Estrogen-2/4-hydroxylase activities in rat brain and liver microsomes exhibit different substrate preferences and sensitivities to inhibition

Seasonal variation in tissue estrogen-2/4-hydroxylases (EH) and in vitro effects of steroids on ovarian EH activity in the catfish Heteropneustes fossilis

A radiometric assay was used to measure microsomal EH activity from tritiated H(2)O formed during the conversion of [2,4 (3)H] estradiol-17β into catecholestrogens in the microsomal fractions of liver, brain and ovary of the catfish Heteropneustes fossilis. The validation data show that enzyme activity increased with incubation time, and substrate and cofactor (NADPH) concentrations, elicited temperature optima of 30-37°C and pH optima of 6.8-7.8. EH activity was strongly NADPH-dependent and in its absence only 13.48% activity was recorded. Liver recorded the highest enzyme activity, followed by brain and ovary. EH activity showed a significant seasonal variation with the peak activity in spawning phase and the lowest activity in resting phase. In the ovary, the follicular layer (theca and granulosa) elicited the highest activity over that of the denuded oocytes. Modulatory effects of steroids on ovarian enzyme activity were further demonstrated. The incubation of postvitellogenic follicles with 1, 10 or 100 nM concentrations of various steroids for 24 h produced varied effects on EH activity. Progesterone and 2-hydroxyestradiol-17β elicited strong suppressive effects on enzyme activity. Estrogens (E(1), E(2) and E(3)) suppressed the activity in a concentration-dependent manner. Among the progestins tested, 17,20α-dihydroxy-4-pregnen-3-one, the isomer of 17,20β-dihydroxy-4-pregnen-3-one (a teleost maturation-inducing steroid) showed the lowest depressing effect. Among androgens, the testosterone metabolite 11-ketotestosterone (functional teleost androgen) showed a high suppressing effect. Corticosteroids elicited low activity with cortisol suppressed the activity at higher concentrations. The study will form a basis to understand the physiological role of catecholestrogens in ovarian functions.

Inhibiting effect of ethinylestradiol/levonorgestrel combination on microsomal enzymatic activities in rat liver and kidney

The aim of the study was to evaluate the effects of two therapeutic combinations of ethinylestradiol (EE) and levonorgestrel (LE), which are used in triphasic contraceptives, on the activities of drug-metabolizing enzymes in rat liver and kidney. Sexually mature female Wistar rats were given 0.03 mg EE and 0.05 mg LE, or 0.03 mg EE and 0.125 mg LE for 6 or 18 sexual cycles, i.e. for 30 or 90 days. EE/LE inhibited not only the metabolic capacity of P450, a protein which directly undergoes suicide inhibition, but also the level of rat liver cytochrome b5 (dependent on the heme pool) as well as the activities of NADPH-cytochrome P450 reductase and NADH-cytochrome b5 reductase in the liver and kidney. The majority of these effects were independent of the gestagen dose and of the duration of treatment, suggesting that estrogen is a predominant inhibiting factor in the EE/LE combination. The study has revealed differences in the enzyme activities between the liver and kidney, which may result from the fact that these organs display different sets of P450 isoforms and, therefore, their monooxygenase systems show distinct capacities to metabolize exogenous steroids.

Studies on neurosteroids. IX. Characterization of estrogens in rat brains using gas chromatography-tandem mass spectrometry

The characterization of the classical estrogens (estrone, estradiol, estriol) and guaiacol estrogens (2-hydroxyestrone 3-methyl ether, 4-hydroxyestrone 3-methyl ether) in rat brains was performed using gas chromatography-tandem mass spectrometry (GC-MS-MS). Estrogens were purified from Wistar strain rat brains by some chromatographic pre-treatments, such as solid-phase extraction, preparative thin-layer chromatography or preparative high-performance liquid chromatography. After the derivatization with O-methylhydroxylamine and/or N,O-bis(trimethylsilyl)trifluoroacetamide, estrogens were identified by comparison of their chromatographic behavior during GC-MS-MS operating in the product ion scan mode and comparison with the product ion MS spectra of an authentic sample. These evidences suggested that estrogens exist in rat brains as neurosteroids or neuroactive steroids.

Effects of estrogens on human melanocytes in vitro

Subjects with elevated serum estrogen concentrations, such as those who are pregnant or ingesting estrogen-containing contraceptive medication, may develop increased skin pigmentation. As little information is available on the mechanism(s) underlying this relationship, the in vitro effects of estrogens on melanocytes cultured from normal human skin were examined. Physiological concentrations of 17 beta-estradiol (10(-11) to 10(-9) M) significantly increased the activity of tyrosinase in melanocytes from 15 of 23 subjects. The observed increases ranged from 1.2- to 2.4-fold. Melanin synthesis, which correlated with tyrosinase activity (r = 0.98, P < 0.001) was increased to a similar extent. Melanin extrusion was also increased by 17 beta-estradiol (10(-9) M). The estrogens, estriol (10(-9) M) and estrone (10(-9) M) stimulated tyrosinase activity and melanin extrusion to a lesser extent than 17 beta-estradiol. The analogue 17 alpha-estradiol (10(-9) M) was shown to have effects on melanocyte tyrosinase activity and melanin extrusion that were equivalent to those of 17 beta-estradiol. The pure estrogen antagonist ICI 164384 (10(-6) M) also stimulated tyrosinase activity. Cycloheximide (50 micrograms/ml) inhibited 17 beta-estradiol-induced tyrosinase stimulation (P < 0.001). These results indicate that several aspects of melanocyte function respond directly to estrogenic stimulation. The equivalent effects of the 17 alpha-analogue and a "pure" anti-estrogen suggest that the 17 beta-estradiol response may be mediated through a non-classical mechanism which is similar to that described in other tissues of neural crest origin.

P450 enzymes of estrogen metabolism

Endogenous and exogenous estrogens undergo extensive oxidative metabolism by specific cytochrome P450 enzymes. Certain drugs and xenobiotics have been found to be potent inducers of estrogen hydroxylating enzymes with C-2 hydroxylase induction being greater than that of C-16 hydroxylase. Oxygenated estrogen metabolites have different biological activities, with C-2 metabolites having limited or no activity and C-4 and C-16 metabolites having similar potency to estradiol. Pathophysiological roles for some of the oxygenated estrogen metabolites have been proposed, e.g. 16 alpha-hydroxyestrone and 4-hydroxyestrone. These reactive estrogens are capable of damaging cellular proteins and DNA and may be carcinogenic in specific cells.

Inhibition of rat liver microsomal estrogen 2-hydroxylase by 2-methoxyestrogens

The inhibition of estrogen 2-hydroxylase by 2-methoxyestrogens was demonstrated in screening assays and has been further investigated under initial velocity conditions. The ability of 2-methoxyestradiol and 2-methoxyestrone to block the conversion of estradiol to 2-hydroxyestradiol by male rat liver microsomal preparations was determined by measuring the release of 3H2O from [2-3H]estradiol. The apparent Kis were found to be 34.86 microM for 2-methoxyestradiol and 18.65 microM for the methoxyestrone, with the apparent Km for the substrate estradiol in these essays of 3.21 microM. Mixed inhibition studies with the methoxyestrogens and 2,4-dibromoestradiol, an effective estrogen 2-hydroxylase inhibitor, in male rat liver microsomes resulted in Dixon plots consisting of a series of parallel lines. Thus, methoxyestrogens and 2,4-dibromoestradiol are mutually exclusive inhibitors, i.e., the binding of one compound to the enzyme interferes with the binding of the other. These results indicate that the compounds are interacting at the same enzymatic site. Finally, a method utilized to measure estrogen 2-hydroxylase activity in vitro is a radioenzymatic assay involving addition of catechol o-methyltransferase (COMT) and radiolabeled S-adenosylmethionine, and the amount of catechol estrogens formed is determined by the amount of radiolabeled methoxyestrogens isolated. The results described here demonstrate inhibition of estrogen 2-hydroxylase by methoxyestrogens; however, under enzymatic conditions of low product formation, the estrogen 2-hydroxylase inhibitory effect of catechol estrogen products from the radioenzymatic assay would be insignificant. Thus, these interactions of methoxyestrogens suggest that the steroid hormonal environment be considered in the examination of estrogen 2-hydroxylase and the catechol estrogen products. off