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

Steroid-triazole conjugates: A brief overview of synthesis and their application as anticancer agents

Steroids are biomolecules that play pivotal roles in various physiological and drug discovery processes. Abundant research has been fuelled towards steroid-heterocycles conjugates over the last few decades as potential therapeutic agents against various diseases especially as anticancer agents. In this context various steroid-triazole conjugates have been synthesized and studied for their anticancer potential against various cancer cell lines. A thorough search of the literatures revealed that a concise review pertaining the present topic is not compiled. Therefore, in thus review we summarize the synthesis, anticancer activity against various cancer cell lines and structure activity relationship (SAR) of various steroid-triazole conjugates. This review can lay down the path towards the development of various steroid-heterocycles conjugates with lesser side effects and profound efficacy.

1H, 13C, 15N Backbone and sidechain chemical shift assignments of the C-terminal domain of human UDP-glucuronosyltransferase 2B17 (UGT2B17-C)

UDP-glucuronosyltransferases are the principal enzymes involved in the glucuronidation of metabolites and xenobiotics for physiological clearance in humans. Though glucuronidation is an indispensable process in the phase II metabolic pathway, UGT-mediated glucuronidation of most prescribed drugs (> 55%) and clinical evidence of UGT-associated drug resistance are major concerns for therapeutic development. While UGTs are highly conserved enzymes, they manifest unique substrate and inhibitor specificity which is poorly understood given the dearth of experimentally determined full-length structures. Such information is important not only to conceptualize their specificity but is central to the design of inhibitors specific to a given UGT in order to avoid toxicity associated with pan-UGT inhibitors. Here, we provide the 1H, 13C and 15N backbone (~ 90%) and sidechain (~ 62%) assignments for the C-terminal domain of UGT2B17, which can be used to determine the molecular binding sites of inhibitor and substrate, and to understand the atomic basis for inhibitor selectivity between UGT2B17 and other members of the UGT2B subfamily. Given the physiological relevance of UGT2B17 in the elimination of hormone-based cancer drugs, these assignments will contribute towards dissecting the structural basis for substrate specificity, selective inhibitor recognition and other aspects of enzyme activity with the goal of selectively overcoming glucuronidation-based drug resistance.

Molecular modeling study of the testosterone metabolizing enzyme UDP-glucuronosyltransferase 2B17

The dominant sex hormone testosterone is mainly metabolized by liver enzymes belonging to the uridine-diphospho (UDP) glucuronosyltransferase (UGT) family. These enzymes are the main phase II enzymes, and they have an important role in the detoxification of endogenous and exogenous compounds in humans. The aim of the present study was to improve the understanding of the binding properties of UGT2B17. A homology modelling procedure was used to generate models of the UGT2B17 enzyme based on templates with known crystal structures. Molecular docking of inhibitors was performed to gain further insights in the interactions between ligand and binding site, and to determine which of the models had the best accuracy. ROC curves were made to evaluate the ability of the models to differentiate between binders (inhibitors) and non-binders (decoys). When comparing the four models, which were based on four different crystal structures, the model based on the 4AMG crystal structure was the most accurate in distinguishing between true binders and non-binders. Investigating pharmacological UGT2B17 inhibition may provide novel treatment for patients with low testosterone levels. Such treatment may elevate endogenous testosterone levels and provide a more predictable increase in serum concentrations rather than un-physiological elevation of serum levels through direct treatment with testosterone, and this could be favorable both for giving a predictable treatment regime with reduced chances of serious adverse effects. The present study may serve as a tool in the search for novel drugs aiming for increasing testosterone levels.

Altered metabolism of synthetic cannabinoid JWH-018 by human cytochrome P450 2C9 and variants

Synthetic cannabinoids (SCBs), synonymous with 'K2', 'Spice' or 'synthetic marijuana', are psychoactive drugs of abuse that frequently result in clinical effects and toxicity more severe than those classically associated with Δ⁹-tetrahydrocannabinol such as extreme agitation, hallucinations, supraventricular tachycardia, syncope, and seizures. JWH-018 is one of the earliest compounds identified in various SCB products, and our laboratory previously demonstrated that JWH-018 undergoes extensive metabolism by cytochromes P450 (P450), binds to, and activates cannabinoid receptors (CBRs). The major enzyme involved in the metabolism of JWH-018 is CYP2C9, a highly polymorphic enzyme found largely in the intestines and liver, with *1 being designated as the wild type, and *2 and *3 as the two most common variants. Three different major products have been identified in human urine and plasma: JWH-018 (ω)-OH, JWH-018 (ω-1)-OH(R), and JWH-018 (ω-1)-OH(S). The (ω-1)-OH metabolite of JWH-018 is a chiral molecule, and is thus designated as either (ω-1)-OH(R) or (ω-1)-OH(S). Here, in vitro enzyme kinetic assays performed with human recombinant CYP2C9 variants (*1, *2, and *3) revealed that oxidative metabolism by CYP2C9*3 resulted in significantly less formation of (ω)-OH and (ω-1)-OH metabolites. Surprisingly, CYP2C9*2 was roughly 3.6-fold more efficient as the CYP2C9*1 enzyme based on V_max/K_m, increasing the rate of JWH-018 metabolism and allowed for a much more rapid elimination. These results suggest that genetic polymorphisms of P450 enzymes result in the production of varying levels of biologically active JWH-018 metabolites in some individuals, offering a mechanistic explanation for the diverse clinical toxicity often observed following JWH-018 abuse.

Structure and Protein-Protein Interactions of Human UDP-Glucuronosyltransferases

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

Mutational analysis of the integral membrane methyltransferase isoprenylcysteine carboxyl methyltransferase (ICMT) reveals potential substrate binding sites

The eukaryotic integral membrane enzyme isoprenylcysteine carboxyl methyltransferase (ICMT) methylates the carboxylate of a lipid-modified cysteine at the C terminus of its protein substrates. This is the final post-translational modification of proteins containing a CAAX motif, including the oncoprotein Ras, and therefore, ICMT may serve as a therapeutic target in cancer development. ICMT has no discernible sequence homology with soluble methyltransferases, and aspects of its catalytic mechanism are unknown. For example, how both the methyl donor S-adenosyl-l-methionine (AdoMet), which is water-soluble, and the methyl acceptor isoprenylcysteine, which is lipophilic, are recognized within the same active site is not clear. To identify regions of ICMT critical for activity, we combined scanning mutagenesis with methyltransferase assays. We mutated nearly half of the residues of the ortholog of human ICMT from Anopheles gambiae and observed reduced or undetectable catalytic activity for 62 of the mutants. The crystal structure of a distantly related prokaryotic methyltransferase (Ma Mtase), which has sequence similarity with ICMT in its AdoMet binding site but methylates different substrates, provides context for the mutational analysis. The data suggest that ICMT and Ma MTase bind AdoMet in a similar manner. With regard to residues potentially involved in isoprenylcysteine binding, we identified numerous amino acids within transmembrane regions of ICMT that dramatically reduced catalytic activity when mutated. Certain substitutions of these caused substrate inhibition by isoprenylcysteine, suggesting that they contribute to the isoprenylcysteine binding site. The data provide evidence that the active site of ICMT spans both cytosolic and membrane-embedded regions of the protein.

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

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

Photoaffinity probes for studying carbohydrate biology

Carbohydrates and carbohydrate-containing biomolecules engage in binding events that underlie many essential biological processes. Yet these carbohydrate-mediated interactions are often poorly characterized, due to their low affinities and heterogenous natures. The use of photocrosslinking functional groups offers a way to photochemically capture carbohydrate-containing complexes, which can be isolated for further analysis. Here we survey progress in the synthesis and use of carbohydrate-based photoprobes, reagents that incorporate carbohydrates or their analogs, photocrosslinking moieties, and affinity purification handles. Carbohydrate photoprobes, used in combination with modern mass spectrometry methods, can provide important new insights into the cellular roles of carbohydrates and glycosylated molecules.

Role of human UDP-glucuronosyltransferases in the biotransformation of the triazoloacridinone and imidazoacridinone antitumor agents C-1305 and C-1311: highly selective substrates for UGT1A10

5-Diethylaminoethylamino-8-hydroxyimidazoacridinone, C-1311 (NSC-645809), is an antitumor agent shown to be effective against breast cancer in phase II clinical trials. A similar compound, 5-dimethylaminopropylamino-8-hydroxytriazoloacridinone, C-1305, shows high activity against experimental tumors and is expected to have even more beneficial pharmacological properties than C-1311. Previously published studies showed that these compounds are not substrates for cytochrome P450s; however, they do contain functional groups that are common targets for glucuronidation. Therefore, the aim of this work was to identify the human UDP-glucuronosyltransferases (UGTs) able to glucuronidate these two compounds. High-performance liquid chromatography analysis was used to examine the activities of human recombinant UGT1A and UGT2B isoforms and microsomes from human liver [human liver microsomes (HLM)], whole human intestinal mucosa [human intestinal microsomes (HIM)], and seven isolated segments of human gastrointestinal tract. Recombinant extrahepatic UGT1A10 glucuronidated 8-hydroxyl groups with the highest catalytic efficiency compared with other recombinant UGTs, V(max)/K(m) = 27.2 and 8.8 μl · min⁻¹ · mg protein⁻¹, for C-1305 and C-1311, respectively. In human hepatic and intestinal microsomes (HLM and HIM, respectively), high variability in UGT activities was observed among donors and for different regions of intestinal tract. However, both compounds underwent UGT-mediated metabolism to 8-O-glucuronides by microsomes from both sources with comparable efficiency; V(max)/K(m) values were from 4.0 to 5.5 μl · min⁻¹ · mg protein⁻¹. In summary, these studies suggest that imid azoacridinone and triazoloacridinone drugs are glucuronidated in human liver and intestine in vivo and may form the basis for future translational studies of the potential role of UGTs in resistance to these drugs.

Phenylalanine 93 of the human UGT1A10 plays a major role in the interactions of the enzyme with estrogens

Little is currently known about the substrate binding site of the human UDP-glucuronosyltransferases (UGTs) and the structural elements that affect their complex substrate selectivity. In order to further understand and extend our earlier findings with phenylalanines 90 and 93 of UGT1A10, we have replaced each of them with Gly, Ala, Val, Leu, Ile or Tyr, and tested the activity of the resulting 12 mutants toward eight different substrates. Apart from scopoletin glucuronidation, the F90 mutants other than F90L were nearly inactive, while the F93 mutants' activity was strongly substrate dependent. Hence, F93L displayed high entacapone and 1-naphthol glucuronidation rates, whereas F93G, which was nearly inactive in entacapone glucuronidation, was highly active toward estradiol, estriol and even ethinylestradiol, a synthetic estrogen that is a poor substrate for the wild-type UGT1A10. Kinetic analyses of 4-nitrophenol, estradiol and ethinylestradiol glucuronidation by the mutants that catalyzed the respective reactions at considerable rates, revealed increased K(m) values for 4-nitrophenol and estradiol in all the mutants, whilst the K(m) values of F93G and F93A for ethinylestradiol were lower than in control UGT1A10. Based on the activity results and a new molecular model of UGT1A10, it is suggested that both F90 and F93 are located in a surface helix at the far end of the substrate binding site. Nevertheless, only F93 directly affects the selectivity of UGT1A10 toward large and rigid estrogens, particularly those with substitutions at the D ring. The effects of F93 mutations on the glucuronidation of smaller or less rigid substrates are indirect, however.

Analysis of R- and S-hydroxywarfarin glucuronidation catalyzed by human liver microsomes and recombinant UDP-glucuronosyltransferases

Coumadin (R-, S-warfarin) is a challenging drug to accurately dose, both initially and for maintenance, because of its narrow therapeutic range and wide interpatient variability and is typically administered as a racemic (Rac) mixture, which complicates the biotransformation pathways. The goal of the current work was to identify the human UDP-glucuronosyltransferases (UGTs) involved in the glucuronidation of the separated R- and S-enantiomers of 6-, 7-, and 8-hydroxywarfarin and the possible interactions between these enantiomers. The kinetic and inhibition constants for human recombinant 1A family UGTs toward these separated enantiomers have been assessed using high-performance liquid chromatography (HPLC)-UV-visible analysis, and product confirmations have been made using HPLC-mass spectrometry/mass spectrometry. We found that separated R- and S-enantiomers of 6-, 7-, and 8-hydroxywarfarin demonstrate significantly different glucuronidation kinetics and can be mutually inhibitory. In some cases significant substrate inhibition was observed, as shown by K(m), V(max), and K(i), comparisons. In particular, UGT1A1 and extrahepatic UGT1A10 have significantly higher capacities than other isoforms for S-7-hydroxywarfarin and R-7-hydroxywarfarin glucuronidation, respectively. Activity data generated using a set of well characterized human liver microsomes supported the recombinant enzyme data, suggesting an important (although not exclusive) role for UGT1A1 in glucuronidation of the main warfarin metabolites, including Rac-6- and 7-hydroxywarfarin and their R- and S-enantiomers in the liver. This is the first demonstration that the R- and S-enantiomers of hydroxywarfarins are glucuronidated, with significantly different enzymatic affinity and capacity, and supports the importance of UGT1A1 as the major hepatic isoform involved.

Design and synthesis of (+)-discodermolide-paclitaxel hybrids leading to enhanced biological activity

Potential binding modes of (+)-discodermolide at the paclitaxel binding site of tubulin have been identified by computational studies based on earlier structural and SAR data. Examination of the prospective binding modes reveal that the aromatic pocket occupied by the paclitaxel side chain is unoccupied by (+)-discodermolide. Based on these findings, a small library of (+)-discodermolide-paclitaxel hybrids have been designed and synthesized. Biological evaluation reveals a two- to eight-fold increase in antiproliferative activity compared to the parent molecule using the A549 and MCF-7 cancer cell lines.

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.

A rapid synthesis of lavendustin-mimetic small molecules by click fragment assembly

Lavendustin-mimetic small molecules modifying the linker -CH(2)-NH- with an 1,2,3-triazole ring have been synthesized via a click chemistry. Two pharmacophoric fragments of lavendustin were varied to investigate chemical space and the auxophoric -CH(2)-NH- was altered to an 1,2,3-triazole for rapid click conjugation. The small molecules were evaluated against HCT116 colon cancer and CCRF-CEM leukemia cell lines. Among 28 analogues, 3-phenylpropyl ester 26b inhibited CCRF-CEM leukemia cell growth with GI(50) value of 0.9microM.

Insights on membrane topology and structure/function of UDP-glucuronosyltransferases

The main characteristic of uridine diphosphate (UDP)-glucuronosyltransferases is their potency to glucuronidate a large array of structurally unrelated substances with various nucleophilic groups. The activity of these enzymes strongly depends on their tight association to the membrane of the endoplasmic reticulum. In light of recent data, this review is focused on the membrane-assembly process, which is a prerequisite for activity, and on the amino acids that govern substrate recognition and catalysis at the active site. The major implication of the highly variable N-terminal domain of UDP-glucuronosyltransferases in the substrate specificity of these enzymes is highlighted. In the absence of crystal data of the N-terminal domain, multidisciplinary approaches of genetic-/protein-engineering techniques, homology modeling with glycosyltransferases, and quantitative structure-activity relationships allowed us to point out crucial amino acids. On the basis of these results, possible reaction mechanisms for the glucuronidation of xenobiotics, involving histidine and aspartic acid residues, have been built and are discussed.

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.

How many and which amino acids are responsible for the large activity differences between the highly homologous UDP-glucuronosyltransferases (UGT) 1A9 and UGT1A10?

The amino acid sequences of the human UDP-glucuronosyltransferases (UGTs) 1A9 and 1A10 are 93% identical, yet there are large differences in their activity and substrate selectivity. For example, the regioselectivity in propranolol glucuronidation, the regioselectivity in dobutamine glucuronidation, and the glucuronidation rate of alpha- and beta-estradiol differ greatly between UGT1A9 and UGT1A10. To identify the residue responsible for the activity differences, we divided the N-terminal half of the two UGTs into five comparable segments by inserting four unique restriction sites at identical positions in both genes and constructing chimeras in which segments of UGT1A9 were individually replaced by the corresponding segments from UGT1A10. Activity analyses of the resulting mutants, 910A [1A10((1-83))/1A9((84-285))], 910B [1A9((1-83))/1A10((84-147))/1A9((148-285))], 910C [1A9((1-147))/1A10((148-181))/1A9((182-285))], 910D [1A9((1-181))/1A10((182-235))/1A9((236-285))], and 910E [1A9((1-235))/1A10((236-285))] indicated that more than one residue is responsible for the differences between UGT1A9 and UGT1A10. We next prepared four double chimeras, in which two of the above UGT1A9 segments were replaced simultaneously by the corresponding UGT1A10 segments. However, none of the double chimeras glucuronidated either estradiol, propranolol, or dobutamine at rates that resembled those of UGT1A10. On the other hand, studying the kinetics of 1-naphthol glucuronidation yielded more focused results, indicating that residues within segment B (84-147) contribute directly to the K(m) value for this substrate. Further mutagenesis and activity assays suggested that Phe117 of UGT1A9 participates in 1-naphthol binding. In addition, it appears that residues within segment C of the N-terminal domain, mainly at positions 152 and 169, contribute to the higher glucuronidation rates of UGT1A10.

Influence of N-terminal domain histidine and proline residues on the substrate selectivities of human UDP-glucuronosyltransferase 1A1, 1A6, 1A9, 2B7, and 2B10

An N-terminal domain histidine [corresponding to position 39 of UDP-glucuronosyltransferase (UGT) 1A1] is conserved in all UGT1A and UGT2B subfamily proteins except UGT1A4 (Pro-40) and UGT2B10 (Leu-34). Unlike most UGT1A and UGT2B xenobiotic-metabolizing enzymes, UGT1A4 and UGT2B10 lack the ability to glucuronidate 4-methylumbelliferone (4MU) and 1-naphthol (1NP), both planar phenols, and naproxen (a carboxylic acid). However, only UGT1A4 glucuronidates the tertiary amines lamotrigine (LTG) and trifluoperazine (TFP). In this study, we sought to elucidate the influence of specific N-terminal histidine and proline residues on UGT enzyme substrate selectivity. The conserved N-terminal domain histidine of UGT1A1, UGT1A6, UGT1A9, and UGT2B7 was mutated to proline and leucine 34 of UGT2B10 was substituted with histidine, and the capacity of the wild-type and mutant proteins to glucuronidate 4MU, 1NP, LTG, TFP, and naproxen was characterized. Whereas UGT1A1(H39P), UGT1A6(H38P), and UGT1A9(H37P) lacked the ability to metabolize 4MU, 1NP, and naproxen, all glucuronidated LTG. K(m) values for UGT1A1(H39P) and UGT1A9(H37P) were 774 and 3812 microM, respectively, compared with 1579 microM for UGT1A4. UGT1A1(H39P) also glucuronidated TFP with a V(max)/K(m) value comparable to that of UGT1A4. In contrast to the wild-type enzyme, UGT2B10(L34H) glucuronidated 4MU and 1NP with respective K(m) values of 260 and 118 microM. UGT2B7(H35P) lacked activity toward all substrates. The data confirm a pivotal role for an N-terminal domain proline in the glucuronidation of the tertiary amines LTG and TFP by UGT1A subfamily proteins, whereas glucuronidation reactions involving proton abstraction generally, although not invariably, require a histidine at the equivalent position in both UGT1A and UGT2B enzymes.

Characterization of human hepatic and extrahepatic UDP-glucuronosyltransferase enzymes involved in the metabolism of classic cannabinoids

Tetrahydrocannabinol (Delta(9)-THC), the primary psychoactive ingredient in marijuana, is subject to cytochrome P450 oxidation and subsequent UDP-glucuronosyltransferase (UGT)-dependent glucuronidation. Many studies have shown that CYP2C9 and CYP3A4 are the primary enzymes responsible for these cytochrome P450-dependent oxidations, but little work has been done to characterize phase II metabolic pathways. In this study, we test the hypothesis that there are specific human UGTs responsible for classic cannabinoid metabolism. The activities of 12 human recombinant UGTs toward classic cannabinoids [cannabinol (CBN), cannabidiol (CBD), (-)-Delta(8)-THC, (-)-Delta(9)-THC, (+/-)-11-hydroxy-Delta(9)-THC (THC-OH), and (-)-11-nor-9-carboxy-Delta(9)-THC (THC-COOH)] were evaluated using high-performance liquid chromatography-tandem mass spectrometry and labeling assays. Despite activity by UGT1A1, 1A3, 1A8, 1A9, 1A10, and 2B7 toward CBN, CBD, THC-OH, and THC-COOH, only selected UGTs demonstrate sufficient activity for further characterization of steady-state kinetics. CBN was the most recognized substrate as evidenced by activities from hepatic UGT1A9 and extrahepatic UGT1A7, UGT1A8, and UGT1A10. These results may reflect the introduction of an aromatic ring to Delta(9)-THC, leading to favorable pi stacking with phenylalanines in the UGT active site. Likewise, oxidation of Delta(9)-THC to THC-OH results in UGT1A9 and UGT1A10 activity toward the cannabinoid. Further oxidation to THC-COOH surprisingly leads to a loss in metabolism by UGT1A9 and UGT1A10, while creating a substrate recognized by UGT1A1 and UGT1A3. The resulting glucuronide of THC-COOH is the main metabolite found in urine, and thus these hepatic enzymes play a critical role in the metabolic clearance of cannabinoids. Taken together, glucuronidation of cannabinoids depends on upstream processing including enzymes such as CYP2C9 and CYP3A4.

Dopamine is a low-affinity and high-specificity substrate for the human UDP-glucuronosyltransferase 1A10

The purpose of this work was to identify human UDP-glucuronosyltransferases (UGTs) capable of glucuronidating dopamine. Using a sensitive liquid chromatography-tandem mass spectrometry method, we screened all 19 known human UGTs and found that only one enzyme, UGT1A10, catalyzed dopamine glucuronidation at substantial rates, yielding both dopamine-4-O-glucuronide (37.1 pmol/min/mg) and dopamine-3-O-glucuronide (32.7 pmol/min/mg). Much lower (<2 pmol/min/mg) or no dopamine glucuronidation activity was found for all other UGTs tested at 1 mM dopamine. Evaluation of the UGT1A10 expression pattern in human tissues by quantitative reverse transcription-polymerase chain reaction confirmed that it is mainly expressed in small intestine, colon, and adipose tissue, whereas only low levels were found in trachea, stomach, liver, testis, and prostate but not in brain. Dopamine glucuronidation assays using microsomes from human liver and intestine corroborated these findings because activity in intestinal microsomes was markedly higher than that in liver microsomes. Moreover, the glucuronidation regioselectivity in intestinal microsomes was similar to that of recombinant UGT1A10, and both enzyme sources exhibited sigmoidal kinetics with substrate affinity (K(A)) values in the range of 2 to 3 mM. Examination of four UGT1A10 mutants, F90A, F90L, F93A, and F93L, revealed lower dopamine glucuronidation in all of them, particularly in F90A and F93A. Nonetheless, the substrate affinities of the four mutants were similar to that of UGT1A10. It is interesting to note that mutant F93L exhibited regioselectivity, conjugating dopamine at the 4-hydroxyl (OH) position approximately 3 times more efficiently than at the 3-OH position. These results shed new light on the structure and function of UGT1A10 and indicate that dopamine may be a useful probe substrate for this enzyme.

Identification of hydroxywarfarin binding site in human UDP glucuronosyltransferase 1a10: phenylalanine90 is crucial for the glucuronidation of 6- and 7-hydroxywarfarin but not 8-hydroxywarfarin

Recent studies show that the extrahepatic human UDP-glucuronosyltransferase (UGT)1A10 is capable of phase II glucuronidation of several major cytochrome P450 metabolites of warfarin (i.e., 6-, 7-, and 8-hydroxywarfarin). This study expands on this finding by testing the hypothesis that the UGT1A10 F(90)-M(91)-V(92)-F(93) amino acid motif is important for proper recognition and conjugation of hydroxywarfarin derivatives. Site-directed mutagenesis studies demonstrate that F(90) is critical for 6- and 7-hydroxywarfarin glucuronidation based on the complete loss of enzymatic activity toward these substrates. In contrast, V92A and F93A mutants lead to higher rates of substrate turnover, have minimum changes in K(m) values, and demonstrate substrate inhibition kinetics. A completely different activity profile is observed in the presence of 8-hydroxywarfarin. No change in either activity or affinity is observed with F90A when compared with wild type, whereas F93A and V92A mutants show increases in V(max) (3- and 10-fold, respectively) and minimum changes in K(m). Liquid chromatographytandem mass spectrometry studies show that enzymatic products produced by mutants are identical to wild-type products produced in the presence of 6-, 7-, and 8-hydroxywarfarin. Because F(90) is not critical for the glucuronidation of 8-hydroxywarfarin, there is likely another, different amino acid responsible for binding this compound. In addition, an inhibitory binding site may be formed in the presence of 6- and 7-hydroxywarfarin. This new knowledge and continued characterization of the hydroxywarfarin binding site(s) for UGT1A10 will help elucidate the molecular mechanism of hydroxywarfarin glucuronidation and potentially result in more effective anticoagulant therapies.

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

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

Critical Roles of Residues 36 and 40 in the Phenol and Tertiary Amine Aglycone Substrate Selectivities of UDP-Glucuronosyltransferases 1A3 and 1A4

Despite high sequence identity, UGT1A3 and UGT1A4 differ in terms of substrate selectivity. UGT1A3 glucuronidates the planar phenols 1-naphthol (1-NP) and 4-methylumbelliferone (4-MU), whereas UGT1A4 converts the tertiary amines lamotrigine (LTG) and trifluoperazine (TFP) to quaternary ammonium glucuronides. Residues 45 to 154 (which incorporate 21 of the 35 amino acid differences) and 45 to 535 were exchanged between UGT1A3 and UGT1A4 to generate UGT1A3-4((45-535)), UGT1A3-4((45-154))-3, UGT1A4-3((45-535)), and UGT1A4-3((45-154))-4 hybrid proteins. Although differences in kinetic parameters were observed between the parent enzymes and chimeras, UGT1A4-3((45-535)) and UGT1A4-3((45-154))-4 [but not UGT1A3-4((45-535)) and UGT1A3-4((45-154))-3] retained the capacity to glucuronidate LTG and TFP. Likewise, UGT1A3-4((45-535)) and UGT1A3-4((45-154))-3 retained the capacity to glucuronidate 1-NP and 4-MU, but UGT1A4-3((45-535)) and UGT1A4-3((45-154))-4 exhibited low or absent activity. Within the first 44 residues, UGT1A3 and UGT1A4 differ in sequence at positions 36 and 40. "Reciprocal" mutagenesis was performed to generate the UGT1A3(I36T), UGT1A3(H40P), UGT1A4(T36I), and UGT1A4 (P40H) mutants. The T36I and P40H mutations in UGT1A4 reduced in vitro clearances for LTG and TFP glucuronidation by >90%. Conversely, the I36T and H40P mutations in UGT1A3 reduced the in vitro clearances for 1-NP and 4-MU glucuronidation by >90%. Introduction of the single H40P mutation in UGT1A3 conferred LTG and TFP glucuronidation, whereas the single T36I mutation in UGT1A4 conferred 1-NP and 4-MU glucuronidation. Thus, residues 36 and 40 of UGT1A3 and UGT1A4 are pivotal for the respective selectivities of these enzymes toward planar phenols and tertiary amines, although other regions of the proteins influence binding affinity and/or turnover.

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.

Phenylalanine(90) and phenylalanine(93) are crucial amino acids within the estrogen binding site of the human UDP-glucuronosyltransferase 1A10

Human UDP-glucuronosyltransferase 1A10 has been identified as the major isoform involved in the biotransformation of a wide range of phenolic substrates, including native estrogens and their oxidized metabolites. Our recent studies point to the F(90)-M(91)-V(92)-F(93) amino acid motif of UGT1A10, which was identified using photoaffinity labeling followed by LC-MS/MS analysis, as a key determinant of the binding of phenolic substrates. In this report, we have evaluated the role of F(90), V(92), and F(93) in the recognition of estrogens by UGT1A10 using site-directed mutagenesis. Kinetic studies using five mutants revealed that F(90) and F(93) are critical residues for the recognition of all estrogen substrates. The substitution of F(90) with alanine totally abolished the activity of this enzyme toward all the estrogens investigated. Overall, sequential removal for the aromatic ring (F to L) and of the hydrophobic chain (F to A and V to A) from amino acids 90, 92, and 93 effectively alters estrogen recognition. This demonstrates that individual features of the native and hydroxylated estrogens determine the specific binding properties of the compound within the binding site of the human UGT1A10 and the mutants. The resulting activities are completely abolished, unchanged, increased, or decreased depending on the structures of both the mutant and the substrate. The novel identification of UGT1A10 as the major isoform involved in the glucuronidation of all estrogens and the discovery of the importance of the FMVF motif in the binding of steroids will help to elucidate the molecular mechanism of glucuronidation, resulting in the design of more effective estrogen-based therapies.