Sterochemistry of estrogen biosynthesis

Human aromatase: perspectives in biochemistry and biotechnology

Aromatase (CYP19) is involved in steroidogenesis, catalyzing the conversion of androgens into estrogens through a unique reaction that causes the aromatization of the A ring of the steroid. The enzyme is widely distributed and well conserved among species as it plays a crucial role in physiological processes such as control of reproduction and neuroprotection. It has also been a subject of intense research both at the biotechnological level in drug development due to its involvement in estrogen-dependent tumors and at a fundamental biochemical level because there are numerous questions regarding its reaction mechanism. This review will report the great progress made in this area.

Coupled electron transfer and proton hopping in the final step of CYP19-catalyzed androgen aromatization

Aromatase (CYP19) catalyzes the terminal step in estrogen biosynthesis, which requires three separate oxidation reactions, culminating in an enigmatic aromatization that converts an androgen to an estrogen. A stable ferric peroxo (Fe(3+)O(2)(2-)) intermediate is seen by electron paramagnetic resonance, but its role in this complex reaction remains controversial. Combining molecular dynamics simulation and hybrid quantum mechanics/molecular mechanics, we show that ferric peroxo addition to the 19-aldehyde initiates the reaction. Stepwise cleavage of the C10-C19 and O-O bonds of the peroxohemiacetal extrudes formate and yields Compound II, which in turn desaturates the steroid through successive abstraction of the 1β-hydrogen atom and deprotonation of the 2β-position. Throughout the transformation, a proton is cyclically relayed between D309 and the substrate to stabilize reaction intermediates. This mechanism invokes novel oxygen intermediates and provides a unifying interpretation of past experimental mechanistic studies.

Aromatase, aromatase inhibitors, and breast cancer

Estrogens are known to be important in the growth of breast cancers in both pre and postmenopausal women. As the number of breast cancer patients increases with age, the majority of breast cancer patients are postmenopausal women. Although estrogens are no longer made in the ovaries after menopause, peripheral tissues produce sufficient concentrations to stimulate tumor growth. As aromatase catalyzes the final and rate-limiting step in the biosynthesis of estrogen, inhibitors of this enzyme are effective targeted therapy for breast cancer. Three aromatase inhibitors (AIs) are now FDA approved and have been shown to be more effective than the antiestrogen tamoxifen and are well tolerated. AIs are now a standard treatment for postmenopausal patients. AIs are effective in adjuvant and first-line metastatic setting. This review describes the development of AIs and their current use in breast cancer. Recent research focuses on elucidating mechanisms of acquired resistance that may develop in some patients with long term AI treatment and also in innate resistance. Preclinical data in resistance models demonstrated that the crosstalk between ER and other signaling pathways particularly MAPK and PI3K/Akt is an important resistant mechanism. Blockade of these other signaling pathways is an attractive strategy to circumvent the resistance to AI therapy in breast cancer. Several clinical trials are ongoing to evaluate the role of these novel targeted therapies to reverse resistance to AIs. Article from the special issue on 'Targeted Inhibitors'.

History of aromatase: saga of an important biological mediator and therapeutic target

Aromatase is the enzyme that catalyzes the conversion of androgens to estrogens. Initial studies of its enzymatic activity and function took place in an environment focused on estrogen as a component of the birth control pill. At an early stage, investigators recognized that inhibition of this enzyme could have major practical applications for treatment of hormone-dependent breast cancer, alterations of ovarian and endometrial function, and treatment of benign disorders such as gynecomastia. Two general approaches ultimately led to the development of potent and selective aromatase inhibitors. One targeted the enzyme using analogs of natural steroidal substrates to work out the relationships between structure and function. The other approach initially sought to block adrenal function as a treatment for breast cancer but led to the serendipitous finding that a nonsteroidal P450 steroidogenesis inhibitor, aminoglutethimide, served as a potent but nonselective aromatase inhibitor. Proof of the therapeutic concept of aromatase inhibition involved a variety of studies with aminoglutethimide and the selective steroidal inhibitor, formestane. The requirement for even more potent and selective inhibitors led to intensive molecular studies to identify the structure of aromatase, to development of high-sensitivity estrogen assays, and to "mega" clinical trials of the third-generation aromatase inhibitors, letrozole, anastrozole, and exemestane, which are now in clinical use in breast cancer. During these studies, unexpected findings led investigators to appreciate the important role of estrogens in males as well as in females and in multiple organs, particularly the bone and brain. These studies identified the important regulatory properties of aromatase acting in an autocrine, paracrine, intracrine, neurocrine, and juxtacrine fashion and the organ-specific enhancers and promoters controlling its transcription. The saga of these studies of aromatase and the ultimate utilization of inhibitors as highly effective treatments of breast cancer and for use in reproductive disorders serves as the basis for this first Endocrine Reviews history manuscript.

The Final Catalytic Step of Cytochrome P450 Aromatase: A Density Functional Theory Study

B3LYP density functional theory calculations are used to unravel the mysterious third step of aromatase catalysis. The feasibility of mechanisms in which the reduced ferrous dioxygen intermediate mediates androgen aromatization is explored and determined to be unlikely. However, proton-assisted homolysis of the peroxo hemiacetal intermediate to produce P450 compound I and the C19 gem-diol likely proceeds with a low energetic barrier. Mechanisms for the aromatization and deformylation sequence which are initiated by 1beta-hydrogen atom abstraction by P450 compound I are considered. 1beta-Hydrogen atom abstraction from substrates in the presence of the 2,3-enol encounters strikingly low barriers (5.3-7.8 kcal/mol), whereas barriers for this same process rise to 17.0-27.1 kcal/mol in the keto tautomer. Transition states for 1beta-hydrogen atom abstraction from enolized substrates in the presence of the 19-gem-diol decayed directly to the experimentally observed products. If the C19 aldehyde remains unhydrated, aromatization occurs with concomitant decarbonylation and therefore does not support dehydration of the C19 aldehyde prior to the final catalytic step. On the doublet surface, the transition state connects to a potentially labile 1(10) dehydrogenated product, which may undergo rapid aromatization, as well as formic acid. Ab initio molecular dynamics confirmed that the 1beta-hydrogen atom abstraction and deformylation or decarbonylation occur in a nonsynchronous, coordinated manner. These calculations support a dehydrogenase behavior of aromatase in the final catalytic step, which can be summarized by 1beta-hydrogen atom abstraction followed by gem-diol deprotonation.

Studies on the interactions between drug and estrogen. II. On the inhibitory effect of 29 drugs reported to induce gynecomastia on the oxidation of estradiol at C-2 or C-17

A study was investigated on the inhibitory effect of 29 drugs that have been reported to induce gynecomastia on the 2-hydroxylation of estradiol (E2) by recombinant P450 CYP3A4 and on the 17-oxidation of E2 by hepatic microsomal type II 17beta-hydroxysteroid dehydrogenase (17beta-HSD) of human male. The IC(50) values were determined for each drug relative to the 2-hydroxylation of E2 (catalytic activity: 1.54 nmol/nmol P450/min), and the inhibition constants (K(i)) were determined for 13 drugs of which IC(50) values were 100 microM or less. Ketoconazole exhibited the lowest inhibitory concentration, and IC(50) and K(i) values of 0.007 and 0.01 microM, respectively, were obtained. The IC(50) and K(i) values for each of the 12 remaining drugs were as follows: cyclosporin A (IC(50): 0.064, K(i): 0.30), nicardipine hydrochloride (0.55, 0.29), tacrolimus (0.64, 0.88), mandipine hydrochloride (3.9, 2.6), nisoldipine (10, 3.3), verapamil hydrochloride (10, 20), domperidone (13, 7.2), haloperidol (14, 55), nitrendipine (14, 2.5), chlormadinone acetate (16, 10), flutamide (30, 39) and omeprazole (49, 47). With the exception of cyclosporin A that exhibited a competitive inhibition, the inhibition mechanisms of these drugs were all non-competitive. Next, the percentage inhibition of the above 29 drugs relative to the 17-oxidation of E2 (catalytic activity: 0.47 nmol/mg protein/min) was investigated at the approximate therapeutic concentration (1 microM) and at the non-clinical overdose concentration (100 microM). Although none of the drugs investigated exhibited inhibitory effects at a concentration of 1 microM, spironolactone and ketoconazole at 100 microM demonstrated percentage inhibitions of 96% and 77%, respectively. When the K(i) values were determined for these two drugs, the former had a K(i) value of 2.4 microM and the latter, 41 microM, and both of their inhibition mechanisms were non-competitive. On the basis of the above results, a total of 14 drugs consisting of the above 13 drugs plus spironolactone were found to inhibit the 2-hydroxylation or 17-oxidation of E2 in the liver, and this is presumed to act as a trigger that causes as increase in the estradiol pool, followed by induction of gynecomastia.

Enzymic aromatization of 6-alkyl-substituted androgens, potent competitive and mechanism-based inhibitors of aromatase

To gain insight into the relationships between the aromatase inhibitory activity of 6-alkyl-substituted androgens, potent competitive inhibitors, and their ability to serve as a substrate of aromatase, we studied the aromatization of a series of 6alpha- and 6beta-alkyl (methyl, ethyl, n-propyl, n-pentyl and n-heptyl)-substituted androst-4-ene-3,17-diones (ADs) and their androsta-1,4-diene-3,17-dione (ADD) derivatives with human placental aromatase, by gas chromatography-mass spectrometry. Among the inhibitors examined, ADD and its 6alpha-alkyl derivatives with alkyl functions less than three carbons long, together with 6beta-methyl ADD, are suicide substrates of aromatase. All of the steroids, except for 6beta-n-pentyl ADD and its n-heptyl analogue as well as 6beta-n-heptyl AD, were found to be converted into the corresponding 6-alkyl oestrogens. The 6-methyl steroids were aromatized most efficiently in each series, and the aromatization rate essentially decreased in proportion to the length of the 6-alkyl chains in each series, where the 6alpha-alkyl androgens were more efficient substrates than the corresponding 6beta isomers. The Vmax of 6alpha-methyl ADD was approx. 2.5-fold that of the natural substrate AD and approx. 3-fold that of the parent ADD. On the basis of this, along with the facts that the rates of a mechanism-based inactivation of aromatase by ADD and its 6alpha-methyl derivative are similar, it is implied that alignment of 6alpha-methyl ADD in the active site could favour the pathway leading to oestrogen over the inactivation pathway, compared with that of ADD. The relative apparent Km values for the androgens obtained in this study are different from the relative Ki values obtained previously, indicating that there is a difference between the ability to serve as an inhibitor and the ability to serve as a substrate in the 6-alkyl androgen series.

Studies directed toward a mechanistic evaluation of aromatase inhibition by androst-5-ene-7,17-dione. Time-dependent inactivation by the 19-nor and 5 beta, 6 beta-epoxy derivatives

To gain further insight into the mechanism for inactivation of aromatase by androst-5-ene-7,17-dione (1) and its 19-nor analog 4, 10 beta-oxygenated steroids 5 and 6, delta 1(10)-steroid 7, and 19-oxo-5 beta,6 beta-epoxy compound 8 were synthesized and tested for their ability to inhibit aromatase in human placental microsomes. All of the steroids studied inhibited the enzyme in a competitive manner with apparent Ki values ranging from 1.1 to 35 microM. The delta 1(10)-compound 7 was the most potent inhibitor among them. All of the inhibitors caused a time-dependent inactivation of aromatase in the presence of NADPH in air with the kinact values ranging from 0.036 to 0.190 min-1. The substrate androstenedione protected the inactivation, but a nucleophile, L-cysteine, did not, in each case. In contrast, each inhibitor did not cause the time-dependent inactivation in the absence of NADPH. These results show that the 5 beta,6 beta-epoxide 8 and/or the dienone 7 are not a reactive electrophile involved in the irreversible binding to the active site of aromatase during the mechanism-based inactivation caused by the suicide substrates 1 and/or 4.

Mechanism for aromatase inactivation by a suicide substrate, androst-4-ene-3,6,17-trione. The 4 beta, 5 beta-epoxy-19-oxo derivative as a reactive electrophile irreversibly binding to the active site

Aromatase is a cytochrome P450 enzyme complex that catalyzes the conversion of androst-4-ene-3,17-dione to estrone through three sequential oxygenations of the 19-methyl group. Androst-4-ene-3,6,17-trione (1) is a suicide substrate of aromatase. The inactivation mechanism for steroid 1 has been studied to show that the inactivation reaction proceeds through the 19-oxo intermediate 3. To further clarify the mechanism, 4 beta, 5 beta-epoxyandrosta-3,6,17,19-tetraone (6) was synthesized as a candidate for a reactive electrophile involved in irreversible binding to the active site of aromatase, upon treatment of compound 3 with hydrogen peroxide in the presence of NaHCO3. The epoxide 6 inhibited human placental aromatase in a competitive manner (Ki = 30 microM); moreover, it inactivated the enzyme in an active-site-directed manner in the absence of NADPH (K1 = 88 microM, kinact = 0.071 min-1). NADPH and BSA both stimulated the inactivation rate without a significant change of the K1 in either case (kinact: 0.133 or 0.091 min-1, in the presence of NADPH or BSA, respectively). The substrate androst-4-ene-3,17-dione protected the inactivation, but a nucleophile, L-cysteine, did not. When both the epoxide 6 and its 19-methyl analog 4 were subjected separately to reaction with N-acetyl-L-cysteine in the presence of NaHCO3, the 19-oxo steroid 6 disappeared from the reaction mixture more rapidly (T1/2 = 40 sec) than the 19-methyl analog 4 (T1/2 = 3.0 min). The results clearly indicate that the 4 beta, 5 beta-epoxy-19-oxo compound 6, which is possibly produced from 19-oxo-4-ene steroid 3 through the 19-hydroxy-19-hydroperoxide intermediate, is a reactive electrophile that irreversibly binds to the active site of aromatase.

Metabolic aspects of the 1 beta-proton and the 19-methyl group of androst-4-ene-3,6,17-trione during aromatization by placental microsomes and inactivation of aromatase

Aromatase catalyzes the conversion of androst-4-ene-3,17-dione to estrogen through sequential oxygenations at the 19-methyl group. Androst-4-ene-3,6,17-trione (AT) is a suicide substrate of aromatase, and the mechanism of inactivation of aromatase has been postulated to involve enzymatic oxygenation at the 19-position. [1 beta-3H,4-14C]-, [19-3H3,4-14C]-, and [1 beta-3H,19-14C]ATs, with high specific activities, were synthesized to study metabolic aspects and the inactivation mechanism. Incubation of the labeled AT with human placental microsomes yielded the 19-oxygenated derivatives, 19-hydroxy-AT and 19-oxo-AT, as well as the aromatization products, 6-oxoestrone and 6-oxoestradiol. A stereospecific 1 beta-proton elimination occurred during the aromatization of [1 beta-3H,4-14C]AT, and a marked tritium isotope effect was observed in the first hydroxylation at C-19 of [19-3H3,4-14C]AT. After incubation of the three double-labeled ATs, the solubilized proteins were subjected to SDS-PAGE and the 3H/14C ratio of the aromatase-bound metabolite in a 46-69 kDa fraction was analyzed. A marked decrease of the 3H/14C ratio of the metabolite was observed in the experiment using [19-3H3,4-14C]AT, compared with that of the labeled AT used, but there were no significant changes in the other experiments, indicating that the adduct retains the 1 beta-proton, the 19-carbon, and one of the three 19-methyl protons of AT. Thus, we conclude that further oxygenation of 19-oxo-AT produced by the two initial hydroxylations of AT at C-19 yields not only 6-oxoestrogen (by a mechanism similar to that involved in the aromatization of the natural substrate) but also a reactive electrophile that immediately binds to the active site in an irreversible manner, resulting in inactivation of aromatase.

Different in vitro metabolism of 7 alpha-methyl-19-nortestosterone by human and equine aromatases

The ability of human and equine placental microsomes to aromatize 7 alpha-methyl-19-nortestosterone (MNT) was studied. Kinetic analysis indicates that MNT shares the androgen-binding site of human and equine placental microsomal aromatases. Human placental microsomal estrogen synthetase had about a 2.5-fold higher relative affinity for MNT than the equine placental enzyme (KiMNT/Km androstenedione of 32 versus 87). However, MNT was not metabolized by human placental microsomes, whereas it was very actively metabolized by equine placental microsomes. Further studies using purified equine cytochrome P-450arom indicated that the presence of a 7 alpha-methyl group and the absence of a C19 methyl group did not impair its conversion by the purified enzyme. The product of this reaction was separated and identified as 7 alpha-methylestradiol by gas chromatography coupled to mass spectrometry.

Studies on the mechanism of aromatase and other cytochrome P450 mediated deformylation reactions

Aromatase is a microsomal cytochrome P450 that converts androgens to estrogens by three sequential oxidations. The isolation of the 19-hydroxy and 19-oxo androgens suggests that the first two oxidations occur at the C19 carbon. However, the mechanism of the third oxidation, which results in C10--C19 bond cleavage, has not been determined. Two proposed mechanisms which remain viable involve either initial 1 beta-hydrogen atom abstraction or addition of the ferric peroxy anion from aromatase to the C19 aldehyde. Semiempirical molecular orbital calculations (AM1) were used to study potential reaction mechanisms initiated by initial 1 beta-hydrogen atom abstraction. Initially, the energetics of carbon--carbon bond cleavage of the keto and enol forms of C1-radicals were studied and were found to be energetically similar. A mechanism was proposed in which the 19-oxo intermediate is subject to initial nucleophilic attack by the protein. The geometry of the A-ring in the androgens is between that for the 1-radicals and estrogen, suggesting that some transition state stabilization for the homolytic cleavage reaction can occur. More recently, studies on liver microsomal cytochrome P450 mediated deformylation of xenobiotic aldehydes supports mechanisms involving an alkyl peroxy intermediate formed by addition of the ferric peroxy anion from aromatase to the C19 aldehyde. Although this intermediate could proceed through several different concerted or non-concerted pathways, one non-concerted pathway involves the heterolytic cleavage of the dioxygen bond resulting in an active oxygenating species (iron-oxene) and a diol. The diol could then undergo hydrogen atom abstraction followed by homolytic carbon--carbon bond cleavage as in the mechanisms modeled previously. When this cleavage was modeled for seven aldehydes, a good correlation with reported experimental aldehyde turnover numbers was obtained. However, when dialkoxy derivatives of the aldehydes are subject to microsomal metabolism, the rates of carbon-carbon cleavage products do not approach the rates of deformylation of the aldehyde analog.

Mechanistic studies on aromatase and related C-C bond cleaving P-450 enzymes

Some P-450 systems, notably aromatase and 14 alpha-demethylase catalyse not only the hydroxylate reaction but also the oxidation of an alcohol into a carbonyl compound as well as a C-C bond cleavage process. All these reactions occur at the same active site. A somewhat analogous situation is noted with 17 alpha-hydroxylase-17,20-lyase that participates in hydroxylation as well as C-C bond cleavage process. The C-C bond cleavage reactions catalysed by the above enzymes conform to the general equation: [formula: see text] It is argued that all three types of reaction catalyzed by these enzymes may be viewed as variations on a common theme. In P-450 dependent hydroxylation the initially formed FeIII-O-O. species is converted into FeIII-O-OH and the heterolysis of the oxygen-oxygen bond of the latter then gives the oxo-derivative for which a number of canonical structures are possible; for example FeV = O<==>(+.)FeIV = O<==>FeIV-O.. One of these, FeIV-O. behaves like an alkoxyl radical and participates in hydrogen abstraction from C-H bond to produce FeIV-OH and carbon radical. The latter is then quenched by the delivery of hydroxyl radical from FeIV-OH. The latter species may thus be regarded as a carrier of hydroxyl radical. We have proposed that the C-C bond cleavage reaction occurs through the participation of the FeIII-O-OH species that is trapped by the electrophilic property of the carbonyl compound giving a peroxide adduct that fragments to produce an acyl-carbon cleavage. Scientific developments leading up to this conclusion are considered. In the first author's views, "The study of mechanisms is not a scientific but a cultural activity. Mechanisms do not aim at an absolute truth but are intended to be a "running" commentary on the status of knowledge in a field. As the structural knowledge in a field advances Mechanisms evolve to take note of the new findings. Just as a constructive "running" commentary provides the stimulus for higher standards of performance, so Mechanisms call for better and firmer structural information from their practitioners".

Theoretical studies on the mechanism of conversion of androgens to estrogens by aromatase

Semiempirical molecular orbital calculations (AM1) were used to model several possible reaction mechanisms for the third oxidation of the aromatase-catalyzed conversion of androgens to estrogens. The reaction mechanisms considered are based on the assumption that the third oxidation is initiated by 1 beta-hydrogen atom abstraction. Homolytic cleavage of the C10-C19 bond was modeled for both the 3-keto and 2-en-3-ol forms of the androgen 1-radicals. The addition of a protein nucleophile to the 19-oxo intermediate was also considered, and -OCH3, -SCH3, and -NHCH3 were used to represent the Ser, Cys, and Lys adducts. The transition states were estimated and optimized from the reaction coordinates obtained by constraining and increasing the C10-C19 bond lengths. The enthalpies of activation range from 14 to 21 kcal and are approximately 2 kcal lower for cleavage of the enol form. Given the tendency for AM1 to overestimate activation energies, all reactions may be energetically accessible. Other reactions modeled include a homolytic cleavage reaction from a thioether radical cation and the direct additions of oxygen radical compounds to the carbonyl of the 1-radical-2-en-3-ol-19-oxo androgen. A mechanism is proposed in which the 19-oxo intermediate is subject to initial nucleophilic attack by the protein. Since rotation of the 19-carbonyl can bring the oxygen within 2.1 A of the 2 beta-hydrogen, the formation of a tetrahedral intermediate can occur with concomitant removal of the 2 beta-proton. Enolization activates the C1-position for hydrogen atom abstraction, since the resulting radical is resonance stabilized.(ABSTRACT TRUNCATED AT 250 WORDS)

Can a single androgen receptor fill the bill?

If the above two hypotheses are correct, they would require at least one more specific nuclear receptor for T, and at least one membrane receptor to account for the very rapid effects induced by androgens on certain target tissues. If this is the case, clearly a single androgen receptor will not fill the bill.

Conversion of 19-oxo[2 beta-2H]androgens into oestrogens by human placental aromatase. An unexpected stereochemical outcome

Aromatase is a cytochrome P-450 enzyme that catalyzes the conversion of androgens into oestrogens via sequential oxidations at the 19-methyl group. Despite intensive investigation, the mechanism of the third step, conversion of the 19-aldehydes into oestrogens, has remained unsolved. We have previously found that a pre-enolized 19-al derivative undergoes smooth aromatization in non-enzymic model studies, but the role of enolization by the enzyme in transformations of 19-oxoandrogens has not been previously investigated. The compounds 19-oxo[2 beta-2H]testosterone and 19-oxo[2 beta-2H]androstenedione have now been synthesized. Exposure of either of these compounds to microsomal aromatase, in the absence of NADPH, for an extended period led to no significant 2H loss or epimerization at C-2, leaving open the importance of an active-site base. However, in the presence of NADPH there was an unexpected substrate-dependent difference in the stereoselectivity of H loss at C-2 in the enzyme-induced aromatization of 19-oxo[2 beta-2H]-testosterone versus 19-oxo[2 beta-2H]androstenedione. The aromatization results for 17 beta-ol derivative 19-oxo[2 beta-2H]-testosterone correspond to about 1.2:1 2 beta-H/2 alpha-H loss from unlabelled 19-oxotestosterone. In contrast, aromatization results for 19-oxo[2 beta-2H]androstenedione correspond to at least 11:1 2 beta-H/2 alpha-H loss from unlabelled 19-oxoandrostenedione. This substrate-dependent stereoselectivity implies a direct role for an enzyme active-site base in 2-H removal. Furthermore, these results argue against the proposal that 2 beta-hydroxylation is the obligatory third step in aromatase action.

Studies on estrogen biosynthesis using radioactive and stable isotopes

The conversion of androgens into estrogen involves three distinct generic reactions which are catalyzed by a single P450 enzyme (aromatase or P450(aromatase)). The first step in the process is the conversion of 19-methyl into a hydroxymethyl group which requires NADPH + O2, thus representing the well-known hydroxylation process. The next stage, converting the -CH2OH into -CHO, also requires NADPH + O2 and may be rationalized either through a second hydroxylation reaction producing a gem-diol, CH(OH)2 (which dehydrates to the aldehyde), or via another route. The final stage in the process again uses NADPH + O2, culminating in the release of C-19 as formate. Our extensive studies using precursors containing 2H, 3H, and 18O have shown that the carbonyl oxygen of the 19-aldehyde group is the one that was introduced in the first step as the hydroxyl group. The aldehydic oxygen along with another, from O2, used in the third step of the process, is incorporated into the released formate. It was found that at each stage of the process, oxygen atoms were introduced or transferred as "whole numbers." In light of these data, mechanisms in which H2O is used to promote the C-10-C-19 bond cleavage or those in which the conversion of the 19-oxoandrostenedione into estrogen is considered to occur via the sequence -CHO----(-)CH(OH)2----estrogen are eliminated. In addition, our mechanistic analysis makes it unlikely that 1 beta-, 2 beta-, or 10 beta-hydroxysteroids serve as intermediates in estrogen biosynthesis. We consider a free radical mechanism for the hydroxylation process.

Conversion of a 3-desoxysteroid to 3-desoxyestrogen by human placental aromatase

Human placental aromatase is a cytochrome P-450 enzyme system which converts androgens to estrogens by three successive oxidative reactions. The first two steps have been shown to be hydroxylations at the androgen 19-carbon, but the third step remains unknown. A leading theory for the third step involves ferric peroxide attack on the 19-oxo group to produce a 19,19-hydroxyferric peroxide intermediate and subsequent collapse to estrogen. We had previously developed a nonenzymatic peroxide model reaction which was based on the above-mentioned theory, and we demonstrated the importance of 3-ketone enolization in facilitating aromatization. This study discusses the synthesis and nonenzymatic and enzymatic study of a 3-desoxy-2,4-diene-19-oxo androgen analogue. This compound was found to be a potent nonenzymatic model substrate and competitive inhibitor of aromatase (Ki = 73 nM). Furthermore, in an unprecedented event, this compound served as a substrate for aromatase, with conversion to the corresponding 3-desoxyestrogen.

Androgen and 19-norandrogen aromatization by equine and human placental microsomes

The ability of equine and human placental microsomes to aromatize testosterone and 19-nortestosterone was studied. When 3 microM [1 beta,2 beta-3H]testosterone was used as substrate, the specific activity of equine placental microsomal aromatase was 2.5 times higher than that of the human microsomal enzyme. Although 19-nortestosterone was aromatized 67 times more rapidly by equine than by human aromatase, we found that equine aromatase exhibited a markedly weaker affinity for this substrate than did the human enzyme. Competitive inhibition of testosterone aromatization by 19-nortestosterone occurred with both equine and human aromatases. While having no effect on mare placental microsomes, Na+ and K+ (500 mM) stimulated testosterone aromatization by human placental microsomes by 73 and 52% respectively. If indeed a single enzyme is responsible for the aromatization of testosterone and 19-nortestosterone, which seems to be the case in both equine and human placental aromatase, our results show that differences in the structure of the active sites exist between equine and human aromatases.

Aromatase

Aromatase catalyzes the conversion of androgens to estrogens through a series of monooxygenations to achieve the 19-desmolation and aromatization of the neutral steroid ring-A structure. We have separated two forms of aromatase, a major (P2a) and a minor (P3) form, from human term placenta through solubilization and chromatography. Partially purified aromatase in each form was immunoaffinity chromatographed to give a single band (SDS-PAGE) cytochrome P-450 of 55 kDa, utilizing a mouse monoclonal anti-human placental aromatase cytochrome P-450 IgGi (MAb3-2C2) which is capable of suppressing placental aromatase activity. The purified cytochrome P-450 showed specific aromatase activity of 25-30 nmol/min per mg with Km of 20-30 nM for androstenedione on reconstitution with NADPH-cyt P-450 reductase and dilauroyl L-alpha-phosphatidylcholine. This one step represents a higher than 100-fold purification with maintenance of the same Km. The stability analysis showed a half-life of more than 5 yr for solubilized aromatase and 2 months for the aromatase cytochrome P-450 on storage at -90 degrees C. Contrary to the recent claim that estrogen biosynthesis by reconstituted human placental cytochrome P-450 is by trans-diaxial 1 alpha,2 beta-hydrogen elimination, all of our partially purified forms and reconstituted aromatase synthesized estrogens by cis-1 beta, 2 beta-hydrogen elimination. Use of purified aromatase and [19-3H3, 4-14C]androstenedione led us to discover a metabolic switching by aromatase to 2 beta-hydroxylation of androgen. Results of the MAb3-2C2 suppression of aromatase activity in different species and tissues including human, baboons, horses, cows, pigs and rats indicated the presence of various isozymes of aromatase.

Aromatization of testosterone to estrogen varies between strains of mice

The nuclear uptake and retention of [3H]testosterone or one of its metabolites and the aromatization of testosterone to estrogen were examined in the Swiss--Webster mouse. Castrated male mice were injected with 0.2 micrograms of either [1 alpha, 2 alpha-3H(N)]testosterone or [1 beta, 2 beta-3H(N)]testosterone per 100 g of body weight and killed one and one-half hours later. The brains were removed and processed for autoradiography. A nuclear localization of testosterone or one of its metabolites was found in the nucleus (n) interstitialis striae terminalis, n. preopticus medialis, n. premamillaris ventralis and n. amygdaloideus medialis in animals injected with [1 alpha, 2 alpha-3H(N)]testosterone. In animals injected with [1 beta, 2 beta-3H(N)]testosterone a nuclear localization was found in only n. interstitialis striae terminalis, n. premamillaris ventralis and n. amygdaloideus medialis. The results suggest testosterone is aromatized to estrogen in n. preopticus medialis ventralis in the Swiss--Webster mouse. Together with previous data, these data suggest (1) the uptake and retention of testosterone or one of its androgenic metabolites and the aromatization of testosterone to estrogen varies between strains of mice and (2) there are two separate uptake and retention systems (receptors?) for testosterone and dihydrotestosterone in the brain in all animals studied thus far with autoradiographic techniques.

Mechanistic studies on C-19 demethylation in oestrogen biosynthesis

Mechanistic aspects of the biosynthesis of oestrogen have been studied with a microsomal preparation from full-term human placenta. The overall transformation, termed the aromatization process, involves three steps using O(2) and NADPH, in which the C-19 methyl group of an androgen is oxidised to formic acid with concomitant production of the aromatic ring of oestrogen: [Formula: see text] To study the mechanism of this process in terms of the involvement of the oxygen atoms, a number of labelled precursors were synthesized. Notable amongst these were 19-hydroxy-4-androstene-3,17-dione (II) and 19-oxo-4-androstene-3,17-dione (IV) in which the C-19 was labelled with (2)H in addition to (18)O. In order to follow the fate of the labelled atoms at C-19 of (II) and (IV) during the aromatization, the formic acid released from C-19 was benzylated and analysed by mass spectrometry. Experimental procedures were devised to minimize the exchange of oxygen atoms in substrates and product with oxygens of the medium. In the conversion of the 19-[(18)O] compounds of types (II) and (IV) into 3-hydroxy-1,3,5-(10)-oestratriene-17-one (V, oestrone), it was found that the formic acid from C-19 retained the original substrate oxygen. When the equivalent (16)O substrates were aromatized under (18)O(2), the formic acid from both substrates contained one atom of (18)O. It is argued that in the conversion of the 19-hydroxy compound (II) into the 19-oxo compound (IV), the C-19 oxygen of the former remains intact and that one atom of oxygen from O(2) is incorporated into formic acid during the conversion of the 19-oxo compound (IV) into oestrogen. This conclusion was further substantiated by demonstrating that in the aromatization of 4-androstene-3,17-dione (I), both the oxygen atoms in the formic acid originated from molecular oxygen. 10beta-Hydroxy-4-oestrene-3,17-dione formate, a possible intermediate in the aromatization, was synthesized and shown not to be converted into oestrogen. In the light of the cumulative evidence available to date, stereochemical aspects of the conversion of the 19-hydroxy compound (II) into the 19-oxo compound (IV), and mechanistic features of the C-10-C-19 bond cleavage step during the conversion of the 19-oxo compound (IV) into oestrogen are discussed.

A short efficient synthesis of 16-oxygenated estratriene 3-sulfates

A novel synthesis of sodium 17-oxo-16 alpha-hydroxy-1,3,5(10)-estratrien-3-yl sulfate (4), sodium 16 alpha, 16 beta-dihydroxy-1,3,5(10)-estratrien-3-yl sulfate (5) and sodium 16-oxo-17 beta-hydroxy-1,3,5(10)-estratrien-3-yl sulfate (6) is described. 16 alpha-Bromo-3-hydroxy-1,3,5(10)-estratrien-17-one (1) was efficiently synthesized in one step with 70-97% yield by bromination of 3-hydroxy-1,3,5(10)-estratrien-17-one with cupric bromide. 3,16 alpha-Dihydroxy-1,3,5(10)-estratrien-17-one (3) was quantitatively obtained by controlled stereospecific hydrolysis of the bromoketone 1 with sodium hydroxide in aqueous pyridine. The bromoketone 1 was converted to the 16 alpha-hydroxy-17-ketone 3-sulfate 4 by sulfation with chlorosulfonic acid in pyridine and a subsequent controlled hydrolysis in a high yield without formation of the other ketols. Treatment of the sulfate 4 with sodium borohydride have the triol sulfate 5. The sulfate 4 was also rearranged to the 17 beta-hydroxy-16-ketone 6 with sodium hydroxide in water in a quantitative yield.

FSH induction of aromatase in cultured rat granulosa cells measured by a radiometric assay

The effects of FSH on the aromatase activity of rat granulosa cells in culture were studied by measuring the stereospecific transfer of 3H from [1,2,6,7-3H]testosterone or [1 beta-3H]testosterone into 3H2O. The use of 3H2O release as a specific assay for aromatization in granulosa cells was validated by various means. 2 days after plating, cultures of granulosa cells released only minimal amounts of 3H2O from [1 beta-3H]testosterone during a 3-h incubation, whereas cells which had been treated with FSH, or with (Bu)2cAMP plus 3-isobutyl-1-methylxanthine (MIX), from the time of plating released considerable amounts of 3H2O into the culture medium. The release of 3H2O from [1 beta-3H]testosterone by cultured cells was inhibited by the aromatizable androgens, 19-hydroxytestosterone and 19-hydroxyandrostenedione, indicating the specificity of the method for aromatization. In order to study the mechanism by which FSH enhanced the release of 3H2O, optimal conditions for aromatization by cell-free sonicates were determined. Optimal aromatase activity was achieved by incubating cell-free sonicates at 37 degrees C in the presence of O2 in 20 mM phosphate buffer (pH 7.4) containing 5 mM dithiothreitol, 20 mM MgCl2, 0.5 mM NADP+, 20 mM glucose 6-phosphate and 2 U/ml glucose-6-phosphate dehydrogenase. A concentration of 0.25 microM testosterone and 0.1 microCi tritium was used in the standard assay. Under these conditions the assay was linear for 1 h with up to 150 microgram protein from granulosa cells having maximal aromatase activity. When cells in culture were stimulated with purified FSH for 48 h from the time of plating, the aromatase activity in cell-free sonicates increased in a dose-dependent fashion. The ED50 for Sairam's FSH S-1528 C2 was 12 ng/ml. It was concluded from these studies that FSH increases estrogen levels primarily by inducing an active aromatase rather than by influencing secretion or availability of substrate or cofactor for the aromatization reaction.

Kinetics of testosterone metabolism in normal postmenopausal women and women with breast cancer

The constant infusion and single injection techniques were utilized to study the kinetics of 3H-testosterone (T) metabolism in posmenopausal women with and without breast cancer. The metabolic clearance rates (mean +/- SEM) for normal postmenopausal women were 578 +/- 82 and 644 +/- 128 1/24 has obtained by the constant infusion and single injection techniques, respectively. The corresponding results for the women with breast cancer (patients) are 644 +/- 25 and 617 +/- 106 1/24 h. The single injection technique yielded values for rate constants (units) and volumes of distribution (1); K1 = 37.5 +/- 1.6 for the normals and 34.5 +/- 1.9 for the patients, K = 76.6 +/- 5.1 for the normals and 71.1 +/- 1.6 for the patients, V1 = 7.9 +/- 2.2 for the normals and 8.7 +/- 1.4 for the patients and V2 = 7.0 +/- 1.5 for the normals and 6.4 +/- 1.2 for the patients. The constant infusion technique yielded values for the conversion ratios for the transformation of T to several products; 4-androstene-3,17-dione/T of 0.02 +/- 0.003 for normals and 0.03 +/- 0.002 for patients, 5alpha-dihydrotestosterone/T of 0.02 +/- 0.002 for normals and 0.03 +/- 0.002 for patients, estrone/T of 0.04 +/- 0.01 for normals and 0.04 +/- 0.01 for patients, estradiol-17beta/T of 0.02 +/- 0.005 for normals and 0.03 +/- 0.005 for patients and estrone sulfate/T of 0.16 +/- 0.02 for normals and 0.24 +/- 0.06 for patients. The T plasma concentrations and production rates were similar for the two groups of subjects. Hence there were no significant differences between the normals and the patients for all the kinetic parameters. It was determined that all the estradiol being produced in postmenopausal women could be coming from circulating T.

Participation of a nonenzymatic transformation in the biosynthesis of estrogens from androgens

The biosynthesis of estrogens from androgens proceeds via three enzymatic hydroxylations, of which the first two take place on the C-19 methyl group and convert it to aldehyde. The final and rate-determining hydroxylation occurs at the 2 beta position, and the product rapidly and nonenzymatically collapses to an estrogen.

Tritium nuclear magnetic resonance spectroscopy. Part VI (1). Tritiated steroid hormones

The regio- and stereo-specificity of the labelling in a series of tritiated steroid hormones has been examined by 3H n.m.r., which also yields quantitative information on the distribution of the tritium between the labelled sites. Complete analysis is thus readily achieved non-destructively. Hydrogen chemical shifts for various skeletal sites are provided for the first time. The specificity of the methods of labelling steroids with tritium by catalytic reduction, catalysed exchange, and tritiodehalogenation are discussed.

The metabolism of the epoxide of testosterone by human placental microsomes

Although 19-hydroxy-4beta,5-oxido-5beta-androstane-3,17 dione (2a) is converted to estradiol-17beta by human placental microsomes, the incubation of 17beta-hydroxy-4beta,5-oxido-5beta-androstan-3-one (2b) under the same conditions produces only metabolites which are more polar than 17beta-estradiol. The metabolites have been isolated and identified as 3alpha-hydroxy-4beta,5-oxido-5beta-androstan-17-one (4a), 4beta,5-oxido-5beta-androstane-3beta, 17beta-diol (5a) and 4beta,5-oxido-5beta-androstane-3alpha,17beta-diol (6a). These results indicate that functionalization at C-19 is a prerequisite for the biological aromatization of such androgen epoxides.