Kinetic properties of the rat intestinal microsomal 1-naphthol:UDP-glucuronosyl transferase. Inhibition by UDP and UDP-N-acetylglucosamine

The kinetic properties of the rat intestinal microsomal 1-naphthol:UDPglucuronosyltransferase (EC 2.4.1.17) were investigated in fully activated microsomes prepared from isolated mucosal cells. The enzyme appeared to follow an ordered sequential bireactant mechanism in which 1-naphthol and UDP-glucuronic acid (UDPGlcUA) are the first and second binding substrates and UDP and 1-naphthol glucuronide the first and second products, respectively. Bisubstrate kinetic analysis yielded the following kinetic constants: Vmax = 102 +/- 6 nmol/min per mg microsomal protein, Km (UDPGlcUA) = 1.26 +/- 0.10 mM, Km (1-naphthol) = 96 +/- 10 microM and Ki (1-naphthol) = 25 +/- 7 microM. The rapid equilibrium random or ordered bireactant mechanisms, as well as the iso-Theorell-Chance mechanism, could be excluded by endproduct inhibition studies with UDP.UDP-N-acetylglucosamine (UDPGlcNAc), usually found to be an activator of UDP glucuronosyltransferase in liver microsomes, acted as a full competitive inhibitor towards UDPGlcUA in rat intestinal microsomes. With regard to 1-naphthol UDPGlcNAc exhibited a dual effect: both inhibition and activation was observed. The effect of activation by MgCl2 and Triton X-100 on the kinetic constants and the inhibition patterns of UDP and UDPGlcNAc were investigated. The results obtained suggest that latency in rat intestinal microsomes may be due to endproduct inhibition by UDP. This endproduct inhibition could be abolished by in vitro treatment with MgCl2 and Triton X-100.

Separation, purification and characterization of three isoenzymes of UDP-glucuronyltransferase from rat liver microsomes

Three isoenzymes of UDP-glucuronyltransferase (UDPGT) have been separated and purified from liver microsomes of untreated female rats or female rats pretreated with 3-methylcholanthrene. The UDPGT isoenzymes were purified utilizing Chromatofocusing, column isoelectric focusing, and UDP-hexanolamine Sepharose 4B affinity chromatography. UDPGT activities could also be separated during UDP-hexanolamine affinity chromatography by elution with different UDPGA (UDP-glucuronic acid) concentrations. One isoenzyme exhibits a subunit molecular weight of 56,000 and is capable of conjugating p-nitrophenol, 1-naphthol, and 4-methylumbelliferone. This isoenzyme is inducible by 3-methylcholanthrene treatment and requires high UDPGA concentrations for elution from the UDP-hexanolamine affinity column in contrast to the other UDPGT isoenzymes. A second isoenzyme was purified and displayed a subunit molecular weight of 50,000. This isoenzyme was not induced by 3-methylcholanthrene and was active towards testosterone, the 17-OH position of beta-estradiol, p-nitrophenol, and 1-naphthol. A third isoenzyme was also purified and exhibited a subunit molecular weight of 52,000. This isoenzyme conjugated androsterone and etiocholanolone and was not induced by 3-methylcholanthrene treatment. This study reports the purification of two separate and distinct rat liver UDPGT isoenzymes capable of conjugating p-nitrophenol, only one of which is inducible by 3-methylcholanthrene treatment. Also, this is the first report of the purification of a UDPGT isoenzyme active towards the 3-OH position of androgens.

The effect of the tumor promoter, phorbol myristate acetate (PMA), on hyaluronic acid (HA) synthesis by chicken embryo fibroblasts

Treatment of chicken embryo fibroblasts (CEF) with the tumor promoter, phorbol myristate acetate (PMA), resulted in a rapid increase in their ability to synthesize the glycosaminoglycan, hyaluronic acid (HA), whereas the parent compound, phorbol, had no effect. CEF cultures incubated with PMA (100 ng/ml) for 6 h resulted in a 15-fold increase in HA synthetase activity compared with phorbol-treated control cultures. The incorporation of [3H]acetate into HA and chemical determination of this polymer also demonstrated increased synthesis of HA by cells treated with PMA. We have previously shown that CEF infected with a temperature-sensitive mutant of Rous sarcoma virus, LA24, exhibit increased synthesis of HA upon transformation. PMA treatment of cells transformed with RSV-LA24 results in a further 1.5-fold stimulation of HA synthesis. These data indicate that PMA causes an increased synthesis of HA in CEF which is analogous to the increased synthesis of HA found in virally transformed CEF.

Identification of increased amounts of UDP-glucuronyltransferase protein in phenobarbital-treated chick-embryo liver cells

UDP-glucuronyltransferase activity of neonatal-chick liver or phenobarbital-treated chick-embryo liver catalysed the glucuronidation of 1-naphthol, 4-nitrophenol and 2-aminophenol. Only low transferase activity towards testosterone was detected, and activity towards bilirubin was not detectable. Liver microsomal transferase activity towards the three phenols was increased approx. 20-50-fold by phenobarbital treatment of chick embryos or by transfer of liver cells into tissue culture. A single form of UDP-glucuronyltransferase, which appears to catalyse the glucuronidation of these three phenols, was purified to near homogeneity from phenobarbital-treated chick-embryo liver microsomal fraction for the first time. The use of this purified enzyme as a standard protein facilitated the identification of this protein in chick-embryo liver microsomal fraction. Further, the accumulation of this microsomal protein was observed following phenobarbital treatment of chick embryos and during tissue culture of chick-embryo liver cells. The value of this model system for the study of the induction of UDP-glucuronyltransferase by drugs and hormones is discussed.

Purification of a form of mouse liver UDP glucuronosyltransferase which glucuronidates androgens

A form of UDP glucuronosyltransferase active in the glucuronidation of the androgens, testosterone, androsterone and dihydrotestosterone has been purified to apparent homogeneity as judged by sodium dodecylsulfate polyacrylamide gel electrophoresis from the livers of phenobarbital-treated C57BL/6N mice. This UDP glucuronosyltransferase is inactive towards estrone as substrate. Data from chromatofocusing and purification experiments suggest that testosterone and androsterone are glucuronidated primarily by this enzyme form and to a lesser extent by an enzyme form which has a slightly higher isoelectric point. However, this major form is only responsible for about half the capacity to glucuronidate dihydrotestosterone.

A comparison of the kinetic properties of two different forms of microsomal UDPglucuronyltransferase

Two forms of UDPglucuronyltransferase (EC 2.4.1.17) have been purified from microsomes of pig liver. One form is free of phospholipids and the other contains a small amount of residual phospholipids. Each form, however, is responsive to activation on addition of purified phospholipids. Comparison of kinetic properties of these enzymes, after reconstitution with identical phospholipid environments, indicate that these are unique functional forms of UDPglucuronyltransferase. The two differ by as much as 100-fold in their rates of conjugation at Vm of p-nitrophenol. Relative rates of glucuronidation of a variety of phenolic aglycones are different for the two enzymes, which suggests different reaction mechanisms. The energetic basis for binding of UDP-glucuronic acid to the active sites is different for the two forms of UDPglucuronyltransferase. Moreover, one form, but not the other, binds Mn2+, which leads to modulation of kinetic properties.

Electroimmunochemical quantification of UDP-glucuronosyltransferase in rat liver microsomes

Microsomal UDPglucuronosyltransferase(1-naphthol), an enzyme form previously shown to be selectively inducible in rat liver by 3-methylcholanthrene-type inducers, was purified to apparent homogeneity. Rabbit antibodies against this enzyme form precipitated UDPglucuronosyltransferase activities towards 1-naphthol and 4-methylumbelliferone faster and to greater extents than enzyme activities towards bilirubin, oestrone and 4-hydroxybiphenyl. Ouchterlony double-diffusion analysis showed immunochemical similarity of the rat liver enzyme with the enzymes from other organs of the rat (kidney, testes) and the mouse liver but not with the enzyme from cat and human liver. Electroimmunochemical quantification of the enzyme indicated that its level was enhanced 1.3-fold and 2.5-fold in liver microsomes from phenobarbital-treated and 3-methylcholanthrene-treated rats, respectively. The results indicate that 3-methylcholanthrene treatment increases the enzyme level of rat liver microsomal UDPglucuronosyltransferase(1-naphthol). Despite phospholipid-dependence of its catalytic activity microsomal enzyme activity appears to be a good index of the enzyme level.

Isolation and characterization of rat liver microsomal UDP-glucuronosyltransferase activity toward chenodeoxycholic acid and testosterone as a single form of enzyme

Microsomal UDP-glucuronosyltransferase activity toward chenodeoxycholic acid and testosterone has been isolated from rat liver and appears to be homogeneous in sodium dodecyl sulfate gel electrophoresis and polyacrylamide gradient gel electrophoresis. The conjugating activities toward chenodeoxycholic acid and testosterone co-purified and showed identical mobilities in disc gel electrophoresis, indicating that chenodeoxycholic acid and testosterone are glucuronidated by a single form of enzyme. UDP-glucuronosyltransferase activities toward estrone, bilirubin, 4-nitrophenol, and morphine did not co-purify with the activity toward chenodeoxycholic acid and testosterone and were not detectable in the pure enzyme in the presence or absence of phospholipids. In addition to glucuronic acid conjugation, the enzyme is able to catalyze galacturonic acid conjugation of chenodeoxycholic acid and testosterone. The enzyme has a subunit Mr approximately 54,000 and in the presence of the stabilizing detergent chenodeoxycholic acid, it appears to exist as an aggregate with an apparent Mr = 318,000 as estimated by gel filtration and 316,000 by polyacrylamide gradient gel electrophoresis.

A procedure for the rapid separation and purification of UDP-glucuronosyltransferases from rabbit liver microsomes

A technique has been developed which rapidly separates and purifies UDP-glucuronosyltransferases from liver microsomes of untreated rabbits. by use of this method, highly purified estrone and p-nitrophenol UDP-glucuronosyltransferases can be obtained in good yield in about 48 hr. Microsomes were solubilized with the nonionic detergent Emulgen 911 in low ionic strength buffers and applied to a DEAE-cellulose column equilibrated with low ionic strength buffers. UDP-glucuronosyltransferase activities were then eluted in a stepwise fashion with increasing concentration of KCl. Three fractions were studied. The first two fractions contained only estrone UDP-glucuronosyltransferase activity while a third contained p-nitrophenol UDP-glucuronosyltransferase activity. Each fraction was directly applied to a UDP-hexanolamine Sepharose-4B column, which was then washed extensively with KCl, and the transferases were eluted with UDP-glucuronic acid. A method for separating the transferases on the affinity column is presented. Testosterone and morphine could not be conjugated by any of the purified enzymes.

The phospholipid-dependence of UDP glucuronosyltransferase. Purification, delipidation and reconstitution of microsomal enzyme from guinea-pig liver

In order to study the interaction of liver microsomal UDPglucuronosyltransferase and microsomal phospholipids under closely defined conditions, guinea-pig enzyme was purified to homogeneity (as judged by sodium dodecyl sulphate gel electrophoresis) by detergent-solubilisation, salt precipitation, chromatography on DEAE-cellulose and DEAE-Sephadex, and affinity chromatography on UDPglucuronosyl-diaminohexanyl--Sepharose 4B. The purified transferase, which catalysed the glucuronidation of p-nitrophenol with high specific activity, was associated with microsomal phospholipids, and phosphatidylcholine was the major species present; the transferase protein had a subunit molecular weight of about 55 000. The enzyme was almost completely inactivated by delipidation of the protein by hydroxyapatite chromatography and efficient reconstitution of high activity was observed only with fluid (microsomal and egg-yolk) phosphatidylcholines. These results confirm that microsomal UDPglucuronosyltransferase is phospholipid-dependent with a specific requirement for phosphatidylcholine.

The separation and purification of rat liver UDP-glucuronyltransferase activities towards testosterone and oestrone

1. Reconstitution of UDP-glucuronyltransferase preparations with phosphataidylcholine liposomes facilitated the purification of testosterone UDP-glucuronyltransferase. 2. Transferase activity towards testosterone co-purifies with that towards 4-nitrophenol. 3. UDP-glucuronyltransferase activity towards oestrone was separated from that towards testosterone. 4. These results suggest that testosterone and 4-nitrophenol may be glucuronidated by a different form of UDP-glucuronyltransferase from the one glucuronidating oestrone.

UDP-glucuronyltransferase. Report on the workshop conference of the German Society for Clinical Chemistry held on September 27-28, 1977 in Schloss Auel

A Workshop Conference on UDP-glucuronyltransferase was held in order to bring together different investigators specialising in a particular aspect of the enzyme. The intention was to find a basis for a closer cooperation between the different groups and to exchange ideas and methods making it possible to cross the hurdles facing those who work in the field of glucuronidation of endogenous and exogenous substances. The topics which were chosen for the meeting covered the following aspects: methods of determination of UDP-glucuronyltransferase activity; problems relating to the origin of the enzyme and the nature of the substrate; application of assays of UDP-glucuronyltransferase in clinical chemistry; enzyme-substrate interactions and factors influencing the enzymic reaction; the role of UDP-glucuronyltransferase in the entero-hepatic circulation of glucuronides, regulation and induction of UDP-glucuronyltransferase, with a subsection on solubilisation and purification.

Purification of mouse liver UDPglucuronosyltransferase

The present study describes a method for the purification of hepatic microsomal UDPglucuronosyltransferase from two different strains of mice, by a combination of detergent solubilization, ion-exchange chromatography and affinity chromatography. Using this procedure up to 0.5 mg of UDPglucuronosyltransferase could be obtained from 30 g of mouse liver and this enzyme could be purified from as little as 10 g of mouse liver. The molecular weight of the apparently homogeneous preparation was 57 000 +/- 2 000 when determined by sodium dodecyl sulphate polyacrylamide gel electrophoresis. The purified transferase catalysed the glucuronidation of approx. 0.6 mumol 4-nitrophenol min-1. mg-1 protein. Immunochemical analysis of this preparation further indicates that UDPglucuronosyltransferase has been purified to apparent homogeneity.

Purification and partial characterization of rat liver bilirubin glucuronoside glucuronosyltransferase

Bilirubin glucuronoside glucuronosyltransferase (EC 2.4.1.95) converts bilirubin monoglucuronide to bilirubin diglucuronide and is concentrated in plasma membrane-enriched fractions of rat liver homogenates. The enzyme was purified 2,000-fold to homogeneity from rat liver. The pI of the enzyme is 7.9 +/- 0.2. The enzyme has a molecular weight of 160,000 and is an oligomer of 28,000 dalton subunits. Km for purified enzyme was 35 microM and Vmax was 2.2 mumol of bilirubin diglucuronide formed/min/mg of protein. Freshly biosynthesized bilirubin monoglucuronide was injected intravenously into homozygous Gunn rats which had bile duct cannulation. Gunn rats lack UDP-glucuronate glucuronyltransferase activity (EC 2.4.1.17), have normal bilirubin glucuronoside glucuronosyltransferase activity, cannot form bilirubin monoglucuronide in vitro or in vivo, and do not excrete bilirubin glucuronides after intravenous injection of unconjugated bilirubin. Within 1 h, approximately 75% of the injected conjugated bilirubin was recovered in bile, of which 20% consisted of bilirubin diglucuronide. These results indicate that bilirubin glucuronide glucuronosyltransferase catalyzes conversion of bilirubin monoglucuronide to diglucuronide in vivo.

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.

Phospholipid-dependence of oestrone UDP-glucuronyltransferase and p-nitrophenol UDP-glucuronyltransferase

Hepatic UDP-glucuronyltransferase activity was resolved into two fractions, one exhibiting oestrone glucuronyltransferase activity and the other exhibiting p-nitrophenol glucuronyltransferase activity. Hydroxyapatite-column chromatography removed greater than 95% of the phospholipids from both preparations. The partially purified delipidated enzymes were essentially devoid of catalytic activity, but activities were restored by the addition of phospholipids or phosphatidylcholine mixtures containing various saturated and unsaturated fatty acids. Both oestrone and p-nitrophenol glucuronyl-transferase activities were reconstituted to similar degrees with the phosphatidylcholine mixtures. When purified phospholipids were tested, phosphatidylcholine and lysophosphatidylcholine were most effective in restoring activity, whereas phosphatidylethanolamine was the least effective. These results further suggest that oestrone and p-nitrophenol UDP-glucuronyltransferases are dependent on phospholipids for their activity.

Substrate specificity and properties of uridine diphosphate glucuronyltransferase purified to apparent homogeneity from phenobarbital-treated rat liver

1. The purification to homogeneity of stable highly active preparations of UDP-glucuronyltransferase from liver of phenobarbital-treated rats is briefly described. 2. A single polypeptide was visible after sodium dodecyl sulphate/polyacrylamide-gel electrophoresis, of mol.wt.57000. 3. Antiserum raised against the pure enzyme produces a single sharp precipitin line after Ouchterlony double-diffusion analysis. 4. The pure UDP-glucuronyltransferase isolated from livers of untreated and phenobarbital-pretreated rats appears to be the same enzyme. 5. The Km (UDP-glucuronic acid) of the pure enzyme is 5.4 mM. 6. The activity of the pure enzyme towards 2-aminophenol can still be activated 2-3-fold by diethylnitrosamine. 7. UDP-glucose and UDP-galacturonic acid are not substrates for the purified enzyme. 8. The final preparation catalysed the glucuronidation of 4-nitrophenol, 1-naphthol, 2-aminophenol, morphine and 2-aminobenzoate. 9. Activities towards 4-nitrophenol, 1-naphthol and 2-aminophenol were all copurified. The proposed heterogeneity of UDP-glucuronyltransferase is discussed.

Separation of oestrone UDP-glucuronyltransferase and p-nitrophenol UDP-glucuronyltransferase activities

Rabbit liver UDP-glucuronyltransferase activity was resolved into two separate fractions on DEAE-cellulose, one containing most of the transferase activity toward oestrone and the other most of the activity toward p-nitrophenol. These two activities were completely separated by rechromatography of each fraction on a second DEAE-cellulose column.

Studies on the purification of rat liver uridine diphosphate glucuronyltransferase

1. A stable, more highly purified, preparation of UDP-glucuronyltransferase was obtained than previously reported. 2. Enzyme activity towards o-aminophenyl and p-nitrophenyl was increased 43- and 46-fold respectively. 3. The final preparation contains only three staining polypeptide bands visible after sodium dodecyl sulphate/polyacrylamide-gel electrophoresis. 4. The only known major accompanying protein appears to be epoxide hydratase. 5. The purified enzyme activity towards o-aminophenol can still be activated 3 fold by diethylnitrosamine. 6. On evidence from purification, o-aminophenol and p-nitrophenol appear to be glucuronidated by the same enzyme protein. The possible recognition of the UDP-glucuronyltransferase enzyme is discussed.

Purification and properties of microsomal UDP-glucuronosyltransferase from rat liver

p-Nitrophenol conjugating activity associated with liver microsomal UDP-glucuronosyltransferase (EC 2.4.1.17) was purified 150- to 200-fold from cell-free homogenates. The purification scheme included solubilization with the nonionic detergent Lubrol WX, anion exchange chromatography at pH 6.0 and 7.5, and affinity chromatography with UDP-hexanolamine Sepharose 4B. The enzyme purified as a phospholipid-protein complex and was shown to consist of a single polypeptide chain of molecular weight 59,000 by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Amino acid analysis indicated approximately 531 mol of amino acids/59,000 g of enzyme and a molar ratio of nonpolar to polar residues of 1.08. During fractionation, the enzyme displayed instability with such steps as gel filtration, dialysis, or ultrafiltration of dilute samples; however, upon adsorption to ion exchange resins or storage in concentrated form, the enzyme was reasonably stable. The active lipoprotein complex showed both size and charge heterogeneity as judged by gel filtration and electrofocusing. Three forms of the enzyme resolved by isoelectric focusing had isoelectric points which averaged pH 6.68, 6.56, and 6.31. Polypeptide compositions of these electrophoretically distinct phospholipid protein complexes were indistinguishable on the basis of sodium dodecyl sulfate-polyacryl-amide gel electrophoresis, suggesting that the charge heterogeneity may be the result of differences in the phospholipid content of the lipoprotein complex.

Species differences in the conjugation of 4-hydroxy-3-methoxyphenylethanol with glucuronic acid and sulphuric acid

The biosynthesis of the glucuronide and sulphate conjugates of 4-hydroxy-3-methoxyphenylethanol was demonstrated in vitro by using the high-speed supernatant and microsomal fractions of liver respectively. These two conjugates were also produced simultaneously by using the post-mitochondrial fraction of rat, rabbit or guinea-pig liver. In contrast only the glucuronide was synthesized by human liver and only the sulphate by mouse and cat livers. Neither of these conjugates was formed by the kidney or the small or large intestine of the rat. A high sulphate-conjugating activity was observed in mouse kidney; the rate of sulphation of 4-hydroxy-3-methoxyphenylethanol with kidney homogenate and high-speed supernatant preparations was 1.8 times greater than with liver preparations. The sulpho-conjugates of 4-hydroxy-3-methoxyphenylethanol and 4-hydroxy-3-methoxy-phenylglycol were also formed by enzyme preparations of rabbit adrenal and rat brain; the glycol was the better substrate in the latter system. Mouse brain did not possess any sulphotransferase activity. For the conjugation of 4-hydroxy-3-methoxyphenylethanol by rabbit liver, the Km for UDP-glucuronic acid was 0.22 mM and that for Na2SO4 was 3.45 mM. The sulphotransferase has a greater affinity for 4-hydroxy-3-methoxyphenyl-ethanol than has glucuronyltransferase, as indicated by their respective Km values of 0.036 and 1.3 mM. It was concluded that sulphate conjugation of 4-hydroxy-3-methoxyphenylethanol predominates in most species of animals.

Delipidation and reactivation of UDPglucuronosyltransferase from rat liver

UDPglucuronosyltransferase was solubilized by treating Wistar rat liver microsomes with deoxycholate. Chromatography of this preparation on Bio-Gel P-30 resulted in extraction of 92% of phospholipids and complete loss of enzyme activity. UDPglucuronosyltransferase was reactivated by dialysing this delipidated preparation in the presence of lecithin, a mixture of liver microsomal lipids or microsomal preparations from livers of UDPglucuronosyltransferase-deficient Gunn rats. Virtually complete enzyme reactivation was obtained with regard to glucuronidation and glucosidation of bilirubin; however, the inactivation of UDPglucuronosyltransferase with p-nitrophenol as substrate was irreversible. These findings demonstrate that UDPglucuronosyltransferase with bilirubin as substrate is a lipid-requiring enzyme.