Mechanism of cadmium-decreased glucuronidation in the rat

Evidence-based strategies for the characterisation of human drug and chemical glucuronidation in vitro and UDP-glucuronosyltransferase reaction phenotyping

Enzymes of the UDP-glucuronosyltransferase (UGT) superfamily contribute to the elimination of drugs from almost all therapeutic classes. Awareness of the importance of glucuronidation as a drug clearance mechanism along with increased knowledge of the enzymology of drug and chemical metabolism has stimulated interest in the development and application of approaches for the characterisation of human drug glucuronidation in vitro, in particular reaction phenotyping (the fractional contribution of the individual UGT enzymes responsible for the glucuronidation of a given drug), assessment of metabolic stability, and UGT enzyme inhibition by drugs and other xenobiotics. In turn, this has permitted the implementation of in vitro - in vivo extrapolation approaches for the prediction of drug metabolic clearance, intestinal availability, and drug-drug interaction liability, all of which are of considerable importance in pre-clinical drug development. Indeed, regulatory agencies (FDA and EMA) require UGT reaction phenotyping for new chemical entities if glucuronidation accounts for ≥25% of total metabolism. In vitro studies are most commonly performed with recombinant UGT enzymes and human liver microsomes (HLM) as the enzyme sources. Despite the widespread use of in vitro approaches for the characterisation of drug and chemical glucuronidation by HLM and recombinant enzymes, evidence-based guidelines relating to experimental approaches are lacking. Here we present evidence-based strategies for the characterisation of drug and chemical glucuronidation in vitro, and for UGT reaction phenotyping. We anticipate that the strategies will inform practice, encourage development of standardised experimental procedures where feasible, and guide ongoing research in the field.

On the mechanisms of cadmium stress alleviation in Medicago truncatula by arbuscular mycorrhizal symbiosis: a root proteomic study

The arbuscular mycorrhizal (AM) symbiosis belongs to the strategies plants have developed to cope with adverse environmental conditions including contamination by heavy metals such as cadmium (Cd). In the present work, we report on the protective effect conferred by AM symbiosis to the model legume Medicago truncatula grown in presence of Cd, and on the 2-D-based proteomic approach further used to compare the proteomes of M. truncatula roots either colonised or not with the AM fungus Glomus intraradices in Cd-free and Cd-contaminated substrates. The results indicated that at the proteome level, 9 out of the 15 cadmium-induced changes in nonmycorrhizal roots were absent or inverse in those Cd-treated and colonized by G. intraradices, including the G. intraradices-dependent down-accumulation of Cd stress-responsive proteins. Out of the twenty-six mycorrhiza-related proteins that were identified, only six displayed changes in abundance upon Cd exposure, suggesting that part of the symbiotic program, which displays low sensitivity to Cd, may be recruited to counteract Cd toxicity through the mycorrhiza-dependent synthesis of proteins having functions putatively involved in alleviating oxidative damages, including a cyclophilin, a guanine nucleotide-binding protein, an ubiquitin carboxyl-terminal hydrolase, a thiazole biosynthetic enzyme, an annexin, a glutathione S-transferase (GST)-like protein, and a S-adenosylmethionine (SAM) synthase.

Galactosamine prevents ethinylestradiol-induced cholestasis

Ethinylestradiol (EE) induces intrahepatic cholestasis in experimental animals being its derivative, ethinylestradiol 17beta-glucuronide, a presumed mediator of this effect. To test whether glucuronidation is a relevant step in the pathogenesis of cholestasis induced by EE (5 mg/kg b.wt. s.c. for 5 consecutive days), the effect of simultaneous administration of galactosamine (200 mg/kg b.wt. i.p.) on biliary secretory function was studied. A single injection of this same dose of galactosamine was able to decrease hepatic UDP-glucuronic acid (UDP-GA) levels by 85% and excretion of EE-17beta-glucuronide after administration of a tracer dose of [3H]EE by 40%. Uridine (0.9 g/kg b.wt. i.p.) coadministration reverted the effect of galactosamine on hepatic UDP-GA levels and restored the excretion of [3H]EE-17beta-glucuronide. When administered for 5 days, galactosamine itself did not alter any of the serum markers of liver injury studied (aspartate aminotransferase, alanine aminotransferase, and alkaline phosphatase) or biliary secretory function. When coadministered with EE, galactosamine partially prevented the impairment induced by this estrogen in total bile flow, the bile-salt-independent fraction of bile flow, basal bile salt secretion, and the secretory rate maximum of tauroursodeoxycholate. Uridine coadministration partially prevented galactosamine from exerting its anticholestatic effects. In conclusion, galactosamine administration partially prevented EE-induced cholestasis by a mechanism involving decreased UDP-GA availability for subsequent formation of EE 17beta-glucuronide. The evidence thus supports the hypothesis that EE 17beta-glucuronide is involved in the pathogenesis of EE cholestasis.

Differential inhibition of morphine glucuronidation in the 3- and 6-position by ranitidine in isolated hepatocytes from guinea pig

The influence of ranitidine on morphine metabolism, with special emphasise on the ratio between morphine-3-glucuronide and morphine-6-glucuronide was studied in isolated guinea pig hepatocytes. Ranitidine reduced the Kel of morphine dose-dependently with a maximum effect of 50%, and increased the relative concentration of morphine-6-glucuronide to morphine-3-glucuronide. These effects could be due to a direct or indirect effect on the conjugation enzymes involved, or an effect on the transport of morphine or glucuronides across cell membranes. The latter explanation was rejected on the basis of the observation that the ratios between intra- and extracellular concentrations of morphine, morphine-3-glucuronide and morphine-6-glucuronide were not influenced by ranitidine. Increasing concentrations of ranitidine gradually decreased the morphine-3-glucuronide/morphine-6-glucuronide ratio by up to 21%. This could stem from interference of energy or co-substrate supply, or through direct effects on the different UDPGTases involved. The observation that the present effect on morphine glucuronidation was the opposite of that observed when administering a known co-substrate (UDPGA) depletor, indicated that in all probability the effect of ranitidine was a direct inhibition on the uridine 5'-diphosphate glucuronyltransferases involved, with a more pronounced effect for the isoenzymes responsible for the 3'-glucuronidation.