Introduction

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Glucuronidation is a major conjugation reaction that is catalyzed by numerous isoforms of UGT¹ (Mackenzie et al., 1997). These enzymes are localized primarily in the endoplasmic reticulum and participate in the metabolic elimination of many endogenous compounds and xenobiotics (Clarke and Burchell, 1994). Compounds with a wide variety of chemical structures, such as amines, hydroxylated compounds, and carboxylic acids, are substrates for UGT isoforms.

Human UGT1A3 is transcribed from the human UGT1 gene complex. This gene complex has been shown to encode at least 12 different isoforms of UGT, which share common second through fifth exons, whereas each has its own unique first exon (Ritter et al., 1992; Cho et al., 1995). The amino-terminal portion of the human UGT1A3 protein is encoded by the third exon 1 (exon 1C) of the UGT1 gene complex (Ritter et al., 1992). Of the 12 first exons in the human UGT1 gene complex, three have been identified as encoding pseudo-gene products (UGT1A2, -1A11, and -1A12); a recent study has suggested that only UGT1A1, -1A3, -1A4, -1A6, and -1A9 are expressed in human liver (Strassburg et al., 1997a,b).

The most important physiological substrate for human UGT1A1 is bilirubin (Ritter et al., 1991). Indeed, UGT1A1 is probably the only physiologically relevant enzyme that catalyzes the glucuronidation of bilirubin (Bosma et al., 1994). In addition to bilirubin, human UGT1A1 has been shown to catalyze the glucuronidation of phenolic compounds, certain estrogens, oripavine opioids, coumarins, flavonoids, and anthraquinones (Senafi et al., 1994; King et al., 1996). Previously, we identified human UGT1A4 as an important catalyst in the glucuronidation of tertiary amines to quaternary ammonium-linked glucuronides and in the glucuronidation of secondary and primary amines (Green et al., 1995). In addition to amines, human UGT1A4 catalyzes the glucuronidation of monoterpenoid alcohols, sapogenins, androgens, and progestins (Green and Tephly, 1996). Human UGT1A6 preferentially catalyzes the glucuronidation of planar phenols, whereas UGT1A9 catalyzes the glucuronidation of bulky phenols, anthraquinones, flavonoids, certain aliphatic alcohols, and NSAIDs (Ebner and Burchell, 1993). Mojarrabi et al. (1996) have shown that expressed human UGT1A3 catalyzes the glucuronidation of estrone, 2-hydroxyestrone, hydroxylated benzo[a]pyrene metabolites, and 2-acetylaminofluorene metabolites. However, a comprehensive study of the substrate specificity of expressed human UGT1A3 has not been conducted. In the present study, the reactivity of the expressed enzyme toward amines, opioids, and other compounds was determined. The results show that expressed human UGT1A3, which is 93% identical to UGT1A4 in primary amino acid sequence, also catalyzes the glucuronidation of amines. However, unlike expressed human UGT1A4, expressed human UGT1A3 catalyzes glucuronidation at carboxylic acid and aromatic hydroxyl moieties but not at aliphatic hydroxyl groups, such as those found in androgens, monoterpenoid alcohols, and sapogenins.

	Materials and Methods

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Chemicals. Aglycone substrates for glucuronidation assays were of the highest purity available and were purchased from Sigma Chemical Co. (St. Louis, MO) or Aldrich Chemical Co. (Milwaukee, WI). Saccharolactone, UDP-glucuronic acid, and L--phosphatidylcholine (type XVI-E from egg yolk) were obtained from Sigma Chemical Co. UDP-[U-¹⁴C]glucuronic acid (225 mCi/mmol) was purchased from ICN Radiochemicals (Costa Mesa, CA). Protein assay reagents were obtained from Bio-Rad (Richmond, CA).

Expression of Human UGT1A3 and UGT1A4 Proteins. Construction of the pCMV5-UGT1A3 expression vector was described previously (Mojarrabi et al., 1996). HK293 cells were transfected at 10-20% confluency using a calcium phosphate method (Chen and Okayama, 1988). After 12-16 hr, the cells were washed twice with phosphate-buffered saline and fresh medium was applied. HK293 cells were grown in Dulbecco's modified Eagle's medium containing 4.5 mM glucose, 10 mM N-(2-hydroxyethyl)piperazine-N'-(2-ethanesulfonic acid), and 10% fetal bovine serum, in a humidified incubator with an atmosphere of 5% CO₂, at 37°C. Forty-eight hours after the fresh medium had been applied, the cells were harvested by scraping, washed in phosphate-buffered saline, and frozen at 80°C until used. Development of an HK293 cell line stably expressing human UGT1A4 protein was described previously (Green et al., 1995; Green and Tephly, 1996).

UGT Assays. HK293 cells expressing UGT1A3 were suspended in Tris-buffered saline (pH 7.4) containing 0.5 mM dithiothreitol and were subjected to three rounds of freeze-thawing before homogenization. Glucuronidation activities toward aglycone substrates were determined using [¹⁴C]UDP-glucuronic acid. Briefly, assay mixtures (final volume, 0.1 ml) contained 50 mM Tris or bis-Tris buffer, 10 mM MgCl₂, 100 µg/ml phosphatidylcholine, 8.5 mM saccharolactone, 2.0 mM UDP-glucuronic acid (0.25 µCi/assay), and 0.5 mM aglycone substrate, unless otherwise indicated. Reaction blanks were produced by omitting aglycone. Amine glucuronidation assays were conducted at pH 8.4 using Tris buffer, whereas opioid glucuronidation rates were determined at pH 8.0. Glucuronidation assays for all other substrates were conducted at pH 7.5. For kinetic studies, the pH optimum for each substrate was determined and used. Glucuronidation assays in the pH 6.0-7.0 range were conducted using bis-Tris buffer, whereas assays in the pH 7.0-8.4 range used Tris buffer. All enzymatic assays were conducted at 37°C under conditions that produced linear product formation with respect to time (10 min to 2 hr) and protein concentration (up to 150 µg/0.1-ml assay). Opioid glucuronidation assays were analyzed using the method of Puig and Tephly (1986). HPLC analysis for morphine-3- and -6-glucuronide formation was conducted using the method of Svensson et al. (1982). TLC analysis of amine glucuronide formation was as described previously (Green et al., 1995). Steroid glucuronidation assays were analyzed using the extraction method of Matern et al. (1994), and all other substrates were analyzed using the TLC method of Bansal and Gessner (1980), modified as described (Green et al., 1994). Glucuronidation activities for the substrates used in the present studies were not detected in nontransfected HK293 cell homogenates. Glucuronidation rates were calculated assuming that all substrates formed monoglucuronides.

	Results

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Expression of Human UGT1A3 in HK293 Cells. Previously, Mojarrabi et al. (1996) described the expression of the pCMV5-UGT1A3 construct using COS-7 cells and showed that the expressed protein catalyzed the glucuronidation of estrone and 2-hydroxyestrone. Expression of pCMV5-UGT1A3 in HK293 cells resulted in the production of a protein with a relative subunit molecular mass of about 55 kDa (data not shown), as was observed in COS-7 cells (Mojarrabi et al., 1996). Estrone and 2-hydroxyestrone glucuronidation activities in cell homogenates of HK293 cells transfected with pCMV5-UGT1A3 were monitored to assess enzyme activity and variability during transient expression of the protein. For five different sets of transfections, the estrone glucuronidation activities were 57 ± 5 pmol of glucuronide formed/min/mg of cell protein (mean ± SD), compared with glucuronidation rates of 8 pmol of glucuronide formed/min/mg of cell protein obtained using COS-7 cells (Mojarrabi et al., 1996). 2-Hydroxyestrone glucuronidation activities were 223 ± 26 pmol of glucuronide formed/min/mg of cell protein (mean ± SD). These results indicated that transient UGT1A3 expression in HK293 cells was reproducible, and they suggested that higher levels of the protein were expressed in HK293 cells, compared with COS-7 cells.

Glucuronidation of Amines and Opioids Catalyzed by Human UGT1A3. Expressed human UGT1A3 catalyzes the N-glucuronidation of many primary, secondary, and tertiary amines (table 1). Glucuronidation of amines catalyzed by expressed UGT1A3 exhibited a pH optimum of 8.4. In general, low to moderate glucuronidation rates were observed for all of the amine substrates. Among the primary amines, 2-aminofluorene and 4-aminobiphenyl glucuronidation rates were higher than those observed for the naphthylamines, 2-aminobiphenyl, and benzidine. With the exception of amitriptyline and cyproheptadine, expressed UGT1A3 catalyzed the glucuronidation of tertiary amines at very low rates. Kinetic analysis of amine conjugation (table 2) showed that the glucuronidation efficiencies (V_max/K_M) of UGT1A3 for these amines were low. For expressed UGT1A3, the apparent K_M values for 2- and 4-aminobiphenyl were in the millimolar range, in contrast to those observed (47 and 66 µM, respectively) using expressed UGT1A4. Likewise, the apparent K_M values for the tertiary amines for UGT1A3 tended to be higher than those determined for expressed UGT1A4 (table 2). These data suggest that UGT1A3 might have only minor significance for the conjugation of amines in vivo.

Glucuronidation of Other Xenobiotics Catalyzed by Human UGT1A3. Expressed human UGT1A3 catalyzes the glucuronidation of many phenolic compounds and carboxylic acid-containing drugs (table 4). In general, 7-hydroxylated coumarins, flavonoids, and anthraquinones exhibited high glucuronidation rates, whereas simple phenolic compounds were glucuronidated at lower rates. Compounds containing aliphatic hydroxyl groups, such as sapogenins and aliphatic and monoterpenoid alcohols, were not substrates for UGT1A3. In addition, a number of androgens and progestins, which have only aliphatic hydroxyl groups, were examined and did not react with expressed UGT1A3 (data not shown), in agreement with a previous study (Mojarrabi et al., 1996). These results suggest that human UGT1A3 catalyzes glucuronidation preferentially at phenolic hydroxyl groups. Many NSAIDs and profens were also glucuronidated, at the carboxyl position, by expressed UGT1A3. In addition, two fatty acids, i.e. decanoic acid and dodecanoic acid, were substrates for expressed UGT1A3. Glucuronidation of bilirubin by expressed UGT1A3 was not detected. Stereoselective glucuronidation of ibuprofen was observed using expressed UGT1A3, with the R-stereoisomer being glucuronidated at a higher rate, compared with the S-isomer.

	Discussion

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Previously, Mojarrabi et al. (1996) showed that expressed human UGT1A3 catalyzed the glucuronidation of estrone, 2-hydroxyestrone, and hydroxylated benzo[a]pyrene and 2-acetylaminofluorene metabolites. In this study, we show the reactivity of expressed human UGT1A3 toward a number of xenobiotics of diverse chemical classes. UGT1A3 catalyzes the N-glucuronidation of primary, secondary, and tertiary amines, the O-glucuronidation of opioids, coumarins, flavonoids, anthraquinones, and phenols, and the glucuronidation of NSAIDs and some other carboxylic acid-containing compounds at the carboxyl moiety. However, in the present study, we found that bilirubin, which is glucuronidated at a carboxyl group, is not a substrate for expressed UGT1A3. Androgens (Mojarrabi et al., 1996), progestins, sapogenins, monoterpenoid alcohols, and other compounds conjugated at aliphatic hydroxyl moieties are not substrates for human UGT1A3.

The reactivity of expressed UGT1A3 toward amines is similar to that of UGT1A4, in that both enzymes catalyze the glucuronidation of primary, secondary, and tertiary amines. Although other human UGTs (e.g. UGT1A6 and UGT1A9) have been shown to catalyze the glucuronidation of primary and secondary amines (Huskey et al., 1994; Orzechowski et al., 1994), only human UGT1A3 and UGT1A4 (Green et al., 1995; Green and Tephly, 1996) have been shown to catalyze the formation of quaternary ammonium-linked glucuronides. Metabolic elimination of primary amines is important, because many primary aromatic amines are carcinogenic. Xenobiotics with tertiary amine moieties are commonly used as antihistamines, tricyclic antidepressants, and antipsychotic agents and, in many cases, quaternary ammonium-linked glucuronide formation represents the major means of disposition for these therapeutic agents (Chaudhuri et al., 1976). UGT1A3 and UGT1A4 differ in their glucuronidation efficiencies for amine substrates. For the primary amines 2-aminobiphenyl and 4-aminobiphenyl, N-glucuronide formation catalyzed by expressed human UGT1A4 is characterized by low apparent K_M values and high V_max values (Green and Tephly, 1996). In the present study, we show that N-glucuronide formation catalyzed by expressed UGT1A3 is characterized by high apparent K_M values and low V_max values. These results suggest that UGT1A3 may have limited significance for the in vivo glucuronidation of carcinogenic primary amines. On the other hand, whereas the apparent K_M values for tertiary amines tended to be higher for expressed UGT1A3 than for expressed UGT1A4, the values were only moderately different (with the exception of cyproheptadine).

Breyer-Pfaff et al. (1997) recently described biphasic kinetics of quaternary ammonium-linked glucuronide formation for amitriptyline and diphenhydramine in human liver microsomes and suggested that two UGTs are present that catalyze the N-glucuronidation of these compounds. The low apparent K_M values for tertiary amines glucuronidated by expressed UGT1A4 and the higher apparent K_M values for amines glucuronidated by expressed UGT1A3 are consistent with these data, although the 2-fold difference in apparent K_M values we obtained for amitriptyline is different from the 200-fold difference observed by Breyer-Pfaff et al. (1997) in human liver microsomes. Recently, Bruck et al. (1997) found that two expressed rabbit UGTs (tentatively identified as rabbit UGT1A4 and rabbit UGT1A7) catalyze the glucuronidation of tertiary amines to form quaternary ammonium-linked glucuronides. Similar to our results, expressed rabbit UGT1A4 catalyzed imipramine glucuronidation with a low apparent K_M and a high V_max, compared with expressed rabbit UGT1A7, which had a 3-fold higher apparent K_M for imipramine and a lower V_max. If human UGT1A7, like the putative rabbit UGT1A7, catalyzes the glucuronidation of tertiary amines, it would be unlikely to play a major role in the metabolic elimination of tertiary amines, because UGT1A7 expression has not been detected in human liver (Strassburg et al., 1997a,b).

Unlike expressed human UGT1A4, expressed human UGT1A3 catalyzes the glucuronidation of opioids, phenolic compounds, and carboxylic acids. The substrate specificity of expressed UGT1A3 for opioids is similar to that observed for expressed human UGT1A1 (King et al., 1996). Both enzymes catalyze the glucuronidation of oripavine opioids at higher rates and with high efficiencies, compared with morphinan opioids. When the glucuronidation of morphine was examined, it was found that UGT1A3 catalyzes formation of only morphine-3-glucuronide. Thus, UGT1A3 does not contribute to the formation of morphine-6-glucuronide in humans. Surprisingly, norbuprenorphine was a better substrate for expressed UGT1A3 than was buprenorphine. It is possible that, in addition to glucuronidation at the 3-hydroxyl position, UGT1A3 catalyzes the N-glucuronidation of norbuprenorphine, but further studies are needed to determine whether desmethylated opioids are N-glucuronidated.

7-Hydroxylated coumarins, anthraquinones, and flavonoids are glucuronidated by expressed human UGT1A3 at high rates. Coumarins form a large class of phenolic compounds occurring in green plants, fungi, and bacteria (Murray et al., 1992). Flavonoids are found in many plants, including citrus fruits, berries, leafy vegetables, roots, herbs, spices, cereal grains, tea, and cocoa (Brown, 1980). The common chemical features of these compounds are that they are highly planar molecules that possess an aromatic hydroxyl group that can be glucuronidated. Other highly planar compounds with aromatic hydroxyl groups (e.g. estrogens, hydroxylated benzpyrene metabolites, and hydroxylated 2-acetylaminofluorene metabolites) have also been shown to be substrates for expressed human UGT1A3 (Mojarrabi et al., 1996). It is probable that estrogens and 2-hydroxyestrogen catechols are the important endogenous substrates for UGT1A3. Thus, it is possible that there may be important natural product-estrogen interactions for glucuronidation. Considering that the average American dietary consumption of flavonoids is about 1 g/day (Pierpoint, 1986), these possible interactions for glucuronidation cannot be ignored.

The high reactivity of expressed human UGT1A3 with naturally occurring flavonoids and coumarins is similar to that observed for expressed human UGT2B15 (Green et al., 1994). The apparent K_M values of UGT1A3 and UGT2B15 for naringenin are similar (36 and 28 µM, respectively), but the glucuronidation efficiency of expressed UGT1A3 for naringenin is 10-fold higher than that found for UGT2B15. Therefore, depending on the relative levels of hepatic expression of the two proteins, UGT1A3 might be a more important enzyme in the elimination of naringenin than is UGT2B15. On the other hand, the glucuronidation efficiencies of UGT1A3 and UGT2B15 for 4-methylumbelliferone are similar, even though the apparent K_M of UGT1A3 is higher than that of UGT2B15 (960 and 78 µM, respectively). Therefore, it would be expected that UGT1A3 contributes to the elimination of coumarins only at high concentrations of the compounds.

In addition to amine and aromatic hydroxyl groups, expressed human UGT1A3 catalyzes the glucuronidation of certain carboxylic acid groups. Bilirubin is not a substrate for human UGT1A3; therefore, only human UGT1A1 appears to actively catalyze bilirubin glucuronidation (Bosma et al., 1994). However, drugs containing a carboxylic acid moiety, such as profen NSAIDs and fibrates, are efficiently glucuronidated by expressed human UGT1A3. Intermediate-chain-length fatty acids are also substrates for UGT1A3. This was surprising, because only human UGT2B7 and UGT1A9 were previously shown to glucuronidate carboxylic acid-containing compounds (Jin et al., 1993; Ebner and Burchell, 1993).

Human UGT1A4 and UGT1A3 are 93% identical in amino acid sequence; of the 535 amino acids in the two proteins, only 36 are different. Therefore, it might be expected that, based on their amino acid sequence identity, the substrate specificities of the two proteins would be similar. However, our studies have shown that, with the exception of amine substrates, the two proteins exhibit very different substrate specificities. Expressed human UGT1A3 does not catalyze the glucuronidation of aliphatic hydroxyl groups; monoterpenoid alcohols, androgens, and sapogenins are not substrates for the expressed enzyme. In addition, morphine is not glucuronidated at the aliphatic 6-hydroxyl position. In contrast, monoterpenoid alcohols, androgens, and sapogenins are good substrates for expressed human UGT1A4, and it is possible that sapogenins are specific substrates for human UGT1A4 (Green and Tephly, 1996). With the exception of a few simple phenols, expressed human UGT1A4 does not catalyze the glucuronidation of aromatic hydroxyl groups (Green and Tephly, 1996). Likewise, coumarins, flavonoids, anthraquinones, and estrogens are not substrates for expressed human UGT1A4. However, these compounds, as well as hydroxylated benzpyrene and acetylaminofluorene metabolites (Mojarrabi et al., 1996), are very good substrates for expressed human UGT1A3. It is interesting to note that, of the 36 amino acid differences between UGT1A3 and UGT1A4, 20 of the differences occur in a 54-amino acid stretch between Glu-72 and Cys-128. It has been postulated that the aglycone binding site for UGTs is contained within the first 250-260 amino acid residues of the molecule (Tephly and Burchell, 1990). The dissimilarity in the substrate specificities of UGT1A3 and UGT1A4, given their high degree of amino acid identity, supports this hypothesis and suggests that the aglycone binding site may be restricted to a stretch of amino acids between Glu-72 and Cys-128.

It has been suggested that in normal human liver the only members of the UGT1 gene complex that are constitutively expressed are UGT1A1, -1A3, -1A4, -1A6, and -1A9 (Strassburg et al., 1997a,b). It is believed that upstream from each unique first exon of the UGT1 gene is a separate regulatory region that regulates the expression of the individual UGT1 isoforms (Ritter et al., 1992). Therefore, the variability in diphenhydramine N-glucuronidation by healthy subjects observed by Breyer-Pfaff et al. (1997) may reflect interindividual differences in the levels of UGT1A3 and UGT1A4 expression in human liver. Northern blot analysis of four human liver samples showed variable expression of UGT1A3 mRNA (Mojarrabi et al., 1996). Because estrone and 2-hydroxylated estrogen catechols are good substrates for expressed human UGT1A3, it is possible that endogenous or exogenous estrogens or estrogen antagonists might regulate UGT1A3 expression in hepatic or extrahepatic tissues. For example, it was recently shown that dihydrotestosterone, a good substrate for human UGT2B15 and UGT2B17, could down-regulate UGT2B17 expression in human prostate cancer LNCaP cells but the level of UGT2B15 mRNA was not affected (Guillemette et al., 1997). It is also possible that dietary flavonoids could regulate UGT1A3 expression and contribute to interindividual differences observed for compounds metabolized by UGT1A3. Further studies are needed to investigate the regulation of UGT1 gene expression.