Aromatase Reaction of 3-Deoxyandrogens: Steric Mode of the C-19 Oxygenation and Cleavage of the C10−C19 Bond by Human Placental Aromatase

A review of mechanistic studies on aromatase (CYP19) and 17α-hydroxylase-17,20-lyase (CYP17)

In the conventional P-450 dependent hydroxylation reaction, the Fe(III) resting state of the enzyme, by a single electron transfer, is reduced to Fe(II), which reacts with O(2) to produce a Fe(III)-O-O intermediate. The latter following the transfer of another electron furnishes a ferric-peroxyanion, Fe(III)-O-O(-), which after protonation leads to the fission of the O-O bond resulting in the formation of Fe(V)O, the key player in the hydroxylation process. Certain members of the P-450 family, including CYP17 and CYP19, catalyze, at the same active site, not only the hydroxylation process but also an acyl-carbon bond cleavage reaction which has been interpreted to involve the nucleophilic attack of the ferric-peroxyanion, Fe(III)-O-O(-), on the acyl carbon to furnish a tetrahedral intermediate which fragments, leading to acyl-carbon cleavage. Evidence is presented to show that in the case of CYP17 the attack of Fe(III)-O-O(-) on the target carbon is promoted by cytochrome b(5), which acts as a conformational regulator of CYP17. It is this regulation of CYP17 that provides a safety mechanism which ensures that during corticoid biosynthesis, which involves 17α-hydroxylation by CYP17, androgen formation is avoided. Finally, a brief account is presented of the inhibitors, of the two enzymes, which have been designed on the basis of their mechanism of action. Article from the Special issue on 'Targeted Inhibitors'.

Molecular mechanisms of aromatase inhibition by new A, D-ring modified steroids

A recent approach for treatment and prevention of estrogen-dependent breast cancer focuses on the inhibition of aromatase, the enzyme that catalyzes the final step of estrogen biosynthesis. Some synthetic steroids, such as formestane and exemestane, resembling the natural enzyme substrate androstenedione, revealed to be potent and useful aromatase inhibitors (AIs) and were approved for the treatment of estrogen-dependent breast cancer in postmenopausal women. Recently, we found that five newly synthesized steroids with chemical features in the A- and D-rings considered important for drug-receptor interaction efficiently inhibit aromatase derived from human placental microsomes. In this work, these steroids showed a similar pattern of anti-aromatase activity in several aromatase-expressing cell lines. 5alpha-androst-3-en-17-one and 3alpha,4alpha-epoxy-5alpha-androstan-17-one were revealed to be the most potent inhibitors. These compounds induced a time-dependent inhibition of aromatase, showing to be irreversible AIs. The specific interactions of these compounds with aromatase active sites were further demonstrated by site-directed mutagenesis studies and evaluated by computer-aided molecular modeling. Both compounds were able to suppress hormone-dependent proliferation of MCF-7aro cells in a dose-dependent manner. These findings are important for the elucidation of a structure-activity relationship on aromatase, which may help in the development of new AIs.

Mass spectrometric analysis of oxygenations in aromatization of androst-4-ene-3,6,17-trione, a suicide substrate of aromatase, by placental microsomes. Isotope effect and stereochemistry

Aromatase catalyzes the conversion of androstenedione (AD) to estrone through three sequential oxygenations of the 19-methyl group. 6-OxoAD (1) is one of the typical suicide substrates of aromatase, which is converted by aromatase to 6-oxoestrone through 19-alcohol (19-ol) and 19-aldehyde (19-al) intermediates 2 and 3. To study the deuterium isotope effect on the conversion of 19-ol 2 to 19-al 3 as well as the stereochemistry of the 19-hydrogen removal in this conversion, we initially synthesized [19,19-(2)H(2)] and [19S- or 19R-(2)H] 19-ols 2, starting from the corresponding deuterium-labeled 19-hydroxyAD derivatives. In incubation of non-labeled and [19,19-(2)H(2)]-labeled 19-ol 2 or that of their 1:1 mixture with human placental microsomes in the presence of NADPH under air, there was no significant deuterium-isotope effect on the production of the aromatized product 6-oxoestrone or on the conversion of 19-ol 2 to 19-al 3, based on gas chromatography-mass spectrometric analysis of the estrogen product or liquid chromatography-mass spectrometric (LC-MS) analysis of the deuterium contents of the product 19-al 3 and the recovered 19-ol 2. Moreover, in the incubations of [19S-(2)H] 19-ol 2 and its 19R isomer, LC-MS analysis of the product 3 demonstrated that the 19-pro-R hydrogen atom was stereospecifically removed in the conversion of 19-ol 2 to 19-al 3. These findings indicate that the 19-oxygenation of 19-ol 2 would proceed in the same mechanism as that involved in the AD aromatization.

Biochemical aromatization of 2-methyleneandrostenedione: stereochemistry of hydrogen removal at the C-1 position

To explore a stereochemistry of hydrogen removal at C-1 of the powerful aromatase inhibitor 2-methyleneandrostenedione (1), of which the A-ring conformation is markedly different from that of the natural substrate androstenedione (AD), in the course of the aromatase-catalyzed A-ring aromatization producing 2-methylestrone (2), we synthesized [1alpha-2H]labeled steroid 1 and its [1beta-2H]stereoisomer, and the metabolic fate of the C-1 deuterium in aromatization was analyzed by gas chromatography-mass spectrometry (GC-MS) in each. Parallel experiments with the natural substrates [1alpha-2H] and [1beta-2H]ADs were also carried out. The GC-MS analysis indicated that 2-methyl estrogen 2 produced from [1alpha-2H]labeled substrate 1 retained completely the 1alpha-deuterium (1beta-H elimination), while product 2 obtained from [1beta-2H]isomer 1 lost completely the 1beta-deuterium. Stereospecific 1beta-hydrogen elimination was also observed in the parallel experiments with the labeled ADs as established previously. The results indicate that biochemical aromatization of the 2-methylene steroid 1 proceeds through the 1beta-hydrogen removal concomitant with cleavage of the C(10)-C(19) bond, yielding 1(10),4-dienone 9, in a similar manner to that involved in AD aromatization. This would give additional evidence for the stereomechanisms for the last step of aromatization of AD, requiring the stereospecific 1beta-hydrogen abstraction and cleavage of the C(10)-C(19) bond, and for the enolization of a carbonyl group at C-3 in the A-ring aromatization.

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.

Reduction of 1,4-Dien-3-one Steroids with LiAl2H4 or NaB2H4: Stereospecific Deuterium-Labeling at the C-1.ALPHA. Position of a 4-En-3-one Steroid

Reduction of a double bond at C-1 of 1,4-dien-3-one steroids 7 and 8 with LiAl2H4 in THF or NaB2H4 in MeOH and H2O gave stereospecifically [1alpha-2H]-labeled 4-en-3-one steroids 9 and 10, respectively. When the deuterated solvents, MeO2H and 2H2O, were used for the reaction of steroid 8 with NaB2H4, [1alpha,2xi-2H2]-labeled compound 10 was produced. This indicates that the reaction proceeds through the initial hydride attack at the C-1alpha position, followed by ketonization of the 2-en-3-ol intermediate.