed Mutagenesis

4-Azido-2-hydroxybenzoic acid (4-AzHBA), a novel photoactive benzoic acid derivative, has been synthesized and used as a photoprobe to identify the phenol binding site of UDP-glucuronosyltransferases (UGTs). Analysis of recombinant His-tag UGTs from the 1A family for their ability to glucuronidate p-nitrophenol (pNP) and 4-methylumbelliferone (4-MU) revealed that UGT1A10 shows high activity toward phenols and phenol derivatives. Purified UGT1A10 was photolabeled with 4-AzHBA, digested with trypsin, and analyzed by matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF)-mass spectrometry. A single modified peak corresponding to amino acid residues 89-98 (EFMVFHAQWK) of UGT1A10 was identified. The attachment site of the 4-AzHBA probe was localized to the quadruplet Phe(90)-Met(91)-Val(92)-Phe(93) using ESI LC-MS/MS. Sequence alignment revealed that the Phe(90) and Phe(93) are conserved in UGT1A7-10. Site-directed mutagenesis of these two amino acids was then followed by kinetic analysis of the mutants with two phenolic substrates, pNP and 4-MU, containing one and two planar rings, respectively. Using the combination of photoaffinity labeling, enzymatic digestion, MALDI-TOF and LC-MS mass spectrometry, and site-directed mutagenesis, we have determined for the first time that Phe(90) and Phe(93) are directly involved in the catalytic activity of UGT1A10 toward 4-MU and pNP.

Structure of UDP‐Glucuronosyltransferases in Membranes

This chapter presents the most recent experimental approaches to the investigation of UDP-glucuronosyltransferase (UGTs) in membranes. The first topic described is the subcellular localization of UGTs with special emphasis on the association of these proteins with the endoplasmic reticulum (ER). Experimental methods include subfractionation of tissue for microsome preparation, evaluation of the purity of the membrane fraction obtained, and measurement of UGT activity in the presence of detergents. Next, the recently demonstrated formation of UGT homo- and heterodimer formation and its functional relevance is discussed and the appropriate methods used to characterize such interactions are given (radiation inactivation, size exclusion chromatography, immunopurification, cross-linking, two-hybrid system). The structural determinants of UGTs in relation to membrane association, residency, and enzymatic activity are the next topic, supplemented by a description of the appropriate methods, including the design and expression of chimeric proteins, membrane insertion, and subcellular localization by immunofluorescence. Also presented is new information on the structure and function of UGTs obtained by molecular modeling, bioinformatics (sequence alignment), and comparison with selected crystallized glycosyltransferases. Finally, we discuss the important, and still not fully developed, issue of UGT active site architecture and organization within the ER. This is addressed from two perspectives: (1) chemical modification of UGT active sites by amino acid-specific probes and (2) photoaffinity labeling of UGTs. The detailed synthesis of a photoaffinity probe for an aglycon-binding site is provided and the use of this probe and direct photoaffinity labeling with retinoids is discussed. The application of proteomics techniques, including proteolytic digestion and protein sequencing by liquid chromatography/tandem mass spectrometry and matrix-assisted laser desorption ionization/time of flight, to the identification of crucial amino acids of the active sites, and subsequent site-directed mutagenesis of identified amino acids, is discussed in detail.

Human UGT1A6 pharmacogenetics

BACKGROUND

UDP-glucuronosyltransferase (UGT) enzymes catalyze the glucuronidation and typically inactivation of endogenous and exogenous molecules including steroid hormones, bilirubin and many drugs. The UGT1A6 protein is expressed predominantly in liver and metabolizes small phenolic drugs including acetaminophen, salicylates and many beta-blockers. Interindividual variation in the capacity of humans to glucuronidate drugs has been observed.

RESULTS

We have identified a novel common single nucleotide polymorphism (SNP) in the human UGT1A6 gene resulting in a Ser7Ala change in encoded amino acid. We have further functionally characterized that polymorphism in the context of two previously reported polymorphisms, Thr181Ala and Arg184Ser. These non-synonymous cSNPs define four common haplotypes. Alleles appear with similar frequencies in Caucasian and African-American populations with distributions adhering to Hardy-Weinberg equilibrium. UGT1A6 genotype, rate of substrate glucuronidation and level of immunoreactive UGT1A6 protein was determined. A 25-fold variation in the rate of substrate glucuronidation and an 85-fold variation in level of immunoreactive protein were measured. Liver tissue samples that were homozygous for UGT1A6*2 exhibited a high rate of glucuronidation relative to tissues with other genotypes. Biochemical kinetic studies of recombinant UGT1A6 expressed in HEK293 cells indicated that the UGT1A6*2 allozyme, expressed homozygously, had almost two-fold greater activity toward p-nitrophenol than UGT1A6*1 and when expressed heterozygously (UGT1A6*1/*2) it was associated with low enzyme activity.

CONCLUSIONS

These data suggest that common genetic variation in human UGT1A6 confers functionally significant differences in biochemical phenotype as assessed in human tissue and cultured cells expressing recombinant allozymes. This genetic variation might impact clinical efficacy or toxicity of drugs metabolized by UGT1A6.

An Active and Water-Soluble Truncation Mutant of the Human UDP-Glucuronosyltransferase 1A9

The UDP-glucuronosyltransferases (UGTs) are integral membrane proteins, and previous attempts to generate a water-soluble UGT by removing the single trans-membrane helix yielded inactive and membrane-bound proteins. We have now replaced the 45 C-terminal amino acids of the human UGT1A9, including its trans-membrane helix, with a fusion peptide ending with six His residues. Detergent-free extraction of insect cells expressing this mutant, UGT1A9Sol, released scopoletin glucuronidation activity into the supernatant, and subsequent ultracentrifugation did not sediment that activity. UGT1A9Sol was purified by immobilized metal affinity chromatography (IMAC) in the absence of detergents throughout the entire process. The IMAC purification increased somewhat the apparent K(m) of UGT1A9 toward scopoletin and rendered the enzyme sensitive to freezing. The activity of UGT1A9Sol in the cell extract was partly inhibited by Triton X-100, irrespective of the presence or absence of phospholipids. UGT1A9Sol exhibited a relatively high rate of scopoletin glucuronidation, whereas its activity toward 1-naphthol, entacapone, umbelliferone, and 4-nitrophenol was much lower. The kinetics and substrate specificity of UGT1A9Sol resembled the detergent-suspended full-length UGT1A9 rather than the membrane-bound UGT1A9. The apparent K(m) value of UGT1A9Sol for scopoletin was similar to that of the full-length UGT1A9 in the presence of detergent, but much higher than the respective value in the membrane-bound enzyme. The results suggest that either the detergent binding to the trans-membrane helix within the full-length UGT1A9, or the removal of this helix by gene manipulation, affect the interaction of the enzyme with its aglycone substrate in a similar manner.

Metabolic assessment in liver microsomes by co-activating cytochrome P450s and UDP-glycosyltransferases

A "dual-activity" microsomal system in which both CYPs and UGTs were active was evaluated for studies of metabolic stability and in-vitro metabolite profiling. In this "dual-activity" system, alamethicin, a pore-forming peptide, was used to activate UGTs in human liver microsomes without affecting CYP activity. Interference studies indicated that CYP cofactors had little effect on UGT surrogate activity as measured by glucuronidation of acetaminophen and trifluoperazine. Further, UGT cofactor, UDPGA (< 2 mM), did not inhibit the marker activity of five major CYPs including 1A2, 2C9, 2C19, 2D6 and 3A4, suggesting that both oxidation and glucuronidation can be co-activated in microsomes. In a comparison study, compounds with significant glucuronidation showed distinct stability profiles in the "dual-activity" system, compared to the conventional microsomal incubation in which only CYPs were active. For compounds with minor or no glucuronidation, the metabolic stability remained similar between the "dual-activity" system and the conventional microsomal incubation. The feasibility of this "dual-activity" system utilized for metabolite profiling was also investigated using tramadol as a model drug. It was found that oxidative metabolites of tramadol generated in the "dual-activity" system matched those detected in the conventional microsomal incubation. However, tramadol glucuronide was observed in the "dual-activity" system but not in the conventional micromosal incubation. Results clearly suggest that the "dual-activity" system is a valuable in vitro model for metabolism studies in drug discovery.

Expression and characterization of recombinant human UDP-glucuronosyltransferases (UGTs). UGT1A9 is more resistant to detergent inhibition than other UGTs and was purified as an active dimeric enzyme

Eight human liver UDP-glucuronosyltransferases (UGTs) were expressed in baculovirus-infected insect cells as fusion proteins carrying a short C-terminal extension that ends with 6 histidine residues (His tag). The activity of recombinant UGT1A1, UGT1A3, UGT1A4, UGT1A6, UGT2B4, UGT2B7, and UGT2B15 was almost fully inhibited by 0.2% Triton X-100. In the case of UGT1A9, however, glucuronidation of alpha-naphthol and scopoletin was resistant to such inhibition, whereas glucuronidation of entacapone and several other aglycones was sensitive. His-tagged UGT1A9 was purified by immobilized metal-chelating chromatography (IMAC). Purified UGT1A9 glucuronidated scopoletin at a high rate, whereas its glucuronidation activity toward entacapone was low and largely dependent on phospholipid addition. Recombinant UGT1A9 in which the His tag was replaced by hemagglutinin antigenic peptide (HA tag) was also prepared. Insect cells were co-infected with baculoviruses encoding both HA-tagged and His-tagged UGT1A9. Membranes from the co-infected cells, or a mixture of membranes from separately infected cells, were subjected to detergent extraction and IMAC, and the resulting fractions were analyzed for the presence of each type of UGT1A9 using tag-specific antibodies. In the case of separate infection, the HA-tagged UGT1A9 did not bind to the column. When co-infected with His-tagged UGT1A9, however, part of the HA-tagged enzyme was bound to the column and was eluted by imidazole concentration gradient together with the His-tagged UGT1A9, suggesting the formation of stable dimers that contain one His-tagged and one HA-tagged UGT1A9 monomers.

Characterization of truncated mutants of human microsomal short-chain dehydrogenase/reductase RoDH-4

Human NAD(+)-dependent microsomal short-chain dehydrogenase/reductase RoDH-4 oxidizes all-trans-retinol, 13-cis-retinol and 3alpha-hydroxysteroids to corresponding retinaldehydes and 3-ketones. RoDH-4 behaves as an integral membrane protein, but its topology in the membrane is not known. Analysis of RoDH-4 polypeptide using algorithms for secondary structure predictions suggests that RoDH-4 contains four potential membrane-spanning domains: the N-terminal, the C-terminal, and the two central hydrophobic segments. To determine the role of each segment in association of RoDH-4 with the membrane, we prepared several expression constructs coding for truncated RoDH-4 polypeptides that lacked the putative membrane-spanning domains and expressed them in insect Sf9 cells using the Baculovirus system. Association of truncated RoDH-4 constructs with the microsomal membranes was analyzed by alkaline extraction and floatation in sucrose gradient. Catalytic activity of truncated RoDH-4 constructs was assayed using the 3alpha-hydroxysteroid androsterone as substrate. Truncated RoDH-4 that lacked the first thirteen amino acids of the N-terminal segment was partially active and exhibited the apparent K(m) value for androsterone similar to that of the wild-type enzyme. Removal of 23 N-terminal hydrophobic amino acids resulted in significant loss of activity and a 14-fold increase in the apparent K(m) value. Removal of the C-terminal 27 amino acid segment resulted in a approximately 600-fold increase in the apparent K(m) value. Each truncated mutant behaved as an integral membrane protein. Furthermore, protein that lacked all four hydrophobic segments remained associated with the membrane. Thus, the N-terminal and the C-terminal ends are both important for RoDH-4 activity and the removal of the putative transmembrane segments does not convert RoDH-4 into a soluble protein, suggesting additional sites of membrane interaction.

Analysis of Opioid Binding to UDP-Glucuronosyltransferase 2B7 Fusion Proteins Using Nuclear Magnetic Resonance Spectroscopy

The UDP-glucuronosyltransferase UGT2B7 is an important human UGT isoform that catalyzes the conjugation of many endogenous and exogenous compounds, among them opioids, resulting in the formation of D-glucuronides. The binding site of the aglycone is located in the N-terminal half of the protein. In this study, we demonstrate that the opioid binding site in UGT2B7 is within the first 119 amino-terminal amino acids. Two maltose binding protein fusion proteins, 2B7F1 and 2B7F2, incorporating the first 157 or 119 amino acids, respectively, of UGT2B7 were expressed in Escherichia coli and purified by affinity chromatography. NMR spectroscopy using one-dimensional spectra, the inversion recovery method, and the transferred nuclear Overhauser effect spectroscopy was used to study the binding properties of opioids to the fusion proteins. Morphine was found to bind at a single site within the first 119 amino acids and to undergo a conformational change upon binding, as demonstrated by transferred nuclear Overhauser effect spectroscopy. Dissociation constants were obtained for morphine, naloxone, buprenorphine, and zidovudine, and the results were confirmed by equilibrium dialysis determinations. Two possible opioid binding sites, based on the nearest neighbors from opioid binding to the micro-receptor and to cytochrome 2D6, are proposed.

Isolation and characterization of the UGT2B28 cDNA encoding a novel human steroid conjugating UDP-glucuronosyltransferase

UDP-glucuronosyltransferase (UGT) enzymes belonging to the UGT2B subfamily catalyze the transfer of glucuronic acid to a large number of endogenous compounds, particularly steroids, to facilitate their elimination from target cells. A novel human UGT2B cDNA of 1666 bp was isolated and encodes a 529-amino acid protein named UGT2B28 type I. Glucuronidation assays demonstrated that UGT2B28 type I catalyzes the conjugation of endogenous and exogenous compounds. The tissue distribution of UGT2B28 revealed the expression of the type I transcript in the liver, breast, and LNCaP cells. Two other UGT2B cDNAs were isolated, and sequence analysis led to the identification of two truncated UGT2B28 species. UGT2B28 type II differs from type I by a deletion of 308 bp in the cofactor binding domain, whereas UGT2B28 type III lacks 351 bp in the putative substrate binding domain. All UGT2B28 isoforms are encoded by a single UGT2B28 gene which has a genomic organization similar to that of the other UGT2B genes characterized thus far. Although no substrates could be identified for the shorter isoforms, the three subtypes were shown to be located in the endoplasmic reticulum and the perinuclear membrane, demonstrating that the missing domains are not required for the subcellular localization of these UGT2B proteins. However, all the domains remain necessary for observing glucuronidation activity. The expression of UGT2B28 type I in the breast and liver suggests a role of this enzyme in the androgen and estrogen metabolism in these tissues.

Human UDP-glucuronosyltransferases: metabolism, expression, and disease

In vertebrates, the glucuronidation of small lipophilic agents is catalyzed by the endoplasmic reticulum UDP-glucuronosyltransferases (UGTs). This metabolic pathway leads to the formation of water-soluble metabolites originating from normal dietary processes, cellular catabolism, or exposure to drugs and xenobiotics. This classic detoxification process, which led to the discovery nearly 50 years ago of the cosubstrate UDP-glucuronic acid (19), is now known to be carried out by 15 human UGTs. Characterization of the individual gene products using cDNA expression experiments has led to the identification of over 350 individual compounds that serve as substrates for this superfamily of proteins. This data, coupled with the introduction of sophisticated RNA detection techniques designed to elucidate patterns of gene expression of the UGT superfamily in human liver and extrahepatic tissues of the gastrointestinal tract, has aided in understanding the contribution of glucuronidation toward epithelial first-pass metabolism. In addition, characterization of the UGT1A locus and genetic studies directed at understanding the role of bilirubin glucuronidation and the biochemical basis of the clinical symptoms found in unconjugated hyperbilirubinemia have uncovered the structural gene polymorphisms associated with Crigler-Najjar's and Gilbert's syndrome. The role of the UGTs in metabolism and different disease states in humans is the topic of this review.

An internal signal sequence mediates the targeting and retention of the human UDP-glucuronosyltransferase 1A6 to the endoplasmic reticulum

The human UDP-glucuronosyltransferase isoform UGT1A6 is predicted to be a type I transmembrane protein anchored in the endoplasmic reticulum by a single C-terminal transmembrane domain, followed by a short cytoplasmic tail. This topology is thought to be established through the sequential action of a cleavable N-terminal signal peptide and of a C-terminal stop transfer/anchor sequence. We found that the deletion of the signal peptide did not prevent membrane targeting and insertion of this protein expressed in an in vitro transcription/translation system or in yeast Pichia pastoris. Interestingly, the same results were obtained when the protein was depleted of both the signal peptide and the C-terminal transmembrane domain/cytoplasmic tail sequences, suggesting the presence of an internal topogenic element able to translocate and retain UGT1A6 in the endoplasmic reticulum membrane in vitro and in yeast cells. To identify such a sequence, the insertion of several N-terminal deletion mutants of UGT1A6 into microsomal membranes was investigated in vitro. The data clearly showed that the deletion of the N-terminal end did not affect endoplasmic reticulum targeting and retention until residues 140-240 were deleted. The signal-like activity of the 140-240 region was demonstrated by the ability of this segment to confer endoplasmic reticulum residency to the cytosolic green fluorescent protein expressed in mammalian cells. Finally, we show that this novel topogenic sequence can posttranslationally mediate the translocation of UGT1A6. This study provides the first evidence that the membrane assembly of the human UGT1A6 involves an internal signal retention sequence.

Chemical modification of rat hepatic microsomes with N-ethylmaleimide results in inactivation of both UDP-N-acetylglucosamine-dependent stimulation of glucuronidation and UDP-glucuronic acid uptake

Chemical modification of rat hepatic microsomes with N-ethylmaleimide (NEM) resulted in inactivation of UDP-N-acetylglucosamine (UDP-GlcNAc)-dependent stimulation of glucuronidation of p-nitrophenol. Inactivation kinetics and pH dependence were in agreement with the modification of a single sulfhydryl group. NEM also inactivated the uptake of UDP-glucuronic acid (UDP-GlcUA) but not UDP-glucose. With various sulfhydryl-modifying reagents, the inactivation of UDP-GlcUA uptake was linked to that of glucuronidation. UDP-GlcUA protected against NEM-sensitive inactivation of both UDP-GlcNAc-dependent stimulation of glucuronidation and UDP-GlcUA uptake, suggesting that the sulfhydryl group is located within or near the UDP-GlcUA binding site of the microsomal protein involved in the stimulation. Using microsomes labeled with biotin-conjugated maleimide and immunopurification with anti-peptide antibody against UDP-glucuronosyltransferase family 1 (UGT1) isozymes, immunopurified UGT1s were found to be labeled with the maleimide and UDP-GlcUA protected against the labeling as it did with the NEM-sensitive inactivation. These data suggest the involvement of a sulfhydryl residue of microsomal protein in the UDP-GlcNAc-dependent stimulation mechanism via the stimulation of UDP-GlcUA uptake into microsomal vesicles.