Phenobarbital induction of cytochrome P-450 and UDP-glucuronosyltransferase in rabbit liver plasma membranes

Cellular asymmetric catalysis by UDP-glucuronosyltransferase 1A8 shows functional localization to the basolateral plasma membrane

UDP-glucuronosyltransferases (UGTs) are highly expressed in liver, intestine and kidney, and catalyze the glucuronic acid conjugation of both endogenous compounds and xenobiotics. Using recombinant human UGT isoforms, we show that glucuronic acid conjugation of the model substrate, (-)-epicatechin, is catalyzed mainly by UGT1A8 and UGT1A9. In HepG2 cells, pretreatment with polyunsaturated fatty acids increased substrate glucuronidation. In the intestinal Caco-2/HT29-MTX co-culture model, overall relative glucuronidation rates were much higher than in HepG2 cells, and (-)-epicatechin was much more readily conjugated when applied to the basolateral side of the cell monolayer. Under these conditions, 95% of the conjugated product was effluxed back to the site of application, and none of the other phase 2-derived metabolites followed this distribution pattern. HT29-MTX cells contained >1000-fold higher levels of UGT1A8 mRNA than Caco-2 or HepG2 cells. Gene expression of UGT1A8 increased after treatment of cells with docosahexaenoic acid, as did UGT1A protein levels. Immunofluorescence staining and Western blotting showed the presence of UGT1A in the basal and lateral parts of the plasma membrane of HT29-MTX cells. These results suggest that some of the UGT1A8 enzyme is not residing in the endoplasmic reticulum but spans the plasma membrane, resulting in increased accessibility to compounds outside the cell. This facilitates more efficient conjugation of substrate and is additionally coupled with rapid efflux by functionally associated basolateral transporters. This novel molecular strategy allows the cell to carry out conjugation without the xenobiotic entering into the interior of the cell.

Structural identification of novel glucoside and glucuronide metabolites of (-)-epigallocatechin-3-gallate in mouse urine using liquid chromatography/electrospray ionization tandem mass spectrometry

(-)-Epigallocatechin-3-gallate (EGCG), the most abundant and most biologically active polyphenolic compound in tea, has been proposed to have many health beneficial effects. The metabolic fate of EGCG, however, is not well understood. In the present study, we identified a novel EGCG metabolite, 7-O-beta-D-glucopyranosyl-EGCG-4''-O-beta-D-glucupyranoside, in a mouse urine sample using liquid chromatography/electrospray ionization tandem mass spectrometry. The structure of this metabolite was confirmed by analyzing the MSn (n = 1-4) spectra as well as comparing the MS/MS spectra of its product ions with those from EGCG and EGCG-4''-O-beta-D-glucupyranoside standards. To our knowledge, this is the first report of the identification of a glucoside metabolite of EGCG in mammals. Our results indicate that glucosidation represents a novel pathway in the metabolism of EGCG in mice.

Immune response to lentiviral bilirubin UDP-glucuronosyltransferase gene transfer in fetal and neonatal rats

Gene therapy for inherited disorders might cause an immune response to the therapeutic protein. A solution would be to introduce the gene in the fetal or neonatal period, which should lead to tolerization. Lentiviral vectors mediate long-term gene expression, and are well suited for gene therapy early in development. A model for fetal or neonatal gene therapy is the inherited disorder of bilirubin metabolism, Crigler-Najjar disease (CN). The absence of bilirubin UDP-glucoronyltransferase (UGT1A1) activity in CN patients causes high serum levels of unconjugated bilirubin and brain damage in infancy. CN is attractive for the development of gene therapy because the mutant Gunn rat closely mimics the human disease. Injection of UGT1A1 lentiviral vectors corrected the hyperbilirubinemia for more than a year in rats injected as fetuses and for up to 18 weeks in rats injected the day of birth. UGT1A1 gene transfer was confirmed by the presence of bilirubin glucuronides in bile. All animals injected with UGT1A1 lentiviral vectors developed antibodies to UGT1A1. Animals injected with green fluorescent protein (GFP) lentiviral vectors did not develop antibodies to GFP. Our results indicate that fetal and neonatal gene therapy with immunogenic proteins such as UGT1A1 does not necessarily lead to tolerization.

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.

Differential time-course of induction of rat liver gamma-glutamyltransferase and drug-metabolizing enzymes in the endoplasmic reticulum, Golgi and plasma membranes after a single phenobarbital injection. Evaluation of protein variations by two-dimensional electrophoresis

This study was conducted to follow as a function of time the activity of gamma-glutamyltransferase in the various membranes of rat liver cells after a single dose of phenobarbital (PB) (75 mg kg-1 body weight). Gamma-glutamyltransferase induction was maximal 24 h after PB treatment in both the rough endoplasmic reticulum and the plasma membranes. This pattern of induction differed from that of some drug metabolizing enzymes. While total cytochrome P-450 content was enhanced mainly in endoplasmic reticulum until 48 h after PB treatment, UDP-glucuronosyltransferase activity was not greatly altered by PB under the same conditions. The comparison of two-dimensional electrophoretic polypeptide profiles of each subcellular membrane isolated from control and phenobarbital-treated rats revealed important variations induced by PB. In plasma membranes, the heaviest subunit (apparent Mr = 60 x 10(3)) of hepatic gamma-glutamyltransferase was provisionally identified as a collection of polypeptide which differ only by their pI. The concentration of these polypeptides was smaller in the endoplasmic reticulum where they were of lower apparent molecular mass. This suggests that the gamma-glutamyltransferase precursor is already processed at the level of the endoplasmic reticulum but it is still not completely mature or glycosylated. Five days of continuous PB treatment induced by appearance of new gamma-glutamyltransferase isoforms in plasma membranes. We demonstrate that after a single injection of PB, gamma-glutamyltransferase activity increases simultaneously with some drug-metabolizing enzymes, such as total cytochrome P-450 but not with others, such as UDP-glucuronosyltransferases.

The activity of 1-naphthol-UDP-glucuronosyltransferase in the brain

Cerebral microsomes catalysed efficiently the glucuronidation of 1-naphthol, this formation of glucuronide being activated by treatment with Triton X-100 or digitonin. Activated microsomes from the brain of the rat conjugated 1-naphthol with an apparent Km of 95 microM and a Vmax of 5.47 nmol/hr mg protein at 30 degrees C. Microsomal uridine diphosphate (UDP)-glucuronosyltransferase activity in brain towards 1-naphthol was not significantly induced by pretreatment of animals with 3-methylcholanthrene or phenobarbital. These data suggest that UDP-glucuronosyltransferases in brain are different from the hepatic enzymes with regard to biochemical parameters and in response to inducers of drug metabolism. The hepatic UDP-glucuronosyltransferase deficiency in Gunn rats was also observed in the brain.

Heterogeneity of hepatic microsomal UDP-glucuronosyltransferases activities: use and comparison of differential inductions in some mammalian species

1. The co-injection in rats of the inducers 3-methylcholanthrene or phenobarbital and of a protein synthesis inhibitor (cycloheximide) shows that two clusters of hepatic UDP-Glucuronosyltransferases (GT1 and GT2) are under separate genomic expression and differentially regulated. 2. The administration of cycloheximide alone even suggests a distinct turn-over for these two groups of isoenzymes. 3. Indirect evidence for a UDPGT isoform specialized for some structurally-related exogenous substrates, the monoterpenoid alcohols, is brought. Their conjugation exhibits a small deficiency and a marked response to phenobarbital treatment in the Gunn rat and an exclusive inducibility by phenobarbital in the guinea-pig.

Distribution of UDPglucuronosyltransferase in rat tissue

UDPglucuronosyltransferase [UDPglucuronate beta-D-glucuronosyltransferase (acceptor-unspecific), EC 2.4.1.17] is a group of enzymes with distinct but partially overlapping substrate specificity. A rabbit antiserum raised against one purified rat liver UDPglycuronosyltransferase isoform was specific for UDPglucuronosyltransferase and recognized all transferase isoforms by immunodiffusion or immunotransblot analysis. The transferase activity toward all substrates was immunoabsorbed from solubilized rat liver microsomes by IgG purified from the antiserum. The purified IgG was used for immunocytochemical localization of UDP-glucuronosyltransferase in rat liver, jejunum, kidney, and adrenal gland. In the liver, UDPglucuronosyltransferase was present exclusively in hepatocytes and was uniformly distributed within all zones of the hepatic lobule. In the jejunum, the transferase was present exclusively in the epithelial cells and showed a progressive increase in concentration from the crypt to the villar tip. In the kidney, the greatest concentration of the transferase was observed in the epithelial cells of the proximal convoluted tubule. Adrenal medullary cells showed intense immunocytochemical staining; the zona glomerulosa and the zona reticularis of the adrenal cortex were more intensely stained than the zona fasciculata. By light microscopy, UDPglucuronosyltransferase was found in the endoplasmic reticulum and nuclear envelope of all the four organs; this was confirmed in the hepatocyte by electron microscopy. The transferase was not observed in mitochondria, Golgi apparatus, lysosomes, peroxisomes, and plasma membrane, even after 3- to 4-fold induction of various substrate-specific UDPglucuronosyltransferase activities.

Kinetic properties of UDP-glucuronosyltransferase(S) in different membranes of rat liver cells

Glucuronidation of 4-nitrophenol, borneol and morphine occurred in rough and smooth endoplasmic reticulum, Golgi apparatus and plasma membranes of rat liver cells. In all fractions, prior fixation of either substrate (UDP-glucuronic acid or the aglycone) enhanced the affinity for the second substrate. Whatever the membrane, glucuronidation of 4-nitrophenol was characterized by high Vmax and high affinity for UDP-glucuronic acid. On the other hand, glucuronidation of borneol exhibited a lower Vmax and a lower affinity for UDP-glucuronic acid. In the endoplasmic reticulum, conjugation of morphine had a low Vmax, but the enzyme had high affinities for both UDP-glucuronic acid and the aglycone.

Ethanol as an inducer of UDP-glucuronyltransferase: a comparison with phenobarbital and 3-methylcholanthrene induction in rabbit hepatic microsomes

In this report we have examined the ability of ethanol to induce UDP-glucuronyltransferase activity in male rabbits using p-nitrophenol as substrate. The rabbit was found to be an excellent species for studies of ethanol induction since almost 3-fold increases in activity were observed relative to controls. Ethanol induction of p-nitrophenol UDP-glucuronyltransferase activity was even greater than induction by 3-methylcholanthrene, the prototypic inducer of this type of activity. Thus, the rabbit shows promise for studies of UDP-glucuronyltransferase isozymes that are induced by chronic ethanol consumption.