A gas chromatographic/mass spectrometric assay for catechol estrogens in microsomal incubations: comparison with a radiometric assay

Gas chromatographic determination of catecholestrogens following isolation by solid-phase extraction

A sensitive and specific assay for the determination of the catecholestrogens 2-hydroxyestradiol (2-OHE2) and 4-hydroxyestradiol (4-OHE2) using gas chromatography with electron-capture detection (GC-ECD) is described. The formation of 2- and 4-OHE2 was assessed following activation of 17beta-estradiol in the microsomal fraction of female rat livers. The analytes were isolated by solid-phase extraction, derivatized to their heptafluorobutyryl esters with heptafluorobutyric acid anhydride, and subjected to solvent exchange prior to analysis; this resulted in minimal chromatographic interference, long column life, and stable derivatized analytes. Derivatized catechols were separated and confirmed with dual column chromatography (DB-5 and DB-608) and quantitated using GC-ECD. The DB-608 column was preferred for quantitation as it provided better 4-OHE2 resolution from interference. Key validation parameters for the assay include sensitivity, intra- and inter-assay precision, and accuracy. Instrument sensitivity and limits of detection (LOD) and quantitation (LOQ) were determined statistically from fortification data approaching expected limits. For 2-OHE2 and 4-OHE2, respective values for these parameters were; instrument sensitivities of 0.4 and 0.7 pg, LODs of 0.8 and 1.3 ng/mg, and LOQs of 2.6 and 4.3 ng/mg.

Enzymatic O-methylation of catechol estrogens in red blood cells: differences in animal species and strains

Enzymatic O-methylation of catechol estrogens in red blood cells has been investigated with respect to species difference. In the presence of S-adenosylmethionine, 2- or 4-hydroxyestradiol (2-OHE2 or 4-OHE2) was incubated with blood lysate obtained from rats (five strains), guinea pigs, mice, rabbits, dogs, monkeys, and humans, respectively. The yielded guaiacols and unchanged substrate were determined by gas chromatography/mass spectrometry in a selected ion monitoring mode employing the corresponding 2H4-labeled compounds as internal standards. The total amounts of guaiacols formed from 2-OHE2 and 4-OHE2 were different, being the highest (79.6% and 38.1%) in monkeys and the lowest (5.1% and 1.9%) in humans. The ratios of isomeric guaiacols formed from 4-OHE2 (4Me/3Me) were 7.6-71, while those from 2-OHE2 (2Me/3Me) were 1.4-3.2. Thus, marked differences in O-methylation of catechol estrogens were observed among animal species, but no significant strain difference was detected in rats.

Oxidative metabolism of estrogens in rat intestinal mitochondria

Intestinal epithelial cells are capable of metabolizing a wide variety of exogenous substrates. To determine how this metabolic capacity may affect endogenous substances such as steroid hormones, we examined the ability of rat gut epithelial preparations to hydroxylate estradiol at the C-2 position. Utilizing a site-specific tritium exchange assay, an active estrogen 2-hydroxylase system was shown to be localized to gut mitochondria throughout the intestine, with enzymatic activities comparable to the activity in crude hepatic homogenates of non-induced animals (0.2 nmol/min/mg protein). Gas chromatography-mass spectrometry confirmed the formation of C-2 hydroxylated estrogens by these mitochondrial preparations. The enzyme system was shown to involve a saturable monooxygenase, utilizing NADH (preferably) or NADPH in a protein- and time-dependent fashion. The Michaelis-Menten constant for this pathway was approximately 150 microM. Enzyme activity decreased by 20% in the presence of carbon monoxide, and was largely unaffected by organic P450 inhibitors such as alpha-naphthoflavone, metyrapone, and SKF-525A. These studies suggest that intestinal mitochondria are able to contribute to the oxidative metabolism of endogenous estrogens circulating within the enterohepatic pool.

Determination of estradiol 2- and 4-hydroxylase activities by gas chromatography with electron-capture detection

A highly sensitive assay has been developed for measuring the rate of formation of 2-hydroxyestradiol and 4-hydroxyestradiol from estradiol by microsomal preparations. Catechol estrogens were converted to heptafluorobutyryl esters, which were separated by capillary column gas chromatography and quantified using electron-capture detection. 2-Hydroxyestradiol 17-acetate was used as an internal standard. The identity of catechol estrogen derivatives was verified by gas chromatography-mass spectrometry using negative-ion chemical ionization. Estrogens were identified by negative molecular ions and/or by characteristic fragments. This procedure permits quantification of catechol estrogens at the subpicogram level. The assay was validated by comparing estrogen 2- and 4-hydroxylase activities in microsomes from hamster and rat liver with values reported previously.

2,3,7,8-Tetrachlorodibenzo-p-dioxin causes an extensive alteration of 17 beta-estradiol metabolism in MCF-7 breast tumor cells

MCF-7 breast tumor cells form multicellular foci in vitro when supplemented with 17 beta-estradiol (E2). In the presence of E2 and the aryl hydrocarbon-receptor agonist 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), MCF-7 cells grow to confluence but do not form foci. To investigate the role of E2 metabolism in this antiestrogenic effect of TCDD, analyses were performed by capillary GC/MS. The results revealed that pretreatment of MCF-7 cultures with TCDD (10 nM) rapidly depletes E2. In untreated cultures supplemented with 10 nM E2, the concentration of free E2 decreased to 4 nM in the first 12 hr, followed by a slower rate of decline. After 3 days most E2 in the medium was in conjugated form(s); 1.7 nM was present as free E2, and 2.9 nM was released by treatment with glucuronidase/sulfatase. In TCDD-treated cultures, E2 declined to 290 pM in 12 hr and after 2 days was not detected (less than 100 pM) either as free steroid or after treatment with glucuronidase/sulfatase. Intracellular E2 and estrone were likewise depleted by pretreatment with TCDD. Microsomes from TCDD-treated cells showed highly elevated aryl hydrocarbon-hydroxylase activity and catalyzed hydroxylations of E2 at C-2, C-4, C-15 alpha, and C-6 alpha with a combined rate of 0.85 nmol/min per nmol of cytochrome P-450 at saturating E2. These results suggest that depletion of E2 by enhanced metabolism accounts for the antiestrogenic activity of TCDD in MCF-7 cells.

Peroxidase-catalyzed displacement of tritium from regiospecifically labeled estradiol and 2-hydroxyestradiol

Estradiol and 2-hydroxyestradiol with 3H at different positions in rings A, B or D were incubated with lactoperoxidase without added H2O2 and their oxidative transformation was followed by transfer of 3H into 3H2O. With estradiol, 3H loss from different positions in the aromatic ring was almost equal and also occurred to a lesser extent from the alicyclic portion of the molecule. Glutathione had less effect on the formation of 3H2O for the aromatic ring of estradiol than from that of the catechol estrogen where it increased the yield 6-fold. The rate of 3H loss was also very much greater from tritiated 2-hydroxyestradiol than from estradiol and NADPH was inhibitory with both steroids. Conditions for the release of 3H from estradiol and 2-hydroxyestradiol by peroxidase as well as the effect of some biochemical inhibitors were also investigated. The possible contribution of peroxidative formation of 3H2O during the radiometric assay for catechol estrogen biosynthesis by tissue monooxygenases is discussed.

Comparison of assays for catechol estrogen synthase activity: product isolation vs radioenzymatic catechol-O-methyltransferase-coupled procedures

Reported values for the activity of enzymes mediating catechol estrogen formation by hamster kidney and liver, measured by catechol-O-methyltransferase-coupled radioenzymatic assay, have been uniformly low and there have been marked discrepancies in values reported from different laboratories. Therefore, we examined the validity of the radioenzymatic assay used in these studies. NADPH-dependent estrogen 2- and 4-hydroxylase activity of hamster liver microsomes measured by radioenzymatic assay was comparable to that reported in the literature but at least one order of magnitude lower than that obtained with a direct product isolation assay. Several features of the radioenzymatic assay were identified which, together, contribute to the underestimation of enzyme activity. They include, incomplete protection from oxidative degradation of both the catechol estrogens generated and of the catechol-O-methyltransferase and assay conditions which are suboptimal for O-methylation of the catechol estrogens. We conclude that results obtained using the catechol-O-methyltransferase-based radioenzymatic assay can only be considered valid if a consistent stoichiometric relationship can be demonstrated between the amounts of catechol estrogens and their O-methylated products.

Opposite effects of estrogen and catecholestrogen on hormone-sensitive breast cancer cell growth and differentiation

Catecholestrogens and especially 2-hydroxyestrone (2OH-E1) are estradiol metabolites locally formed in breast cancer cells. The present study demonstrates that the two parent compounds, estradiol (E2) and its metabolite 2OH-E1, exert opposite effects on hormone-sensitive breast cancer cell growth assessed by cell counts and transferrin receptor levels, and also on cell differentiation assessed by secreted proteins such as alpha-lactalbumin and gross cystic disease fluid protein (GCDFP-15). The present findings may highlight estradiol regulation in hormone-sensitive breast cancer cells.