Enzyme-activated inhibitors of steroidal hydroxylases

Structural insights into aldosterone synthase substrate specificity and targeted inhibition

Aldosterone is a major mineralocorticoid hormone that plays a key role in the regulation of electrolyte balance and blood pressure. Excess aldosterone levels can arise from dysregulation of the renin-angiotensin-aldosterone system and are implicated in the pathogenesis of hypertension and heart failure. Aldosterone synthase (cytochrome P450 11B2, CYP11B2) is the sole enzyme responsible for the production of aldosterone in humans. Blocking of aldosterone synthesis by mediating aldosterone synthase activity is thus a recently emerging pharmacological therapy for hypertension, yet a lack of structural information has limited this approach. Here, we present the crystal structures of human aldosterone synthase in complex with a substrate deoxycorticosterone and an inhibitor fadrozole. The structures reveal a hydrophobic cavity with specific features associated with corticosteroid recognition. The substrate binding mode, along with biochemical data, explains the high 11β-hydroxylase activity of aldosterone synthase toward both gluco- and mineralocorticoid formation. The low processivity of aldosterone synthase with a high extent of intermediates release might be one of the mechanisms of controlled aldosterone production from deoxycorticosterone. Although the active site pocket is lined by identical residues between CYP11B isoforms, most of the divergent residues that confer additional 18-oxidase activity of aldosterone synthase are located in the I-helix (vicinity of the O(2) activation path) and loops around the H-helix (affecting an egress channel closure required for retaining intermediates in the active site). This intrinsic flexibility is also reflected in isoform-selective inhibitor binding. Fadrozole binds to aldosterone synthase in the R-configuration, using part of the active site cavity pointing toward the egress channel. The structural organization of aldosterone synthase provides critical insights into the molecular mechanism of catalysis and enables rational design of more specific antihypertensive agents.

Construction of 3D models of the CYP11B family as a tool to predict ligand binding characteristics

Aldosterone is synthesised by aldosterone synthase (CYP11B2). CYP11B2 has a highly homologous isoform, steroid 11beta-hydroxylase (CYP11B1), which is responsible for the biosynthesis of aldosterone precursors and glucocorticoids. To investigate aldosterone biosynthesis and facilitate the search for selective CYP11B2 inhibitors, we constructed three-dimensional models for CYP11B1 and CYP11B2 for both human and rat. The models were constructed based on the crystal structure of Pseudomonas Putida CYP101 and Oryctolagus Cuniculus CYP2C5. Small steric active site differences between the isoforms were found to be the most important determinants for the regioselective steroid synthesis. A possible explanation for these steric differences for the selective synthesis of aldosterone by CYP11B2 is presented. The activities of the known CYP11B inhibitors metyrapone, R-etomidate, R-fadrazole and S-fadrazole were determined using assays of V79MZ cells that express human CYP11B1 and CYP11B2, respectively. By investigating the inhibitors in the human CYP11B models using molecular docking and molecular dynamics simulations we were able to predict a similar trend in potency for the inhibitors as found in the in vitro assays. Importantly, based on the docking and dynamics simulations it is possible to understand the enantioselectivity of the human enzymes for the inhibitor fadrazole, the R-enantiomer being selective for CYP11B2 and the S-enantiomer being selective for CYP11B1.

Gas chromatography-mass spectrometric study of 19-oxygenation of the aromatase inhibitor 19-methylandrostenedione with human placental microsomes

To gain insight into the catalytic function of aromatase, we studied 19-oxygenation of 19-methyl-substituted derivative of the natural substrate androstenedione (AD), compound 1, with human placental aromatase by use of gas chromatography-mass spectrometry (GC-MS). Incubation of the 19-methyl derivative 1 with human placental microsomes in the presence of NADPH under an aerobic condition did not yield a detectable amount of [19S]19-hydroxy product 2 or its [19R]-isomer 3 when the product was analyzed as the bis-methoxime-trimethylsilyl (TMS) derivative by GC-MS; moreover, the production of estrogen was not detected as the bis-TMS derivative of estradiol (detection limit: about 3 ng and 10 pg per injection for the 19-ol and estradiol, respectively). The results reveal that the 19-methyl steroid 1 does not serve as a substrate of aromatase, although it does serve as a powerful inhibitor of the enzyme.

Gonadotropin-releasing hormone analogues added to adjuvant chemotherapy protect ovarian function and improve clinical outcomes in young women with early breast carcinoma

BACKGROUND

The objective of the current study was a retrospective evaluation of 100 consecutive premenopausal women with high-risk, early breast carcinoma who received a gonadotropin-releasing hormone (Gn-RH) analogue as ovarian protection during adjuvant chemotherapy.

METHODS

After surgery, patients received a Gn-RH analogue and adjuvant chemotherapy, which was tailored to their peculiar biologic features. The median patient age was 43 years (range, 27-50 yrs). Fifty-two women had positive estrogen receptor (ER) status, and 48 women had negative ER status. There were 64 women with Stage II breast carcinoma and 36 women with UICC Stage III breast carcinoma. All patients had their serum estradiol suppressed to values<40 pg/mL. The chemotherapy regimens administered included cyclophosphamide, methotrexate, and 5-fluorouracil (n=26 patients) and anthracycline-based regimens (n=74 patients, including 9 patients who had >10 positive axillary lymph nodes, who also received high-dose chemotherapy with autologous peripheral blood progenitor cell transplantation). Patients with positive c-erb-2 status also received a taxane. Eighty patients received radiation therapy. During therapy with the Gn-RH analogue, patients who had a positive ER status after chemotherapy received an aromatase inhibitor.

RESULTS

After a median follow-up of 75 months, normal menses were resumed by all patients younger than age 40 years and by 56% of patients older than age 40 years. Three pregnancies were observed that resulted in two normal deliveries and one voluntary abortion. The projected recurrence-free survival rates at 5 years and 10 years were 84% and 76%, respectively; and the projected overall survival rates at 5 years and 10 years were 96% and 91%, respectively.

CONCLUSIONS

The current data showed that, in premenopausal women with early breast carcinoma, the addition of a Gn-RH analogue to adjuvant therapy and temporary total estrogen suppression in patients with ER-positive disease was tolerated well, protected long-term ovarian function, and appeared to improve the expected clinical outcome.

Extra-adrenal production of corticosteroids

1. The major corticosteroids aldosterone and cortisol (corticosterone in rodents) are secreted from the adrenal cortex under the regulation of the renin-angiotensin system and the hypothalamic-pituitary-adrenal axis. 2. In addition to their accepted roles in such processes as blood pressure regulation, glycogenesis, hepatic glyconeogenesis and immunosuppression, the corticosteroids have been implicated in the development of cardiac fibrosis, modulation of hippocampal neuron excitability, memory formation and neurodegeneration. 3. The advent of sensitive molecular biological techniques has produced a wealth of evidence to support the existence of extra-adrenal corticosteroidogenic systems. Most attention has been paid to the cardiovascular system and the central nervous system, where the full array of enzymes required for the de novo synthesis of corticosteroids from cholesterol has been identified. 4. Although the evidence for local corticosteroid production is strong, the quantities of steroid would be small compared with adrenal production. Therefore, it is still a matter of debate as to whether extra-adrenal corticosteroids are of any physiological significance. This will depend on factors such as local concentration, proximity to target cells and, possibly, to tissue-specific control mechanisms.

Goserelin as ovarian protection in the adjuvant treatment of premenopausal breast cancer: a phase II pilot study

The aim of the present trial was to investigate the protective effects on ovarian function, and the efficacy and tolerability of goserelin added to adjuvant chemotherapy for early breast cancer. Following surgical treatment, 64 premenopausal patients with early breast cancer received goserelin 3.6 mg (every 28 days for 1 year) and an adjuvant treatment which was chosen according to the patient's prognosis. Median age was 42 years (range 27-50). ECOG performance status was 0-1 in all patients. Twenty-eight patients (44%) had estrogen receptor (ER)+ tumors and 36 (56%) patients had ER- tumors. Fifty-two (81%) patients had stage II disease and 12 (19%) had stage III disease. Eighteen patients received cyclophosphamide, methotrexate and 5-fluorouracil chemotherapy, 46 patients received an anthracycline-based regimen, and nine of them received high-dose chemotherapy and autologous peripheral blood progenitor cell transplantation. Fifty-one patients (80%) were irradiated. ER+ patients also received tamoxifen for 5 years. Serum estradiol was suppressed to values below 40 pg/ml in all patients. After a median follow-up of 55 months, 86% of patients had resumed normal menses, 84% of patients were disease-free and 94% were alive. The 1-, 3- and 5-year projected recurrence-free survival rates were 100, 81 and 75%, respectively. Five years after treatment one patient had a pregnancy that ended with a normal childbirth. No unexpected adverse events were reported. These data show that the addition of goserelin to adjuvant therapy of premenopausal patients with early breast cancer is well tolerated and protects long-term ovarian function.

Inhibitors of abscisic acid 8'-hydroxylase

Structural analogues of the phytohormone (+)-abscisic acid (ABA) have been synthesized and tested as inhibitors of the catabolic enzyme (+)-ABA 8'-hydroxylase. Assays employed microsomes from suspension-cultured corn cells. Four of the analogues [(+)-8'-acetylene-ABA, (+)-9'-propargyl-ABA, (-)-9'-propargyl-ABA, and (+)-9'-allyl-ABA] proved to be suicide substrates of ABA 8'-hydroxylase. For each suicide substrate, inactivation required NADPH, increased with time, and was blocked by addition of the natural substrate, (+)-ABA. The most effective suicide substrate was (+)-9'-propargyl-ABA (K(I) = 0.27 microM). Several analogues were competitive inhibitors of ABA 8'-hydroxylase, of which the most effective was (+)-8'-propargyl-ABA (K(i) = 1.1 microM). Enzymes in the microsomal extracts also hydroxylated (-)-ABA at the 7'-position at a low rate. This activity was not inhibited by the suicide substrates, showing that the 7'-hydroxylation of (-)-ABA was catalyzed by a different enzyme from that which catalyzed 8'-hydroxylation of (+)-ABA. Based on the results described, a simple model for the positioning of substrates in the active site of ABA 8'-hydroxylase is proposed. In a representative physiological assay, inhibition of Arabidopsis thaliana seed germination, (+)-9'-propargyl-ABA and (+)-8'-acetylene-ABA exhibited substantially stronger hormonal activity than (+)-ABA itself.

Mechanism-based inactivation of cytochrome P450 2B1 by 7-ethynylcoumarin: verification of apo-P450 adduction by electrospray ion trap mass spectrometry

7-Ethynylcoumarin was synthesized as a potential mechanism-based inhibitor, and it was found to be an effective inactivator of 7-ethoxy-4-(trifluoromethyl)coumarin (7EFC) O-deethylation catalyzed by purified, reconstituted P450 2B1. In contrast, 7-ethynylcoumarin demonstrated minimal inactivation of P450 2A6-mediated 7-hydroxycoumarin formation. The inactivation of P450 2B1 demonstrated pseudo-first-order kinetics and was NADPH- and inhibitor-dependent. The maximal rate constant for the inactivation of 2B1 was 0.39 min(-)(1) at 30 degrees C, and thus, the time required to inactivate 50% of the P450 2B1 that was present (t(1/2)) was 1.8 min. The estimated concentration which led to half-maximal inactivation (K(I)) was 25 microM. No protection from inactivation was seen in the presence of nucleophiles (glutathione and sodium cyanide), an iron chelator (deferroxamine), or superoxide dismutase and catalase. Addition of the substrate (7EFC) protected P450 2B1 from inactivation, in a concentration-dependent manner. The partition ratio for P450 2B1 was 25; i.e., the number of metabolic events was 25-fold higher than the number of inactivating events. Incubations of 7-ethynylcoumarin with P450 2B1 for 10 min resulted in an 80% loss in enzymatic activity, while 90% of the ability to form a reduced-CO complex remained. This activity loss was not recovered following dialysis, indicative of irreversible inactivation. Covalent attachment of the entire inhibitor and oxygen to apo-P450 2B1, in a 1:1 ratio, was shown via electrospray ion trap mass spectrometry. This method also verified the absence of modification to the heme or the cytochrome P450 reductase. Taken together, the characterization of the inhibition seen with P450 2B1 and 7-ethynylcoumarin was consistent with all of the criteria required to distinguish a mechanism-based inactivator. In addition, electrospray ion trap mass spectrometry has the potential to be applied to protein adducts above and beyond those associated with the mechanism-based inactivation of cytochrome P450s.

18-Vinyldeoxycorticosterone: a potent inhibitor of the bovine cytochrome P-450(11) beta

18-Vinylprogesterone (18-VP) and 18-ethynylprogesterone (18-EP) have proved to be potent suicide inhibitors of P-450(11) beta, the last enzyme of aldosterone biosynthesis (Delorme, C.; Piffeteau, A.; Viger, A.; Marquet, A. Eur. J. Biochem. 1995, 232, 247; Delorme, C.; Piffeteau, A.; Sobrio, F.; Marquet, A. Eur. J. Biochem. 1997, 248, 252). This paper describes the synthesis of 18-vinyldeoxycorticosterone (18-VDOC), an analogue of deoxycorticosterone (DOC), the physiological substrate of the enzyme, and the evaluation of its reversible inhibiting properties for deoxycorticosterone and corticosterone oxidation by the bovine enzyme. 18-VDOC has been obtained by hydroxylation at C-21 of a 18-VP precursor. Its reversible Ki values are, respectively, 0.3 microM for the 11 beta-hydroxylation and 0.8 microM for the 18-hydroxylation. Hence, 18-VDOC is the strongest competitive inhibitor of bovine P-450(11) beta described so far, but in contrast with 18-VP, it does not inhibit more efficiently the 18-hydroxylation than the 11-hydroxylation.

Central hypertensive effects of aldosterone

The soluble mineralocorticoid receptor bound to an agonist acts as a transcription factor for several genes relevant to ion transport by kidney and colon epithelial cells and is a major regulator of electrolyte and fluid homeostasis. Mineralocorticoids, the most prominent of which is aldosterone, also influence the activity of nonepithelial target cells, including vascular smooth muscle cells, by altering intracellular ion transport and content. Evidence is summarized for mineralocorticoid modulation of neuronal activity in a center or centers within the brain, probably in the periventricular area of the anterior hypothalamus, where information on electrolyte, fluid, and cardiovascular status is received and integrated, resulting in alterations in central sympathetic efferent activity. These functions are distinct from central aldosterone effects on salt appetite and peripheral trophic effects on cardiovascular tissue. The isolated mineralocorticoid receptor binds several adrenal steroids, including aldosterone and the major glucocorticoids, with equal affinity. Ligand specificity for the mineralocorticoid receptor differs between tissues, including different organs in the brain. Specificity is conferred extrinsically by the 11-beta-hydroxysteroid dehydrogenase enzymes in transport epithelia, but mechanisms for mineralocorticoid ligand specificity have not been completely defined in the brain. The functional interaction between the mineralocorticoid receptor bound to different ligands and between the mineralocorticoid and glucocorticoid receptors is complex and as yet unresolved. Evidence is presented for the de novo synthesis of adrenal corticosteroids in the brain which may, by paracrine regulation of central control mechanisms, be relevant for certain clinical and experimental forms of hypertension characterized by low circulating levels of mineralocorticoids which respond to mineralocorticoid receptor antagonists.

Mechanism-based inactivation of bovine cytochrome P-450(11beta) by 18-unsaturated progesterone derivatives

Two 18-unsaturated progesterone derivatives, 18-vinylprogesterone (18-VP) and 18-ethynylprogesterone (18-EP) have proved to be potent inhibitors of the bovine cytochrome P-450(11beta), the enzyme involved in the last steps of aldosterone biosynthesis [Delorme, C., Piffeteau, A., Viger, A. & Marquet, A. (1995) Eur. J. Biochem. 232, 247-256]. In the present study, we demonstrate that these two compounds exhibit the characteristics of mechanism-based inactivators of this enzyme. Inactivation followed pseudo-first-order and saturation kinetics. The kinetic parameters of inactivation were k(i) = 0.11 min(-1) and Ki = 4 microM for 18-VP, and k(i) = 0.12 min(-1) and 22 microM for 18-EP. Inactivation of P-450(11beta) activity was strictly dependent on the presence of NADPH. Protection by the substrate deoxycorticosterone was observed, demonstrating a selective modification at the substrate-binding site. With radiolabeled 18-VP, inactivation was shown to be irreversible with a stoichiometry of 1.4 mol bound [3H]18-VP/mol inactivated cytochrome P-450(11beta). SDS/PAGE analysis of the [3H]18-VP-inactivated enzyme showed that, under conditions preventing heme dissociation, the P-450(11beta) band was labeled, while no labeling of the apoprotein was observed under denaturating conditions. Furthermore, the loss of catalytic activity could be correlated with the destruction of the P-450 chromophore evaluated by the FeII-CO versus FeII difference spectra. These arguments led us to propose that 18-vinylprogesterone inactivates cytochrome P-450(11beta) by heme destruction rather than by protein modification.

1,2,3-Thiadiazole: a novel heterocyclic heme ligand for the design of cytochrome P450 inhibitors

The 1,2,3-thiadiazole heterocycle has been explored as a heme ligand and mechanism-based inactivator for the design of cytochrome P450 inhibitors. One 4,5-fused bicyclic and three 4,5-disubstituted monocyclic 1,2,3-thiadiazoles have been examined for their spectral interactions, inhibition, mechanism-based inactivation, and oxidation products by the versatile microsomal P450s 2B4, 2E1, and 1A2. The compounds generally show heteroatom coordination to the heme iron; however, the binding mode is influenced by the architecture of the active site. For example, 4,5-diphenyl-1,2,3-thiadiazole shows type I and type II difference spectra with P450s 2B4 and 2E1, respectively, and no spectral perturbation with P450 1A2. Except for the fused bicyclic compound, the spectral dissociation constants are in the 2-50 microM range. The effectiveness as an inhibitor depends on the substituents at the 4- and 5- positions and on the P450 examined. Inhibition of the P450-catalyzed 1-phenylethanol oxidation to acetophenone by the thiadiazoles does not correlate with either the type of binding spectra or the spectral dissociation constants of the compounds. P450s 2E1 and 2B4 are inactivated by the 4,5-fused bicyclic 1,2,3-thiadiazole in a mechanism-based manner. Inactivation of the P450 correlates with loss in absorbance at 450 nm for the ferrous-CO complex. The monocyclic 1,2,3-thiadiazoles do not inactivate any of the P450s examined. The 1,2,3-thiadiazole ring is oxidized by the P450 system. Oxidation of the monocyclic compounds results in extrusion of the three heteroatoms and formation of the corresponding acetylenes, whereas oxidation of the fused bicyclic compound does not yield an acetylenic product.

Inhibition of steroidogenesis in rat adrenal cells by 18-ethynyldeoxycorticosterone: evidence for an alternative pathway of aldosterone biosynthesis

The effect of the mechanism-based inhibitor 18-ethynyldeoxycorticosterone (18-E-DOC) on the late steps of the aldosterone biosynthetic pathway was examined in freshly isolated cells of the zona glomerulosa (ZG) and fasciculata (ZF) from rat adrenal glands. ZG synthesis of aldosterone was inhibited by 18-E-DOC in a time- and concentration-dependent manner with a Ki of approximately 0.05 microM. The maximal degree of inhibition of ZG production of aldosterone and 18-hydroxycorticosterone (18-OH-B) was approximately 80%. ZF cells, perhaps surprisingly, were found to secrete 18-OH-B at levels approximately one-third to one-fourth those of ZG cells and the Ki of 18-E-DOC inhibition of 18-OH-B secretion was approximately 10 microM for ZF cells, 200-fold higher than for ZG cells. The inhibitor had no effect on the secretion of corticosterone by either ZG or ZF, and the secretion of 18-hydroxydeoxycorticosterone (18-OH-DOC) by both the ZG and ZF was inhibited only to a minor degree. 18-E-DOC inhibited the biosynthesis of aldosterone by ZG cells incubated with 10 microM added DOC or 18-OH-DOC by approximately 75%, similar to the degree of inhibition of aldosterone biosynthesis from endogenous substrate, whereas ZF biosynthesis of 18-OH-B from either substrate was inhibited by less than 40%. ZF cells do not express aldosterone synthase, the only enzyme known to convert 18-OH-DOC into 18-OH-B. Incubation of MA-10 cells stably transfected with the cDNA of the rat aldosterone synthase with 18-E-DOC resulted in a complete inhibition of the conversion of DOC to aldosterone with a Ki of approximately 0.02 microM. In addition, transfected cells expressing 11beta-hydroxylase convert DOC to 18-OH-B in very small quantities only and cannot convert 18-OH-DOC to 18-OH-B. These data suggest that neither 11beta-hydroxylase nor aldosterone synthase are responsible for the biosynthesis of 18-OH-B by ZF cells from DOC or 18-OH-DOC, that 20% of aldosterone synthesis appears not to be attributable to the actions of aldosterone synthase and that an unknown CYP11B enzyme is also involved in the biosynthesis of 18-OH-B.

New steroidal diazo ketones as potential photoaffinity labeling reagents for the mineralocorticoid receptor: synthesis and biological activities

Three diazo ketones in the progesterone series were synthesized as potential photoaffinity reagents. The diazo ketone group was introduced at the C17 (21-diazopregn-4-ene-3,20-dione, 1) or C13 (18-(diazomethyl)-20-hydroxypregn-4-ene-3,18-dione, 2, 18-(diazomethyl)pregn-4-ene-3, 18,20-trione, 3) position of the pregnene skeleton. Whereas compound 1 could be easily obtained from the corresponding acid chloride, preparation of 2 and 3 required a less straightforward route involving reaction of tosyl azide on the formyl derivative of methyl ketone 5. The affinity of the diazo ketones for the human mineralocorticoid receptor (hMR), expressed in Sf9 insect cells using the Baculovirus system, was estimated by competition experiments using [3H]aldosterone as specific ligand. The affinity of 1 for hMR was almost identical with that of aldosterone. The affinities of 2 and 3 were 1, order of magnitude lower than that of aldosterone. The mineralocorticoid activity of the diazo ketones was measured in cis-trans cotransfection assays in CV-1 cells with the mouse mammary tumor virus as DNA target sequence. Compound 1 exhibits an agonist activity (ED50 = 6 x 10(-9) M) with no antagonist activity. In contrast 2 and 3 behave as antagonists, displaying an IC50 of approximately 10(-6) M whether the substituent at the C20 position is a hydroxy (2) or an oxo (3) group.

Synthesis and structure-activity relationships of 6-substituted androst-4-ene analogs as aromatase inhibitors

Series of 6 alpha- and 6 beta-alkyl-substituted androst-4-en-17-ones (18 and 19) and their 17 beta-reduced derivatives (14 and 15)(alkyl: methyl, ethyl, n-propyl, n-pentyl, n-octyl) were synthesized and evaluated as aromatase inhibitors. Androst-4-en-17-ones having an oxygen function (hydroxy, acetoxy, or methoxy group) at C-6 alpha and C-6 beta (4 and 5) were also tested for their abilities to inhibit aromatase. All of the steroids studied inhibited human placental aromatase in a competitive manner. The inhibitory activities of the 6 alpha- and 6 beta-methyl-17-keto steroids 18a and 19a (Ki = 3.1 and 5.3 nM, respectively) as well as the 6 beta- alcohol 5a (Ki = 6.0 nM) were high, and their apparent Ki values were lower than that of the parent 6-unsubstituted 3-deoxy steroid 1 (Ki = 6.8 nM). Elongation of the methyl group decreased affinity for aromatase in relation to carbon number of the alkyl chain in each series, in which the 6 alpha- alkyl steroids 18 essentially had higher affinity for the enzyme than the corresponding 6 beta- isomers 19. The inhibitory activities of the 17 beta-hydroxy analogs 14 and 15 were less potent than those of the corresponding 17-keto steroids. The 6 alpha-ethyl compound 18b, the 6 alpha-oxygenated derivatives 4, and the 6 beta-acetoxy and 6 beta-methoxy analogs 5b and 5c were powerful inhibitors (Ki = 12-24 nM). The methyl steroids (18a and 19a) produced "type I" difference spectra upon interaction with aromatase. These results along with molecular modeling with the PM3 method suggest that compounds 18a and 19a may produce a thermodynamically stable enzyme-inhibitor complex in the hydrophobic binding pocket with a limited accessible volume. A carbonyl group at C-17 of the 6-alkylandrost-4-enes is essential for the tight binding. Moreover, the binding pocket also tolerates a polar hydroxy group at the 6 beta-position rather than at the 6 alpha-position.

Specific inhibition of the last steps of aldosterone biosynthesis by 18-vinylprogesterone in bovine adrenocortical cells

18-Vinylprogesterone (18-VP), designed for mechanism-based specific inhibition of the last steps of the aldosterone biosynthesis, was used to characterize the mechanism of the 11 - and 18-hydroxylase activities of bovine cytochrome P450(11beta). In the present work, its action was studied by observations on a primary culture of bovine adrenocortical cells. First, we investigated the effects of 18-VP on the different enzymatic steps of the biosynthesis of cortisol and aldosterone. The production of cortisol, baseline or hormone-stimulated (ACTH or AII), was inhibited by 18-VP in a dose-dependent manner with a maximal inhibition at 5 microM. Supply of different exogenous substrates to support steroidogenesis revealed an inhibition of the last step of cortisol or corticosterone biosynthesis. We then used specific blockers to measure individual activities and conclude that 11beta-hydroxylation was the only enzymatic activity affected. Aldosterone, as well as 18-hydroxycorticosterone, was also measured following addition of corticosterone. The 18-hydroxylation of corticosterone was inhibited by 18-VP, with 50% inhibition occurring at 0.04 microM compared with the 50% inhibition value of 0.3 microM obtained for 11-hydroxylation. Surprisingly, 18-ethynyl-progesterone (18-EP), which has a structure very similar to 18-VP, only weakly inhibits 11beta-hydroxylation. The inhibition of aldosterone formation was also much lower with 18-EP than with 18-VP. These studies demonstrate that 18-VP inhibits only the later steps of aldosterone biosynthesis and more specifically 18- than 11-hydroxylation activity.