The Configuration of the 17-Hydroxy Group Variably Influences the Glucuronidation of β-Estradiol and Epiestradiol by Human UDP-Glucuronosyltransferases

Abstract

ABSTRACT:The glucuronidation of 17␤-estradiol (␤-estradiol) and 17␣-estradiol (epiestradiol) was studied to elucidate how the orientation of the 17-OH group affects conjugation at the 3-OH or the 17-OH of either diastereomer. Recombinant human UDP-glucuronosyltransferases (UGTs) UGT1A1, UGT1A3, UGT1A7, UGT1A8, and UGT1A10 conjugated one or both diastereomers, mainly at the 3-OH. The activity of UGT1A4 was low and unique because it was directed merely toward the 17-OH of both aglycones. UGT1A10 exhibited particularly high estradiol glucuronidation activity, the rate and affinity of which were significantly higher in the case of ␤-estradiol than with epiestradiol. UGT1A9 did not catalyze estradiol glucuronidation, but UGT1A9-catalyzed scopoletin glucuronidation was competitively inhibited by ␤-estradiol. UGT2B4, UGT2B7, and UGT2B17 exclusively conjugated the estradiols at the 17-OH position in a highly stereoselective fashion. UGT2B4 was specific for epiestradiol; UGT2B7 glucuronidated both diastereomers, with high affinity for epiestradiol, whereas UGT2B17 only glucuronidated ␤-estradiol. UGT2B15 glucuronidated both estradiols at the 3-OH, with a strong preference for epiestradiol. Human UGT2A1 and UGT2A2 glucuronidated both diastereoisomers at both hydroxyl groups. Microsomal studies revealed that human liver mainly yielded epiestradiol 17-O-glucuronide, and human intestine primarily yielded ␤-estradiol 3-O-glucuronide, whereas rat liver preferentially formed ␤-estradiol 17-O-glucuronide. Of the three recombinant rat UGTs that were examined in this study, rUGT2B1 was specific for the 17-OH of ␤-estradiol, rUGT2B2 did not catalyze estradiol glucuronidation, whereas rUGT2B3 exhibited high activity toward the 17-OH in both diastereoisomers. The results show that although many UGTs can catalyze estradiol glucuronidation, there are marked differences in their kinetics, regioselectivity, and stereoselectivity.

UDP-Glucuronosyltransferases (UGTs) 2B7 and UGT2B17 Display Converse Specificity in Testosterone and Epitestosterone Glucuronidation, whereas UGT2A1 Conjugates Both Androgens Similarly

“…In the case of UGT2B15 we have noticed that the specific activity of our recombinant enzyme is significantly lower than that of commercially available UGT2B15 (Itäaho et al, 2008). Therefore, here we tested both our preparation (the expression level of which can be compared with that of the other recombinant UGTs) and UGT2B15 from a commercial source.…”

“…A series of studies by Bichlmaier et al (2007) on the interactions of selected UGTs with different chiral compounds led to the development of high-affinity and high-specificity inhibitors for UGT2B7 and set us on the route to the present work. In addition, we have recently shown that the configuration of C17 in another important steroid, estradiol, has a major effect on its glucuronidation by different UGTs (Itäaho et al, 2008).…”

Dopamine Is a Low-Affinity and High-Specificity Substrate for the Human UDP-Glucuronosyltransferase 1A10

“…Based on the latter, it might be speculated that low expression levels of UGT1A10 that were not detected previously could also occur in the brain. Such a finding, for a highly active enzyme like UGT1A10 (Xiong et al, 2006;Starlard-Davenport et al, 2007;Itäaho et al, 2008), would certainly have functional implications. Therefore, in the present study we have reexamined the expression of UGT1A10 in various human tissues, including the brain.…”

“…In previous works focusing on structure-function relationships of the UGTs, we have investigated the activities of UGT1A10 toward phenol compounds (Xiong et al, 2006) and different estrogens (Starlard-Davenport et al, 2007;Itäaho et al, 2008). In particular, it was suggested that Phe90 and Phe93 of UGT1A10 participated in the binding of phenolic substrates through ring stacking (Xiong et al, 2006).…”

Identification of Enzymes Responsible for the N-Oxidation of Darexaban Glucuronide, the Pharmacologically Active Metabolite of Darexaban, and the Glucuronidation of Darexaban N-Oxides in Human Liver Microsomes

“…These UGT substrates were set at effective concentrations according to previous reports. [15][16][17][18] Bilirubin was dissolved in 0.1 M sodium hydroxide. β-Estradiol, imipramine hydrochloride, propofol, and troglitazone were dissolved in methanol.…”