Gene/Protein Disease Symptom Drug Enzyme Compound
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Query: EC:3.2.1.31 (beta-glucuronidase)
 7,680 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

Isolated livers from male Sprague-Dawley rats were perfused at 20 ml/min for 3 h at 37 degrees C with 100 ml of an oxygenated, recirculating solution of 20% rat blood in Krebs bicarbonate buffer containing 20 micrograms/ml [3H]etoposide. Ninety % of administered radioactivity was eliminated in bile over a 3-h collection period. The clearance of etoposide was 3.56 ml/min indicating that, in the rat, it is not highly extracted. Its clearance is, therefore, independent of hepatic blood flow. Etoposide was both excreted into the bile and metabolized by the liver. Perfusate and bile samples analyzed by reverse-phase high-performance liquid chromatography techniques were found to contain three peaks of radioactivity. Positive and negative ion fast atom bombardment mass spectrometry identified the first two peaks as etoposide glucuronides and the third peak as parent drug. Following the i.v. administration of etoposide to rabbits, etoposide glucuronide was also identified in rabbit urine. The recovery of etoposide both from rabbit urine and rat bile was increased by preincubation with glucuronidase. However, the glucuronides were relatively resistant to the action of glucuronidase and showed varying sensitivity to the type of glucuronidase and the reaction conditions used. These studies document the presence of etoposide glucuronide as an etoposide metabolite in two mammalian species and suggest that previous clinical studies using beta-glucuronidase to quantitate glucuronide formation may have underestimated this metabolite due to its relative resistance to some glucuronidase preparations.
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PMID:Identification of etoposide glucuronide as a major metabolite of etoposide in the rat and rabbit. 334 61

Etoposide, an important anticancer agent, undergoes glucuronidation both in vitro and in vivo. In this study, three isomeric glucuronides of etoposide, including one phenolic (EPG) and two alcoholic glucuronides (EAG1 and EAG2), were biosynthesized in vitro with human liver microsomes (HLMs), and identified by liquid chromatography-electrospray ionization-mass spectrometry and confirmed by beta-glucuronidase cleavage. In vitro UDP-glucuronosyltransferase (UGT) reaction screening with 12 recombinant human UGTs demonstrated that etoposide glucuronidation is mainly catalyzed by UGT1A1. Although UGT1A8 and 1A3 also catalyzed the glucuronidation of etoposide, their activities were approximately 10 and 1% of UGT1A1. Enzyme kinetic study indicated that the predominant form of etoposide glucuronide in HLMs and human intestinal microsomes (HIMs) was EPG, whereas EAG1 and EAG2 were the minor metabolites, with approximately an 8 to 10% glucuronidation rate of EPG. For the formation of EPG, the V(max) of HLMs (110 pmol/min/mg protein) was very similar to that of recombinant UGT1A1 (124 pmol/min/mg protein), whereas the V(max) of HIMs (54.4 pmol/min/mg protein) was 2-fold lower than those of HIMs and UGT1A1. The K(m) values of HLMs (530 microM) and HIMs (608 microM) were 2-fold higher than that of UGT1A1 (285 microM). The V(max)/K(m) values for the formation of EPG were 0.21 and 0.09 microl/min/mg protein for HLMs and HIMs, respectively. The data indicated that UGT1A1 is principally responsible for the formation of etoposide glucuronides, mainly in the form of phenolic glucuronide, suggesting that etoposide can be used as a highly selective probe substrate for human UGT1A1 in vitro.
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PMID:UDP-glucuronosyltransferase 1A1 is the principal enzyme responsible for etoposide glucuronidation in human liver and intestinal microsomes: structural characterization of phenolic and alcoholic glucuronides of etoposide and estimation of enzyme kinetics. 1715 Nov 91