The chemical modification of human liver UDP-glucuronosyltransferase UGT1*6 reveals the involvement of a carboxyl group in catalysis

Understanding substrate selectivity of human UDP-glucuronosyltransferases through QSAR modeling and analysis of homologous enzymes

The UDP-glucuronosyltransferase (UGT) enzyme catalyzes the glucuronidation reaction which is a major metabolic and detoxification pathway in humans. Understanding the mechanisms for substrate recognition by UGT assumes great importance in an attempt to predict its contribution to xenobiotic/drug disposition in vivo. Spurred on by this interest, 2D/3D-quantitative structure activity relationships and pharmacophore models have been established in the absence of a complete mammalian UGT crystal structure. This review discusses the recent progress in modeling human UGT substrates including those with multiple sites of glucuronidation. A better understanding of UGT active site contributing to substrate selectivity (and regioselectivity) from the homologous enzymes (i.e. plant and bacterial UGTs, all belong to family 1 of glycosyltransferase (GT1)) is also highlighted, as these enzymes share a common catalytic mechanism and/or overlapping substrate selectivity.

In Vitro Studies in Microsomes from Rat and Human Liver, Kidney, and Intestine Suggest That Perfluorooctanoic Acid Is Not a Substrate for Microsomal UDP-Glucuronosyltransferases

Perfluorooctanoic acid (PFOA) is a fluorinated fatty acid analogue used as a surfactant in the manufacture of fluoropolymers. Previous studies have indicated that PFOA was metabolically inert in mammals, but recent metabolism studies with related fluorochemicals suggested that PFOA might form a glucuronide conjugate. [(14)C(1)]-PFOA was incubated with male and female human and rat liver, kidney, and small intestine microsomes. Incubations were carried out in the presence of alamethicin and beta-saccharolactone to increase access of PFOA to the enzyme active site and to inhibit potential hydrolysis of PFOA-glucuronide by microsomal beta-glucuronidase, respectively. Although positive control experiments using p-nitrophenol demonstrated significant UDP-glucuronosyltransferase (UDPGT) activity in all of the tested microsomal preparations, no evidence for formation of a PFOA-glucuronide was obtained, either by high-sensitivity radiochromatography or by LC/MS. These data suggest that PFOA is not a substrate for human or rodent microsomal UDPGTs.

Identification of aspartic acid and histidine residues mediating the reaction mechanism and the substrate specificity of the human UDP-glucuronosyltransferases 1A

The human UDP-glucuronosyltransferase UGT1A6 is the primary phenol-metabolizing UDP-glucuronosyltransferase isoform. It catalyzes the nucleophilic attack of phenolic xenobiotics on UDP-glucuronic acid, leading to the formation of water-soluble glucuronides. The catalytic mechanism proposed for this reaction is an acid-base mechanism that involves an aspartic/glutamic acid and/or histidine residue. Here, we investigated the role of 14 highly conserved aspartic/glutamic acid residues over the entire sequence of human UGT1A6 by site-directed mutagenesis. We showed that except for aspartic residues Asp-150 and Asp-488, the substitution of carboxylic residues by alanine led to active mutants but with decreased enzyme activity and lower affinity for acceptor and/or donor substrate. Further analysis including mutation of the corresponding residue in other UGT1A isoforms suggests that Asp-150 plays a major catalytic role. In this report we also identified a single active site residue important for glucuronidation of phenols and carboxylic acid substrates by UGT1A enzyme family. Replacing Pro-40 of UGT1A4 by histidine expanded the glucuronidation activity of the enzyme to phenolic and carboxylic compounds, therefore, leading to UGT1A3-type isoform in terms of substrate specificity. Conversely, when His-40 residue of UGT1A3 was replaced with proline, the substrate specificity shifted toward that of UGT1A4 with loss of glucuronidation of phenolic substrates. Furthermore, mutation of His-39 residue of UGT1A1 (His-40 in UGT1A4) to proline led to loss of glucuronidation of phenols but not of estrogens. This study provides a step forward to better understand the glucuronidation mechanism and substrate recognition, which is invaluable for a better prediction of drug metabolism and toxicity in human.

Comparative modelling of the human UDP-glucuronosyltransferases: insights into structure and mechanism

UDP-glucuronosyltranferases (UGTs) affect the disposition of drugs and other xenobiotics by catalysing the conjugation of glucuronic acid to available oxygen, nitrogen, and sulfur atoms. Several related mammalian isoforms of UGT are expressed that have different binding affinities and turnover rates for the substrates they encounter in the liver and other tissues. Because no high-resolution structural information is available to dissect the enzyme-substrate interactions that give rise to different specificities, a search was conducted to find the best available templates to use for comparative protein modelling. Sequence identity analysis was used to identify some recently crystallized plant UGTs as homologues of microsomal UGTs. Because UGTs contain a Rossman fold motif predicted to bind the UDP-containing sugar donor substrate, this consensus sequence was used to aid sequence alignment, as were other conserved residues thought to be involved in catalysis or substrate binding, and the predicted secondary structure. Docking of UDP-glucuronic acid to a model of UGT1A1 resulted in a root mean square deviation of only 0.37 angstroms vs. UDP co-crystallized with the plant UGT71G1 template. The significance of a comparative model generated for UGT1A1 with respect to both the sugar donor and aglycone binding sites, and mechanism, is discussed.

Cytolysin-dependent delay of vacuole maturation in macrophages infected with Listeria monocytogenes

The bacterial pathogen Listeria monocytogenes (Lm) evades the antimicrobial mechanisms of macrophages by escaping from vacuoles to the cytosol, through the action of the cytolysin listeriolysin O (LLO). Because of heterogeneities in the timing and efficiency of escape, important questions about the contributions of LLO to Lm vacuole identity and trafficking have been inaccessible. Expression of cyan fluorescent protein (CFP)-labelled endocytic membrane markers in macrophages along with a yellow fluorescent protein (YFP)-labelled indicator of Lm entry to the cytosol identified compartments lysed by bacteria. Lm escaped from Rab5a-negative, lysosome-associated membrane protein-1 (LAMP1)-negative, Rab7-positive, phosphatidylinositol 3-phosphate [PI(3)P]-positive vacuoles. Lm vacuoles did not label with YFP-Rab5a unless the bacteria were first opsonized with IgG. Wild-type Lm delayed vacuole fusion with LAMP1-positive lysosomes, relative to LLO-deficient Lm. Bacteria prevented from expressing LLO until their arrival into LAMP1-positive lysosomes escaped inefficiently. Thus, the LLO-dependent delay of Lm vacuole fusion with lysosomes affords Lm a competitive edge against macrophage defences by providing bacteria more time in organelles they can penetrate.

Inhibition and active sites of UDP-glucuronosyltransferases 2B7 and 1A1

Two human UDP-glucuronosyltransferases (UGTs), UGT2B7 and UGT1A1, catalyze the glucuronidation of many endo- and xenobiotics. Although UGT1A1 uniquely catalyzes the glucuronidation of the endobiotic, bilirubin, and UGT2B7 uniquely catalyzes the glucuronidation of morphine to both the 3-0 glucuronide and the 6-0 glucuronide, both catalyze the glucuronidation of the mixed opioid agonist/antagonist buprenorphine with high efficiency. Etonitazenyl, a mu opioid receptor antagonist, was found to inhibit competitively opioid, steroid, and other substrate glucuronidation reactions catalyzed by UGT2B7. Data showing several benzodiazepines and alternative substrates interacting competitively support previous work, which indicates a single binding domain within UGT2B7. Etonitazenyl also competitively inhibited the glucuronidation of buprenorphine catalyzed by UGT1A1. However, neither etonitazenyl nor buprenorphine inhibited bilirubin glucuronidation except at very high concentrations. Therefore, it is unlikely that buprenorphine therapy for opioid or other drug addiction would influence bilirubin glucuronidation and lead to hyperbilirubenmia. Anthraflavic acid and catechol estrogen glucuronidation, catalyzed by UGT1A1, was also not inhibited by etonitazenyl or buprenorphine. Reactions catalyzed by UGT1A6 were not affected by etonitazenyl. These studies indicate that UGT2B7 has one binding site and that UGT1A1 has two or more binding sites for xenobiotics and endobiotics.

Role of LAMP-2 in lysosome biogenesis and autophagy

In LAMP-2-deficient mice autophagic vacuoles accumulate in many tissues, including liver, pancreas, muscle, and heart. Here we extend the phenotype analysis using cultured hepatocytes. In LAMP-2-deficient hepatocytes the half-life of both early and late autophagic vacuoles was prolonged as evaluated by quantitative electron microscopy. However, an endocytic tracer reached the autophagic vacuoles, indicating delivery of endo/lysosomal constituents to autophagic vacuoles. Enzyme activity measurements showed that the trafficking of some lysosomal enzymes to lysosomes was impaired. Immunoprecipitation of metabolically labeled cathepsin D indicated reduced intracellular retention and processing in the knockout cells. The steady-state level of 300-kDa mannose 6-phosphate receptor was slightly lower in LAMP-2-deficient hepatocytes, whereas that of 46-kDa mannose 6-phosphate receptor was decreased to 30% of controls due to a shorter half-life. Less receptor was found in the Golgi region and in vesicles and tubules surrounding multivesicular endosomes, suggesting impaired recycling from endosomes to the Golgi. More receptor was found in autophagic vacuoles, which may explain its shorter half-life. Our data indicate that in hepatocytes LAMP-2 deficiency either directly or indirectly leads to impaired recycling of 46-kDa mannose 6-phosphate receptors and partial mistargeting of a subset of lysosomal enzymes. Autophagic vacuoles may accumulate due to impaired capacity for lysosomal degradation.

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.

Woodward's reagent K inactivation of Escherichia coli L-threonine dehydrogenase: increased absorbance at 340-350 nm is due to modification of cysteine and histidine residues, not aspartate or glutamate carboxyl groups

L-Threonine dehydrogenase (TDH) from Escherichia coli is rapidly inactivated and develops a new absorbance peak at 347 nm when incubated with N-ethyl-5-phenylisoxazolium-3'-sulfonate (Woodward's reagent K, WRK). The cofactors, NAD+ or NADH (1.5 mM), provide complete protection against inactivation; L-threonine (60 mM) is approximately 50% as effective. Tryptic digestion of WRK-modified TDH followed by HPLC fractionation (pH 6.2) yields four 340-nm-absorbing peptides, two of which are absent from enzyme incubated with WRK and NAD+. Peptide I has the sequence TAICGTDVH (TDH residues 35-43), whereas peptide II is TAICGTDVHIY (residues 35-45). Peptides not protected are TMLDTMNHGGR (III, residues 248-258) and NCRGGRTHLCR (IV, residues 98-108). Absorbance spectra of these WRK-peptides were compared with WRK adducts of imidazole, 2-hydroxyethanethiolate, and acetate. Peptides III and IV have pH-dependent lambda max values (340-350 nm), consistent with histidine modification. Peptide I has pH-independent lambda max (350 nm) indicating that a thiol is modified. WRK, therefore, does not react specifically with carboxyl groups in this enzyme, but rather modifies Cys-38 in the active site of TDH; modification of His-105 and His-255 does not affect enzyme activity. These results are the first definitive proof of WRK modifying cysteine and histidine residues of a protein and show that enzyme inactivation by WRK associated with the appearance of new absorptivity at 340-350 nm does not establish modification of aspartate or glutamate residues, as has been assumed in numerous earlier reports.