Affinity alkylation of the active site of C21 steroid side-chain cleavage cytochrome P-450 from neonatal porcine testis: a unique cysteine residue alkylated by 17-(bromoacetoxy)progesterone

Steroidal 5α-reductase and 17α-hydroxylase/17,20-lyase (CYP17) inhibitors useful in the treatment of prostatic diseases

The role of steroidal inhibitors of androgen biosynthesis as potential weapons in the treatment of prostatic diseases, such as benign prostatic hyperplasia and prostatic cancer will be reviewed. Two enzymes have been targeted in the development of inhibitors that potentially could be useful in the management of such conditions. 5α-Reductase is primarily of interest in benign prostatic disease, though some role in the chemoprevention of prostatic carcinoma have been considered, whereas the 17α-hydroxylase/17,20-lyase (CYP17) enzyme is of interest in the treatment of malignant disease. An overview of the main achievements obtained during the past years will be presented, however special focus will be made on steroidal molecules that reached clinical trials or have been commercially launched. Relevant examples of such drugs are finasteride, dutasteride, abiraterone acetate and galeterone (TOK-001, formerly known as VN/124-1). This article is part of a Special Issue entitled "Synthesis and biological testing of steroid derivatives as inhibitors".

Inactivation of cytochrome P450 2B1 by benzyl isothiocyanate, a chemopreventative agent from cruciferous vegetables

A series of arylalkyl isothiocyanates were evaluated for their ability to inactivate purified cytochrome P450 2B1 in a reconstituted system. Benzyl isothiocyanate (BITC) and phenethyl isothiocyanate (PEITC) occur naturally in several cruciferous vegetables, and the inhibition of cytochrome P450 (P450) enzymes has been implicated in their chemopreventative abilities. The naturally occurring isothiocyanates BITC and PEITC inactivated P450 2B1 in a time- and concentration-dependent manner, whereas the synthetic isothiocyanates phenylpropyl and phenylhexyl isothiocyanate did not result in inactivation, but were potent competitive inhibitors of P450 2B1 activity. The kinetics of inactivation of P450 2B1 by BITC were characterized. The 7-ethoxy-4-(trifluoromethyl)coumarin O-deethylation activity of P450 2B1 was inactivated in a mechanism-based manner. The loss of O-deethylation activity followed pseudo-first-order kinetics, was saturable, and required NADPH. The BITC concentration required for half-maximal inactivation (K(I)) was 5.8 microM, and the maximal rate constant for inactivation was 0.66 min(-)(1) at 23 degrees C. BITC was a very efficient inactivator of P450 2B1 with a partition ratio of approximately 9. The mechanism of BITC-mediated inactivation of P450 2B1 was also investigated. More than 80% of the catalytic activity was lost within 12 min with a concomitant loss of approximately 45% in the ability of the reduced enzyme to bind CO. The magnitude of the UV/visible absorption spectrum of the inactivated protein did not decrease significantly, and subsequent HPLC analysis indicated no apparent modification of the heme. HPLC and protein precipitation analyses indicated that the P450 apoprotein was covalently modified by a metabolite of BITC. Determination of the binding stoichiometry indicated that 0.90 +/- 0. 16 mol of radiolabeled metabolite was bound per mole of enzyme that was inactivated, suggesting the modification of a single amino acid residue per molecule of enzyme that was inactivated. The results reported here indicate that BITC is a mechanism-based inactivator of P450 2B1 and that inactivation occurs primarily through protein modification.

Familial male pseudohermaphroditism

Familial male pseudohermaphroditism (MPH) due to 17,20-desmolase deficiency is rare. Here we present two siblings with MPH possibly due to 17,20-desmolase deficiency. The first patient presented with unambiguous female external genitalia and hypergonadotrophic hypogonadism. Chromosomal analysis revealed 46 XY. Ultrasound evaluation of pelvis revealed gonads in the inguinal canal, and no uterus. These findings were confirmed on laparotomy. Histology revealed the gonads to be testes. The second patient had ambiguous genitalia (perineoscrotal hypospadias, bifid scrotum with palpable gonads) with a 46 XY chromosomal pattern. Both patients had high plasma 17-hydroxy progestrone (17 OHP), low normal dehydro epiandrosterone sulphate (DHEAS) and low plasma testosterone. Plasma testosterone and DHEAS showed no response to ACTH or HCG. These features are compatible with the diagnosis of 17,20-desmolase deficiency.

cDNA and deduced protein sequence of CYP6D1: the putative gene for a cytochrome P450 responsible for pyrethroid resistance in house fly

A microsomal cytochrome P450 from the house fly (Musca domestica), termed P450lpr, is involved in P450 monooxygenase-mediated pyrethroid resistance and is expressed at 8-fold higher levels in the insecticide resistant LPR strain compared to a susceptible strain. An internal cDNA sequence was amplified by polymerase chain reaction (PCR) using degenerate primers based on known P450lpr polypeptide sequences, and the remainder of the sequence was amplified by single side-specific PCR. A 1.8 kb cDNA sequence was obtained from 3 overlapping PCR products, with an open reading frame encoding a P450 protein of 516 residues (M(r) 59,182). This gene has been designated CYP6D1 within the P450 gene superfamily. CYP6D1 exhibits most similarity (28.2-29.8% positional identity) to butterfly CYP6B1, house fly CYP6A1 and Drosophila CYP6A2, and also exhibits comparable similarity (24.7% identity) to rat CYP3A1. The deduced protein sequence contains a hydrophobic N-terminal region and conserved sequences thought to be involved in heme-binding and electron donor-protein interactions. Comparison of CYP6D1 with its four most similar proteins (CYP6B1, CYP6A1, CYP6A2 and CYP3A1) reveals the presence of extensive stretches of residues in an alignment row in 2 possible substrate-binding regions. Three introns of 74, 66 and 64 bp, having 5'-GT and AG-3' ends, split the CYP6D1 coding region in genomic DNA. Results indicate that CYP6D1 is likely the P450lpr gene.

Endocrine findings in male pseudohermaphroditism

Recent discoveries in molecular biology have much clarified the regulation and function of steroid converting enzymes. Most progress has been made in the area of cytochromes, which regulate the side chain cleavage of cholesterol (P-450 SCC) and the 17 alpha-hydroxylase- and 17,20-desmolase (or 17,20-lyase) activities (P-450 17 alpha), as well as in 3 beta-hydroxysteroid dehydrogenase. Nevertheless, there are some discrepancies between fundamental knowledge and clinical experience, which are difficult to understand: why is it possible, e.g., that cases with "pure" 17 alpha-hydroxylase or 17,20-desmolase deficiency exist, when there is only one cytochrome regulating both steps? After a brief review of clinical and biochemical findings in the various defects of testosterone biosynthesis, a case is discussed which is of interest in this respect.

3 alpha-hydroxysteroid dehydrogenase activity catalyzed by purified pig adrenal 20 alpha-hydroxysteroid dehydrogenase

In earlier studies, two distinct molecules, 20 alpha-HSD-I and 20 alpha-HSD-II, responsible for 20 alpha-HSD activity of pig adrenal cytosol were purified to homogeneity and characterized [S. Nakajin et al., J. Steroid Biochem. 33 (1989) 1181-1189]. We report here that the purified 20 alpha-HSD-I, which mainly catalyzes the reduction of 17 alpha-hydroxyprogesterone to 17 alpha,20 alpha-dihydroxy-4-pregnen-3-one, catalyzes 3 alpha-hydroxysteroid oxidoreductase activity for 5 alpha (or 5 beta)-androstanes (C19), 5 alpha (or 5 beta)-pregnanes (C21) in the presence of NADPH as the preferred cofactor. The purified enzyme has a preference for the 5 alpha (or 5 beta)-androstane substrates rather than 5 alpha (or 5 beta)-pregnane substrates, and the 5 beta-isomers rather than 5 alpha-isomers, respectively. Kinetic constants in the reduction for 5 alpha-androstanedione (Km; 3.3 microM, Vmax; 69.7 nmol/min/mg) and 5 beta-androstanedione (Km; 7.7 microM, Vmax; 135.7 nmol/min/mg) were demonstrated for comparison with those for 17 alpha-hydroxyprogesterone (Km; 26.2 microM, Vmax; 1.3 nmol/min/mg) which is a substrate for 20 alpha-HSD activity. Regarding oxidation, the apparent Km and Vmax values for 3 alpha-hydroxy-5 alpha-androstan-17-one were 1.7 microM and 43.2 nmol/min/mg, and 1.2 microM and 32.1 nmol/min/mg for 3 alpha-hydroxy-5 beta-androstan-17-one, respectively. 20 alpha-HSD activity in the reduction of 17 alpha-hydroxyprogesterone catalyzed by the purified enzyme was inhibited competitively by addition of 5 alpha-DHT with a Ki value of 2.0 microM. Furthermore, 17 alpha-hydroxyprogesterone inhibited competitively 3 alpha-HSD activity with a Ki value of 150 microM.

Cytochrome P-450 C21scc: one enzyme with two actions: hydroxylase and lyase

Testis, adrenal, ovary and placenta contain a microsomal cytochrome P-450 that is capable of converting progesterone to androstenedione and pregnenolone to dehydroepiandrosterone. This conversion requires 17 alpha-hydroxylation followed by C17,20-lyase activity which are both catalyzed by this one protein. Gene cloning and Northern blotting reveal that, at least in man, the same gene is responsible for both testicular and adrenal enzymes. The enzyme was first purified from neonatal pig testis. Both the testicular and adrenal enzymes show a marked preference for the 5-ene substrate (pregnenolone) in keeping with the extensive use of the 5-ene pathway in that species. Affinity alkylation with 17 alpha-bromoacetoxyprogesterone reveals a conserved cysteine at the active site of the enzyme and confirms the conclusion that a single enzyme catalyzes both reactions. Under some circumstances the enzyme catalyzes only 17 alpha-hydroxylation to permit the formation of the C21 steroid cortisol. The regulation of lyase activity, i.e. the determination of the extent to which the second activity is expressed, results from the availability of P-450 reductase. No doubt the greater concentration of this protein in testicular as opposed to adrenal microsomes (x 3.5) is responsible for the production of androgens in the testis and cortisol in the adrenal. Testicular cytochrome b5 also specifically stimulates lyase activity and also causes the porcine enzyme to catalyze a new reaction, i.e. delta 16-synthetase, resulting in synthesis of the important pheromone androsta-4,16-dien-3-one from progesterone.

Inhibition of testicular steroidogenesis in the neonatal rat following acute ethanol exposure

We have previously reported detrimental effects of in utero ethanol exposure on testicular steroidogenic enzyme activity in newborn rats. It is now reported that in utero ethanol exposure during Day 12 of gestation through birth has no apparent morphological effect on the testes of Day 1 neonatal rats. It appears that the detrimental effects of ethanol on testicular steroidogenesis can be manifested at the biochemical level in the absence of morphological effects. However, it remained unknown as to whether acute exposure to ethanol would elicit similar biochemical effects as chronic ethanol exposure on testicular steroidogenesis. To test this possibility ethanol was injected at 0, 1, or 2 g/kg intraperitoneally (IP) into rats of various postnatal ages. Plasma ethanol and testosterone levels as well as testicular 17 alpha-hydroxylase and C17,20-lyase activities were measured. The results indicate that acute exposure to ethanol significantly (p less than 0.05) inhibits the catalytic activity of testicular 17 alpha-hydroxylase in the newborn rat testis. This inhibition was specific since the activity of testicular C17,20-lyase was not affected. In conjunction with the reduction in testicular enzyme activity, plasma testosterone levels were reduced to 30% of the control levels in newborn animals receiving ethanol. In older animals, i.e., postnatal Day 20 and 40 rats, plasma testosterone levels were reduced, but not significantly, following ethanol treatment. Furthermore, testicular enzyme activity was not significantly reduced following ethanol treatment in these same older animals. These results suggest that the newborn rat testis is especially sensitive to the effects of ethanol.

Fetal lamb 3 beta, 20 alpha-hydroxysteroid oxidoreductase: dual activity at the same active site examined by affinity labeling with 16 alpha-(bromo[2'-14C]acetoxy)progesterone

3 beta,20 alpha-Hydroxysteroid oxidoreductase was purified to homogeneity from fetal lamb erythrocytes. The Mr 35,000 enzyme utilizes NADPH and reduces progesterone to 4-pregnen-20 alpha-ol-3-one [Km = 30.8 microM and Vmax = 0.7 nmol min-1 (nmol of enzyme)-1] and 5 alpha-dihydrotestosterone to 5 alpha-androstane-3 beta, 17 beta-diol [Km = 74 microM and Vmax = 1.3 nmol min-1 (nmol of enzyme)-1]. 5 alpha-Dihydrotestosterone competitively inhibits (Ki = 102 microM) 20 alpha-reductase activity, suggesting that both substrates may be reduced at the same active site. 16 alpha-(Bromoacetoxy)progesterone competitively inhibits 3 beta- and 20 alpha-reductase activities and also causes time-dependent and irreversible losses of both 3 beta-reductase and 20 alpha-reductase activities with the same pseudo-first order kinetic t1/2 value of 75 min. Progesterone and 5 alpha-dihydrotestosterone protect the enzyme against loss of the two reductase activities presumably by competing with the affinity alkylating steroid for the active site of 3 beta,20 alpha-hydroxysteroid oxidoreductase. 16 alpha-(Bromo[2'-14C]acetoxy) progesterone radiolabels the active site of 3 beta,20 alpha-hydroxysteroid oxidoreductase wherein 1 mol of steroid completely inactivates 1 mol of enzyme with complete loss of both reductase activities. Hydrolysis of the 14C-labeled enzyme with 6 N HCl at 110 degrees C and analysis of the amino acid hydrolysate identified predominantly N pi-(carboxy[2'-14C]methyl)histidine [His(pi-CM)].(ABSTRACT TRUNCATED AT 250 WORDS)

Cytochromes P-450 in steroidogenesis: are these enzymes more specific than those of drug metabolism?

1. In contrast to drug-metabolizing cytochromes P-450, the corresponding steroidogenic enzymes appear to be highly specific with respect to substrates, reactions catalysed, and the sites in the substrate molecule that are attacked. 2. Recent studies have shown that important exceptions to this generalization are encountered. 3. The conversion of 11-deoxycorticosterone to aldosterone requires three enzymatic reactions, the last of which (aldehyde synthetase), like the first two, requires a P-450 mono-oxygenation. The involvement of P-450 in this third step (from alcohol to aldehyde) was demonstrated by photochemical action spectra and determination of stoichiometry. The three reactions were shown to be catalysed by a single enzyme as demonstrated by immunoprecipitation and the use of a homogenous enzyme. 4. The conversion of pregnenolone and progesterone to the corresponding delta 16-C19 steroids, which are pheromones, is catalysed by a cytochrome P-450 that also catalyses the conversion of progesterone to androstenedione (that is, C21 side-chain cleavage; hydroxylase/lyase). The synthesis of the delta 16 compounds requires cytochrome b5. 5. The conversion of testosterone and epitestosterone to androstenedione is catalysed by P-450b. Studies of kinetic isotope effects using deuterium and 18O2 show that P-450b catalyses this reaction with epitestosterone via the formation of a gem diol, whereas, with testosterone as substrate, one-third of the product is formed via the same mechanism while the remainder results from the mechanism described as dual hydrogen abstraction.

Differential inhibition of estrogen and antiestrogen binding to the estrogen receptor by diethylpyrocarbonate

Diethylpyrocarbonate differentially inhibited the specific binding, in lamb uterine cytosol, of estradiol (inhibition approximately 90% with 4 mM reagent) and 4-hydroxytamoxifen (inhibition approximately less than 50% with 4-16 mM reagent), a potent triphenylethylene antiestrogen. Saturation analysis experiments indicated that the effects of diethylpyrocarbonate were due to progressive but differing decreases in the concentration of binding sites for the two ligands, with no apparent change in the affinity constants. However, competitive binding and dissociation experiments evidenced that steroidal and nonsteroidal estrogens still bound, but with very low affinities, to diethylpyrocarbonate-modified receptor (greater than 1000-fold decrease in affinity) whereas the affinities of triphenylethylene antiestrogens were much less affected (less than 10-fold decrease). Both ligands prevented the inactivation of the estrogen receptor by diethylpyrocarbonate, estradiol being more efficient than 4-hydroxytamoxifen. These data indicate that the action of diethylpyrocarbonate results in the formation of two populations of estrogen receptor that are quantitatively nearly equivalent: the first does not bind estrogens or antiestrogens; the second does not bind estrogens significantly but still interacts with antiestrogens at a high affinity. The simplest interpretation is that these two populations arise from mutually exclusive modifications by diethylpyrocarbonate of at least two aminoacid residues located at or close to the ligand binding site; modification of one residue totally prevents the binding of estrogens and antiestrogens; the modification of the second impairs only the binding of estrogens. Considering that (i) hydroxylamine, which specifically reverses the diethylpyrocarbonate-induced modification of histidine and tyrosine residues, restored a large part (greater than 80%) of the estradiol- and 4-hydroxytamoxifen-binding capacity of diethylpyrocarbonate-inactivated cytosol, and that (ii) similar differential inhibition of estrogen and antiestrogen binding was observed following the action of tetranitromethane, it is likely that these residues are histidine(s) and/or tyrosine(s). These results evince a marked difference in the interaction of estrogens and triphenylethylene antiestrogens with the estrogen receptor, which could account for the altered activation of the receptor by triphenylethylene antiestrogens. Consequently, the screening of ligands with modified steroid receptors could be a useful method for distinguishing between potential hormone agonists and antagonists.

Isolation of 3 beta,20 alpha-hydroxysteroid oxidoreductase from sheep fetal blood

3 beta,20 alpha-Hydroxysteroid oxidoreductase has been isolated from ovine fetal blood by a 2,370-fold purification scheme of ammonium sulfate fractionation, calcium phosphate gel adsorption, affinity chromatography, and fast performance liquid chromatography. A new high performance liquid chromatography-based assay for measuring 20 alpha-reductase activity is described. The enzyme is a monomer with a molecular weight of 35,000 and uses NADPH as a cofactor for reductase activity. It reduces progesterone to 4-pregnen-20 alpha-ol-3-one or 5 alpha-dihydrotestosterone to 5 alpha-androstan-3 beta,17 beta-diol with kinetic characteristics of Km = 30.8 microM and Vmax = 0.7 nmol min-1 (nmol of enzyme)-1 or Km = 74 microM and Vmax = 1.3 nmol min-1 (nmol of enzyme)-1, respectively. 5 alpha-Dihydrotestosterone competitively inhibits 20 alpha-reductase activity with a Ki value of 102 microM.