Structure of UDP‐Glucuronosyltransferases in Membranes

Knockout of two uridine diphosphate-glycosyltransferase genes increases the susceptibility of Spodoptera litura to Bacillus thuringiensis toxins

Uridine diphosphate-glycosyltransferases (UGTs) catalyze sugar conjugation of endogenous and exogenous molecules in insects. In this study, 45 putative UGT genes in 11 families were identified from the genome of S. litura. Exposure to Bt toxins in 5th-instar larvae of the WT strain led to a significant upregulation of midgut UGT40 expression, particularly of SlUGT40D20, SlUGT40D22, and SlUGT40F25. This upregulation was not observed following exposure to chemical pesticides. Knockout of the UGT genes SlUGT40D20 and SlUGT40D22 in S. litura (mutant strains SlUGT40D20-KO and SlUGT40D22-KO) via CRISPR/Cas9-mediated mutagenesis increased susceptibility of S. litura to Bacillus thuringiensis (Bt) insecticidal proteins. However, in comparison with the wild-type (WT) strain, the mutants did not change susceptibility to chemical pesticides. Observations of 5th-instar larval midgut by electron microscopy revealed severe damage to the midgut epithelium caused by Cry1Ac toxin at 10 μg/g in the SlUGT40D20-KO strain compared to the WT. SDS-PAGE and LC MS/MS analyses identified a specific protein band corresponding to putative proteoglycans in the peritrophic matrix of the WT strain, which was absent in the SlUGT40D20-KO strain. Our study suggests an inverse correlation between expression of some UGTs and the susceptibility of S. litura larvae to some Bt toxins.

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.

Mutual Influence of Human Cytochrome P450 Enzymes and UDP-Glucuronosyltransferases on Their Respective Activities in Recombinant Fission Yeast

Cytochromes P450 (CYPs) and UDP-glucuronosyltransferases (UGTs) are the most important human drug metabolizing enzymes, but their mutual interactions are poorly understood. In this study, we recombinantly co-expressed of each one of the 19 human members of the UGT families 1 and 2 with either CYP2C9, CYP2D6, or CYP4Z1 in fission yeast. Using these strains, we monitored a total of 72 interactions: 57 cases where we tested the influence of UGT co-expression on CYP activity and 15 cases of the opposite approach. In the majority of cases (88%), UGT co-expression had a statistically significant (p < 0.05) effect on P450 activity (58% positive and 30% negative). Strong changes were observed in nine cases, including one case with an activity increase by a factor of 23 (CYP2C9 activity in the presence of UGT2A3) but also four cases with a complete loss of activity. When monitoring the effect of CYP co-expression on the activity of five UGTs, activity changes were generally not so pronounced and, if observed, always detrimental. UGT2B7 activity was not influenced by CYP co-expression, while the other UGTs were affected to varying degrees. These data suggest the notion that mutual influence of CYPs and UGTs on each other's activity is a widespread phenomenon.

The Glycosyltransferase Pathway: An Integrated Analysis of the Cell Metabolome

Nucleotide sugar-dependent glycosyltransferases (UGTs) are critical to the homeostasis of endogenous metabolites and the detoxification of xenobiotics. Their impact on the cell metabolome remains unknown. Cellular metabolic changes resulting from human UGT expression were profiled by untargeted metabolomics. The abundant UGT1A1 and UGT2B7 were studied as UGT prototypes along with their alternative (alt.) splicing-derived isoforms displaying structural differences. Nineteen biochemical routes were modified, beyond known UGT substrates. Significant variations in glycolysis and pyrimidine pathways, and precursors of the co-substrate UDP-glucuronic acid were observed. Bioactive lipids such as arachidonic acid and endocannabinoids were highly enriched by up to 13.3-fold (p < 0.01) in cells expressing the canonical enzymes. Alt. UGT2B7 induced drastic and unique metabolic perturbations, including higher glucose (18-fold) levels and tricarboxylic acid cycle (TCA) cycle metabolites and abrogated the effects of the UGT2B7 canonical enzyme when co-expressed. UGT1A1 proteins promoted the accumulation of branched-chain amino acids (BCAA) and TCA metabolites upstream of the mitochondrial oxoglutarate dehydrogenase complex (OGDC). Alt. UGT1A1 exacerbated these changes, likely through its interaction with the OGDC component oxoglutarate dehydrogenase-like (OGDHL). This study expands the breadth of biochemical pathways associated with UGT expression and establishes extensive connectivity between UGT enzymes, alt. proteins and other metabolic processes.

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.

A review of species differences in the control of, and response to, chemical-induced thyroid hormone perturbations leading to thyroid cancer

This review summarises the current state of knowledge regarding the physiology and control of production of thyroid hormones, the effects of chemicals in perturbing their synthesis and release that result in thyroid cancer. It does not consider the potential neurodevelopmental consequences of low thyroid hormones. There are a number of known molecular initiating events (MIEs) that affect thyroid hormone synthesis in mammals and many chemicals are able to activate multiple MIEs simultaneously. AOP analysis of chemical-induced thyroid cancer in rodents has defined the key events that predispose to the development of rodent cancer and many of these will operate in humans under appropriate conditions, if they were exposed to high enough concentrations of the affecting chemicals. There are conditions however that, at the very least, would indicate significant quantitative differences in the sensitivity of humans to these effects, with rodents being considerably more sensitive to thyroid effects by virtue of differences in the biology, transport and control of thyroid hormones in these species as opposed to humans where turnover is appreciably lower and where serum transport of T4/T3 is different to that operating in rodents. There is heated debate around claimed qualitative differences between the rodent and human thyroid physiology, and significant reservations, both scientific and regulatory, still exist in terms of the potential neurodevelopmental consequences of low thyroid hormone levels at critical windows of time. In contrast, the situation for the chemical induction of thyroid cancer, through effects on thyroid hormone production and release, is less ambiguous with both theoretical, and actual data, showing clear dose-related thresholds for the key events predisposing to chemically induced thyroid cancer in rodents. In addition, qualitative differences in transport, and quantitative differences in half life, catabolism and turnover of thyroid hormones, exist that would not operate under normal situations in humans.

Alternative promoters control UGT2B17-dependent androgen catabolism in prostate cancer and its influence on progression

BACKGROUND

Perturbation of the major UGT2B17-dependent androgen catabolism pathway has the potential to affect prostate cancer (PCa) progression. The objective was to evaluate UGT2B17 protein expression in primary tumours in relation to hormone levels, disease characteristics and cancer evolution.

METHODS

We conducted an analysis of a high-density prostate tumour tissue microarray consisting of 239 localised PCa cases treated by radical prostatectomy (RP). Cox proportional hazard ratio analysis was used to evaluate biochemical recurrence (BCR), and a linear regression model evaluated variations in circulating hormone levels measured by mass spectrometry. The transcriptome of UGT2B17 in PCa was established by using RNA-sequencing data.

RESULTS

UGT2B17 expression in primary tumours was associated with node-positive disease at RP and linked to circulating levels of 3α-diol-17 glucuronide, a major circulating DHT metabolite produced by the UGT2B17 pathway. UGT2B17 was an independent prognostic factor linked to BCR after RP, and its overexpression was associated with development of metastasis. Finally, we demonstrated that distinctive alternative promoters dictate UGT2B17-dependent androgen catabolism in localised and metastatic PCa.

CONCLUSIONS

The androgen-inactivating gene UGT2B17 is controlled by overlooked regulatory regions in PCa. UGT2B17 expression in primary tumours influences the steroidome, and is associated with relevant clinical outcomes, such as BCR and metastasis.

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.

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.

Characterization of a membrane-bound C-glucosyltransferase responsible for carminic acid biosynthesis in Dactylopius coccus Costa

Carminic acid, a glucosylated anthraquinone found in scale insects like Dactylopius coccus, has since ancient times been used as a red colorant in various applications. Here we show that a membrane-bound C-glucosyltransferase, isolated from D. coccus and designated DcUGT2, catalyzes the glucosylation of flavokermesic acid and kermesic acid into their respective C-glucosides dcII and carminic acid. DcUGT2 is predicted to be a type I integral endoplasmic reticulum (ER) membrane protein, containing a cleavable N-terminal signal peptide and a C-terminal transmembrane helix that anchors the protein to the ER, followed by a short cytoplasmic tail. DcUGT2 is found to be heavily glycosylated. Truncated DcUGT2 proteins synthesized in yeast indicate the presence of an internal ER-targeting signal. The cleavable N-terminal signal peptide is shown to be essential for the activity of DcUGT2, whereas the transmembrane helix/cytoplasmic domains, although important, are not crucial for its catalytic function.

Carminic acid is a widely applied red colorant that is still harvested from insects because its biosynthesis is not fully understood. Here, the authors identify and characterize a membrane-bound C-glucosyltransferase catalyzing the final step during carminic acid biosynthesis.

Cytochrome P450 and Non-Cytochrome P450 Oxidative Metabolism: Contributions to the Pharmacokinetics, Safety, and Efficacy of Xenobiotics

The drug-metabolizing enzymes that contribute to the metabolism or bioactivation of a drug play a crucial role in defining the absorption, distribution, metabolism, and excretion properties of that drug. Although the overall effect of the cytochrome P450 (P450) family of drug-metabolizing enzymes in this capacity cannot be understated, advancements in the field of non-P450-mediated metabolism have garnered increasing attention in recent years. This is perhaps a direct result of our ability to systematically avoid P450 liabilities by introducing chemical moieties that are not susceptible to P450 metabolism but, as a result, may introduce key pharmacophores for other drug-metabolizing enzymes. Furthermore, the effects of both P450 and non-P450 metabolism at a drug's site of therapeutic action have also been subject to increased scrutiny. To this end, this Special Section on Emerging Novel Enzyme Pathways in Drug Metabolism will highlight a number of advancements that have recently been reported. The included articles support the important role of non-P450 enzymes in the clearance pathways of U.S. Food and Drug Administration-approved drugs over the past 10 years. Specific examples will detail recent reports of aldehyde oxidase, flavin-containing monooxygenase, and other non-P450 pathways that contribute to the metabolic, pharmacokinetic, or pharmacodynamic properties of xenobiotic compounds. Collectively, this series of articles provides additional support for the role of non-P450-mediated metabolic pathways that contribute to the absorption, distribution, metabolism, and excretion properties of current xenobiotics.

Transport of estradiol-17β-glucuronide, estrone-3-sulfate and taurocholate across the endoplasmic reticulum membrane: evidence for different transport systems

Important reactions of drug metabolism, including UGT mediated glucuronidation and steroidsulfatase mediated hydrolysis of sulfates, take place in the endoplasmic reticulum (ER) of hepatocytes. Consequently, UGT generated glucuronides, like estradiol-17β-glucuronide, have to be translocated back into the cytoplasm to reach their site of excretion. Also steroidsulfatase substrates, including estrone-3-sulfate, have to cross the ER membrane to reach their site of hydrolysis. Based on their physicochemical properties such compounds are not favored for passive diffusion and therefore likely necessitate transport system(s) to cross the ER membrane in either direction.

The current study aims to investigate the transport of taurocholate, estradiol-17β-glucuronide, and estrone-3-sulfate in smooth (SER) and rough (RER) endoplasmic reticulum membrane vesicles isolated from Wistar and TR⁻ rat liver.

Time-dependent and bidirectional transport was demonstrated for taurocholate, showing higher uptake rates in SER than RER vesicles. For estradiol-17β-glucuronide a fast time-dependent efflux with similar efficiencies from SER and RER but no clear protein-mediated uptake was shown, indicating an asymmetric transport system for this substrate. Estrone-3-sulfate uptake was time-dependent and higher in SER than in RER vesicles. Inhibition of steroidsulfatase mediated estrone-3-sulfate hydrolysis decreased estrone-3-sulfate uptake but had no effect on taurocholate or estradiol-17β-glucuronide transport.

Based on inhibition studies and transport characteristics, three different transport mechanisms are suggested to be involved in the transport of taurocholate, estrone-3-sulfate and estradiol-17β-glucuronide across the ER membrane.

ATP serves as an endogenous inhibitor of UDP-glucuronosyltransferase (UGT): a new insight into the latency of UGT

We have suggested that adenine-related compounds are allosteric inhibitors of UGT in rat liver microsomes (RLM) treated with detergent. To clarify whether the same occurs with a pore-forming peptide, alamethicin, the effects of adenine-related compounds on 4-metylumbelliferone (4-MU) glucuronidation were examined using RLM and human liver microsomes (HLM). ATP inhibited 4-MU glucuronidation when polyoxyethylene cetyl alcohol ether (Brij-58)-treated RLM were used (IC(50) = approximately 70 μM). However, alamethicin-treated RLM exhibited a lower susceptibility (IC(50) = approximately 460 μM) than Brij-58-treated RLM. A similar phenomenon was observed when pooled HLM were used. Then, the endogenous ATP content of RLM was determined in the presence and absence of alamethicin or detergent, and although no ATP remained in the microsomal pellets after Brij-58 treatment, more than half of the microsomal ATP remained even after treatment with alamethicin. Furthermore, the V(max) in the absence of an adenine-related compound was approximately three times higher in Brij-58-treated than in alamethicin-treated RLM. The difference in the inhibitory potency observed was due to the difference in remaining endogenous ATP and the accessibility of exogenous ATP to the luminal side of the endoplasmic reticulum (ER), where the active site of UDP-glucuronosyltransferase (UGT) is located. Gefitinib (Iressa), a protein tyrosine kinase inhibitor, markedly inhibited human UGT1A9 activity. It is interesting to note that AMP antagonized Gefitinib-provoked inhibition of UGT1A9, and ATP exhibited an additive inhibitory effect at a lower concentration. Therefore, Gefitinib inhibits UGT1A9 at the common ATP-binding site shared with ATP and AMP. Releasing adenine nucleotide from the ER is suggested to be one of the mechanisms that explain the "latency" of UGT.

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.

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

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

Regioselective glucuronidation of flavonols by six human UGT1A isoforms

PURPOSE

Glucuronidation is a major barrier to flavonoid bioavailability; understanding its regiospecificity and reaction kinetics would greatly enhance our ability to model and predict flavonoid disposition. We aimed to determine the regioselective glucuronidation of four model flavonols using six expressed human UGT1A isoforms (UGT1A1, 1A3, 1A7, 1A8, 1A9, 1A10).

METHODS

In vitro reaction kinetic profiles of six UGT1A-mediated metabolism of four flavonols (all with 7-OH group) were characterized; kinetic parameters (K(m), V(max) and CL(int) = V(max)/K(m)) were determined.

RESULTS

UGT1A1 and 1A3 regioselectively metabolized the 7-OH group, whereas UGT1A7, 1A8, 1A9 and 1A10 preferred to glucuronidate the 3-OH group. UGT1A1 and 1A9 were the most efficient conjugating enzymes with K(m) values of ≤1 μM and relative catalytic efficiency ratios of ≥5.5. Glucuronidation by UGT1As displayed surprisingly strong substrate inhibition. In particular, K(si) values (substrate inhibition constant) were less than 5.4 μM for UGT1A1-mediated metabolism.

CONCLUSION

UGT1A isoforms displayed distinct positional preferences between 3-OH and 7-OH of flavonols. Differentiated kinetic properties between 3-O- and 7-O- glucuronidation suggested that (at least) two distinct binding modes within the catalytic domain were possible. The existence of multiple binding modes should provide better "expert" knowledge to model and predict UGT1A-mediated glucuronidation.

An evolutionary view of functional diversity in family 1 glycosyltransferases

Glycosyltransferases (GTs) (EC 2.4.x.y) catalyze the transfer of sugar moieties to a wide range of acceptor molecules, such as sugars, lipids, proteins, nucleic acids, antibiotics and other small molecules, including plant secondary metabolites. These enzymes can be classified into at least 92 families, of which family 1 glycosyltransferases (GT1), often referred to as UDP glycosyltransferases (UGTs), is the largest in the plant kingdom. To understand how UGTs expanded in both number and function during evolution of land plants, we screened genome sequences from six plants (Physcomitrella patens, Selaginella moellendorffii, Populus trichocarpa, Oryza sativa, Arabidopsis thaliana and Arabidopsis lyrata) for the presence of a conserved UGT protein domain. Phylogenetic analyses of the UGT genes revealed a significant expansion of UGTs, with lineage specificity and a higher duplication rate in vascular plants after the divergence of Physcomitrella. The UGTs from the six species fell into 24 orthologous groups that contained genes derived from the common ancestor of these six species. Some orthologous groups contained multiple UGT families with known functions, suggesting that UGTs discriminate compounds as substrates in a lineage-specific manner. Orthologous groups containing only a single UGT family tend to play a crucial role in plants, suggesting that such UGT families may have not expanded because of evolutionary constraints.

Activation of morphine glucuronidation by fatty acyl-CoAs and its plasticity: a comparative study in humans and rodents including chimeric mice carrying human liver

The formation of morphine-3-glucuronide (M-3-G, pharmacologically inactive) and morphine-6-glucuronide (M-6-G, active metabolite) by liver microsomes from humans and rodents, including chimeric mice carrying human liver, was evaluated in the presence of fatty acyl-CoAs. Medium- to long-chain fatty acyl-CoAs, including oleoyl-CoAs, at a physiologic level (around 15 microM) markedly enhanced M-3-G formation catalyzed by rat liver microsomes. A separate experiment indicated that 15 microM oleoyl-CoA enhanced (14)C-UDP-glucuronic acid (UDPGA) uptake by microsomes. The activation by acyl-CoAs disappeared or was greatly reduced by either pre-treating microsomes with detergent or freezing/thawing the rat liver before preparation. Many of the microsomes prepared from frozen human livers (N=14) resisted oleoyl-CoA-mediated activation of UDP-glucuronosyltransferase (UGT) activity, including M-6-G formation, which is highly specific to humans. In sharp contrast, the activity of M-6-G and M-3-G formation in freshly-prepared hepatic microsomes from chimeric mice with humanized liver was potently activated by oleoyl-CoA. Thus, acyl-CoAs activate morphine glucuronidation mediated by human as well as rat UGTs. This activation is assumed to be due to the acyl-CoA-facilitated transportation of UDPGA, and microsomes need to maintain the intact conditions required for the activation. The function of UGT appears to be dynamically changed depending on the cellular acyl-CoA level in many species.

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.

Combination of several bioinformatics approaches for the identification of new putative glycosyltransferases in Arabidopsis

Approximately 450 glycosyltransferase (GT) sequences have been already identified in the Arabidopsis genome that organize into 40 sequence-based families, but a vast majority of these gene products remain biochemically uncharacterized open reading frames. Given the complexity of the cell wall carbohydrate network, it can be inferred that some of the biosynthetic genes have not yet been identified by classical bioinformatics approaches. With the objective to identify new plant GT genes, we designed a bioinformatic strategy that is based on the use of several remote homology detection methods that act at the 1D, 2D, and 3D level. Together, these methods led to the identification of more than 150 candidate protein sequences. Among them, 20 are considered as putative glycosyltransferases that should further be investigated since known GT signatures were clearly identified.

Silkworm as a model animal to evaluate drug candidate toxicity and metabolism

To evaluate the feasibility of using the silkworm as a model animal for screening drug candidates, we examined whether the lethal dose of cytotoxic chemicals in silkworm, Bombyx mori, were consistent with those in mammals, and compared the metabolic pathways of these drugs between silkworms and mice. The lethal dose levels of cytotoxic chemicals in silkworms were consistent with those in mammals. We examined the fate of model drugs, 4-methyl umbelliferone, umbelliferone, and 7-ethoxycoumarine, in silkworm larvae. The half-life of 4-methyl umbelliferone in the silkworm larvae hemolymph was 7.0+/-0.1 min, similar to that in mouse blood. In silkworm larvae, 4-methyl umbelliferone was conjugated with glucose, whereas in mammals it is conjugated with glucuronate or sulfate. These results are consistent with a previous report that UDP-glucosyltransferase catalyzes the conjugation of 4-methyl umbelliferone. The glucose-conjugation reaction of 4-methyl umbelliferone was observed in microsomal fractions of fat bodies isolated from silkworms. Furthermore, most umbelliferone and 7-ethoxycoumarine injected into the hemolymph of silkworms was eliminated through the feces in the glucose-conjugated form. These findings suggest that chemicals are metabolized through a pathway common to both mammals and silkworms: reaction with cytochrome P450, conjugation with hydroxylated compounds, and excretion.

Biotransformation of the chemopreventive agent 2',4',4-trihydroxychalcone (isoliquiritigenin) by UDP-glucuronosyltransferases

2',4',4-trihydroxychalcone (isoliquiritigenin), a chalcone found in licorice root and shallots, exhibits antioxidant, estrogenic, and antitumor activities. To complement our previous studies concerning the phase 1 metabolism of isoliquiritigenin, the phase 2 transformation of isoliquiritigenin by human hepatocytes and pooled human liver microsomes (HLMs) was investigated using liquid chromatography/tandem mass spectrometry and UV absorbance. Five glucuronides were detected corresponding to monoglucuronides of isoliquiritigenin and liquiritigenin, but no sulfate conjugates were observed. The UDP-glucuronosyltransferases (UGTs) involved in the formation of the major glucuronide conjugates were identified using recombinant human UGTs in combination with liquid chromatography/mass spectrometry. UGT1A1 and UGT1A9 were the major enzymes responsible for the formation of the most abundant conjugate, isoliquiritigenin 4'-O-glucuronide (MG5), with Km values of 4.30+/-0.47 and 3.15+/-0.24 microM, respectively. UGT1A1 and UGT1A10 converted isoliquiritigenin to the next most abundant phase 2 metabolite, isoliquiritigenin 2'-O-glucuronide (MG4), with Km values of 2.98+/-0.8 and 25.8+/-1.3 microM, respectively. In addition, isoliquiritigenin glucuronides MG4 and MG5 were formed by pooled human intestine and kidney microsomes, respectively. Based on the in vitro determination of a 25.3-min half-life for isoliquiritigenin when incubated with HLMs, the intrinsic clearance of isoliquiritigenin was estimated to be 36.4 ml/min/kg. These studies indicate that isoliquiritigenin will be conjugated rapidly in the liver to form up to five monoglucuronides.

Glucuronidation of polychlorinated biphenylols and UDP-glucuronic acid concentrations in channel catfish liver and intestine

Polychlorinated biphenylols (OH-PCBs) are potentially toxic polychlorinated biphenyl metabolites that can be eliminated by glucuronidation, catalyzed by UDP-glucuronosyltransferases (UGTs). OH-PCBs with a 3,5-dichloro-4-hydroxy substitution pattern have been detected in blood from humans and wildlife, suggesting slow elimination. In this study we assessed the glucuronidation of 4-OH-PCBs with zero, one, or two chlorine atoms flanking the 4-hydroxyl group and zero to four chlorine atoms in the aphenolic ring in microsomes from channel catfish liver and proximal intestine. Product formation was quantitated with [(14)C]UDP-glucuronic acid (UDPGA). Physiological concentrations of UDPGA were measured in preparations of liver and intestine. When the OH-PCB concentrations were varied in the presence of saturating UDPGA concentrations, glucuronidation V(max) values were higher in hepatic than in intestinal microsomes (0.40-3.4 and 0.12-0.78 nmol/min/mg of protein, respectively), whereas the K(m) values were generally lower for intestine (0.042-0.47 mM) than for liver (0.11-1.64 mM). In both tissues V(max) values with 3,5-dichloro-4-OH-PCBs were lower than with the corresponding 3-chloro-4-OH-PCBs. Varying the UDPGA concentrations in the presence of saturating concentrations of OH-PCB showed that the K(m) for UDPGA was lower in intestine (27 microM) than in liver (690 microM). The measured concentration of UDPGA in catfish liver (246-377 nmol/g) was lower than the K(m) for UDPGA, suggesting that in vivo rates of glucuronidation may be suboptimal, whereas in intestine the measured UDPGA concentration (71-258 nmol/g) was higher than the K(m) for UDPGA. Although liver has a greater glucuronidation capacity than proximal intestine, the properties of intestinal UGTs in channel catfish enable them to efficiently glucuronidate low concentrations of OH-PCBs.

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

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

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.

The human UDP-glucuronosyltransferase: identification of key residues within the nucleotide-sugar binding site

UDP-glucuronosyltransferases (UGTs) play important roles in the metabolism, detoxification,and clearance of many different xenobiotics, including drugs and endogenous compounds. Structural information about these membrane-bound enzymes of the endoplasmic reticulum is limited. We do not know the identity or the location of the key residues for catalysis and binding of the aglycone substrate and the cosubstrate UDP-glucuronic acid (UDPGA). One suggestion was that His371 (UGT1A6 numbering) is the "catalytic base" that deprotonates the phenol group. We have now re-examined this hypothesis by analyzing the activities of the corresponding mutants, 6H371A (in UGT1A6) and the 9H369A (in UGT1A9). The K(m) values of mutant 6H371A for scopoletin and UDPGA were higher by 4- and 11-fold, respectively, than in UGT1A6. The K(d) for the enzyme-UDPGA complex, derived from bisubstrate kinetics, was about 9-fold higher in 6H371A than in UGT1A6, indicating severely impaired cosubstrate binding by the mutant. The effect of mutation on V(max) was large in UGT1A6 but variable in UGT1A9, suggesting that His371 does not play the catalytic role previously hypothesized. In both UGTs, the E379A mutation (UGT1A6 numbering) had an effect similar to that of the H371A mutations. A homology model of the putative UDPGA binding region of UGT1A6 was built using distant homologous protein structures from the "GT1 class." The combined results of activity determinations, kinetic analyses, and modeling strongly suggest that His371 and Glu379 are directly involved in UDPGA binding but are not the general acid or general base.

Potent Inhibitors of the Human UDP-Glucuronosyltransferase 2B7 Derived from the Sesquiterpenoid Alcohol Longifolol

The tricyclic sesquiterpenol (+)-longifolol served as a lead structure for the design of inhibitors of the human UDP-glucuronosyltransferase (UGT) 2B7. Twenty-four homochiral and epimeric longifolol derivatives were synthesized and screened for their ability to inhibit the enzyme. The absolute configuration at the stereogenic center C1' was determined by X-ray crystallography and 2D NMR spectroscopy (gHSQC, gNOESY). The phenyl-substituted secondary alcohol 16 b (beta-phenyllongifolol) displayed the highest affinity toward UGT2B7, and its inhibitory dissociation constant was 0.91 nM. The mode of inhibition was rapidly reversible and competitive. The inhibitor was not glucuronidated by UGT2B7 or other hepatic UGTs, presumably as a result of the high steric demand exerted by the phenyl group. Inhibition assays employing 14 other UGT isoforms suggested that inhibitor 16 b was highly selective for UGT2B7.

High Throughput Screening Assay for UDP-Glucuronosyltransferase 1A1 Glucuronidation Profiling

Development of high throughput screening (HTS) assays for evaluation of a compound's toxicity and potential for drug-drug interactions is a critical step towards production of better drug candidates and cost reduction in the drug development process. HTS assays for drug metabolism mediated by cytochrome P450s are now routinely used in compound library characterization and for computer modeling studies. However, development and application of HTS assays involving UDP-glucuronosyltransferases (UGTs) are lagging behind. Here we describe the development of a fluorescence-based HTS assay for UGT1A1 using recombinant enzyme and fluorescent substrate in the presence of an aqueous solution of PreserveX-QML (QBI Life Sciences, Madison, WI) polymeric micelles, acting as a stabilizer and a blocker of nonspecific interactions. The data include assay characteristics in 384-well plate format obtained with robotic liquid handling equipment and structures of hits (assay modifiers) obtained from the screening of a small molecule library at the University of Wisconsin HTS screening facility. The application of the assay for predicting UGT-related drug-drug interactions and building pharmacophore models, as well as the effects of polymeric micelles on the assay performance and compound promiscuity, is discussed.

Isoform-selective inhibition of the human UDP-glucuronosyltransferase 2B7 by isolongifolol derivatives

A set of 48 derivatives of the tricyclic sesquiterpenol alcohol isolongifolol was synthesized. The set comprised homochiral and diastereomeric alcohols, amines, chlorohydrins, as well as carboxylic acids, phosphonic acids, and their corresponding esters. The absolute configuration of the epimeric compounds was assigned by 2D NMR experiments [gradient heteronuclear single quantum correlation (gHSQC) and gradient nuclear Overhauser enhancement spectroscopy (gNOESY)] in agreement with crystallographic data. The tricyclic derivatives were assessed as inhibitors of the human UDP-glucuronosyltransferase (UGT) 2B7. The phenyl-substituted secondary alcohol 26b was the best inhibitor in this series and its competitive inhibition constant was 18 nM. Compound 26b was not glucuronidated by UGT2B7 and other hepatic UGT enzymes, presumably due to the high steric hindrance exerted by its bulky phenyl substituent. Its inhibitory activity toward 14 other UGT isoforms of subfamily 1A and 2B was determined, and the data indicated that the tricyclic secondary alcohol 26b was highly selective for UGT2B7 (true selectivity >1000).

Adaptive evolution of multiple-variable exons and structural diversity of drug-metabolizing enzymes

BACKGROUND

The human genome contains a large number of gene clusters with multiple-variable-first exons, including the drug-metabolizing UDP glucuronosyltransferase (UGT1) and I-branching beta-1,6-N-acetylglucosaminyltransferase (GCNT2, also known as IGNT) clusters, organized in a tandem array, similar to that of the protocadherin (PCDH), immunoglobulin (IG), and T-cell receptor (TCR) clusters. To gain insight into the evolutionary processes that may have shaped their diversity, we performed comprehensive comparative analyses for vertebrate multiple-variable-first-exon clusters.

RESULTS

We found that there are species-specific variable-exon duplications and mutations in the vertebrate Ugt1, Gcnt2, and Ugt2a clusters and that their variable and constant genomic organizations are conserved and vertebrate-specific. In addition, analyzing the complete repertoires of closely-related Ugt2 clusters in humans, mice, and rats revealed extensive lineage-specific duplications. In contrast to the Pcdh gene clusters, gene conversion does not play a predominant role in the evolution of the vertebrate Ugt1, Gcnt2 and Ugt2 gene clusters. Thus, their tremendous diversity is achieved through "birth-and-death" evolution. Comparative analyses and homologous modeling demonstrated that vertebrate UGT proteins have similar three-dimensional structures each with N-terminal and C-terminal Rossmann-fold domains binding acceptor and donor substrates, respectively. Molecular docking experiments identified key residues in donor and acceptor recognition and provided insight into the catalytic mechanism of UGT glucuronidation, suggesting the human UGT1A1 residue histidine 39 (H39) as a general base and the residue aspartic acid 151 (D151) as an important electron-transfer helper. In addition, we identified four hypervariable regions in the N-terminal Rossmann domain that form an acceptor-binding pocket. Finally, analyzing patterns of nonsynonymous and synonymous nucleotide substitutions identified codon sites that are subject to positive Darwinian selection at the molecular level. These diversified residues likely play an important role in recognition of myriad xenobiotics and endobiotics.

CONCLUSION

Our results suggest that enormous diversity of vertebrate multiple variable first exons is achieved through birth-and-death evolution and that adaptive evolution of specific codon sites enhances vertebrate UGT diversity for defense against environmental agents. Our results also have interesting implications regarding the staggering molecular diversity required for chemical detoxification and drug clearance.