On the substrate specificity of the digitoxigenin monodigitoxoside conjugating UDP-glucuronyltransferase in rat liver

Effect of spironolactone on the expression of rat hepatic UDP-glucuronosyltransferase

Spironolactone (SL) increases the glucuronidation rate of several compounds. We analyzed the molecular basis of changes occurring in major rat liver UDP-glucuronosyltransferase (UGT) family 1 isoforms and in UGT2B1, a relevant isoform of family 2, in response to SL. UGT activity toward bilirubin, ethynylestradiol and p-nitrophenol was assayed in native and activated microsomes. Protein and mRNA levels were determined by Western and Northern blotting. The lipid composition and physicochemical properties of the microsomal membrane were also analyzed. Glucuronidation rates of bilirubin and ethynylestradiol (at both 3-OH and 17 beta-OH positions), determined in UDP-N-acetylglucosamine-activated membranes, were increased in SL group. Western blot analysis revealed increased levels of UGT1A1 and 1A5 (bilirubin and 3-OH ethynylestradiol conjugation), and 2B1 (17 beta-OH ethynylestradiol conjugation). Northern blot studies suggested transcriptional regulation by the steroid. Analysis of UGT activity in native vs. alamethicin-activated microsomes indicated increased latency, which was not associated to changes in physicochemical properties of the microsomal membrane. p-Nitrophenol glucuronidation rate and mRNA and protein levels of UGT1A6, the main isoform conjugating planar phenols, were not affected by the inducer. The data suggest transcriptional regulation of specific isoforms of hepatic UGT by SL, thus explaining previously reported increases in UGT activity toward selective substrates.

The influence of age and some inducers on UDP-glucuronyltransferase activity

UDP-glucuronyltransferase (UDP-GT) activity was examined in male Wistar rats aged 0.5, 1, 2, 4, 8, 12, 20, and 28 months. The rats were treated with phenobarbital (75 mg/kg, 72 and 48 h before death), beta-naphthoflavone or dexamethasone (40 mg/kg and 20 mg/kg, respectively, for three days before death). Prior to decapitation the rats were fasted for 12 h. Hepatic microsomes were prepared according to the method of Dallner. UDP-GT activity was determined by the method of Burchell and Weatherill. p-Nitrophenol was used as an aglucone. UDP-GT activity decreased rapidly in the control rats aged from two weeks to four months. In the older control rats the activity tended to increase. Two-week-old rats treated with phenobarbital showed a slightly increased UDP-GT activity. In the older animals (up to one year) UDP-GT activity increased to 150% of the control value and stayed at this level in the remaining age groups. beta-Naphthoflavone was a more potent inducer of UDP-GT than phenobarbital. The activity of beta-naphthoflavone-induced UDP-GT was low in the youngest rats. It was about 180% in two-month-old rats and reached 260% of the control value in eight-month-old rats. Although the activity decreased in the older rats, it still exceeded 200%. Dexamethasone did not affect UDP-GT activity. Only in two-week-old and two-month-old rats did we observe a slight increase in the activity of UDP-GT.

Effect of age and sex on the inducibility of hepatic microsomal 4-nitrophenol UDP-glucuronyltransferase activity in rats

In this report, we examined the effect of age and sex of Long-Evans rats on the inducibility of 4-nitrophenol glucuronyltransferase activity in liver microsomes of animals treated with phenobarbital, dexamethasone, or 3-methylcholanthrene. Glucuronyltransferase activity in control male liver microsomes was approximately 50-75% higher than that in control female liver microsomes. Treatment with sodium phenobarbital (i.p., 75 mg/kg/day) for 3-4 days increased glucuronyltransferase activity by approximately 100% and 60% in liver microsomes from adult female and male rats, respectively, but did not influence this activity in liver microsomes from immature female and male rats. Treatment with dexamethasone (i.p., 75 mg/kg/day) for 3-4 days increased glucuronyltransferase activity by approximately 4-fold in liver microsomes from immature female rats but markedly decreased this enzyme activity in liver microsomes from adult female rats, adult male rats, and immature male rats. Treatment with 3-methylcholanthrene (i.p., 25 mg/kg/day) for 3-4 days increased glucuronyltransferase activity by approximately 10- to 13-fold in liver microsomes from immature female rats, adult female rats, and adult male rats, but only increased this enzyme activity by approximately 1-fold in liver microsomes from immature male rats. In conclusion, the extent of induction of hepatic microsomal glucuronyltransferase activity for 4-nitrophenol is affected dramatically by the age and sex of the animals.

Heterogeneity of hepatic UDP-glucuronosyltransferase activities: investigations of isoenzymes involved in p-nitrophenol glucuronidation

1. UDP-Glucuronosyltransferase (UGT, EC 2.1.4.17) has been measured routinely with para-nitrophenol (pNP) because the UGT assay using this substrate is easy to assess and run. 2. This compound has been used in several studies as a substrate for purification of the enzyme. 3. In the present work, we characterize the para-nitrophenol-conjugating activity. 4. An analysis of kinetics of para-nitrophenol conjugation obtained from various biological sources (various tissues and various species) leads us to the conclusion that at least three isoenzymes are responsible for this activity in the rat. 5. Both UGT-(testosterone) and the 3-methylcholanthrene-inducible form previously described in the literature, may be responsible for the activity, whilst a highly specific form (UGT-phenol) is reported here for the first time. 6. This work is intended to lay down the basis of further investigations, including purification of the highly specific isoform.

Hydroxysteroid sulfotransferase and a specific UDP-glucuronosyltransferase are involved in the metabolism of digitoxin in man

In vitro experiments were performed with cytosolic and microsomal fractions of human liver specimens in order to investigate which enzyme forms of sulfotransferase (ST) and UDP-glucurosyltransferase (GT) are involved in the metabolism of digitoxin (dt-3) and/or its cleavage products. It was found that the cytosolic STs preferentially react with digitoxigenin (dt-0) whereas microsomal GTs conjugate digitoxigenin-monodigitoxoside (dt-1) and in traces the bisdigitoxoside (dt-2). Dt-3 and dt-0 cannot be glucuronidated. By separation of different sulfotransferases it was found that the hydroxysteroid-ST is responsible for dt-0 and 3-epidigitoxigenin (epi-dt-0) sulfation. The hydroxysteroid-ST could be purified and characterized (apparent Km and Vmax for dt-0 sulfation: approx. 17 mumol/l and 2.7 nmol/min mg protein, respectively). Of various model substrates and endogenous compounds (steroids, bilirubin) none caused a competitive inhibition of the microsomal dt-1 glucuronidation except dt-2 and dt-3. Therefore it can be supposed that a new GT form catalyses this reaction. It is characterized by an extraordinarily high affinity towards dt-1 with Km values ranging between 0.7 and 27 mumol/l.

Male rat hepatic UDP-glucuronosyltransferase activity toward thyroxine. Activation and induction properties--relation with thyroxine plasma disappearance rate

Detergent-activation of UDP-glucuronosyltransferase (UGT) isoenzyme(s) involved in thyroxine (T4) glucuronidation in control, phenobarbital (PB)- and 3-methylcholanthrene (3-MC)-treated rats showed that between the four tested detergents, i.e. Triton X-100, Brij 58, Lubrol Px and 3-(3-cholamidopropyl)dimethylammonio-1-propanesulfonic acid (CHAPS), optimal activation of T4 UGT was displayed by the zwitterion CHAPS. "Native" versus optimal detergent-activated T4 UGT activity determination revealed that the latency of T4 UGT in microsomes from 3-MC-treated rats was decreased while the latency of T4 UGT in microsomes from PB-treated rats was increased compared to control, and suggest that the UGT isoenzyme(s) involved in the hepatic glucuronidation of T4 is (are) different in PB-treated rats than in 3-MC-treated rats. After a 7-day treatment with 20 mg/kg 3-MC, the activity of T4 UGT was increased 5-fold when determined in "native" and 4-fold when determined in optimal detergent-activated microsomes compared to controls. After a 7-day treatment with 75 mg/kg PB, T4 UGT was equivalent to the control when determined in "native", and increased 1.3-fold when determined in optimal detergent-activated microsomes. The results thus extend evidence that both 3-MC and PB induce the synthesis of UGT protein(s) involved in the glucuronidation of T4, 3-MC being a strong and PB a weak inducer. Hyperthyroid and hypothyroid status, achieved respectively by a 7-day treatment with 100 microns/kg T4 or a 7-day treatment with 10 mg/kg of one of the antithyroid drugs propylthiouracile or methymazole, did not modify T4 UGT activity, suggesting that the isoenzyme(s) conjugating T4 in microsomes from control rats is (are) unlikely to be either 4-nitrophenol or bilirubin UGT isoenzymes. After 14 days of treatment with 75 mg/kg PB, the hepatic glucuronidation rate of T4 was not different from the control when enzyme activity was expressed per mg microsomal protein but was significantly increased 1.4-fold when expressed per whole liver. A significant (1.5-fold) increase in the 125I-T4 plasma elimination rate was also observed in PB-treated rats compared to controls. A strong (3.6-fold) increase in the T4 glucuronidation rate was observed in rats treated with 5 mg/kg 3-MC for 14 days while the 125I-T4 plasma elimination rate was equivalent to the controls. These results demonstrate that there is no direct relation between T4 UGT activity (and subsequent biliary secretion of T4-glucuronides) and T4 plasma clearance and suggest an important contribution of the intestinal exchangeable thyroid hormone pool to the maintenance of blood thyroid hormone levels.

Metabolism of digoxin, digoxigenin digitoxosides and digoxigenin in human hepatocytes and liver microsomes

In vitro metabolism of digoxin and its cleavage-related compounds was investigated using hepatocytes in primary culture and microsomal fractions both isolated from human livers. On these models, digoxin (DG3) and digoxigenin bisdigitoxoside (DG2) were not shown to be significantly metabolized in vitro in man. Therefore, it appeared that the stepwise cleavage of DG3 and DG2 sugars was not cytochrome P450 dependent. This enzymatic system probably plays a minor role in humans for this particular reaction. However, digoxigenin monodigitoxoside (DG1) and digoxigenin (DG0) which are known to be formed after intra-gastric hydrolysis of DG3, were extensively converted to polar compounds (mainly glucuronides). In addition, using human liver microsomes, a wide variability in UDP-glucuronyl transferase (UDPGT) activities responsible for DG1 glucuronidation was demonstrated. These results suggest that two main factors may contribute to the overall interindividual variability of digoxin biotransformation: 1), the individual intra-gastric pH which influences the sugar cleavage leading to DG1 and DG0; ii), a variability in the level of the hepatic UDPGT specific for digitalis compounds conjugation.

Purification of a rat liver cytosolic sulfotransferase responsible for the conjugation of digitoxigenin

Previous investigations on the digitoxin metabolism hardly considered the role of the sulfate ester conjugation. Therefore, this study examined whether digitoxin (dt-3) or one of its cleavage products might be sulfated in vitro. It was proven that digitoxigenin (dt-0) is by far the best substrate for the cytosolic sulfotransferases (ST). Digitoxigenin-monodigitoxoside (dt-1) and digitoxigenin-bisdigitoxoside (dt-2) are sulfated in trace amounts whereas dt-3 is not sulfated at all. The purification of the responsible enzyme was performed by liquid chromatography on Q-Sepharose and hydroxyapatite. During the purification procedure this enzymatic activity corresponded exactly to that towards dehydroepiandrosterone (DHEA). The 134-fold purified and gel electrophoretically homogeneous enzyme protein (Mr 33,000) showed a Vmax of 12.5 nmoles dt-0 sulfate/min mg protein and a KM of 37 mumol/L. The purified enzyme conjugated dt-1 and dt-2 in trace amounts only and was inhibited competitively by DHEA. It can be concluded that in the rat a 3 beta-hydroxy-steroid sulfotransferase is responsible for the sulfation of dt-0. The purified enzyme reacts with dt-1, dt-2 and digoxigenin (dg-0) in traces only, a sulfation of dt-3 is not detectable.

The enantioselective glucuronidation of morphine in rats and humans. Evidence for the involvement of more than one UDP-glucuronosyltransferase isoenzyme

The formation of morphine glucuronides is enantio- and regioselective in rats and humans. In rat liver microsomes, natural (-)-morphine formed only the 3-O-glucuronide, whereas the unnatural (+)-morphine formed glucuronides at both the 3-OH and 6-OH positions, with the 6-O-glucuronide being the principal product. In human liver microsomes, both the 3-OH-and 6-OH positions were glucuronidated with each of the enantiomers, with the 3-O-glucuronide being the major product with (-)-morphine, and the 6-OH position preferred with the (+)-enantiomer. By using a series of biochemical and biological situations such as induction by xenobiotics, ontogeny, selective inhibition and genetic deficiencies, which are considered to be diagnostic of UDP-glucuronosyltransferase heterogeneity, we determined that two UDP-glucuronosyltransferase isoenzymes were responsible for the glucuronidation of morphine in rat liver. One isoenzyme (the so-called "morphine UDP-glucuronosyltransferase") was responsible for the glucoronidation at the (-)-3-OH and (+)-6-OH positions of morphine, whereas the other formed only the (+)-morphine-3-glucuronide. Evidence from enzyme induction and the genetically deficient deficient Gunn rat suggested that bilirubin UDPGT may be responsible for the (+)-morphine-3-UDP-glucuronosyltransferase activity. In human kidney, glucuronidation of both (-)- and (+)-enantiomers at the 6-OH position was deficient, whereas the activity at the 3-OH positions was still present, which indicated the involvement of two UDP-glucuronosyltransferases in the glucuronidation of morphine in man, as well as rats.

Studies on the pregnenolone-16 alpha-carbonitrile-inducible form of rat liver microsomal cytochrome P-450 and UDP-glucuronosyltransferase

Treatment of rats with pregnenolone-16 alpha-carbonitrile (PCN) markedly induces rat liver microsomal cytochrome P-450p and UDP-GT-dt1, a glucuronosyltransferase active towards the digitoxin metabolite, digitoxigenin monodigitoxoside. The present study characterizes the regulation of these two enzymes in rats treated with different xenobiotics. Like PCN, treatment of rats with dexamethasone, spironolactone, troleandomycin or erythromycin estolate markedly induced both UDP-GT-dt1 and cytochrome P-450p (measured as erythromycin demethylase and testosterone 2 beta-, 6 beta-, 15 beta-, and 18-hydroxylase activities). However, compared to PCN and dexamethasone, both troleandomycin and erythromycin estolate preferentially induced cytochrome P-450p, whereas spironolactone preferentially induced UDP-GT-dt1. Treatment of rats with the polychlorinated biphenyl mixture, Aroclor 1254, increased both cytochrome P-450p and UDP-GT-dt1 activity to about 40% of that in liver microsomes from rats induced with PCN or dexamethasone. Treatment of rats with phenobarbital or chlordane caused a relatively small increase in cytochrome P-450p and UDP-GT-dt1 activity. Neither enzyme was induced by treatment of rats with 3-methylcholanthrene, rifampin or digitoxin. The induction of cytochrome P-450p and UDP-GT-dt1 by PCN followed similar dose-response curves. Although cytochrome P-450p and UDP-GT-dt1 are differentially affected by the age and the sex of rats, the enzymes responded similarly, but not identically, to xenobiotic treatment. This suggests that cytochrome P-450p and UDP-GT-dt1 are co-inducible but not coordinately regulated.

Evidence for a digitoxin conjugating UDP-glucuronosyltransferase in the dog

Liver microsomes of male Beagle dogs contain a form of UDP-glucuronyltransferase which is capable of conjugating digitoxin and its cleavage products digitoxigenin-bisdigitoxoside and digitoxigenin-monodigitoxoside. The highest reaction rates (Vmax 236 pmoles/mg microsomal protein min) were found for digitoxin and digitoxigenin-monodigitoxoside whereas the lowest Km was obtained for digitoxigenin-bisdigitoxoside (29 microM). Digoxin cannot be glucuronidated and digitoxigenin is glucuronidated only in traces. The result may explain the fast digitoxin elimination in dogs. Mutual induction experiments utilizing cardenolides and model substrates of UDP-glucuronyltransferase result in the conclusion that a specific form of UDP-glucuronyltransferase is responsible for glucuronidating digitoxigenin glycosides.

Digitoxin metabolism by liver microsomal cytochrome P-450 and UDP-glucuronosyltransferase and its role in the protection of rats from digitoxin toxicity by pregnenolone-16 alpha-carbonitrile

The aim of the present study was to investigate whether the mechanism by which pregnenolone-16 alpha-carbonitrile (PCN) protects rats from digitoxin toxicity was dependent on the induction of liver microsomal cytochrome P-450p and/or the UDP-glucuronosyltransferase active toward digitoxigenin monodigitoxoside (UDP-GT-dt1). Evidence is presented that suggests troleandomycin is a selective inhibitor of cytochrome P-450p in vivo, based on the pattern of inhibition observed when zoxazolamine paralysis time and hexobarbital sleeping time were measured in rats treated with different cytochrome P-450 inducers. A single dose of troleandomycin completely reversed the ability of PCN to protect rats from digitoxin toxicity, establishing the importance of cytochrome P-450p induction in the protective effect of PCN. The postpubertal decline in constitutive cytochrome P-450p levels in female but not male rats was paralleled by a female-specific, age-dependent decline in the rate of digitoxin sugar cleavage (i.e., digitoxosyl oxidation of digitoxin to 15'-dehydrodigitoxin and digitoxosyl cleavage to digitoxigenin bisdigitoxoside). This resulted in a marked sex difference in the rate of digitoxin sugar cleavage catalyzed by liver microsomes from mature rats (male/female approximately 6). However, no sex difference in digitoxin toxicity was observed in either immature or mature rats. In contrast to cytochrome P-450p, liver microsomal UDP-GT-dt1 activity increased dramatically with age in both male and female rats (mature/immature approximately 10). However, no age differences in digitoxin toxicity were observed in rats of either sex. The results indicate that cytochrome P-450p and UDP-GT-dt1 can be independently regulated in rat liver and that large changes in the constitutive levels of these microsomal enzymes have no effect on digitoxin toxicity. This suggests that the induction of cytochrome P-450p and UDP-GT-dt1 does not fully account for the mechanism by which PCN protects rats from digitoxin toxicity.