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Estradiol glucuronide
Chemical compound From Wikipedia, the free encyclopedia
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Estradiol glucuronide, or estradiol 17β-D-glucuronide, is a conjugated metabolite of estradiol.[1] It is formed from estradiol in the liver by UDP-glucuronyltransferase via attachment of glucuronic acid and is eventually excreted in the urine by the kidneys.[1] It has much higher water solubility than does estradiol.[1] Glucuronides are the most abundant estrogen conjugates.[1]
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When exogenous estradiol is administered orally, it is subject to extensive first-pass metabolism (95%) in the intestines and liver.[2][3] A single administered dose of estradiol is absorbed 15% as estrone, 25% as estrone sulfate, 25% as estradiol glucuronide, and 25% as estrone glucuronide.[2] Formation of estrogen glucuronide conjugates is particularly important with oral estradiol as the percentage of estrogen glucuronide conjugates in circulation is much higher with oral ingestion than with parenteral estradiol.[2] Estradiol glucuronide can be converted back into estradiol, and a large circulating pool of estrogen glucuronide and sulfate conjugates serves as a long-lasting reservoir of estradiol that effectively extends its elimination half-life of oral estradiol.[2] In demonstration of the importance of first-pass metabolism and the estrogen conjugate reservoir in the pharmacokinetics of estradiol,[2] the elimination half-life of oral estradiol is 13 to 20 hours[4] whereas with intravenous injection its elimination half-life is only about 1 to 2 hours.[5]
Approximately 7% of estradiol is excreted in the urine as estradiol glucuronide.[6]
Estradiol glucuronide is transported into prostate gland, testis, and breast cells by OATP1A2, OATP1B1, OATP1B3, OATP1C1, and OATP3A1.[7] The ABC transporters MRP2, MRP3, MRP4, and BCRP, as well as several other transporters, have been found to transport estradiol glucuronide out of cells.[7][8]
The circulating concentrations of estrogen glucuronides are generally more than 10-fold lower than those of estrone sulfate, the most abundant estrogen conjugate in the circulation.[8]
Estradiol glucuronide has been identified as an agonist of the G protein-coupled estrogen receptor (GPER), a membrane estrogen receptor.[9] This may be involved in estradiol glucuronide-induced cholestasis.[9]
Estrogen glucuronides can be deglucuronidated into the corresponding free estrogens by β-glucuronidase in tissues that express this enzyme, such as the mammary gland.[10] As a result, estrogen glucuronides have estrogenic activity via conversion into estrogens.[10]
Estradiol glucuronide shows about 300-fold lower potency in activating the estrogen receptors relative to estradiol in vitro.[11]
The positional isomer of estradiol glucuronide, estradiol 3-glucuronide, also occurs as a major endogenous metabolite of estradiol, circulating at two-thirds of the levels of estrone sulfate when it reaches its maximal concentrations just before ovulation and during the peak in estradiol levels that occurs at this time.[12]
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