UDP-glucuronosyltransferases and their role in metabolism and disposition of carcinogens

Drug glucuronidation in clinical psychopharmacology

Glucuronidation is a phase II metabolic process and one of the most common pathways in the formation of hydrophilic drug metabolites. At least 33 families of uridine diphosphate-glucuronosyltransferases have been identified in vitro, and specific nomenclature similar to that used to classify the cytochrome (CYP) P450 system has been established. The UGT1 and UGT2 subfamilies represent the most important of these enzymes in human drug metabolism. Factors affecting glucuronidation include the following: cigarette smoking, obesity, age, and gender. In addition, several drugs have been found in vitro to be substrates, inhibitors, or inducers of UGT enzymes. Induction or inhibition of both UGT and CYP isoforms may occur simultaneously. Some important drug interactions involving glucuronidation have been documented and others can be postulated. This review summarizes the relevant literature pertaining to drug glucuronidation and its implications for clinical psychopharmacology.

Identification of cyclosporine A and tacrolimus glucuronidation in human liver and the gastrointestinal tract by a differentially expressed UDP-glucuronosyltransferase: UGT2B7

BACKGROUND/AIMS

The oral administration of the major transplant immunosuppressants cyclosporine A and tacrolimus leads to unpredictable drug levels requiring drug monitoring. Hepatic and extrahepatic metabolism of cyclosporine A and tacrolimus by cytochrome P450 proteins has been analyzed but metabolism and inactivation by glucuronidation has not been investigated.

METHODS

Cyclosporine A and tacrolimus glucuronidation was measured in hepatic and gastrointestinal microsomal protein, and with 11 recombinant hepatic and extrahepatic family 1 and 2 UDP-glucuronosyltransferases. UDP-glucuronosyltransferase transcripts were determined by polymerase chain reaction.

RESULTS

Significant cyclosporine and tacrolimus glucuronidation activity was present in endoplasmic reticulum from liver, duodenum, jejunum, ileum and colon, but was absent in stomach. Specific cyclosporine A glucuronidation activity was highest in liver and colon, tacrolimus glucuronidation was highest in liver. Analyses using recombinant UDPglucuronosyltransferases identified UGT2B7 as a human UDP-glucuronosyltransferase with specific activity toward cyclosporine A and tacrolimus. The hepato-gastrointestinal distribution of immunosuppressant glucuronidation activity corresponded to the differential expression pattern of UGT2B7 mRNA.

CONCLUSIONS

This study provides conclusive evidence of hepatic and extrahepatic immunosuppressant glucuronidation by human UGT2B7 which was identified to be differentially expressed in the human hepatogastrointestinal tract. Hepatic and extrahepatic glucuronidation may influence the therapeutic efficacy of transplant immunosuppressants.

Metabolic activity of fresh and cryopreserved cynomolgus monkey (Macaca fascicularis) hepatocytes

1. The effect of cryopreservation on the metabolic capacity of monkey hepatocytes over 4 h in suspension and 24 h in culture was determined. Hepatocytes were diluted in a buffer containing 10% DMSO and frozen in a computer-controlled chamber. 2. Initial ethoxyresorufin and ethoxycoumarin O-deethylase (ECOD) activities were the same in fresh and cryopreserved (CP) hepatocytes. ECOD activity in suspensions declined over 4 h but was the same in fresh and CP hepatocytes. 3. The formation of testosterone hydroxy (OHT) metabolites (namely 6beta-OHT, 2beta-OHT, 16beta-OHT, 16alpha-OHT, 15beta-OHT, 2alpha-OHT and 6beta-OHT) was unaffected by cryopreservation. The loss of OHT activities over 4 h in CP and fresh whole cell suspensions was attributed to a loss of cofactor. CP hepatocyte cultures had equivalent OHT activities to freshly isolated hepatocytes. 4. Initial UDP-glucuronyltransferase (UGT) activities, using the substrates 4-methylumbelliferone, ethoxycoumarin and hydroxycoumarin, were equivalent in fresh and CP whole hepatocytes. At later times, UGT activity was lower in CP than fresh hepatocytes but this was due to a loss of UDPGA. Initial sulphotransferase (SULT) activities, using the substrates 2-naphthol, ethoxycoumarin and hydroxycoumarin, were equivalent in fresh and CP hepatocytes. SULT activities were less stable than UGT activities but were the same in fresh and CP hepatocytes throughout the 4-h incubation. 5. Initial glutathione S-transferase activities (using 1-chloro-2,4-dinitrobenzene) were the same in fresh and CP hepatocytes and both did not decrease over 4 h. 6. CP monkey hepatocytes are a useful model for metabolic and cytotoxicity studies. These cells can be can be used either in suspension or in culture.

UDP-glucuronosyltransferase activity in human liver and colon

BACKGROUND & AIMS

The contribution of glucuronidation toward human drug metabolism is carried out by the Super gene family of UDP-glucuronosyltransferases (UGTs). Regulation of the human UGT1A locus is tissue specific, resulting in the unique expression of multiple hepatic and extrahepatic gene products. Studies were undertaken to examine UGT1A expression in human hepatic and colonic tissues.

METHODS

UGT1A messenger RNA, protein, catalytic activity, and substrate kinetics were studied in 5 samples of normal hepatic and sigmoid colon tissue using duplex reverse-transcription polymerase chain reaction (RT-PCR), enzymatic and Western blot analysis, and indirect immunofluorescence analysis.

RESULTS

Specific patterns of UGT1A gene expression occur in the liver and colon, which were consistent with different banding patterns as detected by Western blot analysis using a UGT1A-specific antibody. However, microsomal UGT activities in colon were up to 96-fold lower for many phenolic substrates, a finding that was not concordant with RT-PCR and Western blot analysis. Interestingly, UGT activity toward tertiary amines and some steroid hormones was equal.

CONCLUSIONS

Differences of glucuronidation activity between human liver and colon suggest that UGT1A activity may be regulated as a result of the relative presence of individual isoforms with differing catalytic activities or by tissue-specific modulators after gene expression.

Glucuronidation: a dual control

1. Glucuronidation is a major detoxication process catalyzed by uridine diphosphate glucuronosyltransferases. 2. The amount of enzyme can be modulated by numerous foreign compounds, such as common chemical inducers already implicated in the induction of other detoxication enzymes. 3. Hormones such as thyroid hormones or growth hormone also are implicated in the control of glucuronidation. 4. Because glucuronidation enzymes (isozymes) are anchored in the endoplasmic reticulum membrane, with their active site likely being located on the lumenal side of the membrane, the membrane environment of these enzymes was shown to modulate their functional state as evaluated by the conjugating activity per enzymatic molecular unit. 5. In accord with a first, previously proposed model, it seems that this modulation can be attributed to different conformational states of the enzymes, depending on the physicochemical state of the membrane. 6. In accord with a second model, the membrane may act as a barrier between the enzymes and the cosubstrate UDP-glucuronic acid, which is a polar and charged molecule synthesized in the cytosol. This would imply a transporting process for this molecule through the reticulum membrane, which has been characterized in vitro and could be of importance in vivo. 7. Glucuronidation is under the control of a dual regulation, by means of a specific isozyme expression level and by the modulation of their functional state.

Mitochondrial bioactivation of cysteine S-conjugates and 4-thiaalkanoates: implications for mitochondrial dysfunction and mitochondrial diseases

The toxicity of most drugs and chemicals is associated with their enzymatic conversion to toxic metabolites. Bioactivation reactions occur in a range of organs and organelles, including mitochondria. The toxicity of haloalkene-derived cysteine S-conjugates and related 4-thiaalkanoates is associated with their mitochondrial bioactivation. Toxic cysteine S-conjugates are formed by the glutathione S-transferase-catalyzed addition of glutathione to haloalkenes to give glutathione S-conjugates, which are hydrolyzed by gamma-glutamyltransferase and dipeptidases. Mitochondrial cysteine conjugate beta-lyase-catalyzed bioactivation of cysteine S-conjugates affords unstable alpha-halothiolates. Haloalkene-derived 4-thiaalkanoates, which are analogs of cysteine S-conjugates that lack an alpha-amino group, undergo bioactivation by the enzymes of fatty acid beta-oxidation to give 3-hydroxy-4-thiaalkanoates that eliminate alpha-halothiolates. alpha-Halothiolates yield alkylating and acylating agents that interact with cellular macromolecules and thereby cause cell damage. Mitochondrial dysfunction is the hallmark of cysteine S-conjugate-induced cytotoxicity: decreased respiration, decreased ATP and total adenine nucleotide concentrations, depletion of the mitochondrial glutathione content, perturbations in cellular Ca2+ homeostasis, and damage to the mitochondrial genome are seen with cysteine S-conjugates. Similar changes are observed with cytotoxic 4-thiaalkanoates, but inhibition of the medium-chain acyl-CoA dehydrogenase and hypoglycemia are also observed.