Liver Microsomal UDP-glucuronyltransferase Research Articles

Melatonin protects the cytochrome P450 system through a novel antioxidant mechanism

Melatonin, an endogenous hormone, is used as an antioxidant drug in doses quite higher than the endogenous circulating levels of this hormone. Hepatic endoplasmic reticulum contains the cytochrome P450 (CYP450) system, which catalyzes one biotransformation pathway of melatonin; this organelle is also one of the main sources of reactive oxygen species in cells. Therefore, we proposed that the antioxidant activity of this hormone may have a biological relevance in the organelle where it is biotransformed. To evaluate this postulate, we used Fe(3+)/ascorbate, an oxygen free radical generating system that leads to lipid peroxidation, loss of protein-thiol content, and activation of UDP-glucuronyltransferase in rat liver microsomes. We found that mM concentrations of melatonin prevented all these oxidative phenomena. We also found that Fe(3+)/ascorbate leads to structural alterations in the CYP450 monooxygenase, the enzyme that binds the substrate in the CYP450 system catalytic cycle, probably through direct oxidation of the protein, and also inhibited p-nitroanisole O-demethylation, a reaction catalyzed by the CYP450 system. Notably, melatonin prevented both phenomena at microM concentrations. We provide evidence suggesting that melatonin may be oxidized by oxygen free radicals. Thus, we postulate that melatonin may be acting as an oxygen free radical scavenger, and Fe(3+)/ascorbate-modified melatonin would be directly protecting the CYP450 system through an additional specific mechanism. Pharmacological relevance of this phenomenon is discussed.

Copper modifies liver microsomal UDP-glucuronyltransferase activity through different and opposite mechanisms

Treatment of hepatic microsomes with Fe 3+/ascorbate activates UDP-glucuronyltransferase (UGT), a phenomenon totally prevented and reversed by reducing agents. At μM concentrations, iron and copper ions catalyze the formation of ROS through Fenton and/or Haber–Weiss reactions. Unlike iron ions, indiscriminate binding of copper ions to thiol groups of proteins different from the specialized copper-binding proteins may occur. Thus, we hypothesize that incubation of hepatic microsomes with the Cu 2+/ascorbate system will lead to both UGT oxidative activation and Cu 2+-binding induced inhibition, simultaneously. We studied the effects of Cu 2+ alone and in the presence of ascorbate on rat liver microsomal UGT activity. Our results show that the effects of both copper alone and in the presence of ascorbate were copper ion concentration- and incubation time-dependent. At very low Cu 2+ (25 nM), this ion did not modify UGT activity. In the presence of ascorbate, however, UGT activity was increased. At higher copper concentrations (10 and 50 μM), this ion led to UGT activity inhibition. In the presence of ascorbate, 10 μM Cu 2+ activated UGT at short incubation periods but inhibited this enzyme at longer incubation times; 50 μM Cu 2+ only inhibited UGT activity. Thiol reducing agent 2,4-dithiothreitol prevented and reversed UGT activation while EDTA prevented both, UGT activation and inhibition. Our results are consistent with a model in which Cu 2+-induced oxidation of UGT leads to the activation of the enzyme, while Cu 2+-binding leads to its inhibition. We discuss physiological and pathological implications of these findings.

Androgen UDP-glucuronyl transferase activity is found primarily in liver in the rat.

UDP-glucuronyl transferase (UDPGT) activity was determined for androgens in tissue minces and microsomal fractions from the liver and extrahepatic tissues (kidney, skin, prostate, and preputial glands) of the male rat. Liver microsomes showed the highest UDPGT activity with each of the androgens tested (Vmax = 7, 3, and 10 nmol/minute/mg protein for testosterone, androsterone, and androstanediol, respectively). UDPGT activity (Vmax) for androstanediol in the liver was 10(2)-fold higher than in the kidney and 10(3)-fold higher than in the skin and prostate. UDPGT activity for androgens was not detected in microsomes from preputial glands. Furthermore, no body site distribution was found for androgen UDPGT activity in skin microsomes. The Michaelis-Menten constant (Km) for UDPGT in liver microsomes was 20.4, 12.2, and 2.2 microM, respectively, for testosterone, androstanediol, and androsterone. Kidney microsomes showed a Km of 19.4 and 26.9 microM, respectively, for androstanediol and androsterone. The Km for testosterone was very high in the kidney (138 microM), suggesting that it was a poor substrate. In microsomes from the skin and prostate, the Km was very high (range 43-162 microM) for all three androgen substrates, suggesting that these androgens were not the preferred substrates for UDPGT in these tissues. These results indicate that the liver was the main site of androgen UDPGT activity and the skin and prostate formed little, if any, androgen glucuronides. These results suggest that androstanediol glucuronide was formed primarily in the liver and may not be a reliable marker of peripheral androgen metabolism.

Androgen UDP‐Glucuronyl Transferase Activity Is Found Primarily in Liver in the Rat

ABSTRACT: UDP‐glucuronyl transferase (UDPGT) activity was determined for androgens in tissue minces and microsomal fractions from the liver and extrahepatic tissues (kidney, skin, prostate, and preputial glands) of the male rat. Liver microsomes showed the highest UDPGT activity with each of the androgens tested (Vmax = 7, 3, and 10 nmol/minute/mg protein for testosterone, androsterone, and androstanediol, respectively). UDPGT activity (Vmax) for androstanediol in the liver was 102‐fold higher than in the kidney and 103‐fold higher than in the skin and prostate. UDPGT activity for androgens was not detected in microsomes from preputial glands. Furthermore, no body site distribution was found for androgen UDPGT activity in skin microsomes. The Michaelis‐Menten constant (Km) for UDPGT in liver microsomes was 20.4, 12.2, and 2.2 μ, respectively, for testosterone, androstanediol, and androsterone. Kidney microsomes showed a Km of 19.4 and 26.9 μ, respectively, for androstanediol and androsterone. The Km for testosterone was very high in the kidney (138 μ), suggesting that it was a poor substrate. In microsomes from the skin and prostate, the Km was very high (range 43–162 μ) for all three androgen substrates, suggesting that these androgens were not the preferred substrates for UDPGT in these tissues.These results indicate that the liver was the main site of androgen UDPGT activity and the skin and prostate formed little, if any, androgen glucuronides. These results suggest that androstanediol glucuronide was formed primarily in the liver and may not be a reliable marker of peripheral androgen metabolism.

Topological Disposition of UDP-Glucuronyltransferase in Rat Liver Microsomes

The topological disposition of a form of UDP-glucuronyltransferase (called GT-1) in rat liver microsomes was examined. Concanavalin A-Sepharose failed to bind microsomal vesicles even though GT-1 has sugar chains of "high mannose" type, indicating that mannose-containing sugar chains of microsomal glycoproteins including GT-1 are not exposed to the outer surface of microsomal vesicles. Polyclonal antibodies raised against purified GT-1 could bind to microsomal vesicles, indicating that at least part of the GT-1 polypeptide chain is extruded to the outside of the microsomal membrane. Intact microsomal vesicles were digested with carboxypeptidase Y and then subjected to immunoblot analysis using the anti-GT-1 antibodies. It was thus found that the digestion resulted in cleavage of a C-terminal, 2-kDa fragment, leaving a 52-kDa fragment of GT-1 still tightly bound to the membrane. From these results, it is concluded that GT-1 is a transmembrane protein, which extrudes its C-terminal end (at least 2 kDa) to the outside of the membrane, whereas most of its polypeptide chain together with the sugar chains are located on the luminal side of the membrane.

Oltipraz-induced amelioration of acetaminophen hepatotoxicity in hamsters: II. Competitive shunt in metabolism via glucuronidation

Previously, we demonstrated that oltipraz [OTP: 5-(2-pyrazinyl)-4-methyl-1,2-dithiol-3-thione] prevented the hepatotoxicity of acetaminophen (AAP) in hamsters and that the observed protection was not related to increases in hepatic reduced glutathione (GSH) levels. These experiments were designed to elucidate the mechanism of OTP-induced protection with respect to an apparent non-GSH-dependent system. Marked differences in the relative amounts of hepatic GSH content depleted by AAP in control vs OTP-treated hamsters occurred. Urinary recoveries of AAP and metabolites indicated that more AAP-glucuronide was formed at the expense of other major metabolites (AAP-GSH, - N-acetylcysteine, and -sulfate) in OTP-treated hamsters, while plasma toxicokinetic modeling suggested a greater rate of AAP systemic clearance. An increased apparent formation rate constant for AAP glucuronidation (135%), in concert with significantly lower apparent formation rate constants for those metabolites which reflect the production of the reactive intermediate from AAP (glutathione and N-acetylcysteine), provide the rationale for this shift of metabolism. The biochemical basis for metabolic shunting is significantly elevated hepatic UDP-glucuronic acid content, an increased calculated UDP-glucuronic acid synthetic rate, and an increased liver microsomal UDP-glucuronyl transferase activity in OTP-treated animals. These changes in AAP conjugation were concomitant with decreased fractional clearance of AAP via bioactivation and less in vivo AAP covalent binding. These data support the hypothesis that OTP provides a protecting effect from AAP hepatotoxicity due to an augmented and predisposing glucuronidation capacity.

Effect of tocotrienols on hepatocarcinogenesis induced by 2-acetylaminofluorene in rats

The effects of tocotrienols on hepatocarcinogenesis in rats fed with 2-acetylaminofluorene (AAF) were followed morphologically and histologically for a period of 20 wk. No differences between treated and control rats in the morphology and histology of their livers was observed. Cell damage was extensive in the livers of AAF-treated rats but less extensive in the AAF-tocotrienols-treated rats when compared with normal and tocotrienols-treated rats. 2-Acetylaminofluorene significantly increases the activities of both plasma and liver microsomal gamma-glutamyltranspeptidase (GGT) and liver microsomal UDP-glucuronyltransferase (UDP-GT). Tocotrienols administered together with AAF significantly decrease the activities of plasma GGT after 12 and 20 wk (P less than 0.01, P less than 0.002, respectively) and liver microsomal UDP-GT after 20 wk (P less than 0.02) when compared with the controls and with rats treated only with tocotrienols. Liver microsomal GGT also showed a similar pattern to liver microsomal UDP-GT but the decrease was not significant. These results suggest that tocotrienols administered to AAF-treated rats reduce the severity of hepatocarcinogenesis.

Diabetes in female rats; changes in liver microsomal aminopyrine N-demethylase and UDP-glucuronyl transferase activities

Short or long term diabetes in female rats produced remarkable activation of aminopyrine N-demethylation, inhibition of oestrone and p-nitrophenol glucuronidation and no changes in morphine UDP-glucuronyltransferase activity in vitro. Km and Vmax for these reactions were determined. Insulin treatment partially antagonized diabetes activation of aminopyrine N-demethylation: it restored decreased UDP-glucuronyltransferase activities for oestrone and p-nitrophenol only in long term and short term diabetes, respectively. Insulin also markedly inhibited morphine glucuronidation. Triton X-100 also displayed a differential pattern of activation for the glucuronidation reactions in liver microsomes of diabetic rats. Results suggest that diabetes in female rats may increase the actual amount of enzyme protein for aminopyrine metabolism and to decrease that for oestrone and p-nitrophenol.

Induction of liver microsomal UDP-glucuronyltransferase in the rat administered with a plant phenol, eugenol.

UDP-glucuronyltransferase activity toward xenobiotics in rat liver microsomes was increased about 2.6-fold by administration of a eugenol (4-allyl-2-methoxyphenol). Km value of the induced enzyme toward UDP-glucuronic acid, however, did not change. Immunoblotting analysis revealed that the amount of UDP-glucuronyltransferase protein was increased in the microsomes of eugenol-treated rat liver. In vitro translation assay showed that the level of translatable mRNA encoding this enzyme increased in the liver. These results indicate that mRNA specific for production of UDP-glucuronyltransferase has accumulated, presumably by de novo synthesis in response to a plant phenol, eugenol.

Induction of liver microsomal epoxide hydrolase, UDP-glucuronyl transferase and cytosolic glutathione transferase in different rodent species by 2-acetylaminofluorene or 3-methylcholanthrene.

Control activities vary 12-fold for microsomal epoxide hydrolase, two-fold for UDP-glucuronyl transferase and five-fold for cytosolic glutathione (GSH) transferase among the different rodents (rat, hamster, guinea-pig, mouse) examined. For all three enzymes the activities in rat liver are towards the lower values. In these rodents, except for a 100% increase in microsomal epoxide hydrolase in guinea-pig liver, 2-acetylaminofluorene induces the three phase 2 enzymes only in rat. Treatment with 3-methylcholanthrene also produces the largest effects on these three enzyme activities in rat liver; exceptions are its failure to induce microsomal epoxide hydrolase in female rat and the large induction of cytosolic GSH transferase in hamster liver. Quantitatively, hepatic microsomal epoxide hydrolase, UDP-glucuronyl transferase and cytosolic GSH transferase activities, and their inducibility by 2-acetylaminofluorene or 3-methylcholanthrene, in male Sprague-Dawley rats are not representative for other rodent species or even, in all cases, for female rat.

The inducibility and ontogeny of rat liver UDP- glucuronyltransferase toward furosemide

Furosemide (F) conjugation with glucuronic acid is the main pathway of F metabolism in humans and experimental animals. In order to study rat liver microsomal UDP-glucuronyltransferase (UDP-GT) activity towards F we developed an in vitro assay in which the conjugation product, furosemide 1-0-acyl glucuronide (FG) was separated and quantitatively determined by reverse phase high pressure liquid chromatography. The optimal conditions of the reaction were established and the apparent Km for F and UDP-glucuronic acid (UDPGA) were 0.22 and 1.76 mM, respectively. Substrate inhibition of UDP-GT toward F occured at F concentrations higher than 1.5 mM. Developmental changes in F glucuronidation were compared to the ontogeny of UDP GT activity toward two other acceptors, 1-napthol and esterone that are known to have different patterns of maturation. F glucuronidation was 26% of adult activity at 18 days of gestation, reached 48% at birth and gradually increased to 250% of adult activity at 22 days of age. Glucuronidation of 1-napthol and estrone attained 87% and 44% of adult activity at 22 days of gestation, 37% and 66% in six-day-old rats and 100% and 427% of adult activity in 22-day-old rats, respectively. The effect of 3-methylcholantrene (3-MC), phenobarbital (PB) and pregnenolone-16α-carbonitrile (PCN) on F UDP-GT was studied and compared to their effect on 1-napthol and estrone glucuronidation. PB, 3-MC and PCN increased F-UDP-GT activity to 208%, 282% and 342% of vehicle-treated animals, respectively, while F pretreatment did not affect the conjugation of F. In comparison, 1-napthol glucuronidation was preferentially induced by 3-MC (4.4-fold of control) while estrone glucuronidation was induced by PB and PCN (4.9- and 2.5- fold of control, respectively). These studies suggest that several forms of UDP-GT activities, which differ in their ontogeny and inducibility patterns, are involved in the glucuronidation of F in vitro.

Microsomal UDP-glucuronyltransferase-catalyzed bilirubin diglucuronide formation in human liver

Human liver microsomal bilirubin UDP-glucuronyltransferase catalyzes formation of bilirubin mono- and diglucuronide. KmUDPGA and Vmax of the enzyme are 0.6 mM and 1.69 nmol/mg protein X min. In vitro, bilirubin readily dissolves in the microsomal lipid phase. Taking this into account a Kmbilirubin of 60.6 microM was found, which is much higher than the in vivo microsomal UCB concentration of human liver (2.9-11.4 microM). The total capacity of human liver to form bilirubin mono- and diglucuronide in vitro exceeds the in vivo mono- and diglucuronide production rates by a factor 8 to 10. Radiation-inactivation studies reveal that human liver microsomal bilirubin UDP-glucuronyltransferase is a tetrameric enzyme with a molecular mass of 209 000 +/- 20 000 Da. The complete tetrameric enzyme catalyzes both glucuronidation steps, formation of bilirubin monoglucuronide and conversion of mono- to diglucuronide. In its monomeric form, the enzyme with molecular mass of 55 000 +/- 1 500 Da catalyzes only the first step of bilirubin glucuronidation, the formation of bilirubin monoglucuronide.

Evidence that UDP-glucuronyltransferase in liver microsomes at 37 degrees C is in a gel phase lipid environment.

UDP-glucuronyltransferase (EC 2.4.1.17) in intact, untreated microsomes from pig liver is activated by relatively low concentrations of UDP-N-acetylglucosamine. This property is absent after treatment of microsomes with detergents or phospholipases, and also is not a characteristic of pure, delipidated enzyme. Sensitivity to activation by UDP-N-acetylglucosamine was reconstituted, however, by incorporation of pure, delipidated enzyme into unilamellar bilayers of phosphatidylcholine that were in a gel phase. Warming of these bilayers to just above the temperature for the gel to liquid crystalline phase transition led to an abrupt loss of sensitivity of UDP-glucuronyltransferase to activation by UDP-N-acetylglucosamine. These experiments establish that sensitivity to activation by UDP-N-acetylglucosamine is an inherent property of UDP-glucuronyltransferase. The data also suggest that the lipid environment of UDP-glucuronyltransferase in intact, untreated microsomes can modulate the sensitivity of this enzyme to allosteric activation by UDP-N-acetylglucosamine, and that this lipid environment is in a gel phase in intact microsomes at 37 degrees C.

Effects of Sorghum Grain Tannins and Dietary Protein on the Activity of Liver UDP-Glucuronyltransferase

The activity of liver microsomal UDP-glucuronyltransferase (EC 2.4.1.17), an enzyme known to detoxify phenolic compounds, was measured in chicks and rats fed high- (HTS) and low-tannin sorghums (LTS). In an initial investigation, activity was significantly elevated in chicks fed HTS-soybean meal diets over those fed the LTS control diet. Other studies were designed to differentiate between the effects due to tannin and those resulting from a protein deficiency which had previously been reported to increase the activity of this enzyme. In general, only a relatively small part of the increased activity observed by feeding HTS to chicks could be attributed to a tannin-induced protein deficiency. The same phenomenon of elevated activity produced by feeding HTS to chicks was not observed in the rat. These results would suggest that sorghum tannins, or their breakdown products, are absorbed and activating UDP-glucuronyltransferase in the chick, but not the rat.

Purification of rat-liver microsomal UDP-glucuronyltransferase. Separation of two enzyme forms inducible by 3-methylcholanthrene or phenobarbital.

Glucuronidation reactions catalysed by rat liver microsomal UDP-glucuronyltransferase are differentially inducible by 3-methylcholanthrene and phenobarbital. To elucidate the molecular basis of this functional heterogeneity the enzyme was purified from livers of rats pretreated with the inducing agents. Using cholate solubilization, chromatography on Bio-Gel A-1.5m and on DEAE-cellulose in the presence of the nonionic detergent Brij 58, two enzyme forms could be separated. Both forms were subsequently purified to apparent homogeneity by affinity chromatography on UDP-hexanolamine Sepharose 4B, 3-Methylcholanthrene-inducible enzyme activity towards 1-naphthol, 4-nitrophenol, 3-hydroxybenzo(a)pyrene and N-hydroxy-2-naphthylamine copurified with one enzyme form (enzyme 1). In contrast phenobarbital-inducible enzyme activity towards morphine, chloramphenicol and 4-hydroxybiphenyl was associated with the other enzyme fraction (enzyme 2). Sodium dodecylsulfate/polyacrylamide gels showed similar molecular weights of 54000 for enzyme 1 and 56000 for enzyme 2. The results suggest the presence of at least two forms of UDP-glucuronyltransferase in rat liver. Factors affecting enzyme activity in purified and membrane-bound states are discussed.

Activation and induction of rat liver microsomal UDP-glucuronyltransferase with 3-hydroxybenzo(a)pyrene and N-hydroxy-2-naphthylamine as substrates

Activation and induction of rat liver microsomal UDP-glucuronyltransferase with 3-hydroxybenzo(a)pyrene and N-hydroxy-2-naphthylamine as substrates

Formula omitted]-Stilbene oxide: A new inducer of rat liver microsomal UDP-glucuronyltransferase

formula omitted]-Stilbene oxide: A new inducer of rat liver microsomal UDP-glucuronyltransferase

Phospholipid dependence of UDP-glucuronyltransferase

Very extensive hydrolysis of phospholipids with pure Bacillus cereus phospholipase C at 5°C greatly inhibited the maximum demonstrable rate of glucuronidation of p- nitrophenol by UDPglucuronyltransferase in guinea pig liver microsomes. Lysophosphatidylcholine restored much of the inhibited activity but non-phospholipid surfactants or hydrolysis of diglycerides failed to reactivate. Phospholipid depletion likewise inhibited o- aminophenol glucuronidation and phospholipids restored activity. It is concluded that glucuronyltransferase specifically requires phospholipids for optimal activity. It seems unlikely that these phospholipids only serve to dissolve aglycones, or that they are direct physiological regulators of the transferase. Instead, a permissive role is ascribed to phosphlipids, allowing glucuronyltransferase to be regulated by other means.

Determination of microsomal UDP-glucuronyltransferase in needle-biopsy specimens of human liver.

Sensitive, reliable and convenient assays are described for the study of human liver microsomal UDP-glucuronyltransferase. Using 14C-1-naphthol as substrate about 2 mg of a liver biopsy specimen and for 14C-morphine about 20 mg of tissue will suffice for enzyme estimation. Lack of inhibition of 1-naphthol glucuronidation by morphine suggests that the substrates are glucuronidated by different forms of the enzyme. Enzyme levels in the native and activated state were studied in biopsies from patients grouped according to histopathological and clinical criteria. The enzyme assays may help to characterize UDP-glucuronyltransferases in human tissues and their induction by drugs and environmental chemicals, as well as their alteration in various diseases.

The inhibition of morphine: UDP-glucuronyltransferase in rabbit liver microsomes by cyproheptadine

The reactivity of the non-narcotic substances, cyproheptadine and N-desmethylcyproheptadine with morphine: UDP-glucuronyltransferase was studied in rabbit hepatic microsomal preparations. Cyproheptadine produced a potent competitive inhibition of morphine glucuronidation in vitro (K i=0.08 mM) whereas its N-desmethyl derivative was significantly less effective (K i=0.4 mM). No cyproheptadine glucuronide was formed in these reactions suggesting that cyproheptadine acts as a dead-end inhibitor. Results indicate that the mechanism of the inhibition of morphine: UDP-glucuronyltransferase by cyproheptadine is similar to that produced by opioids and is related to the presence of the N-alkyl group in it structure.