Irreversible inhibition of delta 5-3-oxosteroid isomerase by 2-substituted progesterones

Mechanisms responsible for progesterone's protection against lordosis-inhibiting effects of restraint II. Role of progesterone metabolites

When ovariectomized Fischer female rats are hormonally primed with 10 μg estradiol benzoate, a 5 min restraint experience rapidly inhibits lordosis behavior. Addition of progesterone to the hormonal priming prevents this restraint-induced inhibition. In prior work, we reported evidence that progesterone receptors (PR) may contribute to this protective effect of progesterone. In the current manuscript, we provide evidence that progesterone metabolites may also contribute to progesterone's ability to reduce the effects of restraint. Ovariectomized female rats were hormonally primed with 10 μg estradiol benzoate followed 2 days later with 4.0 mg/kg of the progesterone metabolite, allopregnanolone. Allopregnanolone, administered either 4 h or 2 h before the restraint experience, was as effective as progesterone in reducing the lordosis-inhibitory effects of restraint. In the second experiment, progesterone metabolism was blocked with 50 mg/kg of the 5α-reductase inhibitor, finasteride. Surprisingly, finasteride did not prevent progesterone from reducing the effects of restraint. In a third experiment, we tested the possibility that allopregnanolone acted through metabolism to dihydroprogesterone. Rats were treated with allopregnanolone or with allopregnanolone plus the 3α-hydroxysteroid dehydrogenase inhibitor, indomethacin. Indomethacin did not prevent allopregnanolone from reducing the effects of restraint. Mechanisms are discussed whereby cross-talk between PR-mediated and metabolite-mediated events may converge in producing progesterone's attenuation of the effect of restraint.

Clues to the development of mechanism-based inactivators of 3 alpha-hydroxysteroid dehydrogenase: comparison of steroidal and nonsteroidal Michael acceptors and epoxides

A series of steroidal and nonsteroidal Michael acceptors that represent reaction products for 3 alpha-hydroxysteroid dehydrogenase were synthesized and evaluated as potential enzyme-generated inactivators. Introduction of exocyclic olefins either at C-2 or C-6 produced inhibitors with high affinity for the enzyme (0.05 to 5.0 microM). However, despite this affinity, none of these compounds produced time-dependent inactivation of the enzyme. By contrast, analogs based on 1-phenyl-2-propen-1-one were stoichiometric inactivators of the enzyme and ease of turnover of the parent latent Michael acceptor depended on the presence of an electron-withdrawing substituent at the para position. A series of steroidal and nonsteroidal epoxides in which the oxiranyl oxygen could be substituted for the 3-ketone (the acceptor carbonyl of a steroid substrate) were also synthesized and evaluated as potential mechanism-based inactivators. Steroidal 2 alpha,3 alpha-, and 3 alpha,4 alpha-epoxides as well as 3 alpha- and 3 beta-spiroepoxides did not bind to the enzyme and were unable to cause enzyme inactivation in either the presence or absence of pyridine nucleotide. In contrast, nitrostyrene oxides produced time-dependent inactivation, the rate of which was governed by the presence of an electron withdrawing group at the para position. These data indicate that the design of mechanism-based inactivators for 3 alpha-hydroxysteroid dehydrogenase requires the incorporation of electron-withdrawing groups adjacent to the latent enzyme-activated group and, as a result, the turnover and/or reactivity of these compounds is increased. Moreover, these compounds can be modeled on nonsteroids.

Inhibition of progesterone synthesis in normal and transformed human placental cells by tight binding inhibitors of 3 beta-hydroxysteroid dehydrogenase

The biosynthesis of progesterone from [3H]pregnenolone was curvilinear over a 6 h time course in human placenta cytotrophoblasts and in human placenta choriocarcinoma cells (JEG-3 cells). Mass measurements determined independently by radioimmunoassay indicate that the progesterone synthesized by cytotrophoblasts (21.0 +/- 5.20 ng/6 h/mg protein) is substantially higher than that synthesized by the JEG-3 cells (4.48 +/- 0.56 ng/6 h/mg protein). Two tight binding inhibitors of 3 beta-hydroxysteroid dehydrogenase (2 alpha-cyanoprogesterone I and cyanoketone II), and a potent inhibitor of the microsomal conversion of pregnenolone to progesterone (2 alpha-bromo-5 alpha-androstan-3-one-17 beta-acetate III) were compared as inhibitors of progesterone synthesis in the two cell-types. Compounds I and II were very potent inhibitors yielding IC50 values of between 10 and 20 nM. At higher concentrations (100 nM - 1,000 nM) compound I promoted a complete cessation of progesterone synthesis which could be reversed by washing the cells free of inhibitor. By contrast compound III was ineffectual as an inhibitor yielding an IC50 value greater than 10 microM. This 1,000-fold difference in inhibitory potency suggests that 2 alpha-cyano-substituted steroids display an unusual capacity to inhibit progesterone biosynthesis and secretion in normal and transformed human cells.

Detection of a transient enzyme-steroid complex during active-site-directed irreversible inhibition of 3-oxo-delta 5-steroid isomerase

The reaction of the active-site-directed irreversible inhibitor (17S)-spiro[estra-1,3,5(10),6,8-pentaene-17,2'-oxiran]-3-ol (5 beta) with 3-oxo-delta 5-steroid isomerase has been monitored by repetitive scanning ultraviolet spectroscopy of a solution of 5 beta plus isomerase against a blank containing only 5 beta. Upon initial mixing of 5 beta with the isomerase an absorbance maximum at ca. 250 nm appears. With time, this peak decreases and is replaced with a new peak near 280 nm. These results directly demonstrate the existence of a transient enzyme-steroid intermediate in the inactivation reaction. The ultraviolet spectrum suggests that the steroid in the transient complex resembles the ionized phenol, while the phenolic group in the irreversibly bound complex is un-ionized. These spectral studies support our previous proposal that there are two enzyme-steroid complexes that are related by a 180 degree rotation about an axis perpendicular to the plane of the steroid nucleus. This hypothesis offers an explanation for the reaction of 17 beta-oxiranes with the same residue (Asp-38) that is thought to be involved in the catalytic mechanism. Two new oxiranes, (17S)-spiro[estra-1,3,5(10)-triene-17,2'-oxiran]-3 beta-ol (6 beta) and (17S)-spiro[5 alpha-androstane-17,2'-oxiran]-3-one (8 beta), were also found to be potent active-site-directed irreversible inhibitors of the isomerase (k3/KI = 31 M-1 s-1 and 340 M-1 s-1, respectively). The relationship of these results to the nature of the active site of the isomerase is discussed.

Potent inhibition of mammalian progesterone synthesis by 2 alpha-cyanoprogesterone

2 alpha-Cyanoprogesterone potently inhibits the conversion of [3H]pregnenolone into progesterone catalysed by bovine corpora lutea, bovine adrenal cortex and human term placenta microsomes (microsomal fractions), yielding IC50 (concentration causing 50% inhibition) values of 66 nM, 120 nM and 700 nM respectively. By contrast, it is an exceedingly poor inhibitor of the isomerization of pregn-5-ene-3,20-dione, yielding IC50 values between 50 and 70 microM. On this basis, 2 alpha-cyanoprogesterone would appear to be an extraordinarily selective inhibitor of the 3 beta-hydroxysteroid dehydrogenase. Dixon plots indicate that it is a very-tight-binding competitive inhibitor of the corpus-luteum enzyme, yielding a Ki of 15 nM. In the bovine adrenal cortex and human placenta the steroid is less potent and inhibits the dehydrogenase non-competitively with Ki values of 150 nM and 1.0 microM respectively. Thus 2 alpha-cyanoprogesterone inhibits the corpus-luteum dehydrogenase with substantial selectivity. Because of its high affinity for the ovarian enzyme, the presence of low-micromolar concentrations of 2 alpha-cyanoprogesterone can promote a complete cessation of progesterone synthesis in corpora-lutea microsomes for several hours. Since this effect is observed in the presence of saturating concentrations of pregnenolone (50 microM), it is predicted that this inhibitor may be even more potent in vivo. 2 alpha-Cyanoprogesterone displays very low affinity for the human progesterone receptor, yielding a Kd of 600 nM as against a Kd of 1.6 nM for progesterone. It is suggested that 2 alpha-cyanoprogesterone may be a selective inhibitor of ovarian progesterone synthesis and may act as an effective anti-gestational agent in vivo.