Key amino acid residues responsible for the differences in substrate specificity of human UDP-glucuronosyltransferase (UGT)1A9 and UGT1A8

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.

Effect of polyethylene glycol 400 on the pharmacokinetics and tissue distribution of baicalin by intravenous injection based on the enzyme activity of UGT1A8/1A9

Baicalin (BG) is a bioactive flavonoid extracted from the dried root of the medicinal plant, Scutellaria radix (SR) (dicotyledonous family, Labiatae), and has several biological activities. Polyethylene glycol 400 (PEG400) has been used as a suitable solvent for several traditional Chinese medicines (TCM) and is often used as an excipient for the compound preparation of SR. However, the drug-excipient interactions between BG and PEG400 are still unknown. Herein, we evaluated the effect of a single intravenous PEG400 administration on the BG levels of rats using pharmacokinetic and tissue distribution studies. A liver microsome and recombinant enzyme incubation system were used to further confirm the interaction mechanism between PEG400 and UDP-glucuronosyltransferases (UGTs) (UGT1A8 and UGT1A9). The pharmacokinetic study demonstrated that following the co-intravenous administration of PEG400 and BG, the total clearance (CL_z) of BG in the rat plasma decreased by 101.60% (p < 0.05), whereas the area under the plasma concentration-time curve (AUC)_0-t and AUC_0-inf increased by 144.59% (p < 0.05) and 140.05% (p < 0.05), respectively. Additionally, the tissue distribution study showed that the concentration of BG and baicalein-6-O-β-D-glucuronide (B6G) in the tissues increased, whereas baicalein (B) in the tissues decreased, and the total amount of BG and its metabolites in tissues altered following the intravenous administration of PEG400. We further found that PEG400 induced the UGT1A8 and UGT1A9 enzyme activities by affecting the maximum enzymatic velocity (V_max) and Michaelis-Menten constant (K_m) values of UGT1A8 and UGT1A9. In conclusion, our results demonstrated that PEG400 interaction with UGTs altered the pharmacokinetic behaviors and tissue distribution characteristics of BG and its metabolites in rats.

CRISPR screen identified that UGT1A9 was required for bisphenols-induced mitochondria dyshomeostasis

Exposure to bisphenols chemicals could cause various adverse health effects, including non-alcoholic fatty liver disease (NAFLD), which have been associated with cellular mitochondria stress. However, the biological mechanism underlying the mitochondria stress-mediated cell death by bisphenols was poorly understood. Here, CRISPR screens were performed to identify the critical genes which were involved in cell death caused by exposure to four bisphenols (BPA, BPB, BPE and BPS). Results of CRISPR screens showed that UGT1A9 was the primary genetic factor facilitating cell death induced by all of the four bisphenols. Systematic toxicological tests demonstrated that UGT1A9 was required for BPA-induced mitochondria dyshomeostasis in vitro and in vivo, and UGT1A9-mediated mitochondria dyshomeostasis was an important cause of facilitating cell death. Liver injury caused by exposure to BPA in wild-type mice was accompanied with suppression of mitophagy and increased expression of C-Caspase 3, but UGT1A9 knockout attenuated these adverse effects induced by BPA. Finally, molecular epidemiology analysis suggested that the five genetic variants of UGT1A9 could be potential genetic risk factors of NAFLD when people were exposed to BPA. The biological mechanism uncovered here provided mechanistic evidence for identification of susceptible populations of liver injury associated with exposure to BPA.

The pharmaceutical excipient PEG400 affect the absorption of baicalein in Caco-2 monolayer model by interacting with UDP-glucuronosyltransferases and efflux transport proteins

The bioavailability of drugs is often related to intestinal metabolism and transport mechanisms. In previous studies, pharmaceutical excipients were recognized as inert substances in clinical safety evaluations. However, a large number of studies have shown that pharmaceutical excipients regulate the metabolism and transport of drugs in the body and improve the bioavailability. The pharmaceutical excipient polyethylene glycol 400 (PEG400) as a good solubilizer and surfactant has the potential to improve the bioavailability of drugs. The combined action of UDP-glucuronosyltransferases (UGTs) and efflux transport proteins is responsible for the intestinal disposition and poor bioavailability of baicalein. Our aim is to study the effect of PEG400 on the absorption of baicalein on the Caco-2 monolayer, and confirm the interaction of PEG400 with UGTs (UGT1A8 and UGT1A9) and efflux transports. We initially found that baicalein in the Caco-2 monolayer would be metabolized into glucuronide conjugates BG and B6G under the action of UGT1A8 and UGT1A9 on the endoplasmic reticulum membrane, and then mainly excreted to different sides by acting of MRP and BCRP. The addition of PEG400 significantly accelerated the metabolism of B in Caco-2 cells and increased the penetration of BG and B6G. Furthermore, PEG400 also significantly decreased the efflux ratio of BG and B6G, which was the evidence of the interaction with the efflux transporters. In the in vitro intestinal microsome regeneration system, low concentration PEG400 decreased the K_m value of UGT1A8 and UGT1A9 (key enzymes that mediate the production of BG and B6G); high concentration PEG400 enhanced the V_max value of UGT1A8 and UGT1A9. In conclusion, our results determined that PEG400 interacted with some UGTs and efflux transporters, which were the main factors affecting the absorption of baicalein.

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.

Targeted screen for human UDP-glucuronosyltransferases inhibitors and the evaluation of potential drug-drug interactions with zafirlukast

Inhibition of drug metabolizing enzymes is a major mechanism in drug-drug interactions (DDIs). A number of cases of DDIs via inhibition of UDP-glucuronosyltranseferases (UGTs) have been reported, although the changes in pharmacokinetics are relatively small in comparison with drugs that are metabolized by cytochrome P450s. Most of the past studies have investigated hepatic UGTs, although recent studies have revealed a significant contribution of UGTs in the small intestine to drug clearance. To evaluate potential DDIs caused by inhibition of intestinal UGTs, we assessed inhibitory effects of 578 compounds, including drugs, xenobiotics, and endobiotics, on human UGT1A8 and UGT1A10, which are major contributors to intestinal glucuronidation. We identified 29 inhibitors by monitoring raloxifene glucuronidation with recombinant UGTs. All of the inhibitors potently inhibited UGT1A1 activity, as well. We found that zafirlukast is a potent general inhibitor of UGT1As and a moderate inhibitor of UGT2Bs because it monitors 4-methylumbelliferone glucuronidation by recombinant UGTs. However, zafirlukast did not potently inhibit diclofenac glucuronidation, suggesting that the inhibitory effects might be substrate specific. Inhibitory effects of zafirlukast on some UGT substrates were further investigated in human liver and human small intestine microsomes in order to evaluate potential DDIs. The R values (the ratios of intrinsic clearance with and without an inhibitor) revealed that zafirlukast has potential to cause clinical DDIs in the small intestine. Although we could not identify specific UGT1A8 and UGT1A10 inhibitors, zafirlukast was identified as a general inhibitor for UGTs in vitro. The present study suggests that the inhibition of UGT in the small intestine would be an underlying mechanism for DDIs.

Dietary supplement 4-methylumbelliferone: an effective chemopreventive and therapeutic agent for prostate cancer

BACKGROUND

Prevention and treatment of advanced prostate cancer (PCa) by a nontoxic agent can improve outcome, while maintaining quality of life. 4-methylumbelliferone (4-MU) is a dietary supplement that inhibits hyaluronic acid (HA) synthesis. We evaluated the chemopreventive and therapeutic efficacy and mechanism of action of 4-MU.

METHODS

TRAMP mice (7-28 per group) were gavaged with 4-MU (450mg/kg/day) in a stage-specific treatment design (8-28, 12-28, 22-28 weeks). Efficacy of 4-MU (200-450mg/kg/day) was also evaluated in the PC3-ML/Luc(+) intracardiac injection and DU145 subcutaneous models. PCa cells and tissues were analyzed for HA and Phosphoinositide 3-kinase (PI-3K)/Akt signaling and apoptosis effectors. HA add-back and myristoylated Akt (mAkt) overexpression studies evaluated the mechanism of action of 4-MU. Data were analyzed with one-way analysis of variance and unpaired t test or Tukey's multiple comparison test. All statistical tests were two-sided.

RESULTS

While vehicle-treated transgenic adenocarcinoma of the prostate (TRAMP) mice developed prostate tumors and metastases at 28 weeks, both were abrogated in treatment groups, without serum/organ toxicity or weight loss; no tumors developed at one year, even after stopping the treatment at 28 weeks. 4-MU did not alter the transgene or neuroendocrine marker expression but downregulated HA levels. However, 4-MU decreased microvessel density and proliferative index (P < .0001,). 4-MU completely prevented/inhibited skeletal metastasis in the PC3-ML/Luc(+) model and DU145-tumor growth (85-90% inhibition, P = .002). 4-MU also statistically significantly downregulated HA receptors, PI-3K/CD44 complex and activity, Akt signaling, and β-catenin levels/activation, but upregulated GSK-3 function, E-cadherin, and apoptosis effectors (P < .001); HA addition or mAkt overexpression rescued these effects.

CONCLUSION

4-MU is an effective nontoxic, oral chemopreventive, and therapeutic agent that targets PCa development, growth, and metastasis by abrogating HA signaling.

Episodic positive selection during the evolution of naphthalene dioxygenase to nitroarene dioxygenase

Using different maximum-likelihood models of adaptive evolution, signatures of natural selective pressure, operating across the naphthalene family of dioxygenases, were examined. A lineage- and branch-site specific combined analysis revealed that purifying selection pressure dominated the evolutionary history of the enzyme family. Specifically, episodic positive Darwinian selection pressure, affecting only a few sites in a subset of lineages, was found to be responsible for the evolution of nitroarene dioxygenases (NArDO) from naphthalene dioxygenase (NDO). Site-specific analysis confirmed the absence of diversifying selection pressure at any particular site. Different sets of positively selected residues, obtained from branch-site specific analysis, were detected for the evolution of each NArDO. They were mainly located around the active site, the catalytic pocket and their adjacent regions, when mapped onto the 3D structure of the α-subunit of NDO. The present analysis enriches the current understanding of adaptive evolution and also broadens the scope for rational alteration of substrate specificity of enzyme by directed evolution.

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.

Resveratrol in combination with other dietary polyphenols concomitantly enhances antiproliferation and UGT1A1 induction in Caco-2 cells

AIMS

The only FDA approved medication for colorectal cancer (CRC) prevention is celecoxib. Its adverse effects underline the need for safer drugs. Polyphenols like resveratrol are in clinical trials for this purpose. This study aimed at examining effects of resveratrol alone and in combination with curcumin or chrysin on UGT induction in Caco-2 cells. Phytochemical combinations were selected using drug combination analyses of various anti-proliferation ratios of resveratrol+curcumin and resveratrol+chrysin.

MAIN METHODS

Cell proliferation and UGT1A1 induction assays were carried out with individual polyphenols and combinations. Cell viability was determined with AlamarBlue assays. UGT1A1 mRNA was quantified via real time RT-PCR. UGT activity was determined with 4-methylumbelliferone (4MU) glucuronidation.

KEY FINDINGS

Cell proliferation IC(50) estimates (± SE) for resveratrol, curcumin and chrysin were 20.8 ± 1.2, 20.1 ± 1.1 and 16.3 ± 1.3μM respectively. Combination of anti-proliferative effects showed additivity for resveratrol+chrysin and resveratrol+curcumin. Resveratrol at its IC(50) mediated a four-fold induction of UGT1A1 mRNA in a concentration independent manner. Chrysin at its IC(50) induced UGT1A1 expression seven-fold while Curcumin at its IC(90) mediated a two-fold induction. The 20 μM:40μ M resveratrol+curcumin and 20 μM :32 μM resveratrol+chrysin combinations mediated the greatest increases in mRNA expression (12 and 22 folds respectively). Significant increase in 4-MU glucuronidation was observed with combinations exhibiting maximal mRNA induction.

SIGNIFICANCE

Phytochemical combinations can offer greater chemoprevention than single agents. These chemicals might offer safer options than present synthetic therapeutics for CRC prevention.

Metabolism of parabens (4-hydroxybenzoic acid esters) by hepatic esterases and UDP-glucuronosyltransferases in man

Parabens (alkyl esters of 4-hydroxybenzoic acid) are widely used as preservatives in drugs, cosmetic products, and foodstuffs. Safety concerns have recently increased due to the potential health risks associated to exposure to large amounts of these substances. Biotransformation of parabens mainly includes hydrolysis of the ester bond and glucuronidation reactions. The hydrolysis and glucuronidation of a series of six parabens differing by the nature of the alkyl group were investigated in human liver microsomes and plasma, and the major human UDP-glucuronosyltransferase (UGT) isoforms involved in the reaction were identified. Methyl- and ethylparaben were stable in human plasma, with 95% of the initial concentration remaining after 24 h. On the other hand, propyl-, butyl- and benzylparaben concentrations decreased by 50% under similar conditions. In contrast, rapid hydrolysis was measured with human liver microsomes depending on the alkyl chain length, with t(1/2) varying from 22 min for methylparaben to 87 min for butylparaben. All parabens were actively glucuronidated by liver microsomes, in comparison to 4-hydroxybenzoic acid. They were mainly substrates of human recombinant UGT1A1, UGT1A8, UGT1A9, UGT2B7, UGT2B15 and UGT2B17. In conclusion, the parabens were readily metabolized in human liver through esterase hydrolysis and glucuronidation by several UGT isoforms. These results suggest that these parabens do not accumulate in human tissue.

A molecular model of the human UDP-glucuronosyltransferase 1A1, its membrane orientation, and the interactions between different parts of the enzyme

The vertebrate UDP-glucuronosyltransferases (UGTs) are membrane-bound enzymes of the endoplasmic reticulum that process both endogenous and exogenous substrates. The human UGTs are well known biologically, but biophysical understanding is scarce, largely because of problems in purification. The one resolved crystal structure covers the C-terminal domain of the human UGT2B7. Here, we present a homology model of the complete monomeric human UGT1A1, the enzyme that catalyzes bilirubin glucuronidation. The enzyme can be seen as composed of four different domains: two large ones, the N- and C-terminal domains, and two small ones, the "envelope" helices and the transmembrane segment that includes the cytoplasmic tail. The hydrophobic core of the N-terminal domain and the two envelope helices that connect the large domains are shown to be structurally well conserved even among distant homologs and can thus be modeled with good certainty according to plant and bacterial structures. We consider alternative solutions for the highly variable N-terminal regions that probably contribute to substrate binding. The bilirubin binding site, known pathological mutations in UGT1A1, and other specific residues have been examined in the context of the model with regard to available experimental data. A putative orientation of the protein relative to the membrane has been derived from the location of predicted N-glycosylation sites. The model presents extensive interactions between the N- and C-terminal domains, the two envelope helices, and the membrane. Together, these interactions could allow for a concerted large-scale conformational change during catalysis.

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.

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.

In silico and in vitro approaches to elucidate the thermal stability of human UDP-glucuronosyltransferase (UGT) 1A9

UDP-Glucuronosyltransferases (UGTs) are predominant drug metabolizing enzymes in the liver and extrahepatic tissues. Human UGT1A9 is uniquely stable against heat treatment. To understand the unique properties of UGT1A9, the three-dimensional structure was constructed by homology modeling using a crystal structure of TDP-epi-vancosaminyltransferase as template. Sequence alignment analysis revealed that 13 amino acid residues (Arg42, Lys91, Ala92, Tyr106, Gly111, Tyr113, Asp115, Asn152, Leu173, Leu219, His221, Arg222, and Glu241) are unique to UGT1A9 as compared with UGT1A7, UGT1A8 and UGT1A10. To examine the roles of these residues in the conformational stability of UGT1A9, molecular dynamics simulation of the structures was carried out at 310 K and 360 K in aqueous solution for 3.0 nanoseconds. Root mean square deviation analyses revealed that Arg42, Leu173, Leu219, His221 and Arg222 were responsible for the thermal stability. Root mean square fluctuation analyses and a dynamical cross correlation map revealed that Lys91, Ala92, Tyr106, Gly111, Tyr113, Asp115, Leu219, His221, Arg222 and Glu241 were responsible for the thermal stability. In vitro study using mutants of these residues demonstrated that all these amino acids may be collectively involved in the thermal stability of UGT1A9. The results presented here provide a molecular basis for the thermal stability of human UGT1A9.

Functional characterization of low-prevalence missense polymorphisms in the UDP-glucuronosyltransferase 1A9 gene

The UDP-glucuronosyltransferase (UGT) 1A9 has been shown to play an important role in the detoxification of several carcinogens and clearance of anticancer and pain medications. The goal of the present study was to identify novel polymorphisms in UGT1A9 and characterize their effect on glucuronidation activity. The UGT1A9 gene was analyzed by direct sequencing of buccal cell genomic DNA from 90 healthy subjects. In addition to a previously identified single nucleotide polymorphism (SNP) at codon 33 resulting in an amino acid substitution (Met>Thr), two low-prevalence (<0.02) novel missense SNPs at codons 167 (Val>Ala) and 183 (Cys>Gly) were identified and are present in both white and African-American subjects. Glucuronidation activity assays using HEK293 cell lines overexpressing wild-type or variant UGT1A9 demonstrated that the UGT1A9(33Thr) and UGT1A9(183Gly) variants exhibited differential glucuronidation activities compared with wild-type UGT1A9, but this was substrate-dependent. The UGT1A9(167Ala) variant exhibited levels of activity similar to those of wild-type UGT1A9 for all substrates tested. Whereas the wild-type and UGT1A9(33Thr) and UGT1A9(167Ala) variants formed homodimers as determined by Western blot analysis of native polyacrylamide gels, the UGT1A9(183Gly) variant was incapable of homodimerization. These results suggest that several low-prevalence missense polymorphisms exist for UGT1A9 and that two of these (M33T and C183G) are functional. These results also suggest that although Cys183 is necessary for UGT1A9 homodimerization, the lack of capacity for UGT1A9 homodimerization is not sufficient to eliminate UGT1A9 activity.