Aromatization of 16alpha-hydroxyandrostenedione by human placental microsomes: effect of preincubation with suicide substrates of androstenedione aromatization

Human cytochrome P450 enzymes 5-51 as targets of drugs and natural and environmental compounds: mechanisms, induction, and inhibition - toxic effects and benefits

Cytochrome P450 (P450, CYP) enzymes have long been of interest due to their roles in the metabolism of drugs, pesticides, pro-carcinogens, and other xenobiotic chemicals. They have also been of interest due to their very critical roles in the biosynthesis and metabolism of steroids, vitamins, and certain eicosanoids. This review covers the 22 (of the total of 57) human P450s in Families 5-51 and their substrate selectivity. Furthermore, included is information and references regarding inducibility, inhibition, and (in some cases) stimulation by chemicals. We update and discuss important aspects of each of these 22 P450s and questions that remain open.

Steroid metabolome in fetal and maternal body fluids in human late pregnancy

Despite the extensive research during the last six decades the fundamental questions concerning the role of steroids in the initiation of human parturition and origin and function of some steroids in pregnancy were not definitely answered. Based on steroid metabolomic data found in the literature and our so far unpublished results, we attempted to bring new insights concerning the role of steroids in the sustaining and termination of human pregnancy, and predictive value of these substances for estimation of term. We also aimed to explain enigmas concerning the biosynthesis of progesterone and its bioactive catabolites considering the conjunctions between placental production of CRH, synthesis of bioactive steroids produced by fetal adrenal, localization of placental oxidoreductases and sustaining of human pregnancy. Evaluation of data available in the literature, including our recent findings as well as our new unpublished data indicates increasing progesterone synthesis and its concurrently increasing catabolism with approaching parturition, confirms declining production of pregnancy sustaining 5β-pregnane steroids providing uterine quiescence in late pregnancy, increased sulfation of further neuroinhibiting and pregnancy sustaining steroids. In contrast to the established concept considering LDL cholesterol as the primary substrate for progesterone synthesis in pregnancy, our data demonstrates the functioning of alternative mechanism for progesterone synthesis, which is based on the utilization of fetal pregnenolone sulfate for progesterone production in placenta. Close relationships were found between localization of placental oxidoreductases and consistently higher levels of sex hormones, neuroactive steroids and their metabolites in the oxidized form in the fetus and in the reduced form in the maternal compartment.

Aromatization of androstenedione and 16alpha-hydroxyandrostenedione in human placental microsomes. Kinetic analysis of inhibition by the 19-oxygenated and 3-deoxy analogs

Inhibition of aromatase activity in human placental microsomes with androstenedione (AD) (1a) and its 19-oxygenated derivatives 1b and 1c, their 16alpha-hydroxy compounds 2 and 3, and 3-deoxyandrost-4-ene compounds 5 and 6 was studied using [1beta-(3)H]AD as a substrate and compared to that with [1beta-(3)H]16alpha-hydroxyandrostenedione (16-OHAD). AD series of steroids, compounds 1, inhibited competitively [1beta-(3)H]AD aromatization whereas other 16alpha-hydroxy steroids 2, 3, 5, and 6 inhibited AD aromatization in a non-competitive manner. On the other hand, all of 16-OHAD series, compounds 2, blocked the [1beta-(3)H]16-OHAD aromatization in a competitive manner whereas the AD series steroids 1 as well as the 3-deoxy-16alpha-hydroxy-17-one steroids 5 and 3-deoxy-16alpha,17beta-diol steroids 6 inhibited 16-OHAD aromatization non-competitively. 3-carbonyl and 16alpha-hydroxy functions of 16-OHAD play a critical role of selection of the 16-OHAD binding site. The results suggest that the AD derivatives 1 are kinetically aromatized at a different site from the 16-OHAD derivatives 2. Physical and/or chemical environments around the aromatase protein in the microsomal membrane may play a significant role in the expression of the substrate specificity, and the present results do not exclude the idea that the placental microsomes have a single binding site.

The effect of methadone and buprenorphine on human placental aromatase

Methadone and buprenorphine (BUP) are used for treatment of the pregnant opiate addict. CYP19/aromatase is the major placental enzyme responsible for the metabolism of methadone to 2-ethylidine-1,5-dimethyl-3,3-diphenylpyrrolidine (EDDP) and BUP to norbuprenorphine (norBUP). The aim of this investigation was to determine the effects of methadone and BUP on the activity of placental microsomal aromatase in the conversion of its endogenous substrates testosterone to 17beta-estradiol (E(2)) and 16alpha-hydroxytestosterone (16-OHT) to estriol (E(3)). The conversion of testosterone and 16-OHT by human placental microsomes exhibited saturation kinetics, and the apparent K(m) values were 0.2 +/- 1 and 6 +/- 3 microM, respectively. V(max) values for E(2) and E(3) formation were 70 +/- 16 and 28 +/- 10 pmol/mg proteinmin, respectively. Also, data obtained revealed that methadone and BUP are competitive inhibitors of testosterone conversion to E(2) and 16-OHT to E(3). The K(i) for methadone inhibition of E(2) and E(3) formation were 393 +/-144 and 53 +/- 28 microM, respectively, and for BUP the K(i) was 36 +/- 9 and 6 +/- 1 microM. The higher potency of the two opiates and their metabolites in inhibiting E(3) formation is in agreement with the lower affinity of 16-OHT than testosterone to aromatase. Moreover, the metabolites EDDP and norBUP were weaker inhibitors of aromatase than their parent compounds. The determined inhibition constants of methadone and BUP for E(3) formation by a cDNA-expressed CYP19 preparation were similar to those for placental microsomes. Therefore, data reported here suggest that methadone, BUP, and their metabolites are inhibitors of androgen aromatization in the placental biosynthesis of estrogens.

In vitro models in endocrine disruptor screening

The public and scientific concern that chemicals present in the human diet and the environment and their ability to disrupt the normal hormonal milieu in humans and wildlife have become a high-profile international issue. In 1998, the Endocrine Disruptor Screening and Testing Advisory Committee (EDSTAC) convened by the Environmental Protection Agency (EPA) recommended a tiered testing approach for the evaluation of estrogen, androgen, and thyroid-related effects of some 87,000 commercial chemicals and environmental contaminants. The function of this committee concluded with its final report, and the further implementation of the recommended testing strategy has now been carried forward with the assistance of the Endocrine Disruptor Methods Validation Subcommittee. The function of this body is to provide advice to the EPA on scientific and technical issues related specifically to the conduct of studies required for the validation of assays proposed by the EDSTAC as part of the tiered screening program. The EDSTAC recommended and alternative screening batteries encompass four in vitro mammalian assays. The current methodologies and validation status of the proposed in vitro EDSTAC assays are discussed and consist of estrogen/androgen receptor binding, estrogen/androgen gene transactivation, and minced testis, and one alternate (placental aromatase) in vitro screening assay.

Structure-activity relationships of 3-deoxy androgens as aromatase inhibitors. Synthesis and biochemical studies of 4-substituted 4-ene and 5-ene steroids

As part of our investigation into the structure-activity relationship of a novel class of aromatase inhibitors, two series of 3-deoxy androgens, androst-5-en-17-ones with a non-polar alkoxy (5 and 6), alkyl (20-22), or phenylalkyl (23 and 24) group at C-4beta and 4-acyloxyandrost-4-en-17-ones (29-32, and 34) were synthesized and evaluated. The 4beta-alkyl and 4beta-phenylalkyl compounds were obtained through reaction of 4alpha,5alpha-epoxy steroid (8) with RMgBr (R: alkyl and phenylalkyl) followed by dehydration of the 4beta-substituted 5alpha-hydroxy products (15-19) with SOCl(2) as key reactions. Acylation of 4alpha,5alpha-diol (25) with (RCO)(2)O in pyridine and subsequent dehydration with SOCl(2) gave the 4-acyloxy steroids. All of the steroids studied, except for 4-acetoxy-19-ol (34) that was a non-competitive inhibitor of human placental aromatase, blocked aromatase activity in a competitive manner. 4-Benzoyloxy- and 4-acetoxy steroids (31) and (32) were the most powerful inhibitors of aromatase (K(i)=70 and 60nM, respectively). Elongation of an acetoxy group in a series of 4-acyloxy steroids or a methyl group in a series of 4beta-alkyl steroids decreased affinity for aromatase principally in relation to carbon number of the acyl or alkyl function. The present findings are potentially useful for understanding the spatial and electronic nature of the binding site of aromatase as well as for developing effective aromatase inhibitors.

Kinetic analysis of reversible inhibition of 16alpha-hydroxyandrostenedione aromatization in human placental microsomes by suicide substrates of androstenedione aromatization

To gain insight into the catalytic function of aromatase and its substrate specificity, we studied reversible inhibition of 16alpha-hydroxyandrostenedione (16alpha-OHAD) aromatization in human placental microsomes by several suicide substrates of androstenedione (AD) aromatization, including 4-hydroxyAD (1), 6-oxoAD (2) and its 19-hydroxy analogue 3, androst-5-ene-4,7,17-trione (4), and 10beta-acetoxyandrost-5-en-7,17-dione (5) that, in contrast, do not cause a suicide inactivation of 16alpha-OHAD aromatization. All inhibitors examined blocked 16alpha-OHAD aromatization in a competitive manner with apparent K(i) values ranging from 0.50 to 980 nM. The relative K(i) values between inhibitors 1-5 obtained in the 16alpha-OHAD aromatization experiments were markedly different from those obtained in the AD aromatization experiments. The results predict that all inhibitors examined bind to the 16alpha-OHAD binding site in a manner that does not cause suicide inactivation of 16alpha-OHAD aromatization. These findings would be useful for understanding the active (binding) site structure as well as the catalytic function of aromatase.

Aromatase inhibition by 4 beta,5 beta-epoxides of 16 alpha-hydroxyandrostenedione and its 19-oxygenated analogs, potential precursors of estriol production in the feto-placental unit

To gain insight into the nature of the substrate binding site and the catalytic function of aromatase, we studied the inhibition of androstenedione aromatization by 4beta,5beta-epoxy-16alpha-hydroxyandrostenedione (4) and its 19-hydroxy and 19-oxo derivatives, 5 and 6, as well as the biochemical aromatization of these steroids in human placental microsomes. The 19-methyl and 19-oxo compounds, 4 and 6, were weak competitive inhibitors of aromatase, with apparent K(i) values of 246 microM and 270 microM, respectively, whereas the 19-hydroxy compound 5 inhibited aromatase in a non-competitive manner with the K(i) of 135 microM. The 19-methyl compound 4 inactivated aromatase in a time-dependent manner with k(inact) of 0.213 min(-1) in the presence of NADPH in air, but the other two did not cause it. The conversion of the three epoxides into estrogen, as well as 19-oxygenation of 19-methyl steroid 4 with human placental microsomes in the presence of NADPH in air, were not detected by gas chromatography-mass spectrometry. The present results are consistent with the two binding sites theory in the active site of aromatase.