Coregulation of C3-hydroxyl versus C17-hydroxyl glucuronidation of beta-estradiol in pregnancy and after treatment with phenobarbital or ethinyl-estradiol.

S Connors, M Vore

Index: J. Pharmacol. Exp. Ther. 246(1) , 54-9, (1988)

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Abstract

The glucuronidation of [3H]estradiol-17 beta at the C3 vs. the C17 hydroxyl groups was determined in female Sprague-Dawley rat liver microsomes. A high-performance liquid chromatography method was developed to resolve the glucuronide conjugates which were then quantitated by liquid scintillation counting. The rates of formation of 17 beta-estradiol 3-(beta-D-glucuronide) (E(2)3G) and 17 beta-estradiol 17-(beta-D-glucuronide) (E(2)17G) were 0.49 +/- 0.03 and 0.40 +/- 0.02 nmol/min/mg of protein, respectively. The apparent Km and Vmax of estradiol glucuronidation were determined in control, pregnant (day 19 of gestation), phenobarbital-treated (80 mg/kg/day i.p. for 5 days) and ethinylestradiol-treated (5 mg/kg/day i.p. for 5 days) female rats. The least-squares estimates of Km and Vmax values as well as the confidence contours of the joint sums of squares for the parameter spaces were calculated. The Vmax (nanomoles per minute per milligram of protein) for E(2)3G was significantly decreased from 0.94 to 0.57 in pregnancy and to 0.47 as a result of ethinylestradiol treatment. The Vmax values for E(2)17G were significantly different in control (0.43), pregnant (0.31) and ethinylestradiol-treated (0.27) rats. Phenobarbital treatment slightly increased the Vmax to 0.51 for E(2)17G whereas the Vmax for E(2)3G was unchanged (0.90) compared to controls. The Km (micromolar) for E(2)3G was 144 in the controls, 112 in pregnancy and 86 and 92 as a result of treatment with ethinylestradiol and phenobarbital, respectively. The Km for E(2)17G was 60 in the controls, 40 in pregnancy, 43 and 68 as a result of ethinylestradiol and phenobarbital treatment, respectively. None of the changes in Km were statistically significant.(ABSTRACT TRUNCATED AT 250 WORDS)