Enzyme kinetic and inhibition analyses of cytochrome P450XVII, a protein with a bifunctional catalytic site. Quantification of effective substrate concentrations at the active site and their significance for intrinsic control of the hydroxylase/lyase reaction sequence

Kinetic processivity of the two-step oxidations of progesterone and pregnenolone to androgens by human cytochrome P450 17A1

Cytochrome P450 (P450, CYP) 17A1 plays a critical role in steroid metabolism, catalyzing both the 17α-hydroxylation of pregnenolone and progesterone and the subsequent 17α,20-lyase reactions to form dehydroepiandrosterone (DHEA) and androstenedione (Andro), respectively, critical for generating glucocorticoids and androgens. Human P450 17A1 reaction rates examined are enhanced by the accessory protein cytochrome b₅ (b₅), but the exact role of b₅ in P450 17A1-catalyzed reactions is unclear as are several details of these reactions. Here, we examined in detail the processivity of the 17α-hydroxylation and lyase steps. b₅ did not enhance reaction rates by decreasing the k_off rates of any of the steroids. Steroid binding to P450 17A1 was more complex than a simple two-state system. Pre-steady-state experiments indicated lag phases for Andro production from progesterone and for DHEA from pregnenolone, indicating a distributive character of the enzyme. However, we observed processivity in pregnenolone/DHEA pulse-chase experiments. (S)-Orteronel was three times more inhibitory toward the conversion of 17α-hydroxypregnenolone to DHEA than toward the 17α-hydroxylation of pregnenolone. IC₅₀ values for (S)-orteronel were identical for blocking DHEA formation from pregnenolone and for 17α-hydroxylation, suggestive of processivity. Global kinetic modeling helped assign sets of rate constants for individual or groups of reactions, indicating that human P450 17A1 is an inherently distributive enzyme but that some processivity is present, i.e. some of the 17α-OH pregnenolone formed from pregnenolone did not dissociate from P450 17A1 before conversion to DHEA. Our results also suggest multiple conformations of P450 17A1, as previously proposed on the basis of NMR spectroscopy and X-ray crystallography.

Role of cytochrome b5 in the modulation of the enzymatic activities of cytochrome P450 17α-hydroxylase/17,20-lyase (P450 17A1)

Cytochrome b5 (cyt b5) is a small hemoprotein that plays a significant role in the modulation of activities of an important steroidogenic enzyme, cytochrome P450 17α-hydroxylase/17,20-lyase (P450 17A1, CYP17A1). Located in the zona fasciculata and zona reticularis of the adrenal cortex and in the gonads, P450 17A1 catalyzes two different reactions in the steroidogenic pathway; the 17α-hydroxylation and 17,20-lyase, in the endoplasmic reticulum of these respective tissues. The activities of P450 17A1 are regulated by cyt b5 that enhances the 17,20-lyase reaction by promoting the coupling of P450 17A1 and cytochrome P450 reductase (CPR), allosterically. Cyt b5 can also act as an electron donor to enhance the 16-ene-synthase activity of human P450 17A1. In this review, we discuss the many roles of cyt b5 and focus on the modulation of CYP17A1 activities by cyt b5 and the mechanisms involved.

Structural and kinetic basis of steroid 17α,20-lyase activity in teleost fish cytochrome P450 17A1 and its absence in cytochrome P450 17A2

Cytochrome P450 (P450) 17A enzymes play a critical role in the oxidation of the steroids progesterone (Prog) and pregnenolone (Preg) to glucocorticoids and androgens. In mammals, a single enzyme, P450 17A1, catalyzes both 17α-hydroxylation and a subsequent 17α,20-lyase reaction with both Prog and Preg. Teleost fish contain two 17A P450s; zebrafish P450 17A1 catalyzes both 17α-hydroxylation and lyase reactions with Prog and Preg, and P450 17A2 is more efficient in pregnenolone 17α-hydroxylation but does not catalyze the lyase reaction, even in the presence of cytochrome b5. P450 17A2 binds all substrates and products, although more loosely than P450 17A1. Pulse-chase and kinetic spectral experiments and modeling established that the two-step P450 17A1 Prog oxidation is more distributive than the Preg reaction, i.e. 17α-OH product dissociates more prior to the lyase step. The drug orteronel selectively blocked the lyase reaction of P450 17A1 but only in the case of Prog. X-ray crystal structures of zebrafish P450 17A1 and 17A2 were obtained with the ligand abiraterone and with Prog for P450 17A2. Comparison of the two fish P450 17A-abiraterone structures with human P450 17A1 (DeVore, N. M., and Scott, E. E. (2013) Nature 482, 116-119) showed only a few differences near the active site, despite only ∼50% identity among the three proteins. The P450 17A2 structure differed in four residues near the heme periphery. These residues may allow the proposed alternative ferric peroxide mechanism for the lyase reaction, or residues removed from the active site may allow conformations that lead to the lyase activity.

Dynamic disorder-driven substrate inhibition and bistability in a simple enzymatic reaction

Conformations and catalytic rates of enzymes (biological catalysts) fluctuate over a wide range of time scales. Recent experimental and theoretical investigations demonstrated case studies where the enzymatic catalysis rate follows the Michaelis-Menten (MM) rate law despite molecular fluctuations. In this paper, we investigate deviations from MM law and their effects on the dynamical behavior of the enzymatic network. We consider a simple kinetic scheme for a single substrate enzymatic reaction in which the product release step is treated explicitly. We examine how conformational fluctuations affect the underlying rate law in the quasi-static limit when conformational dynamics is very slow in one of the states. Our numerical results and analytically solvable model indicate that slow conformational fluctuations of the enzyme-substrate complex lead to non-MM behavior, substrate inhibition, and possible bistability of the reaction network.

Allosteric phenomena in cytochrome P450-catalyzed monooxygenations

Allosteric regulation of monooxygenase activity is shown to occur with diverse cytochrome P450 isoforms and is characterized by kinetic patterns deviating from the Michaelis-Menten model. Homotropic and heterotropic phenomena are encountered in both substrate activation and productive coupling of the electron donors NADPH-cytochrome P450 reductase and cytochrome b5, and the lipid environment of the system also appears to play a role as an effector. Circumstantial analysis reveals the components of the electron transfer chain to be mutually beneficial in interactions with each other depending on the substrate used and type of cytochrome P450 operative. It is noteworthy that association of diatomic gaseous ligands may be amenable to allosteric regulation as well. Thus, dioxygen binding to cytochrome P450 displays nonhyperbolic kinetic profiles in the presence of certain substrates; the latter, together with redox proteins such as cytochrome b5, can exert efficient control of the abortive breakdown of the oxyferrous intermediates formed. Similarly, substrates may modulate the structural features of the access channel for solutes such as carbon monoxide in specific cytochrome P450 isozymes to either facilitate or impair ligand diffusion to the heme iron. The in vivo importance of allosteric regulation of enzyme activity is discussed in detail.

Conversion of pregnenolone to DHEA by human 17alpha-hydroxylase/17, 20-lyase (P450c17). Evidence that DHEA is produced from the released intermediate, 17alpha-hydroxypregnenolone

Most previous studies using reconstituted systems and fast kinetics suggest that the conversion of pregnenolone to dehydroepiandrosterone (DHEA; the precursor of androgen and estrogen biosynthesis) by P450c17 does not require the release of the intermediate 17alpha-OHPreg (a precursor of cortisol biosynthesis). With such a mechanism, it is difficult to conceive how high amounts of DHEA may be produced in some cells or tissues, such as the testis and cells from the adrenal reticularis, while in other tissues such as the fasciculata zone, high levels of 17alpha-OHPreg are synthesized. In this report, we address this matter using intact transfected cells, which better reflect the actual cellular conditions. Furthermore, by using transfected cells, we can conveniently analyze human enzymes, as we are not restricted by the availability of human tissues as in the case of methods using purified or partially purified enzymes. Using intact HEK-293 cells transfected with human P450c17 in culture, we showed, in a time course study of the transformation of pregnenolone, that there is an accumulation of 17alpha-OHPreg, and that, subsequently, the accumulated 17alpha-OHPreg decreases with a concomitant increase in DHEA production. The DHEA/17alpha-OHPreg ratio changes from 0.1 :1 after 1 h incubation to 50 : 1 after 20 h. This result strongly suggests that the transformation of Preg to DHEA proceeds through two steps in which DHEA is produced from the released intermediate 17alpha-OHPreg. We also show that high levels of substrate vs. enzyme concentration will lead to high hydroxylase activity whereas the reverse will increase the lyase activity. The result is in good agreement with recent observations suggesting that surrounding enzymes and steroids could modulate the lyase activity. Cotransfection of vectors expressing cytochrome b5 and NADPH cytochrome P450 reductase indicates that both are required for an optimum production of DHEA.

Protein phosphorylation changes ligand-binding efficiency of cytochrome P450c17 (CYP17) and accelerates its proteolytic degradation: putative relevance for hormonal regulation of CYP17 activity

Two novel mechanisms of protein kinase function in the complex gonadotropic regulation of the bifunctional cytochrome P450c17 (CYP17), the rate-limiting enzyme of androgen synthesis within the smooth endoplasmic reticulum of gonadal endocrine cells, are reported. In microsomal membranes from rat testes, the maximal type I optical difference spectrum induced by the physiological CYP17 substrate, progesterone, as a measure of spin state transition due to hydrophobic ligand-protein interaction is enhanced by 24% within 15 minutes in the presence of MgATP; the dissociation constant decreases from 71 to 43 nM. Testicular cytosol does not modify this effect which is completely abolished by the protein kinase inhibitor, bisindolylmaleimide, and which does not occur with ketoconazole as ligand. Furthermore, CYP17 degradation by cytosolic protease(s) is 2.5-fold accelerated by ATP; this action is completely reversed by the protein kinase inhibitors bisindolylmaleimide (half-maximal protective concentration 2.04 microM) and KT5720 (99 nM). The former compound also prevents human choriogonadotropin-induced testicular CYP17 inactivation in situ. It is concluded that protein kinase A-catalyzed target phosphorylation integrates the known biphasic steroidogenic response upon hormonal stimulation by initial improvement of substrate accommodation followed by counter-regulatory promotion of CYP17 proteolysis.

Novel connections between NADPH-induced lipid peroxidation and cytochrome P450 inactivation, and antioxidant and enzyme protective properties of estradiol in gonadal membranes

This study uses microsomal membranes from rat testis tissue, including the cytochrome P450c17 (steroid 17 alpha-monooxygenase/17 alpha-hydroxyprogesterone aldolase, catalyzing the conversion of progesterone to androstenedione), to decipher the possible relation of NADPH-induced (no exogenous iron added) lipid peroxidation and cytochrome P450 inactivation and the protective effect of certain steroids. NADPH (300 microM) causes a 3.6-fold stimulation of malondialdehyde formation (thiobarbituric acid-reactive substances) and a 29% cytochrome P450c17 loss within 1 h at 37 degrees C, but has no effect on lipid peroxidation in the presence of the iron chelator desferrioxamine. Hydrogen peroxide has only marginal effects. The antioxidant efficiency of estradiol (IC50 = 13.9 microM) is higher than its cytochrome P450c17 protective efficiency (IC50 = 33.0 microM), whereas androstenedione does not inhibit lipid peroxidation but protects cytochrome P450c17 completely. The human choriogonadotropin-induced degradation of cytochrome P450c17 in incubated decapsulated testes can not be correlated with a stimulation of lipid peroxidation, and it is partially inhibited by estradiol but completely abolished by androstenedione. It is concluded (I) that NADPH stimulates iron-dependent generation of reactive oxygen species by the monooxygenase system even in the presence of certain P450 ligands in the physiological membrane environment, (II) that membrane lipid peroxidation may be suppressed by hydrophobic steroids acting as antioxidants such as estradiol, (III) that steroid ligands stabilize cytochrome P450c17 against inactivation in the presence of NADPH even if they do not act as substrates and do not possess antioxidant activity, and (IV) that the choriogonadotropin-induced down-regulation of cytochrome P450c17 is not due to accumulating steroids acting as "pseudosubstrates" as occasionally supposed.

Kinetic analysis of duodenal and testicular cytochrome P450c17 in the rat

In the adult rat, the duodenal tissue of both sexes can convert progesterone to 17alpha-hydroxyprogesterone, androstenedione and testosterone. The transition from C21 to C19 steroids is apparently controlled by the same cytochrome P450c17 expressed in the testis, which catalyzes both 17alpha-hydroxylation and C-17,20 bond scission at a single bifunctional active site. The kinetic parameters of this enzyme were measured at the steady state for both reactions using [1,2-3H]progesterone and [1,2-3H]17alpha-hydroxyprogesterone as substrates. In the testis and male and female duodena, the Km values for progesterone 17alpha-hydroxylation were 14.2, 23.8 and 23.2 nM, whereas the Vmax values were 105, 3.5 and 3.1 pmol/mg protein/min, respectively. With respect to C-17,20 lyase activity, the Km values for exogenous 17alpha-hydroxyprogesterone were 525, 675 and 637 nM, whereas the Vmax values were 283, 7.8 and 7.8 pmol/mg protein/min, respectively. However, when the Km values were calculated with respect to intermediate 17alpha-hydroxyprogesterone formed from progesterone, they were similar to the Km values for 17alpha-hydroxylase, being 15, 31.4 and 24.8 nM, whereas the Vmax values were 26.3, 2 and 1.8 pmol/mg protein/min, respectively. The similarity of Km values is due to the fact that the relative androgen formation efficiency (bond scission events/total 17alpha-hydroxylation events ratio) was remarkably constant in both testicular and duodenal incubates, irrespective of progesterone concentration. Efficiency values were 2-fold higher in duodenal tissue (0.54) than in testis (0.25). Estradiol-17beta inhibited 17alpha-hydroxylation but not bond scission on intermediate 17alpha-hydroxyprogesterone, because it did not affect the efficiency value. Rat duodenal P450c17 has the same substrate affinity, a lower specific activity and a higher androgen formation efficiency than testicular P450c17.

Inhibition of bovine cytochrome P-450(11 beta) by 18-unsaturated progesterone derivatives

The last step of aldosterone biosynthesis, an 11 beta-hydroxylation followed by two 18-hydroxylations, are catalyzed, in the bovine system, by the same enzyme, the cytochrome P-450(11 beta) (deoxycorticosterone (DOC)-->corticosterone-->18-hydroxycorticosterone-->aldosterone). The 11 beta- and 18-hydroxylase activities were studied separately with a reconstituted enzymic system, using 11-deoxy[14C]corticosterone and [3H]corticosterone, respectively, as substrates. The inhibition of 11 beta-hydroxylase activity by corticosterone was competitive (Ki = 60 microM) showing that transformation of both substrates occurs at the same site. Double-label/double-substrate experiments, using an equimolar mixture of 11-deoxy[14C]corticosterone and [3H]corticosterone, suggested that 18-hydroxycorticosterone is directly formed from 11-deoxycorticosterone without the intermediate corticosterone leaving the enzyme. Inhibitions by 18-vinylprogesterone and 18-ethynylprogesterone, potent inhibitors of aldosterone biosynthesis [Viger, A., Coustal, S., Pérard, S., Piffeteau, A. & Marquet, A. (1989) J. Steroid Biochem. 33, 119-124], were characterized for both activities (11 beta- and 18-hydroxylase). The value of reversible Ki for the 18-hydroxylation (Ki = 5 microM for 18-vinylprogesterone and 30 microM for 18-ethynylprogesterone) is lower than that for the 11 beta-hydroxylation (30 microM and 100-150 microM, respectively); the former inhibitor is stronger than the latter for both steps. The binding of substrates and inhibitors to the active site was also examined by difference absorption spectroscopy. 18-Vinylprogesterone gave rise to a type I spectrum with a Ks value of 35 microM close to that of progesterone, while 18-ethynylprogesterone showed a reverse type I spectrum with a much higher Ks value (140 microM). Based on these results, a hypothetical model, involving a conformational change of the enzyme for the second step, is proposed.

Modulation of hydroxylase and lyase activities of bovine cytochrome P-450(17) alpha in adrenal and testicular microsomes by a tissue-specific local membrane environment

In steroidogenic tissues, cytochrome P-450(17) alpha catalyzes both steroid 17 alpha-hydroxylation and 17,20-lyase reactions. The ratio of the two activities, hydroxylase over lyase (H/L) depends upon the tissue of origin; this ratio is low in the testis whereas it is high in the adrenal cortex. To examine the factors responsible for this specific regulation, two approaches were followed: (i) the purified enzyme was incorporated into liposomes made of microsomal lipids of testis or adrenal cortex; and (ii) the effects of disorganization of the microsomal membrane on the activities were observed. The results show that the cytochrome 17,20-lyase activity is stimulated by the presence of lipids from testicular origin. In the adrenal microsomes, this activity appears to be dependent upon the local membrane organization. Specific component(s) associated with the neutral fraction of the microsome lipid extract may be responsible for the repression of lyase activity in the adrenal.

Characterization of cytochrome P450TYR, a multifunctional haem-thiolate N-hydroxylase involved in the biosynthesis of the cyanogenic glucoside dhurrin

The haem-thiolate N-hydroxylase cytochrome P450TYR involved in the biosynthesis of the tyrosine-derived cyanogenic glucoside dhurrin in Sorghum bicolor had recently been isolated. Reconstitution of enzyme activity by insertion of cytochrome P450TYR and NADPH-cytochrome P450-reductase into L-alpha-dilauroylphosphatidylcholine micelles and using tyrosine as substrate results in the formation of p-hydroxyphenylacetaldehyde oxime. Quantitative substrate binding spectra demonstrate that tyrosine and N-hydroxytyrosine are mutually exclusive substrates that bind to the same active site of cytochrome P450TYR. The multifunctionality of cytochrome P450TYR has been confirmed in reconstitution experiments using recombinant cytochrome P450TYR expressed in Escherichia coli. It was earlier reported that an in vitro microsomal system catalyzing all but the last step in the biosynthetic pathway for cyanogenic glucosides exhibits catalytic facilitation (channelling). This observation is explained by the multifunctionality of cytochrome P450TYR. The cytochrome P450TYR sequence represents the first amino acid sequence of a functionally characterized cytochrome P-450 enzyme from a monocotyledonous plant and the first sequence of an N-hydroxylase with high substrate specificity. Multifunctional N-hydroxylases of the cytochrome P-450 type have not previously been reported in living organisms.

Regulation mechanism of the catalytic activity of bovine adrenal cytochrome P-450(11)beta

In our previous paper (Ikushiro et al. (1992) J. Biol. Chem. 267, 1464), two catalytic states were proposed for bovine adrenocortical P-450(11)beta at 37 degrees C: one in liposome membranes and the other in liposome membranes containing P-450scc. Similar reaction characteristics were observed at 5 degrees C and all the experiments in this study were performed at 5 degrees C. P-450(11)beta-proteoliposomes had relatively low 11 beta-hydroxylase activity and could catalyze aldosterone formation from 11-deoxycorticosterone. Relatively high 11 beta-hydroxylase activity was observed in P450(11)beta-proteoliposomes containing P-450scc and in Tween-20 solubilized P-450(11)beta, in which no aldosterone formation could be detected. Optical titration indicated binding of corticosterone to P-450(11)beta to be much weaker in the Tween-20 solubilized state than in proteoliposomes. Corticosterone competitively inhibited 11 beta-hydroxylation reaction of P-450(11)beta-proteoliposomes, but neither in P-450(11)beta-proteoliposomes containing P-450scc nor in the Tween-20 solubilized system. The binding of corticosterone to P-450(11)beta was concluded quite weak in proteoliposomes in the presence of P-450scc and in the Tween-20 solubilized state. Aldosterone formation thus was not possible in these systems. Inability of the bovine adrenocortical zonae fasciculata and reticularis to produce aldosterone may be due to the weak binding of corticosterone to P-450(11)beta in these zones.

Binding of the 2',5'-ADP subsite stimulates cyclohydrolase activity of human NADP(+)-dependent methylenetetrahydrofolate dehydrogenase/cyclohydrolase

The bifunctional dehydrogenase/cyclohydrolase domain of the human trifunctional methylenetetrahydrofolate dehydrogenase/methenyltetrahydrofolate cyclohydrolase/formyltetrahydrofolate synthetase catalyzes two sequential reactions with significant channeling of the intermediate, methenyltetrahydrofolate. Equilibrium dialysis established that a single, high-affinity NADP+ binding site exists per monomer of the dimeric enzyme. Kinetic characterization of NADP+ binding to the dehydrogenase using analogs as inhibitors demonstrated that affinity for this substrate is due almost exclusively to binding at the 2',5'-ADP subsite. The same structural specificities for binding are exhibited by these analogs in their effects on the cyclohydrolase. Both NADP+ and its 3-aminopyridine analog AADP partially inhibit the activity of the cyclohydrolase when assayed with added methenyltetrahydrofolate as substrate. However, under the same conditions, the cyclohydrolase is actually activated by 2',5'-ADP; activation requires the presence of the 5'-phosphate since 2'-AMP binds but does not activate. Nicotinamide ribose monophosphate (NMN) has no detectable effect either alone or in combination with 2',5'-ADP. The results are consistent with the existence of a shared dehydrogenase/cyclohydrolase active site proximal to the 2',5'-ADP subsite. NADP+ reduces the rate of the fully activated cyclohydrolase by 2-fold. Inhibition appears to be due to the loosely bound nicotinamide ring interacting with the common folate subsite, resulting in only partial inhibition by NADP+. The interaction of 2',5'-ADP with the cyclohydrolase suggests a potential role for this portion of the molecule in promoting the efficiency of the channeling of endogenously generated methenyltetrahydrofolate.

The substrate-depletion error on the multiple Michaelis-Menten equation with and without an added term linearly dependent on the substrate concentration

Saturation curves for substrate interaction with an acceptor (enzyme, binding or carrier protein) are often analysed on the assumption that the amount of acceptor-bound substrate is negligible compared to its total amount. Analytical criteria permitting one to decide whether the assumption is justified or not for systems described by a single Michaelis-Menten equation have been derived previously. For more complicated systems, error formulae often cannot be obtained in closed form, and, if obtainable, are unwieldy. How this more complicated problem can be tackled is shown for mixtures of acceptor sites described by a sum of several Michaelis-Menten terms, without or with an additional term for 'non-specific' uptake or binding. In particular, it is shown that the maximum error, for which simple analytical expressions are obtained, provides a valid criterion for assessing whether substrate depletion is negligible or not.

Regulation of steroid hydroxylase gene expression: importance to physiology and disease

Steroid hydroxylase gene expression is multifactorial in nature, being regulated by tissue-specific, developmental, constitutive and signal transduction systems. The biochemistry of this complex pattern of regulation is not yet clearly elucidated, but studies in several laboratories have led to an understanding of specific aspects of regulation, particularly that involving signal transduction. The complexity of regulation appears to be necessary for normal human physiology because of the wide variety of steroid hormones produced by these enzymes. Genetic diseases associated with the steroid hydroxylases provide examples of how aberrant physiology can result from alterations in the multifactorial regulation of steroid hydroxylase gene expression.

Characterization of the hydrophobic interaction of steroids with endoplasmic reticulum membranes by quenching of 6,8(14)-bis-dehydro-17 alpha-hydroxyprogesterone fluorescence

The fluorescence behaviour of 4,6,8(14)-trien-3-one steroids, which exhibit fluorescence in protic media but no fluorescence in hydrophobic environments, was used to characterize the molecular nature and temperature-sensitivity of steroid hormone-biomembrane interactions. Since 17 alpha-hydroxyprogesterone as the key intermediate is known to accumulate in smooth endoplasmic reticulum membranes in the course of adrenal and testicular steroid hormone biosynthesis, its fluorescent analogue, 6,8(14)-bis-dehydro-17 alpha-hydroxyprogesterone (BDHP), was used as the probe molecule. With rat testis microsomal membranes and liposomes, fluorescence quenching in the presence of membranes (related to fluorescence in aqueous solution) was independent on steroid concentration but was dependent on membrane lipid concentration in terms of a hyperbolic function. Complete fluorescence loss occurred at infinite lipid concentration at 20 degrees C, indicating complete insertion of the steroid probe into the hydrophobic portion of the membrane compartment. The partition coefficient KP increased with increasing temperature as a consequence of increased membrane fluidity. The result that BDHP fluorescence decreased considerably with elevated temperature in both the aqueous and the membrane milieu was interpreted as the consequence of increasing molecular mobility; this effect was much more pronounced in the aqueous than in the membrane environment. On the basis of local BDHP concentrations within the membrane phase (calculated from KP), relative fluorescence quenching was over-proportional at low temperatures; under that condition, hydrophobic interactions with rigid membrane lipid domains are obviously favoured.

Ketoconazole inhibition of the bifunctional cytochrome P450c17 does not affect androgen formation from the endogenous lyase substrate. The catalytic site remains refractory in the course of intermediary hydroxyprogesterone processing

The inhibition of the bifunctional steroidogenic cytochrome P450c17 (CYP17: steroid-17 alpha-hydroxylase/steroid-17,20-lyase) by the imidazole-type fungicide, [(+/-)-cis-1-acetyl-4-[4-[[2-(2,4-dichlorophenyl)-2-(1H-imidazol-1-yl- methyl)-1,3-dioxolan-4-yl]methoxy]phenyl]piperazine) (ketoconazole), was investigated with the aim of differentiating between effects on androgen formation from exogenously added and endogenously produced 17 alpha-hydroxyprogesterone. Using microsomal membranes from rat testis, turnover of progesterone by P450c17 was competitively inhibited by ketoconazole with KI = 0.40 microM. Ketoconazole did not affect the linear relationship between the ratio of productive events (corresponding to androgen formation rates) versus abortive events (corresponding to 17 alpha-hydroxyprogesterone formation rates) and the sum of catalytic events. This was an indication that this inhibitor did not interfere with intermediate processing by P450c17. Androgen formation from exogenous but not from endogenous 17 alpha-hydroxyprogesterone was competitively inhibited by ketoconazole. The simultaneous conversion of 1 microM each of [3H]progesterone and 17 alpha-hydroxy[14C]progesterone was also reduced by ketoconazole. Calculation of 3H/14C ratios in the 17 alpha-hydroxyprogesterone and androgen fractions revealed that the endogenous 17 alpha-hydroxyprogesterone pool was metabolized to androgens at rates 6.4, 11.6, 17.6 and 21.2-fold faster than the exogenous pool in the presence of 0.5, 1, 2 and 4 microM ketoconazole, respectively; this value was only 4.0 in controls. It is concluded that ketoconazole inhibits turnover of steroid ligands only when they approach the P450c17 active site in a substrate-state and that inhibition of androgen formation from progesterone is due to inhibition of the first catalytic step only. A model is described in which the P450c17 active site is refractory towards ketoconazole when the intermediary steroid is retained and being processed at that site.

Steroidogenic enzymes: structure, function, and role in regulation of steroid hormone biosynthesis

In the pathways of steroid hormone biosynthesis there are two major types of enzymes: cytochromes P450 and other steroid oxidoreductases. This review presents an overview of the function and expression of both types of enzymes with emphasis on steroidogenic P450s. The final part of the review on regulation of steroidogenesis includes a description of the normal physiological fluctuations in the steroid output of adrenal cortex and gonads, and provides an analysis of the relative role of enzyme levels in the determination of these fluctuations. The repertoire of enzymes expressed in a steroidogenic cell matches the cell's capacity for the biosynthesis of specific steroids. Thus, steroidogenic capacity is regulated mainly by tissue and cell specific expression of enzymes, and not by selective activation or inhibition of enzymes from a larger repertoire. The quantitative capacity of steroidogenic cells for the biosynthesis of specific steroids is determined by the levels of steroidogenic enzymes. The major physiological variations in enzyme levels, are generally associated with parallel changes in gene expression. The level of expression of each steroidogenic enzyme varies in three characteristics: (a) tissue- and cell-specific expression, determined during tissue and cell differentiation; (b) basal expression, in the absence of trophic hormonal stimulation; and (c) hormonal signal regulated expression. Each of these three types of expression probably represent the functioning of distinct gene regulatory elements. In adult steroidogenic tissues, the levels of most of the cell- and tissue-specific steroidogenic enzymes depend mainly on trophic hormonal stimulation mediated by a complex network of signal transduction systems.

Affinity labeling of human placental 3 beta-hydroxy-delta 5-steroid dehydrogenase and steroid delta-isomerase: evidence for bifunctional catalysis by a different conformation of the same protein for each enzyme activity

3 beta-Hydroxy-delta 5-steroid dehydrogenase and steroid delta-isomerase copurify from human placental microsomes as a single enzyme protein. The affinity-alkylating secosteroid, 5,10-secoestr-4-yne-3,10,17-trione, inactivates the dehydrogenase and isomerase reactions in a time-dependent manner, but which of the two activities is targeted depends on the concentration of secosteroid. At 2-5 microM secosteroid, the dehydrogenase activity is alkylated in a site-specific manner (pregnenolone slows inactivation) that follows first-order inactivation kinetics (KI = 4.2 microM, k3 = 1.31 x 10(-2) min-1). As the secosteroid level increases from 11 to 30 microM, dehydrogenase is paradoxically inactivated at progressively slower rates, and pregnenolone no longer protects against the alkylator. The inactivation of isomerase exhibits the expected first-order kinetics (KI = 31.3 microM, k3 = 6.42 x 10(-2) min-1) at 11-30 microM secosteroid. 5-Androstene-3,17-dione protects isomerase from inactivation by 15 microM secosteroid, but the substrate steroid unexpectedly fails to slow the inactivation of isomerase by a lower concentration of alkylator (5 microM). A shift from a dehydrogenase to an isomerase conformation in response to rising secosteroid levels explains these results. Analysis of the ligand-induced conformational change along with cofactor protection data suggests that the enzyme expresses both activities at a bifunctional catalytic site. According to this model, the protein begins the reaction sequence as 3 beta-hydroxysteroid dehydrogenase. The products of the first step (principally NADH) promote a change in protein conformation that triggers the isomerase reaction.

Cytochrome P-450(17 alpha,lyase)-mediating pathway of androgen synthesis in bovine adrenocortical cultured cells

Cytochrome P-450(17 alpha,lyase) mediating pathway of dehydroepiandrosterone (DHA) formation from pregnenolone was investigated in primary cultures of bovine adrenocortical fasciculata-reticularis cells. To determine whether DHA formation proceeds predominantly by successive monooxygenase reactions without 17 alpha-hydroxypregnenolone leaving P-450(17 alpha,lyase) the cells were incubated with [14C]pregnenolone and 17 alpha-[3H]hydroxypregnenolone in the presence of Trilostane. Results of the double-substrate double-label experiments indicate that in the presence of high concentration of pregnenolone most of DHA was formed, directly from pregnenolone by the successive reactions. Since the concentration of pregnenolone usually exceeds that of 17 alpha-hydroxypregnenolone in the adrenal glands, DHA is concluded to be formed predominantly by successive reactions from pregnenolone without 17 alpha-hydroxypregnenolone leaving P-450(17 alpha,lyase) in vivo. By chronic ACTH treatment, the activities of 17 alpha-hydroxylation and DHA formation in adrenocortical cultured cells became higher concomitantly with the increase of P-450(17 alpha,lyase) content. Most of DHA was found to be formed by successive reactions from pregnenolone even under such conditions.

Integration of new regulatory strategies into the network of an endocrine control system: limitation of androgen secretion by rat testis is achieved by substrate-dependent modulation of P450XVII enzyme concentration and catalytic efficiency

In addition to the well-known control circuits involved in the regulation and adaptation of testicular androgen biosynthesis, it is proposed that two new control strategies are involved in the maintenance of steady-state testosterone secretion rates by testicular Leydig cells. Cytochrome P450XVII (steroid-17 alpha-monooxygenase/steroid-17,20-lyase), one key enzyme in steroid hormone biosynthesis, responds to external human choriogonadotropin stimulation with an oxygen-dependent and substrate flux-dependent inactivation and decomposition, and increased substrate availability decreases the efficiency of androgen formation in favour of abortive intermediate leakage. These results are discussed as a paradigm of substrate-dependent modulation of cytochrome P450 activities.