N-glucuronidation of perfluorooctanesulfonamide by human, rat, dog, and monkey liver microsomes and by expressed rat and human UDP-glucuronosyltransferases.

N-Alkylperfluorooctanesulfonamides have been used in a range of industrial and commercial applications. Perfluorooctanesulfonamide (FOSA) is a major metabolite of N-alkylperfluorooctanesulfonamides and has a long half-life in animals and in the environment and is biotransformed to FOSA N-glucuronide. The objective of this study was to identify and characterize the human and experimental animal liver UDP-glucuronosyltransferases (UGTs) that catalyze the N-glucuronidation of FOSA. The results showed that pooled human liver and rat liver microsomes had high N-glucuronidation activities. Expressed rat UGT1.1, UGT2B1, and UGT2B12 in HK293 cells catalyzed the N-glucuronidation of FOSA but at rates that were lower than those observed in rat liver microsomes. Of the 10 expressed human UGTs (1A1, 1A3, 1A4, 1A6, 1A9, 2B4, 2B7, 2B15, and 2B17) studied, only hUGT2B4 and hUGT2B7 catalyzed the N-glucuronidation of FOSA. The kinetics of N-glucuronidation of FOSA by rat liver microsomes and by hUGT2B4/7 was consistent with a single-enzyme Michaelis-Menten model, whereas human liver microsomes showed sigmoidal kinetics. These data show that rat liver UGT1.1, UGT2B1, and UGT2B12 catalyze the N-glucuronidation of FOSA, albeit at low rates, and that hUGT2B4 and hUGT2B7 catalyze the N-glucuronidation of FOSA.

Opioids bind to the amino acids 84 to 118 of UDP-glucuronosyltransferase UGT2B7.

The UDP-glucuronosyltransferase UGT2B7 is an important human UGT isoform that catalyzes the conjugation of many endogenous and exogenous compounds, among them opioids, resulting in the formation of D-glucuronides. The binding site of the aglycone is located in the N-terminal half of the protein. Using NMR analysis, we demonstrate that the opioid binding site in UGT2B7 is within the 84 to 118 N-terminal amino acids. Three maltose binding protein-UGT2B7 fusion proteins, 2B7F3 and 2B7F4 incorporating the amino acids 24 to 118 and 24 to 96 of UGT2B7, respectively, and 2B7F5 incorporating amino acids 84 to 118 of UGT2B7 were expressed in Escherichia coli and purified by affinity chromatography. NMR analysis showed that morphine was bound to the fusion protein 2B7F3 with a K(D) value similar to the K(D) values obtained for the previously produced fusion proteins, which included amino acids 24 to 180. Morphine did not bind to 2B7F4, but it did bind to 2B7F5. Both NMR 1-D spectra and NOESY experiments indicated that the 2B7F5 protein was mediating magnetization transfer within the morphine. These results allowed us to predict and model a binding site within the amino acids 96 to 101 of UGT2B7. A mutant fusion protein 2B7F3 with the substitution D99A was produced, and the NMR spectroscopy analysis of the protein supported the model. A marked reduction of morphine binding was observed when the charged aspartate was substituted with alanine.

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.

Aspartame: review of safety.

Over 20 years have elapsed since aspartame was approved by regulatory agencies as a sweetener and flavor enhancer. The safety of aspartame and its metabolic constituents was established through extensive toxicology studies in laboratory animals, using much greater doses than people could possibly consume. Its safety was further confirmed through studies in several human subpopulations, including healthy infants, children, adolescents, and adults; obese individuals; diabetics; lactating women; and individuals heterozygous (PKUH) for the genetic disease phenylketonuria (PKU) who have a decreased ability to metabolize the essential amino acid, phenylalanine. Several scientific issues continued to be raised after approval, largely as a concern for theoretical toxicity from its metabolic components--the amino acids, aspartate and phenylalanine, and methanol--even though dietary exposure to these components is much greater than from aspartame. Nonetheless, additional research, including evaluations of possible associations between aspartame and headaches, seizures, behavior, cognition, and mood as well as allergic-type reactions and use by potentially sensitive subpopulations, has continued after approval. These findings are reviewed here. The safety testing of aspartame has gone well beyond that required to evaluate the safety of a food additive. When all the research on aspartame, including evaluations in both the premarketing and postmarketing periods, is examined as a whole, it is clear that aspartame is safe, and there are no unresolved questions regarding its safety under conditions of intended use.

Expression of UGT2B7, a UDP-glucuronosyltransferase implicated in the metabolism of 4-hydroxyestrone and all-trans retinoic acid, in normal human breast parenchyma and in invasive and in situ breast cancers.

Glucuronidation, mediated by UDP-glucuronosyltransferases (UGTs), affects the actions and disposition of diverse endo- and xenobiotics. In the case of catecholestrogens (CEs), glucuronidation is likely to block their oxidation to quinone estrogens that are the putative mediators of CEs' actions as initiators of cancers. The goal of this study was to determine whether UGT2B7, the isoenzyme with a high affinity for 4-hydroxyestrone, is expressed in human breast parenchyma. Glucuronidation of 4-hydroxyestrone has relevance to breast carcinogenesis because quinone metabolites of 4-hydroxylated CEs can form potentially mutagenic depurinating DNA adducts, and because in breast tissue estrone is likely to be the predominant estrogen available for 4-hydroxylation. Using reverse transcriptase-polymerase chain reaction, immunocytochemistry, immunoblot analyses, and assays of glucuronidation of 4-hydroxyestrone, we show that UGT2B7 is expressed in human mammary epithelium, and that its expression is dramatically reduced in invasive breast cancers. In many in situ carcinomas, however, 4-hydroxyestrone immunostaining was not only preserved but even more intense than in normal mammary epithelium. The finding of reduced UGT2B7 protein and glucuronidation of 4-hydroxyestrone in invasive cancers suggests a tumor-suppressor function for the enzyme. Recent identification of all-trans retinoic acid as a substrate of UGT2B7 suggests that this function includes the generation of retinoyl-beta-glucuronide, a potent mediator of actions of retinoids important for maintaining epithelia in a differentiated state. Current knowledge does not provide any ready explanation for the apparent increase in UGT2B7 expression in carcinomas in situ. However, this finding, together with reduced immunostaining at loci showing breach of the basement membrane (microinvasion), suggests involvement of UGT2B7-catalyzed reaction(s) in protection against invasion of surrounding tissue by cancer cells.

Measurements of UDP- glucuronosyltransferase (UGT) activities.

Mammalian UDP-glucuronosyltransferases are a family of isoenzymes that catalyze the reaction of endobiotics and xenobiotics with glucuronic acid resulting in the formation of hydrophilic glucuronides. This pathway is an important step in the metabolism and subsequent excretion of many compounds that would otherwise have toxic effects. This unit describes three methods for measuring UGT activity. Thin layer chromatography is a powerful screening method and may be used to analyze multiple substrates simultaneously. The Sep-Pak C18 cartridge extraction method has been developed to specifically separate opioid glucuronides from UDP-glucuronic acid. Finally, the ethyl acetate extraction method is used to separate the glucuronides of bilirubin, sterols, and vile acids from UDP-glucuronic acid. These methods may be applied to a microsomal fraction or to cultured cells transformed with cDNA for UGT.