Endogenous Substrates for Udp-Glucuronosyl-Transferases

Tephly, Thomas R.; Green, Mitchell; Puig, Jaime; Irshaid, Yacoub M.

doi:10.3109/00498258809042244

“…UDP-glucuronosyltransferase (UGT) enzymes are important phase-II detoxifying enzymes that conjugate xenobiotics [1] as well as endogenous compounds, such as bilirubin, bile acids, thyroxin, biogenic amines, fat-soluble vitamins and steroids, which have functional groups of oxygen, nitrogen, sulphur or carbon [2][3][4][5]. Glucuronidated compounds are more polar, less toxic and are generally eliminated from the body through the bile or urine.…”

Section: Introductionmentioning

confidence: 99%

Cloning and characterization of a simian UDP-glucuronosyltransferase enzyme UGT2B20, a novel C19 steroid-conjugating protein

Barbier¹,

Bélanger²,

Hum³

1999

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Steroid glucuronidation by UDP-glucuronosyltransferase (UGT) enzymes is a mechanism leading to catabolism and elimination of steroid hormones. To establish an animal model to investigate the conjugation of steroids by UGT enzymes, previous results revealed that simian and human are unique in having high levels of circulating androsterone glucuronide and androstane-3alpha, 17beta-diol (3d-Diol) glucuronide. A cDNA, UGT2B20, was isolated from cynomolgus monkey liver and prostate libraries. The cDNA was 2075 bp in length and contained an open reading frame of 1590 bp, encoding a protein of 530 amino acid residues. The UGT2B20 clone was transfected and stably expressed in the human embryo kidney HK293 cell line, and the transferase activity of UGT2B20 was tested with 73 compounds. This enzyme was shown to be active with androgens, such as testosterone, dihydrotestosterone (DHT) and 3alpha-Diol, and on catecholoestrogens including 1,3,5,10-oestratriene-3, 4-diol-17-one. Kinetic analysis performed with intact cells yielded apparent Km values of 1.1, 2.3 and 4.6 microM for 3alpha-Diol, DHT and testosterone respectively. Reverse transcriptase-PCR analysis demonstrated that UGT2B20 transcript is expressed in several tissues including the liver, prostate, kidney, epididymis and adrenal of the cynomolgus monkey. Amino acid sequence alignment shows that the UGT2B20 protein is 92% identical with UGT2B15. Both enzymes have similar apparent Km values for DHT and 3alpha-Diol, and demonstrate similar transcript tissue distribution. The characterization of simian UGT2B20 as a structural and functional homologue of human UGT2B15 further demonstrates the similarities of steroid glucuronidation in these two species, and indicates the relevance of using the monkey as an animal model to study and understand steroid glucuronidation in extrahepatic-steroid target tissues.

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“…UDP-glucuronosyltransferase (UGT) enzymes are important phase-II detoxifying enzymes that conjugate xenobiotics [1] as well as endogenous compounds, such as bilirubin, bile acids, thyroxin, biogenic amines, fat-soluble vitamins and steroids, which have functional groups of oxygen, nitrogen, sulphur or carbon [2][3][4][5]. Glucuronidated compounds are more polar, less toxic and are generally eliminated from the body through the bile or urine.…”

Section: Introductionmentioning

confidence: 99%

Cloning and characterization of a simian UDP-glucuronosyltransferase enzyme UGT2B20, a novel C19 steroid-conjugating protein

Barbier

¹

,

Bélanger

²

,

Hum

³

1999

Biochemical Journal

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Steroid glucuronidation by UDP-glucuronosyltransferase (UGT) enzymes is a mechanism leading to catabolism and elimination of steroid hormones. To establish an animal model to investigate the conjugation of steroids by UGT enzymes, previous results revealed that simian and human are unique in having high levels of circulating androsterone glucuronide and androstane-3α,17β-diol (3d-Diol) glucuronide. A cDNA, UGT2B20, was isolated from cynomolgus monkey liver and prostate libraries. The cDNA was 2075 bp in length and contained an open reading frame of 1590 bp, encoding a protein of 530 amino acid residues. The UGT2B20 clone was transfected and stably expressed in the human embryo kidney HK293 cell line, and the transferase activity of UGT2B20 was tested with 73 compounds. This enzyme was shown to be active with androgens, such as testosterone, dihydrotestosterone (DHT) and 3α-Diol, and on catecholoestrogens including 1,3,5,10-oestratriene-3,4-diol-17-one. Kinetic analysis performed with intact cells yielded apparent K m

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“…UGT1 has multiple isozymes, all of which have a conserved carboxyl region encoded by exons 2 to 5, with a variable region encoded by various exon 1. UGT1A1, one of the twelve isoforms in the UGT1 family, is a natural substrate for bilirubin and regulates the endogenous bilirubin conjugation [57]. A variant dinucleotide repeat sequence (from 5 to 9 repeats) in the promoter region of UGT1A1 was found to influence the in vitro and in vivo glucuronidation rate of SN-38 [58][59][60].…”

Section: Sn-38 Glucuronidation and Diarrheamentioning

confidence: 99%

Lessons Learned from the Irinotecan Metabolic Pathway

B.K.

¹

,

McLeod²

2003

CMC

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Irinotecan, a camptothecin analogue, is a prodrug which requires bioactivation to form the active metabolite SN-38. SN-38 acts as a DNA topoisomerase I poison. Irinotecan has been widely used in the treatment of metastatic colorectal cancer, small cell lung cancer and several other solid tumors. However, large inter-patient variability in irinotecan and SN-38 disposition, as well as severe but unpredictable diarrhea limits the clinical potential of irinotecan. Intense clinical pharmacology studies have been conducted to elucidate its complicated metabolic pathways and to provide scientific rationale in defining strategies to optimize drug therapy. Irinotecan is subjected to be shunted between CYP3A4 mediated oxidative metabolism to form two inactive metabolites APC or NPC and tissue carboxylesterase mediated hydrolysis to form SN-38 which is eventually detoxified via glucuronidation by UGT1A1 to form SN-38G. The pharmacology of this compound is further complicated by the existence of genetic inter-individual differences in activation and deactivation enzymes of irinotecan (e.g., CYP3A4, CYP3A5, UGT1A1) and sharing competitive elimination pathways with many concomitant medications, such as anticonvulsants, St. John's Wort, and ketoconazole. Efflux of the parent compound and metabolites out of cells by several drug transporters (e.g., Pgp, BCRP, MRP1, MRP2) also occurs. This review highlights the latest findings in drug activation, transport mechanisms, glucuronidation, and CYP3A-mediated drug-drug interactions of irinotecan in order to unlock some of its complicated pharmacology and to provide ideas for relevant future studies into optimization of this promising agent.

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Endogenous Substrates for Udp-Glucuronosyl-Transferases

Cited by 47 publications

References 31 publications

Cloning and characterization of a simian UDP-glucuronosyltransferase enzyme UGT2B20, a novel C19 steroid-conjugating protein

Cloning and characterization of a simian UDP-glucuronosyltransferase enzyme UGT2B20, a novel C19 steroid-conjugating protein

Cloning and characterization of a simian UDP-glucuronosyltransferase enzyme UGT2B20, a novel C19 steroid-conjugating protein

Lessons Learned from the Irinotecan Metabolic Pathway

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