A novel method for the immunoquantification of UDP-glucuronosyltransferases in human tissue.

Glucuronidation is a major pathway of drug and xenobiotic metabolism that is catalyzed by members of the UDP-glucuronosyltransferase (UGT) family. Predicting the contribution of individual UGTs to drug metabolism would be of considerable value in drug development and would be greatly aided by the availability of detailed absolute expression levels of these proteins; this is hampered by the lack of purified protein standards because of the hydrophobic membrane-associated nature of UGTs and the consequential difficulties in expression and purification. Here we describe a novel solution to this problem by expressing UGTs in Escherichia coli as fusion proteins with ribonuclease S-peptide, targeted to the periplasm with the pelB leader sequence. After addition of ribonuclease S-protein to membrane extracts, a functional ribonuclease is reconstituted that provides a direct and absolute quantification of the amount of UGT fusion protein; this is subsequently used to generate standard curves for immunoquantification by immunoblotting. To illustrate the value of the method, we have quantified the expression of UGT1A1 and UGT1A6 in human liver and kidney microsomes using new isoform-specific antibodies developed against peptides from these proteins. Expression levels of both proteins in liver were highly variable (28- and 20-fold, respectively) and correlated strongly with UGT enzyme activity toward the probe substrates bilirubin and 1-naphthol, respectively. The method is broadly applicable and provides a straightforward means of determining the absolute, as opposed to relative, quantities of UGT proteins present in human tissues.

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

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

Identification of the human liver UDP-glucuronosyltransferase involved in the metabolism of p-ethoxyphenylurea (dulcin).

Dulcin (DL), now banned, was once a widely used artificial sweetener. DL possesses an ureido group that is metabolized by direct glucuronidation in rabbit liver microsomes. Dulcin N-glucuronide (DNG) is the only type of ureido N-glucuronide known to date; ureido glucuronidation in humans has not been previously reported. Accordingly, the glucuronidation of DL was studied using human liver microsomes (HLM) and expressed human UDP-glucuronosyltransferase (UGT) enzymes. The average K (m) and V (max) values from nine HLM samples were 2.10 mM and 0.156 nmol/mg/min, respectively. Of the six human UGT isoforms screened for their ability to glucuronidate DL, only UGT1A1 and UGT1A9 showed activity. The apparent K (m) values using UGT1A1 and UGT1A9 were 5.06 and 6.99 mM, and the apparent V (max) values were 0.0461 and 0.106 nmol/min/mg, respectively. Phenolphthalein, a substrate for UGT1A9, inhibited DL glucuronidation in HLM competitively (K (i) = 0.356 mM), but bilirubin, a substrate for UGT1A1, did not. These results suggest that UGT1A9 is a key enzyme catalyzing the glucuronidation of DL.

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

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

Glucuronidation of SN-38 and NU/ICRF 505 in human colon cancer and adjacent normal colon.

BACKGROUND: Glucuronidation represents a novel mechanism of intrinsic drug resistance in colon cancer cells. To safely reverse this mechanism in vivo, it is essential to identify which isoforms of UDP-glucuronosyltransferases are responsible for catalysing this drug metabolism in tumour tissue. MATERIALS AND METHODS: LC-MS was applied to measure rates of glucuronidation of two anticancer compounds (SN-38 and NU/ICRF 505) by patient colon cancer biopsies and paired normal colon. RESULTS: Three independent lines of enquiry indicated that, in the tumour specimens, SN-38 was glucuronidated primarily by UGT1A1, the isozyme generally recognised as being responsible for hepatic detoxification of this compound, while with NU/ICRF 505 two candidate isoforms emerged - UGT1A8 and/or UGT1A10 - both of which are not normally expressed in the liver. CONCLUSION: These data suggest that tumour selective modulation of this drug resistance mechanism in patients may be feasible with NU/ICRF 505 but more difficult to realise with SN-38. De novo drug resistance is recognised as contributing significantly to the poor response rates of colorectal cancer (CRC) to chemotherapy (1). Nonetheless, the underlying mechanisms responsible for drug insensitivity remain

Nomenclature update for the mammalian UDP glycosyltransferase (UGT) gene superfamily.

Several novel UDP glycosyltransferase (UGT) genes, mainly UDP glucuronosyltransferases, have been identified in the human, mouse and rat genomes and in other mammalian species. This review provides an update of the UGT nomenclature to include these new genes and prevent the confusion that arises when the same gene is given different names. The new genes are named following previously established recommendations, taking into consideration evolutionary relatedness and the names already in general usage in the literature. The mammalian UGT gene superfamily currently has 117 members that can be divided into four families, UGT1, UGT2, UGT3 and UGT8. The 5-exon genes of the UGT1 family each contain a unique first exon, plus four exons that are shared between the genes; the exons 1 appear to have evolved by a process of duplication, leading to the synthesis of proteins with identical carboxyl-terminal and variable amino-terminal domains. Exon-sharing is also seen with the 6-exon UGT2A1 and UGT2A2 genes. However, UGT2A3 and those of the UGT2B (six exons), UGT3 (seven exons) and UGT8 gene families (five or six exons) do not share exons and most likely were derived by a process of duplication of all exons in the gene. Most UGT1 and UGT8 enzymes have been characterized in detail; however, the catalytic functions of the UGT3A enzymes and several UGT2 enzymes remain to be characterized.

Application of high-performance liquid chromatography-electrospray ionization-mass spectrometry to measure microsomal membrane transport of glucuronides.

A primary reason for poor characterization of microsomal transport to date is the limitations of the measurement techniques used. Radiodetection provides sufficient sensitivity, but it can be applied only when labeled analogue is available. In this article, we report the novel application of high-performance liquid chromatography and electrospray tandem mass spectrometry (LC-MS/MS) in "rapid filtration" transport assays. The method was developed using glucuronides, but it is adaptable to any compound that can be measured with LC-MS/MS. Because of the high sensitivity and accuracy of this detection technique, the substrates can be used at their physiological concentration in the experiments. The new methodology does not require radiolabeling, so it remarkably widens the range of possible substrates to investigate and allows simultaneous detection as well as monitoring of substrate stability during the experiments.

Substrate specificity of human hepatic udp-glucuronosyltransferases.

Five human hepatic UDP-glucuronosyltransferases (UGTs) catalyze the facilitated excretion of more than 90% of drugs eliminated by glucuronidation. The substrate specificity of these UGTs has been examined using cloned expressed enzymes and liquid chromatography-mass spectrometry assays to determine the intrinsic clearance of drug glucuronidation in vitro. Specific substrates for the five individual UGTs have been identified. These five probe substrates could be used to predict the drug clearance catalyzed by individual UGTs in vivo.

Identification of the rabbit liver UDP-glucuronosyltransferase catalyzing the glucuronidation of 4-ethoxyphenylurea (dulcin).

Dulcin (DL), 4-ethoxyphenylurea, a synthetic chemical about 200 times as sweet as sucrose, has been proposed for use as an artificial sweetener. DL is excreted as a urinary ureido-N-glucuronide after oral administration to rabbits. The phenylurea N-glucuronide is the only ureido conjugate with glucuronic acid known at present; therefore, DL is interesting as a probe to search for new functions of UDP-glucuronosyltransferases (UGTs). Seven UGT isoforms (UGT1A3, UGT1A4, UGT1A6, UGT1A7, UGT2B13, UGT2B14, and UGT2B16) have been identified from rabbit liver, but these UGTs have not been investigated using DL as a substrate. In this work, the identities of UGT isoforms catalyzing the formation of DL glucuronide were investigated using rabbit liver microsomes (RabLM) and cloned/expressed as rabbit UGT isoforms. DL-N-glucuronide (DNG) production was determined quantitatively in RabLM and homogenates of COS-7 cells expressing each UGT isoform by using electrospray liquid chromatography-tandem mass spectrometry. Analysis of DNG formation using RabLM, by Eadie-Hofstee plot, gave a Vmax of 0.911 nmol/min/mg protein and the Km of 1.66 mM. DNG formation was catalyzed only by cloned expressed rabbit UGT1A7 and UGT2B16 (Vmax of 3.98 and 1.16 pmol/min/mg protein and a Km of 1.23 and 1.69 mM, respectively). Substrate inhibition of UGT1A7 by octylgallate confirmed the significant contribution of UGT1A7 to the formation of DNG. Octylgallate was further shown to competitively inhibit DNG production by RabLM (Ki = 0.149 mM). These results demonstrate that UGT1A7 is the major isoform catalyzing the N-glucuronidation of DL in RabLM.

Assessment of catechol induction and glucuronidation in rat liver microsomes.

Catechols are substances with a 1,2-dihydroxybenzene group from natural or synthetic origin. The aim of this study was to determine whether catechols (4-methylcatechol, 4-nitrocatechol, 2,3-dihydroxynaphthalene) and the antiparkinsonian drugs, entacapone and tolcapone, at doses 150 to 300 mg/kg/day, for 3 days, are able to enhance their own glucuronidation. The induction potency of catechols on rat liver UDP-glucuronosyltransferases (UGTs) was compared with that of a standard polychlorinated biphenyl (PCB) inducer, Aroclor 1254. The glucuronidation rate of these catechols was enhanced up to 15-fold in the liver microsomes of PCB-treated rats, whereas treatment with catechols had little effect. Entacapone, tolcapone, 4-methylcatechol, catechol, 2,3-dihydroxynaphthalene, and 4-nitrocatechol were glucuronidated in control microsomes at rates ranging from 0.12 for entacapone to 22.0 nmol/min/mg for 4-nitrocatechol. Using 1-naphthol, entacapone, and 1-hydroxypyrene as substrates, a 5-, 8-, and 16-fold induction was detected in the PCB rats, respectively, whereas the catechol-induced activities were 1.1- to 1.5-fold only. Entacapone was glucuronidated more efficiently by PCB microsomes than by control microsomes (Vmax/Km, 0.0125 and 0.0016 ml/min/mg protein, respectively). Similar kinetic results were obtained for 1-hydroxypyrene. The Eadie-Hofstee plots suggested the contribution of multiple UGTs for the glucuronidation of 1-hydroxypyrene (Km1, Km2, Km3 = 0.8, 9.7, and 63 microM, and Vmax1, Vmax2, Vmax3 = 11, 24, and 55 nmol/min/mg, respectively), whereas only one UGT could be implicated in the glucuronidation of entacapone (Km = 130 microM, Vmax = 1.6 nmol/min/mg). In conclusion, catechols are poor inducers of their own glucuronidation supported by several UGT isoforms. Their administration is unlikely to affect the glucuronidation of other drugs administered concomitantly.

Evidence for multiple glucuronide transporters in rat liver microsomes.

The transport of glucuronides across the endoplasmic reticulum membrane is an important step in the overall process of biotransformation, although the mechanism remains unclear and the participating transporters are unidentified. Using a rapid filtration assay in combination with liquid chromatography-mass spectrometry, we measured the transport of a variety of beta-D-glucuronides in rat liver microsomes and investigated the substrate specificity of the participating transporter(s) by inhibition studies. Time-dependent and bi-directional transport of phenolphthalein glucuronide was detected and the kinetic parameters for transport were determined. The K(m) and V(max) values of high affinity transport were 26microM and 3.9nmol/min/mg protein, respectively. Phenolphthalein glucuronide transport was inhibited by 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid and N-ethylmaleimide. Transport inhibition studies revealed competition between three glucuronides: phenolphthalein glucuronide, estradiol 17-glucuronide and naphthol AS-BI glucuronide indicating that they share a common transporter in the endoplasmic reticulum membrane. Their transport was inhibited by phenolphthalein, but was not affected by p-nitrophenyl glucuronide, naphthyl glucuronide or d-glucuronate. Morphine 3-glucuronide transport was not inhibited by any of the latter four compounds or by phenolphthalein glucuronide. This novel experimental approach has produced data consistent with the presence of multiple (at least three) transporters catalyzing the transport of glucuronides through the endoplasmic reticulum membrane. These data also indicate that the size and/or shape of the aglycone rather than the glucuronic acid moiety per se is an important determinant of transporter specificity.

N-glucuronidation of carbamazepine in human tissues is mediated by UGT2B7.

Carbamazepine (CBZ) is one of the most widely prescribed anticonvulsants despite a high incidence of idiosyncratic side effects. Metabolism of CBZ is complex, and of the more than 30 metabolites identified, one of the most abundant is CBZ N-glucuronide. To date the uridine diphosphate glucuronosyltransferase (UGT) isoform responsible for the N-glucuronidation of CBZ has not been identified. We have developed a sensitive liquid chromatography/mass spectrometry assay to quantify CBZ glucuronidation, and we report that CBZ is specifically glucuronidated by human UGT2B7. Kinetics of CBZ glucuronidation in human liver, kidney, and intestine microsomes were consistent with those of recombinant UGT2B7, which displayed a Km value of 214 microM and Vmax value of 0.79 pmol/mg/min. In addition to revealing the isoform responsible for CBZ glucuronidation, this is the first example of primary amine glucuronidation by UGT2B7.

Farnesol is glucuronidated in human liver, kidney and intestine in vitro, and is a novel substrate for UGT2B7 and UGT1A1.

Farnesol is an isoprenoid found in many aromatic plants and is also produced in humans, where it acts on numerous nuclear receptors and has received considerable attention due to its apparent anticancer properties. Although farnesol has been studied for over 30 years, its metabolism has not been well characterized. Recently, farnesol was shown to be metabolized by cytochromes P450 in rabbit; however, neither farnesol hydroxylation nor glucuronidation in humans have been reported to date. In the present paper, we show for the first time that farnesol is metabolized to farnesyl glucuronide, hydroxyfarnesol and hydroxyfarnesyl glucuronide by human tissue microsomes, and we identify the specific human UGTs (uridine diphosphoglucuronosyltransferases) involved. Farnesol metabolism was examined by a sensitive LC (liquid chromatography)-MS/MS method. Results indicate that farnesol is a good substrate for glucuronidation in human liver, kidney and intestine microsomes (values in nmol/min per mg). Initial analysis using expressed human UGTs indicated that UGTs 1A1 and 2B7 were primarily responsible for glucuronidation in vitro, with significantly lower activity for all the other UGTs tested (UGTs 1A3, 1A4, 1A6, 1A9 and 2B4). Kinetic analysis and inhibition experiments indicate that, in liver microsomes, UGT1A1 is primarily responsible for farnesol glucuronidation; however, in intestine microsomes, UGT2B7 is probably the major isoform involved, with a very-low-micromolar K(m). We also show the first direct evidence that farnesol can be metabolized to hydroxyfarnesol by human liver microsomes and that hydroxyfarnesol is metabolized further to hydroxyfarnesyl glucuronide. Thus glucuronidation may modulate the physiological and/or pharmacological properties of this potent signalling molecule.

Glucuronidation as a mechanism of intrinsic drug resistance in human colon cancer: reversal of resistance by food additives.

Colon cancer exhibits inherent insensitivity to chemotherapy by mechanisms that are poorly characterized. We have shown that human colon cancer cells are efficient in drug conjugation catalyzed by UDP-glucuronosyltransferases (UGTs) and now report on the role of glucuronidation in de novo resistance to two topoisomerase I inhibitors. Identification of the UGT responsible for glucuronidation of SN-38 and the anthraquinone NU/ICRF 505 was achieved by first using a panel of human cDNA-expressed isozymes to measure conjugating activity. HT29 colon cancer cells were then probed by reverse transcriptase-PCR, Western Blot analysis, and liquid chromatography with mass spectrometry for their profile and activity of UGT isozymes and screened for effective inhibitors of glucuronidation. Expression analysis was also conducted in colon cancer biopsies and paired adjacent normal colon specimens. UGT1A9 was identified as the isozyme catalyzing biotransformation of the two compounds in HT29 cells and propofol as an effective competitive inhibitor of this metabolism. Inhibition of glucuronidation resulted in up to a 5-fold enhancement in drug activity. The majority of colon cancer biopsies studies expressed UGT protein at levels greater than in HT29 cells but with marked interpatient variations and proficiently glucuronidated the two anticancer drugs. A range of UGT aglycones were capable of modulating glucuronidation in the biopies with octylgallate being 10-fold more potent (ID(50) 24 microM) than propofol. In a subset of tumors (33%), UGT protein levels and activity exceeded that of paired normal colon. Glucuronidation may represent a mechanism of intrinsic drug resistance in colon cancer open to modulation by a range of food additives and proprietary medicines.

Conjugation of catechols by recombinant human sulfotransferases, UDP-glucuronosyltransferases, and soluble catechol O-methyltransferase: structure-conjugation relationships and predictive models.

Conjugation of a structurally diverse set of 53 catechol compounds was studied in vitro using six recombinant human sulfotransferases (SULTs), five UDP-glucuronosyltransferases (UGT) and the soluble form of catechol O-methyltransferase (S-COMT) as catalyst. The catechol set comprised endogenous compounds, such as catecholamines and catecholestrogens, drugs, natural plant constituents, and other catechols with diverse substituent properties and substitution patterns. Most of the catechols studied were substrates of S-COMT and four SULT isoforms (1A1, 1A2, 1A3, and 1B1), but the rates of conjugation varied considerably, depending on the substrate structure and the enzyme form. SULT1E1 sulfated fewer catechols. Only low activities were observed for SULT1C2. UGT1A9 glucuronidated catechols representing various structural classes, and almost half of the studied compounds were glucuronidated at a high rate. The other UGT enzymes (1A1, 1A6, 2B7, and 2B15) showed narrower substrate specificity for catechols, but each glucuronidated some catechols at a high rate. Dependence of specificity and rate of conjugation on the molecular structure of the substrate was characterized by structure-activity relationship analysis and quantitative structure-activity relationship modeling. Twelve structural descriptors were used to characterize lipophilicity/polar interaction properties, steric properties, and electronic effects of the substituents modifying the catechol structure. PLS models explaining more than 80% and predicting more than 70% of the variance in conjugation activity were derived for the representative enzyme forms SULT1A3, UGT1A9, and S-COMT. Several structural factors governing the conjugation of catechol hormones, metabolites, and drugs were identified. The results have significant implications for predicting the metabolic fate of catechols.

Glucuronidation of HMR1098 in human microsomes: evidence for the involvement of UGT1A1 in the formation of S-glucuronides.

HMR1098, a novel KATP-blocking agent, is metabolized to form an S-glucuronide in rat and dog bile. Synthesis of the S-glucuronide metabolite was studied in human liver and kidney microsomes. Recombinant UPD-glucuronosyltransferases (UGTs) were screened for activity, and kinetic analysis was performed to identify the isoform or isoforms responsible for the formation of this novel S-glucuronide in humans. S-Glucuronidation is relatively rare, but from this study it appears that S-glucuronides are not generated exclusively by a single UGT isoform. From the panel of recombinant isoforms used, both UGT1A1 and UGT1A9 catalyzed the glucuronidation of HMR1098. The Vmax values in both instances were similar, but the Km for UGT1A1 was substantially lower than that measured for UGT1A9, 82 microM compared with 233 microM, respectively. Liver and kidney microsomes displayed similar Km values, but the Vmax in kidney was more than 20-fold less than in liver microsomes, which is suggestive of a significant role for the bilirubin UGT in catalysis of HMR1098, although other UGTs may play a secondary role.

The effect of valproic acid on drug and steroid glucuronidation by expressed human UDP-glucuronosyltransferases.

Valproic acid glucuronidation kinetics were carried our with three human UGT isoforms: UGT1A6, UGT1A9, and UGT2B7 as well as human liver and kidney microsomes. The glucuronidation of valproic acid was typified by high K(m) values with microsomes and expressed UGTs (2.3-5.2mM). The ability of valproic acid to interact with the glucuronidation of drugs, steroids and xenobiotics in vitro was investigated using the three UGT isoforms known to glucuronidate valproic acid. In addition to this the effect of valproic acid was investigated using two other UGT isoforms: UGT1A1 and UGT2B15 which do not glucuronidate valproic acid. Valproic acid inhibited UGT1A9 catalyzed propofol glucuronidation in an uncompetitive manner and UGT2B7 catalyzed AZT glucuronidation competitively (K(i)=1.6+/-0.06mM). Valproate significantly inhibited UGT2B15 catalyzed steroid and xenobiotic glucuronidation although valproate was not a substrate for this UGT isoform. No significant inhibition of UGT1A1 or UGT1A6 by valproic acid was observed. These data indicate that valproic acid inhibition of glucuronidation reactions is not always due to simple competitive inhibition of substrates.

Cloning and characterisation of the first drug-metabolising canine UDP-glucuronosyltransferase of the 2B subfamily.

Glucuronidation is a major route of clearance for a diverse set of both drug and endogenous substrates. The present study was undertaken to redress the lack of molecular information currently available on drug glucuronidation by the dog, a species widely used in metabolism studies by the pharmaceutical industry. A novel dog uridine diphosphate glucuronosyltransferase (UGT), designated UGT2B31 (GenBank Accession Number: AY135176), has been isolated from a dog cDNA library, expressed in V79 cells and characterised using various methods: (i) UGT2B31 sequence has been compared with mammalian UGT sequences using both sequence alignments and phylogenetic analysis; and (ii) the substrate specificity of UGT2B31 has been determined using functional analysis and compared with that obtained using UGT2B7 and dog liver microsomes. The following results were obtained: (i) sequence alignments between UGT2B31 and UGT2B15 gave the greatest degree of identity (76%); however, human UGT2B4, human UGT2B7, monkey UGT2B9 (all 75%), and rat UGT2B1 (73%) also gave a high degree of identity; (ii) phylogenetic analysis determined UGT2B31 to be most closely related to rat UGT2B1; (iii) UGT2B31 displayed a substrate specificity similar to human UGT2B7 and rat UGT2B1, catalysing the glucuronidation of phenols, opioids, and carboxylic acid-containing drugs; and (iv) UGT2B31 only formed morphine-3-glucuronide; however, kinetic analysis determined the K(m) of this reaction to be similar to that observed with UGT2B7 (both approximately 1300 microM). The results suggest that UGT2B31 plays a crucial role in drug detoxification by the dog and may be the canine equivalent of human UGT2B7.

MRP1-transfected cells do not show increased resistance against oxidative stress.

Sensitivity of V79 Chinese hamster cells and V79 cells transfected with human MRP1 gene to several agents inducing oxidative stress was compared. Cells overexpressing MRP1 did not show increased resistance to tert-butyl hydroperoxide, diamide, paraquat, menadione, dichromate and carmustine as estimated by cell survival and DNA damage assessed by comet assay. These findings suggest that overexpression of MRP1 does not confer increased resistance to oxidative stress.

Glucuronidation of catechols by human hepatic, gastric, and intestinal microsomal UDP-glucuronosyltransferases (UGT) and recombinant UGT1A6, UGT1A9, and UGT2B7.

The substrate specificity of human gastric and intestinal UDP-glucuronosyltransferases (UGTs) toward catechols was investigated and compared to that of liver UGTs. Small catechols were efficiently glucuronidated by stomach (0.8-10.2 nmol/mgprotein x min) and intestine (0.9-7.7 nmol/mgprotein x min) with activities in a range similar to those found in liver (2.9-19 nmol/mgprotein x min). Large interindividual variations were observed among the samples. Immunoblot analysis demonstrated the presence of UGT1A6 and UGT2B7 in stomach and throughout the intestine. Recombinant human UGT1A6, 1A9, and 2B7, stably expressed in mammalian cells, all effectively catalyzed catechol glucuronidation. K(m) values (0.09-13.6mM) indicated low affinity for UGTs and V(max) values ranged from 0.51 to 64.0 nmol/mgprotein x min. These results demonstrate for the first time glucuronidation of catechols by gastric and intestinal microsomal UGTs and three human recombinant UGT isoforms.