Endogenous substrates for UDP-glucuronosyltransferases

Consolidation of temperature-dependent toxicity and thermoregulatory behavior into risk assessments of insecticides under thermal scenarios: A prospective study on Eremias argus

In this study, the temperature-dependent chemical toxicity of three insecticides and the resulting thermoregulatory (TR) behavior of the lizard Eremias argus have been consolidated into the current risk assessment framework. According to acute dermal toxicity assays, an increase of ambient temperature from 15 °C to 35 °C decreased the acute dermal toxicity of beta-cyfluthrin (BC) but increased the toxicity of chlorpyrifos (CPF). The toxicity of avermectin (AVM) did not show significant temperature-dependent responses. Based on thermal preference trials, lizards changed their body temperature via TR behavior to adaptively reduce toxicity under sub-lethal doses, which can be understood as a "self-rescue" behavior attenuating lethal effects. However, the risk quotient indicated that the effectiveness of this "self-rescue" behavior is limited. Metabolomics analysis showed that six different metabolites (i.e., creatine, glutamate, succinate, N-acetylaspartate, acetylcholine, and lactate) contributed to TR behavior changes. Biochemical assays and insecticide residue results demonstrated that the temperature-dependent toxicity of BC, CPF, and AVM affected lizards in the three aspects of biotransformation, oxidative stress, and neurometabolic interference. This work clarifies the ecotoxicological impacts of representative insecticides on reptiles from toxicological understanding to risk relevance. This knowledge may improve ecological predictions of agrochemical applications in the context of global climate change.

Expression of UDP-glucuronosyltransferase 1A6 isoform in Caco-2 cells stimulated with lipopolysaccharide

Glucuronidation is an important metabolic process of detoxification in all vertebrates. The reaction is catalyzed by a multigene family of UDP-glucuronosyltransferases (UGTs) able to convert many xenobiotics and endobiotics (hydrophobic substances) to inactive, water-soluble glucuronides. The UGTs play a protective role, facilitating the elimination of potentially toxic metabolites via urine, bile and feces; therefore, impairment of UGTs may have important toxicological consequences. The regulation of UGTs during bacterial infection or inflammation is not well described. In this study, we investigated the in vitro effect of lipopolysaccharide (LPS) on the expression of the UGT1A6 isoform in human colon carcinoma Caco-2 cells. Results demonstrated a significant down-regulation of UGT1A6 expression, both in terms of mRNA and protein levels, and a reduced UGT activity after LPS exposure of cell cultures, suggesting a role for endotoxins on UGT regulation mechanisms.

Pharmacogenetics of uridine diphosphoglucuronosyltransferase (UGT) 1A family members and its role in patient response to irinotecan

Glucuronidation, catalyzed by the glucuronosyltransferase (UGT) superfamily, is a major biotransformation pathway for several drugs, including irinotecan. Irinotecan is commonly used in colorectal cancer chemotherapy. Irinotecan undergoes metabolism in humans and is converted to its active metabolite SN-38, a topoisomerase I inhibitor. SN-38 is inactivated via glucuronidation catalyzed by various hepatic and extrahepatic UGT1A isozymes. Although the role of the UGT1A1 *28 genetic variant has received much attention in altered toxicity upon irinotecan treatment, other UGT1A enzymes also play an important role. This review summarizes pharmacokinetic, toxicologic, and pharmacogenetic studies carried out to date in irinotecan and SN-38 disposition.

Phase I clinical and pharmacogenetic study of weekly TAS-103 in patients with advanced cancer

PURPOSE

TAS-103 is an inhibitor of both topoisomerase I and II enzymes with broad antitumor activity. It is metabolized to TAS-103-glucuronide (TAS-103-G) predominantly by uridine diphosphate glucuronosyltransferase isoform 1A1 (UGT1A1). We conducted a phase I study to determine the maximum-tolerated dose (MTD) and dose-limiting toxicity (DLT) of TAS-103 when administered on a weekly schedule to patients with advanced cancer. In addition, we evaluated the influence of UGT1A1 genotype on the pharmacokinetics and toxicity of TAS-103.

PATIENTS AND METHODS

Thirty-two patients were treated with escalating doses (50 to 200 mg/m(2)) of TAS-103, administered intravenously over 1 hour each week for 3 weeks. Pharmacokinetic analysis was performed at the 130-, 160-, and 200-mg/m(2) dose levels. UGT1A1 genotypes were determined using reverse-transcription polymerase chain reaction techniques.

RESULTS

DLT (grade 3 neutropenia) was observed in 5 of 12 patients at 160 mg/m(2) and in 3 of 6 patients at 200 mg/m(2). At 160 mg/m(2), there was a significant correlation between areas under the curve (AUCs) for TAS-103 and TAS-103-G (r = 0.76, P <.05) and an apparent relationship between TAS-103 AUC and D 15 absolute neutrophil count (r = -0.63, P <.05, n = 11, one outlier excluded). UGT1A1 genotype did not influence clearance of TAS-103.

CONCLUSION

We recommend a dose of 130 to 160 mg/m(2), or 250 to 300 mg administered using the above weekly schedule for phase II studies. Further studies to characterize the pharmacodynamics and pharmacogenetics of TAS-103 are warranted.

UGT2B23, a novel uridine diphosphate-glucuronosyltransferase enzyme expressed in steroid target tissues that conjugates androgen and estrogen metabolites

Glucuronidation is widely accepted as a mechanism involved in the catabolism and elimination of steroid hormones from the body. However, relatively little is known about the enzymes involved, their specificity for the different steroids, and their site of expression and action. To characterize the pathway of steroid glucuronidation, a novel uridine diphosphate glucuronosyltransferase (UGT) enzyme was cloned and characterized. A 1768-bp complementary DNA, encoding UGT2B23 was isolated from a monkey liver library. Stable expression of UGT2B23 in human HK293 cells and Western blot analysis demonstrated the presence of a 51-kDa protein. The UGT2B23 transferase activity was tested with 62 potential endogenous substrates and was demonstrated to be active on 6 steroids and the bile acid, hyodeoxycholic acid. Kinetic analysis yielded apparent Michaelis constant (Km) values of 0.9, 13.5, 1.6, and 5.7 microM for the conjugation of androsterone (ADT), 3alpha-Diol, estriol, and 4-hydroxyestrone, respectively. RT-PCR analysis revealed that UGT2B23 transcript is expressed in several tissues, including the prostate, mammary gland, epididymis, testis, and ovary. Primary structure analysis shows that UGT2B23 is in the same family of enzymes as the previously characterized monkey isoforms UGT2B9 and UGT2B18, which are active on hydroxyandrogens. The characterization of UGT2B23 as a functional enzyme active on 3alpha-hydroxysteroids, and its expression in extrahepatic tissues, indicate that it may potentially play an important role in estrogen and androgen catabolism in peripheral steroid target tissues.

The monkey and human uridine diphosphate-glucuronosyltransferase UGT1A9, expressed in steroid target tissues, are estrogen-conjugating enzymes

Considering the physiologic importance of the steroid response, which is regulated in part by steroid levels in a given tissue, relatively little is known about steroid glucuronidation, which is widely accepted as a major pathway involved in the catabolism and elimination of steroid hormones from the human body. In a previous study, it was ascertained that the monkey may be the most appropriate model in which to examine the role of steroid glucuronidation. Northern blot analysis of simian RNA, hybridized with human UGT complementary DNA (cDNA) probes demonstrate the similarity of the transcripts. The simian UGT1A09 cDNA isolated from a liver library is 2396 bp and contains an open reading frame encoding 530 amino acids. The predicted primary structure is most homologous to the human UGT1A9 (hUGT1A9) enzyme, which share 93% identity. Stable transfection of the monkey UGT1A09 (monUGT1A09) cDNA into HK293 cells, expresses a microsomal protein with an apparent molecular mass of 55 kDa. Of the more than 30 endogenous substrates tested, both proteins show the highest activity on 4-hydroxyestradiol and 4-hydroxyestrone, followed by 2-hydroxyestradiol and estradiol. RT-PCR analysis demonstrate that UGT1A9 transcript is expressed in several tissues, which include the prostate, testis, breast, ovary, and skin of the monkey and humans. The expression of UGT1A9 in extrahepatic estrogen-responsive tissues, and its high activity on estrogens is consistent with this enzyme having a role in estrogen metabolism.

Novel bioartificial liver support system: preclinical evaluation

Preclinical safety and efficacy evaluation of a novel bioartificial liver support system (BLSS) was conducted using a D-galactosamine canine liver failure model. The BLSS houses a suspension of porcine hepatocytes in a hollow fiber cartridge with the hepatocytes on one side of the membrane and whole blood flowing on the other. Porcine hepatocytes harvested by a collagenase digestion technique were infused into the hollow fiber cartridge and incubated for 16 to 24 hours prior to use. Fifteen purpose-bred male hounds, 1-3 years old, 25-30 kg, were administered a lethal dose, 1.5 g/kg, of D-galactosamine. The animals were divided into three treatment groups: (1b) no BLSS treatment (n = 6); (2b) BLSS treatment starting at 24-26 h post D-galactosamine (n = 5); and (2c) BLSS treatment starting at 16-18 h post D-galactosamine (n = 4). While maintained under isoflurane anesthesia, canine supportive care was guided by electrolyte and invasive physiologic monitoring consisting of arterial pressure, central venous pressure, extradural intracranial pressure (ICP), pulmonary artery pressure, urinary catheter, and end-tidal CO2. All animals were treated until death or death-equivalent (inability to sustain systolic blood pressure > 80 mmHg for 20 minutes despite massive fluid resuscitation and/or dopamine administration), or euthanized at 60 hours. All animals developed evidence of liver failure at 12-24 hours as evidenced by blood pressure lability, elevated ICP, marked hepatocellular enzyme elevation with microscopic massive hepatocyte necrosis and cerebral edema, elevated prothrombin time, and metabolic acidosis. Groups 2b and 2c marginally prolong survival compared with Group 1b (pairwise log rank censored survival time analysis, p = 0.096 and p = 0.064, respectively). Since survival times for Groups 2b and 2c are not significantly different (p = 0.694), the groups were combined for further statistical analysis. Survival times for the combined active treatment Groups 2b and 2c are significantly prolonged versus Group 1b (p = 0.047). These results suggest the novel BLSS reported here can have a significant impact on the course of liver failure in the D-galactosamine canine liver failure model. The BLSS is ready for Phase I safety evaluation in a clinical setting.

Pharmacogenetics and cancer chemotherapy

Cancer chemotherapy is limited by significant inter-individual variations in responses and toxicities. Such variations are often due to genetic alterations in drug metabolising enzymes (pharmacokinetic polymorphisms) or receptor expression (pharmacodynamic polymorphisms). Pharmacogenetic screening prior to anticancer drug administration may lead to identification of specific populations predisposed to drug toxicity or poor drug responses. The role of polymorphisms in specific enzymes, such as thiopurine S-methyltransferases (TPMT), dihydropyrimidine dehydrogenase (DPD), aldehyde dehydrogenases (ALDH), glutathione S-transferases (GST), uridine diphosphate glucuronosyl-transferases (UGTs) and cytochrome P450 (CYP 450) enzymes in cancer therapy are discussed in this review.

Characterization and regulation of UDP-glucuronosyltransferases in steroid target tissues

Conjugation of compounds by glucuronidation is a pathway found in all vertebrates studied to date. Although, it is widely recognized that the liver is a major site of glucuronidation, it is now clear that extrahepatic tissues are also involved in the conjugation of compounds to which these tissues are exposed. High levels of androsterone glucuronide and androstane-3alpha,17beta-diol glucuronide found in the human prostate, breast cyst fluid and ovary follicular fluid suggest that glucuronidation of 5alpha-reduced C19 steroids occurs in these tissues. Recently, we have reported the tissue distribution of UGT2B15, which can conjugate steroids in several human extrahepatic steroid target tissues including the skin, breast and prostate. We have also isolated a new UGT2B cDNA encoding UGT2B17, that conjugates ADT which is the major 5alpha-reduced C19 steroid glucuronide in the circulation of humans. UGT2B17 is also widely distributed in several human steroid target tissues. This gene was mapped to human chromosome 4q13 and has an exon/intron structure similar to that of rat UGT2B1 and UGT2B2. Both UGT2B15 and UGT2B17, which are able to catalyze the glucuronidation of DHT, are expressed in LNCaP cells. Interestingly, glucuronidation of steroids is markedly regulated by several factors including androgens and growth factors. Treatment of LNCaP cells with dihydrotestosterone (DHT) and epidermal growth factor (EGF) caused a decrease of DHT glucuronidation and UGT2B mRNA levels. RNase protection assays showed a specific decrease of UGT2B17 transcript in LNCaP cells treated with DHT and EGF however, the level of UGT2B15 mRNA was not affected. As well, Western blot analysis demonstrated a diminution of UGT2B17 protein level in response to DHT and EGF. These results demonstrate a differential regulation of different isoforms of steroid conjugating UGTs present in human prostate LNCaP cells. In addition, UGT2B17 was shown to be more labile than UGT2B15 indicating that regulation of UGT2B17 expression would lead to a more rapid change in the level of glucuronidated steroids. Expression of exogenous UGT2B17 in LNCaP cells by gene transfer led to a significant decrease in the androgen response. This result indicates the ability of UGT enzymes to regulate the androgen response by conjugating androgens which abolishes their interaction with their receptor and facilitates their clearance from the cell. The glucuronidation of steroids by UGT enzymes is an important mechanism by which the levels of steroids is regulated in steroid target tissues.

Genetic predisposition to the metabolism of irinotecan (CPT-11). Role of uridine diphosphate glucuronosyltransferase isoform 1A1 in the glucuronidation of its active metabolite (SN-38) in human liver microsomes

Irinotecan (CPT-11) is a promising antitumor agent, recently approved for use in patients with metastatic colorectal cancer. Its active metabolite, SN-38, is glucuronidated by hepatic uridine diphosphate glucuronosyltransferases (UGTs). The major dose-limiting toxicity of irinotecan therapy is diarrhea, which is believed to be secondary to the biliary excretion of SN-38, the extent of which is determined by SN-38 glucuronidation. The purpose of this study was to identify the specific isoform of UGT involved in SN-38 glucuronidation. In vitro glucuronidation of SN-38 was screened in hepatic microsomes from normal rats (n = 4), normal humans (n = 25), Gunn rats (n = 3), and patients (n = 4) with Crigler-Najjar type I (CN-I) syndrome. A wide intersubject variability in in vitro SN-38 glucuronide formation rates was found in humans. Gunn rats and CN-I patients lacked SN-38 glucuronidating activity, indicating the role of UGT1 isoform in SN-38 glucuronidation. A significant correlation was observed between SN-38 and bilirubin glucuronidation (r = 0.89; P = 0.001), whereas there was a poor relationship between para-nitrophenol and SN-38 glucuronidation (r = 0.08; P = 0.703). Intact SN-38 glucuronidation was observed only in HK293 cells transfected with the UGT1A1 isozyme. These results demonstrate that UGT1A1 is the isoform responsible for SN-38 glucuronidation. These findings indicate a genetic predisposition to the metabolism of irinotecan, suggesting that patients with low UGT1A1 activity, such as those with Gilbert's syndrome, may be at an increased risk for irinotecan toxicity.

Isolation and characterization of a simian UDP-glucuronosyltransferase UGT2B18 active on 3-hydroxyandrogens

A monkey cDNA, UGT2B18, encoding a UDP-glucuronosyltransferase (UGT) active on 3-hydroxyandrogens, has been isolated and characterized. Previous results suggested that the monkey represents the most appropriate animal model for studying the physiologic relevance of steroid UGTs. UGT2B18 was isolated from a cynomolgus monkey prostate cDNA library using human UGT2B7, UGT2B10 and UGT2B15 cDNA as probes. The cDNA is 1748 bp in length and contains an open reading frame of 1587 bp encoding a protein of 529 residues. The UGT2B18 cDNA clone was transfected into HK293 cells and a stable cell line expressing UGT2B18 protein was established. Western blot analysis of the UGT2B18-HK293 cell line using a human UGT2B17 polyclonal antibody (EL-93) revealed high expression of a 53 kDa UGT2B protein. The transferase activity of UGT2B18 was tested with over 60 compounds and was demonstrated to be principally active on C19 steroids having an hydroxyl group at position 3alpha of the steroid molecule. UGT2B18 was also active on planar phenols and bile acids. Kinetic analysis revealed that UGT2B18 glucuronidates 3-hydroxyandrogens with high velocity and affinity. Using cell homogenates, Km values of 5.1, 7.8 and 23 microM for androsterone (ADT), etiocholanolone and androstane-3alpha, 17beta diol (3alpha-diol) were obtained, respectively. Specific RT-PCR analysis demonstrated the expression of UGT2B18 transcripts in several tissues including liver, prostate, kidney, testis, adrenal, bile duct, bladder, colon, small intestine, cerebellum and pancreas suggesting a contribution of this isoenzyme to the high plasma levels of glucuronidated ADT and 3alpha-diol found in the cynomolgus monkey.

Differential effect of hypophysectomy and growth hormone treatment on hepatic glucuronosyltransferases in male rats: evidence for an action at a pretranslational level for isoforms glucuronidating bilirubin

The influence of growth hormone (GH) on 4-nitrophenol, bilirubin, testosterone, androsterone and estrone glucuronidation activities was studied in fully activated male rat hepatic microsomes. Sham-operated and hypophysectomized animals were injected with two different dosages of GH, mimicking either the male or female GH secretion pattern. Half the animals received thyroxine and cortisol in concentrations chosen to compensate for the lack of thyroid hormones and glucocorticoids in hypophysectomized rats. GH induced a decrease in several glucuronidation activities: bilirubin glucuronidation in both sham-operated and cortisol/ thyroxine-treated hypophysectomized rats in a dose-dependent manner, testosterone glucuronidation in hypophysectomized animals, and androsterone and estrone glucuronidation in cortisol/thyroxin-treated hypophysectomized rats. 4-nitrophenol glucuronidation was not affected by GH treatment. A hypothetical "feminizing" effect of GH (due to an almost continuous secretion) could not be invoked to explain these results, contrary to what has been observed elsewhere for other hepatic enzyme activities. Hypophysectomy altered all the activities tested, with bilirubin the most modified (a 200% enhancement). Restoration of control values was achieved in hypophysectomized animals with cortisol/thyroxine replacement together with a low dosage of GH (mimicking a male GH secretion pattern), except for androsterone glucuronidation activity where both GH and cortisol/thyroxine treatments reinforced the decreasing effect of hypophysectomy. Variations in protein amounts were correlated to variations in bilirubin, testosterone and androsterone conjugation activities induced by hypophysectomy and GH treatment. Reverse transcription-polymerase chain reaction (RT-PCR) mRNA analysis of bilirubin cluster isoforms or uridine diphosphate glucuronosyltransferase 1B1 (UGT1B1), UGT1B2 and UGT1B5 showed that GH controlled the different isoforms involved in bilirubin glucuronidation differentially at a pretranslational level.

Effect of dietary protein on hepatic and extrahepatic phase I and phase II drug metabolizing enzymes

Weanling male rats were fed low (LP, 7.5%), standard (SP, 15%) or high protein (HP, 45%) diet for 7 or 14 days ad libitum, and cytochrome c reductase (CYC) and UDP-glucuronosyltransferase (UGT) enzyme activities were determined in intestine, kidney and liver microsomes. HP diet increased CYC activity in intestine and kidney, while LP diet had no effect. Hepatic CYC activity declined with decreasing level of dietary protein. Liver and intestine UGT activities were higher on an LP diet, while kidney enzyme activities were higher on an HP diet. UGT activity toward alpha-naphthol, a UGT1 isoform substrate, was modulated by dietary protein in all tissues, while UGT activity toward 4-hydroxybiphenyl, a substrate for a second UGT1 isoform, was affected only in the intestine. The duration of feeding affected CYC and UGT activities in the intestine. This observation may be explained by the dynamic nature of intestinal tissue. The observation of unique tissue and enzyme responses suggests that generalizations regarding metabolic response to diets based on hepatic studies or single enzymes, may be erroneous.

Pharmacokinetics and bioavailability of zidovudine and its glucuronidated metabolite in patients with human immunodeficiency virus infection and hepatic disease (AIDS Clinical Trials Group protocol 062)

The pharmacokinetics of zidovudine (ZDV) are established in patients with various stages of human immunodeficiency virus (HIV) disease. This study was conducted to determine the pharmacokinetic parameters of ZDV in patients with asymptomatic HIV infection and liver disease. HIV-infected volunteers with normal renal function were stratified according to the severity of liver disease (seven of eight were classified as mild). Each subject received a single intravenous dose of ZDV (120 mg) on the first day, followed by a single oral dose of ZDV (200 mg) on the second day. Blood samples were obtained over a 8-h collection interval, and concentrations of ZDV and its glucuronidated metabolite (GZDV) were determined by high-performance liquid chromatography. The following pharmacokinetic parameters were obtained after oral administration of ZDV to HIV-infected patients with mild hepatic disease; these values were compared with previously reported data in healthy volunteers. The area under the curve (AUC) (1,670 +/- 192 ng.h/ml), maximum concentration of drug in serum (1,751 +/- 180 ng/ml), and half-life (2.04 +/- 0.38 h) of ZDV were increased, while the apparent oral clearance (1.57 +/- 0.31 liter/h/kg of body weight) was decreased; AUC (7,685 +/- 1,222 ng.h/ml) and maximum concentration of drug in serum (5,220 +/- 1,350 ng/ml) of GZDV and the AUC ratio of GZDV to ZDV (2.79 +/- 0.43) after oral administration were decreased. ZDV absolute bioavailability was 0.75 +/- 0.15 in HIV-infected patients with hepatic disease. Although the ZDV apparent oral clearance was not impaired as significantly as in patients with biopsy-proven cirrhosis, our results suggest that ZDV, could accumulate in HIV-infected patients with mild hepatic disease because of impaired formation of GZDV. Patients with mild hepatic disease may require dosage adjustment to avoid accumulation of ZDV after extended therapy.

Expression of transcripts encoding steroid UDP-glucuronosyltransferases in human prostate hyperplastic tissue and the LNCaP cell line

The UDP-glucuronosyltransferase (EC 2.4.1.17) enzymes transform many lipophilic compounds to more water-soluble products via conjugation with glucuronic acid. This conversion is responsible for enhancing the excretion of endogenous aglycones such as steroids. To date, several distinct isoforms of steroid UDP-glucuronosyltransferases (UGTs) have been isolated in the human liver. Among these UGTs, UGT2B7 is specific for estriol and 3,4-catechol estrogens, UGT2B15 glucuronidates 17beta-hydroxy-C19 steroids while UGT2B10 has as yet an undescribed activity. To further demonstrate the presence of UGTs in peripheral tissues we studied the expression of these enzymes in human prostate hyperplastic tissue and the LNCaP cell line. Metabolism studies using intact LNCaP cells in culture indicate the presence of UGT activities involved in the glucuronidation of 3alpha-hydroxysteroids (androsterone) and 17beta-hydroxysteroids (testosterone and dihydrotestosterone). Northern blot analysis of poly(A+) RNA from LNCaP cells and prostate using a UGT2B15 cDNA probe revealed two bands of 2.0 and 2.3 kb. In order to identify more specifically the mRNAs detected in Northern blot analysis we used RNase protection and RT-PCR, although, these approaches did not allow detection of UGT2B7 transcripts. Our studies demonstrate the presence of two UGT activities and at least two types of UGT transcripts in both the human prostate and the LNCaP.

Hormonal influences of detoxication in the rat ovary on enzymes in comparison with the liver

Variations in the total capacity of the rat ovary to metabolize xenobiotics during different phases of the estrous cycle were studied. The level of the conjugating enzymes, phenol UDP-glucuronosyltransferase (pUDPGT; EC 2.4.1.17), phenol sulfotransferase (pST; EC 2.8.2.1) and glutathione transferases (EC 2.5.1.18) was determined in the ovary and compared with the corresponding hepatic activities. In addition, catalase (EC 1.11.1.6) and NAD(P)H: quinone oxidoreductase (EC 1.6.99.2) two other detoxifying enzymes, were assayed. In order to study the hormonal influences on detoxifying enzymes, mature rats were characterized with respect to their stage in the estrous cycle. Immature rats were treated with pregnant mare's serum gonadotropin (PMSG) for 2 or 3 days to enrich the ovaries in preovulatory follicles or corpora lutea, respectively. The present study demonstrates that ovarian pUDPGT and pST activities are increased 936% and 175%, respectively, in ovaries enriched in corpora lutea compared to ovaries from untreated immature rats. Increases in these activities in mature rats during the metestrous stage of the estrous cycle compared to the proestrous stage were also noted. In the liver pUDPGT activity is increased significantly (1.6-fold) in immature rats with ovaries enriched in preovulatory follicles compared to untreated rats. Both ovarian pST and pUDPGT activities increased in mature rats treated with PMSG ("hyperstimulated"), while in the liver only pST was increased by such treatment. Ovarian glutathione transferase activity proved not to be dependent on the hormonal fluctuations associated with the estrous cycle. However, in the liver of mature rats treated with PMSG, this activity increased 2-fold compared to the untreated immature rats. The catalase activity found in the ovarian mitochondrial fraction was approx. 10-fold higher than in the cytosolic fraction, independent of the hormonal status. Moreover, we found a significant 1.4-fold increase in peroxisomal catalase activity in the mitochondrial fraction of immature rats treated with PMSG, both when enriched in preovulatory follicles and in corpora lutea. In the liver cytosolic catalase activity decreased several-fold in immature rats following PMSG treatment. We did not find any variations in ovarian NAD(P)H: quinone oxidoreductase activity during the estrous cycle, whereas in the liver this activity decreased in the luteal phase, as it did in mature rats treated with PMSG. From this study and earlier investigations in our laboratory, we conclude that cyclic variations due to hormones of the estrous cycle of the major 7,12-dimethylbenz(a)anthracene (DMBA)-metabolizing phase I enzymes in the ovary are not accompanied by increases in the activities of the corresponding phase II enzymes.(ABSTRACT TRUNCATED AT 400 WORDS)

Role of hepatic metabolism in the bioactivation and detoxication of amodiaquine

1. The hepatic metabolism of the antimalarial drug amodiaquine was investigated in order to gain further insight into the postulated metabolic causation of the hepatotoxicity, which restricts the use of the drug. After intraportal (i.p.) administration (54 mumol/kg) to the anaesthetized rat, the drug was excreted in bile (23 +/- 3% dose over 5 h; mean +/- SD, n = 6) primarily as thioether conjugates. 2. After i.p. administration, 20% of the dose was excreted into urine over 24 h as parent compound and products of N-dealkylation and oxidative deamination. Desethylamodiaquine accumulated in liver, but was not a substrate for bioactivation as measured by biliary elimination of a glutathione adduct. 3. Prior administration of ketoconazole, an inhibitor of P450, reduced biliary excretion by 50% and effected a corresponding decrease in the amount of drug irreversibly bound to liver proteins. This indicated a role for P450 in the bioactivation of amodiaquine to a reactive metabolite that conjugates with glutathione and protein. 4. De-ethylation and irreversible binding were observed in vitro using male rat liver microsomes, and were again inhibited by ketoconazole. However, no such binding was observed with human (six individuals) hepatic microsomes despite extensive turnover of amodiaquine to desethylamodiaquine. 5. Amodiaquine quinoneimine underwent rapid reduction in the presence of either human or rat liver microsomes. Therefore in vitro studies may underestimate the bioactivation of amodiaquine in vivo. These data indicate that the extent of protein adduct formation in the liver will depend on the relative rates of oxidation of amodiaquine and reduction of its quinoneimine. This in turn may be a predisposing factor in the idiosyncratic hepatotoxicity associated with amodiaquine. 6. Substitution of a fluorine for the phenolic hydroxyl group in amodiaquine blocked bioactivation of the drug in vivo. Insertion of an N-hydroxyethyl function enabled partial clearance of amodiaquine and its deshydroxyfluoro analogue via O-glucuronidation and altered the balance between phase I oxidation and direct phase II conjugation of amodiaquine.

Membrane translocation and regulation of uridine diphosphate-glucuronic acid uptake in rat liver microsomal vesicles

BACKGROUND/AIMS

Hepatic glucuronidation is quantitatively the most important conjugation reaction by which an array of endogenous compounds and xenobiotics undergo biotransformation and detoxification. The active site of the uridine diphosphate (UDP) glucuronosyltransferases, which catalyze glucuronidation reactions, has been postulated to reside in the lumen of the endoplasmic reticulum. The aim of this study was to characterize the process whereby UDP glucuronic acid (UDP-GlcUA), the cosubstrate for all glucuronidation reactions, is transported into microsomal vesicles.

METHODS

The uptake process was analyzed using rapid filtration techniques, radiolabeled UDP-GlcUA, and rat liver microsomes.

RESULTS

Uptake was saturable with respect to time and concentration, inhibited by 4,4'-diisothiocyanato-stilbene-2,2'-disulfonic acid and 4-acetamido-4'-isothio-cyanatostilbene-2-2'-disulfonic acid, and was osmotically sensitive. Transport was stimulated by Mg2+ and guanosine triphosphate (50 mumol/L) but not guanosine 5'-O-(3-thiotriphosphate) or adenosine triphosphate. Luminal UDP-N-acetylglucosamine (1 mmol/L) produced enhanced uptake of UDP-GlcUA (trans stimulation). In contrast to nucleotide sugar transport in the Golgi apparatus, trans uridine monophosphate and UDP did not alter UDP-GlcUA transport in microsomes, indicating distinct processes.

CONCLUSIONS

These data provide unambiguous evidence for the existence of a unique, substrate-specific, regulated, carrier-mediated process that transports UDP-GlcUA into the lumen of hepatocyte microsomes. This transporter may regulate glucuronidation in vivo.

Induction and immunological characterization of the uridine diphosphate-glucuronosyltransferase conjugating 1-naphthol in the rat choroid plexus

The uridine diphosphate-glucuronosyltransferase isoform conjugating 1-naphthol has a very high activity in the rat choroid plexus. We showed that antibodies raised against the main liver isoenzyme cross-reacted with a choroidal protein exhibiting the same molecular weight as the hepatic form. Both enzymes had a similar affinity for 1-naphthol. After an in vivo treatment by 3-methylcholanthrene, a 3-fold increase of the activity in both the liver and choroid plexuses was observed. These results suggest that the choroidal and hepatic enzymes conjugating 1-naphthol are identical. This high metabolic activity suggests a metabolic protection of the brain by the choroid plexus.

Inhibition of rat liver microsomal bilirubin UDP-glucuronosyltransferase by ursodeoxycholic acid

Ursodeoxycholic acid and its endogenous metabolite tauroursodeoxycholic acid inhibited in vitro the microsomal bilirubin UDP-glucuronosyltransferase from rat liver. The magnitude of the inhibition correlated well with the loss of integrity of microsomal vesicles, suggesting that bile salts needed to reach the lumen to exert their inhibitory effects. The endogenous bile acids cholic acid, chenodeoxycholic acid and deoxycholic acid also exhibited inhibitory effects on bilirubin glucuronidation in digitonin-disrupted microsomes. Ursodeoxycholic acid inhibitory capacity was similar to that of chenodeoxycholic acid and deoxycholic acid but greater than that of cholic acid, the major endogenous bile salt. Kinetic studies, performed in detergent-activated preparations, showed that the inhibitions produced by ursodeoxycholic and tauroursodeoxycholic acids were competitive toward both bilirubin and UDP-glucuronic acid. The estimated Ki(app) for both substrates did not differ statistically between ursodeoxycholic and tauroursodeoxycholic acids. Both bile salts were weak inhibitors toward bilirubin but rather strong inhibitors toward UDP-glucuronic acid.

Absence of phenolic glucuronidation and enhanced hydroxylation of diflunisal in the homozygous Gunn rat

1. The disposition of diflunisal (DF) was investigated in bile-exteriorized and intact homozygous Gunn rats given 10 and 50 mg/kg doses i.v. and in Wistar rats given 10 mg/kg doses i.v. 2. In Gunn rats, DF sulphate, DF acyl glucuronide, and a hitherto unidentified metabolite of DF, a conjugate of 3-hydroxy-DF, were identified as the major metabolites, accounting for approximately 37%, 16% and 11% respectively of 10 mg/kg doses and 35%, 24% and 15% respectively of 50 mg/kg doses in bile-exteriorized animals. There was no evidence for formation of DF phenolic glucuronide. 3. Total plasma clearance of DF and formation clearances of DF to DF sulphate and 3-hydroxy-DF were little affected by increase of dose from 10 to 50 mg DF/kg, whereas formation clearance of DF to DF acyl glucuronide was increased, but not significantly. 4. In Gunn rats with undisturbed bile flow into the gut, recoveries of DF sulphate and total 3-hydroxy-DF in urine increased to approximately 48% and 25% dose respectively at the expense of DF acyl glucuronide through enterohepatic recirculation. 5. In bile-exteriorized Wistar rats, DF phenolic glucuronide, DF acyl glucuronide, DF sulphate and 3-hydroxy-DF accounted for 16%, 27%, 14% and 2%, respectively, of 10 mg/kg doses. In intact Wistar rats, urinary recoveries of the metabolites were 15%, 13%, 23% and 5%, respectively. 6. Thus in comparison to Wistar rats, phenolic glucuronidation of DF was absent or negligible in homozygous Gunn rats, acyl glucuronidation was significantly decreased, sulphation was unchanged, and the 3-hydroxylation of DF was significantly enhanced.

Isolation and characterization of hyodeoxycholic-acid: UDP-glucuronosyltransferase from human liver

The enzyme hyodeoxycholic-acid: UDP-glucuronosyltransferase was purified about 230-fold from a solubilized human liver microsomal preparation utilizing anion-exchange chromatography, ampholyte-displacement chromatography and UDP-hexanolamine--Sepharose affinity chromatography. The homogeneity of the final enzyme preparation was judged by two criteria: the appearance of a single band of Mr 52000 in SDS/PAGE; the elution of a single peak in reversed-phase FPLC. The isolated enzyme catalyzed the glucuronidation of the 6 alpha-hydroxy bile acids hyodeoxycholic and hyocholic acids, and of the steroid hormone estriol, with a ratio of relative reaction rates of 13:1:2.7. UDP-glucuronosyltransferase activities toward the 3 alpha-hydroxy bile acid lithocholic acid, androsterone, testosterone, bilirubin and p-nitrophenol were not detectable in the pure enzyme preparation and were shown to be separated from enzyme activity toward hyodeoxycholic acid during ampholyte-displacement chromatography and/or UDP-hexanolamine--Sepharose affinity chromatography. Two-substrate kinetic analysis of hyodeoxycholic-acid-conjugating activity gave a sequential mechanism with apparent Km values of 12 microM and 4 microM for hyodeoxycholic acid and UDP-glucuronic acid, respectively. Phospholipids were required for reconstitution of maximal activity toward hyodeoxycholic acid. Phosphatidylcholine was the most effective activator of enzyme activity.

Glucuronidation of 3'-azido-3'-deoxythymidine catalyzed by human liver UDP-glucuronosyltransferase. Significance of nucleoside hydrophobicity and inhibition by xenobiotics

The enzymatic glucuronidation of 3'-azido-3'-deoxythymidine (AZT) catalyzed by human liver microsomal UDP-glucuronosyltransferase (EC 2.4.1.17, UDPGT) was inhibited by a number of nucleoside analogs. The inhibitory potency of these nucleoside analogs correlated with their hydrophobicity (r2 = 0.90, N = 13). Since similar results were obtained with solubilized UDPGT (r2 = 0.87, N = 7), the affinity of the nucleosides for UDPGT was probably being assessed rather than the ability of the compounds to access the membrane-bound enzyme. Three homologous inhibitors, 3'-azido-2',3'-dideoxyuridine (AzddU), 5-ethyl-AzddU, and 5-propyl-AzddU, were also studied as substrates of UDPGT. The substrate efficiency (Vmax/Km) of these three compounds and AZT also correlated with their hydrophobicity (r2 = 0.94). Sixteen drugs that are structurally unrelated to nucleosides also inhibited the glucuronidation of AZT. The mechanism of inhibition was competitive for seven compounds tested. Ki values were estimated from Dixon plots for nine other less soluble inhibitors; their mechanism of inhibition was assumed to be competitive. Since the peak physiological drug concentrations of the tested inhibitors are considerably less than their Ki values, none of these compounds are expected to strongly inhibit AZT glucuronidation in humans. However, the rank order of these drugs with respect to their inhibitory potential is probenecid greater than chrloramphenicol greater than naproxen greater than phenylbutazone much greater than other drugs tested.

17 Beta-hydroxysteroid UDP-glucuronosyl transferase is expressed in bile ductular epithelial cells under physiological conditions

Using an improved procedure to isolate pure bile ductular epithelial cells from rat liver, we were able to define UDP-glucuronosyl transferase (UDPGT) isozymes expressed in these cells under physiological conditions. The cells contained mRNA for a 17 beta-hydroxysteroid, a 3 alpha-hydroxysteroid and a phenol UDPGT. Concomitantly, a distinct pattern of UDPGT-immunoreactive proteins was expressed, including a putative 17 beta-hydroxysteroid UDPGT. The presence of this UDPGT isozyme was confirmed by a high level of testosterone glucuronidation activity. This is the first demonstration of a metabolic pathway in bile ductular epithelial cells that is primarily dedicated to the processing of endogenous compounds.

Induction of drug-metabolizing enzymes by xenobiotics

1. Isozymes of the cytochromes P-450, UDP-glucuronosyl transferases (UDPGT) and glutathione S-transferases appear to be differentially inducible by prototype inducers, such as 3-methylcholanthrene (MC), phenobarbital, pregnenolone 16 alpha-carbonitrile and clofibrate. 2. Mechanisms of induction include both transcriptional and post-transcriptional control. MC-type inducers (representing a large number of planar polycyclic aromatics, beta-naphthoflavone and polyhalogenated aromatics) bind with high affinity to the Ah receptor which controls gene expression similar to steroid hormone receptors. The Ah receptor controls the expression of several drug metabolizing enzymes. For example, both cytochrome P450 IA1 and UDPGT-1 appear to be co-induced by inducers with widely differing potencies, such as 2,3,7,8-tetrachloro-dibenzo-p-dioxin, 1,2,3,7,8-pentachloro-dibenzo-p-dioxin and benz(a)anthracene. Much less is known about the mechanism of action of other inducer prototypes. 3. Induction and co-induction of drug-metabolizing enzymes are generally considered as adaptive responses leading to more efficient elimination and detoxication of xenobiotics such as benzo(a)pyrene. For example, when the mutagenicity of benzo(a)pyrene and benzo(a)pyrene-3,6-quinone was studied in the Ames test, glucuronidation or glutathione conjugation (concomitant with cytochrome P-450-dependent reactions) markedly decreased their mutagenicity. The protective effect was more pronounced with the homogenate S9 fraction of MC-treated rats. However, at 'non-physiological' levels of exposure enzyme induction may lead to increased toxic risk.

UDP-glucuronosyltransferases: a family of detoxifying enzymes

Glucuronidation is an important process in the metabolism of xenobiotic and endogenous substances leading to enhancement of excretion of these compounds from the body. A multigene family encodes a number of UDP-glucuronosyltransferase enzymes which catalyse this route of metabolism. Recent advances in biochemical and molecular biological approaches, reviewed here by Thomas Tephly and Brian Burchell, have given new insight into the function and structure of UDP-glucuronosyltransferases. These proteins have surprising similarities and yet appear to be capable of conjugating a remarkable number of different chemicals.

Kinetic characteristics of UDP-glucuronosyltransferases towards a dithiol metabolite of malotilate in hepatic microsomes of rats and rabbits

1. The kinetic activity of UDP-glucuronosyltransferases (UDPGT) towards a dithiol metabolite of malotilate, 2,2-di(isopropoxycarbonyl)ethylene-1,1-dithiol, was investigated using rat and rabbit hepatic microsomes. The thio-glucuronide formed was analysed by h.p.l.c. The Km values obtained using rat and rabbit UDPGT were 36.3 +/- 3.3 and 443 +/- 43 microM, respectively. The Vmax values were 7.14 +/- 0.61 and 29.2 +/- 6.4 nmol/min per mg (mean +/- SD, n = 3). 2. Phenobarbital, an inducer of the GT2 isoform of UDPGT, increased rat microsomal UDPGT activity towards the dithiol. In inhibitory studies, menthol and borneol (specific substrates for GT2a isoform) competitively inhibited glucuronidation of the dithiol. Thus it was concluded that formation of the thio-glucuronide was catalysed mainly by the GT2a isozyme of UDPGT, which is involved in glucuronidation of monoterpenoid alcohols.

The role of sulfation in thyroid hormone metabolism

Sulfate conjugation is a significant metabolic reaction for thyroxine and especially so for triiodothyronine and lower iodothyronines in rats. Triiodothyronine sulfation has also been demonstrated in humans. Sulfation accelerates the deiodinative breakdown of iodothyronines by the type I iodothyronine deiodinase in liver and thus represents a rate-limiting step in one of the elimination pathways of thyroid hormone.

The metabolism of 2,4-dibromo-17 alpha-ethynyl[6,7-3H]oestradiol in the rat

1. The metabolism of 2,4-dibromoethynyloestradiol (2,4-DBEE2) in the rat was studied in order to determine the influence of ring-A substituents on the phase I biotransformations of oestrogens. 2. 2,4-Dibromo-17 alpha-ethynyl[6,7-3H]oestradiol was synthesized by the one-stage bromination of 17 alpha-ethynyl[6,7-3H]oestradiol (EE2) with N-bromoacetamide, and administered (30 micrograms/kg, i.v.) to anaesthetized male and female rats. 3. A single metabolite, identified as a glucuronide of 2,4-DBEE2, was rapidly and extensively eliminated in bile by male rats (83% of the dose over 6 h). Females excreted additional minor conjugated metabolites. Neither unchanged 2,4-DBEE2 nor EE2 was detected in bile. 4. The hepatic residues after 6 h (percentage of dose) were 2.7% and 3.4% in male and female rats, respectively, whilst less than 0.1% per organ(s) was found in kidneys, heart, spleen, lungs and brain. 5. 2,4-Dibromo substitution of EE2 effectively blocked all phase I biotransformations whilst not limiting glucuronylation in male rats, but did not entirely preclude phase I metabolism in females. The inertness of 2,4-DBEE2 to ring-A hydroxylation in male rats conforms with the insignificant debromination of 2,4-dibromoestradiol by hepatic microsomes.

Enzymatic oxidation of xenobiotic chemicals

Studies with biomimetic models can yield considerable insight into mechanisms of enzymatic catalysis. The discussion above indicates how such information has been important in the cases of flavoproteins, hemoproteins, and, to a lesser extent, the copper protein dopamine beta-hydroxylase. Some of the moieties that we generally accept as intermediates (i.e., high-valent iron oxygen complex in cytochrome P-450 reactions) would be extremely hard to characterize were it not for biomimetic models and more stable analogs such as peroxidase Compound I complexes. Although biomimetic models can be useful, we do need to keep them in perspective. It is possible to alter ligands and aspects of the environment in a way that may not reflect the active site of the protein. Eventually, the model work needs to be carried back to the proteins. We have seen that diagnostic substrates can be of considerable use in understanding enzymes and examples of elucidation of mechanisms through the use of rearrangements, mechanism-based inactivation, isotope labeling, kinetic isotope effects, and free energy relationships have been given. The point should be made that a myriad of approaches need to be applied to the study of each enzyme, for there is potential for misleading information if total reliance is placed on a single approach. The point also needs to be made that in the future we need information concerning the structures of the active sites of enzymes in order to fully understand them. Of the enzymes considered here, only a bacterial form of cytochrome P-450 (P-450cam) has been crystallized. The challenge to determine the three-dimensional structures of these enzymes, particularly the intrinsic membrane proteins, is formidable, yet our further understanding of the mechanisms of enzyme catalysis will remain elusive as long as we have to speak of putative specific residues, domains, and distances in anecdotal terms. The point should be made that there is actually some commonality among many of the catalytic mechanisms of oxidation, even among proteins with different structures and prosthetic groups. Thus, we see that cytochrome P-450 has some elements of a peroxidase and vice versa; indeed, the chemistry at the prosthetic group is probably very similar and the overall chemistry seems to be induced by the protein structure. The copper protein dopamine beta-hydroxylase appears to proceed with chemistry similar to that of the hemoprotein cytochrome P-450 and, although not so thoroughly studied, the non-heme iron protein P. oleovarans omega-hydroxylase.(ABSTRACT TRUNCATED AT 400 WORDS)

Localization, distribution, and induction of xenobiotic-metabolizing enzymes and aryl hydrocarbon hydroxylase activity within lung

The metabolism of xenobiotics within lung often leads to toxicity, although certain pulmonary cells are more readily damaged than others. This differential susceptibility can result from cell-specific differences in xenobiotic activation and detoxication. The localization and distribution of xenobiotic-metabolizing enzymes (cytochromes P-450, NADPH-cytochrome P-450 reductase, epoxide hydrolase, glutathione S-transferases, UDP-glucuronosyltransferases, and a sulfotransferase) and of aryl hydrocarbon (benzo[a]pyrene) hydroxylase activity determined immunohistochemically and histochemically, respectively, within lung are discussed. Findings reveal that xenobiotics can be metabolized in situ, albeit to different extents, by bronchial epithelial cells, Clara and ciliated bronchiolar epithelial cells, and type II pneumocytes and other alveolar wall cells and that enzymes and activities are not necessarily induced uniformly among these cells.

N-glycosylation of purified rat and rabbit hepatic UDP-glucuronosyltransferases

Five UDP-glucuronosyltransferases (UDPGTs) have been isolated to apparent homogeneity from rat and rabbit liver and have been characterized for their glycoprotein nature by reacting these proteins with commercially available endo- and exoglycosidases. The enzymes studied were rat hepatic p-nitrophenol, 17 beta-hydroxysteroid, and 3 alpha-hydroxysteroid UDPGTs and rabbit hepatic p-nitrophenol and estrone UDPGTs. Hydrolysis of oligosaccharide moieties was evidenced by an increase in the mobility (decreased apparent molecular weight) of the protein subunits after sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Purified rabbit hepatic estrone and p-nitrophenol UDPGTs were hydrolyzed by almond glycopeptidase A and endo-beta-N-acetylglucosaminidase H from Streptomyces plicatus (endo H), but not by endo-beta-N-acetylglucosaminidase D from Diplococus pneumoniae (endo D) suggesting that these transferases are glycoproteins of the high mannose type and not of the complex type. Likewise, purified rat hepatic 3 alpha-hydroxysteroid and p-nitrophenol UDPGTs were substrates for glycopeptidase A and endo H but not for endo D. One enzyme, 17 beta-hydroxysteroid UDPGT, was not glycosylated since it was not hydrolyzed by any of the three endoglycosidases. All four glycosylated UDPGTs could serve as substrates for jack bean alpha-mannosidase, confirming the high mannose nature of the oligosaccharide. Deglycosylation of the purified UDPGTs by endo H did not have an effect on the catalytic activities of these proteins.

Heterogeneous alterations of UDP-glucuronosyltransferases in mouse hepatic foci

UDP-glucuronosyltransferase (UDPGT) was studied immunohistochemically in hepatic foci and nodules of N-nitrosomorpholine-treated mice. Serial sections were stained for glucose-6-phosphatase (G6Pase). It was found that a high percentage of G6Pase-negative liver foci and nodules were also UDPGT-negative (34%). In addition, G6Pase-negative foci without altered UDPGT phenotype (30%) and UDPGT-negative foci without altered G6Pase phenotype (8%) were detected. G6Pase-positive foci were also present (24%). Interestingly, most G6Pase-positive foci were UDPGT-positive (16%). Some G6Pase-positive lesions without altered UDPGT phenotype were also found (8%). The major phenotype observed in rat hepatocarcinogenesis models (UDPGT-positive/G6Pase-negative foci) was not detectable in the mouse model. These results demonstrate heterogeneous alterations of UDPGTs in mouse hepatic foci. They furthermore suggest marked differences between the mouse and the rat in the regulation of UDPGTs in similarly induced rat hepatic foci.

UDP-glucuronosyltransferases in the metabolic disposition of xenobiotics

UDPGT isoenzymes are products of multiple gene families as demonstrated by sequence analysis of purified proteins and by molecular cloning experiments. These isoenzymes are relatively specific for endogenous substrates but have broad substrate specificities for xenobiotic substrates. They are important metabolic enzymes capable of converting exogenous and endogenous substances to more hydrophilic metabolites. Each species has its own pattern of UDPGTs and it is not possible at this time to extrapolate information directly from one species to another.