Thermodynamics and modulation of progesterone microcompartmentation and hydrophobic interaction with cytochrome P450XVII based on quantification of local ligand concentrations in a complex multi-component system

Ligand binding to cytochrome P450 3A4 in phospholipid bilayer nanodiscs: the effect of model membranes

The membrane-bound protein cytochrome P450 3A4 (CYP3A4) is a major drug-metabolizing enzyme. Most studies of ligand binding by CYP3A4 are currently carried out in solution, in the absence of a model membrane. Therefore, there is little information concerning the membrane effects on CYP3A4 ligand binding behavior. Phospholipid bilayer Nanodiscs are a novel model membrane system derived from high density lipoprotein particles, whose stability, monodispersity, and consistency are ensured by their self-assembly. We explore the energetics of four ligands (6-(p-toluidino)-2-naphthalenesulfonic acid (TNS), alpha-naphthoflavone (ANF), miconazole, and bromocriptine) binding to CYP3A4 incorporated into Nanodiscs. Ligand binding to Nanodiscs was monitored by a combination of environment-sensitive ligand fluorescence and ligand-induced shifts in the fluorescence of tryptophan residues present in the scaffold proteins of Nanodiscs; binding to the CYP3A4 active site was monitored by ligand-induced shifts in the heme Soret band absorbance. The dissociation constants for binding to the active site in CYP3A4-Nanodiscs were 4.0 microm for TNS, 5.8 microm for ANF, 0.45 microm for miconazole, and 0.45 microm for bromocriptine. These values are for CYP3A4 incorporated into a lipid bilayer and are therefore presumably more biologically relevant that those measured using CYP3A4 in solution. In some cases, affinity measurements using CYP3A4 in Nanodiscs differ significantly from solution values. We also studied the equilibrium between ligand binding to CYP3A4 and to the membrane. TNS showed no marked preference for either environment; ANF preferentially bound to the membrane, and miconazole and bromocriptine preferentially bound to the CYP3A4 active site.

pH-metric logP 10. Determination of liposomal membrane-water partition coefficients of ionizable drugs

PURPOSE

To investigate a novel approach for the determination of liposomal membrane-water partition coefficients and lipophilicity profiles of ionizable drugs.

METHODS

The measurements were performed by using a pH-metric technique in a system consisting of dioleylphosphatidylcholine (DOPC) unilamellar vesicles in 0.15 M KCl at 25 degrees C. The DOPC unilamellar vesicle suspension was prepared via an extrusion process.

RESULTS

The liposomal membrane-water partition coefficients of eight ionizable drugs: ibuprofen, diclofenac, 5-phenylvaleric acid, warfarin, propranolol, lidocaine, tetracaine and procaine were determined and the values for neutral and ionized species were found to be in the ranges of approximately 4.5 to 2.4 and 2.6 to 0.8 logarithmic units, respectively.

CONCLUSIONS

It has been shown that the liposomal membrane-water partition coefficients as derived from the pH-metric technique are consistent with those obtained from alternative methods such as ultrafiltration and dialysis. It was found that in liposome system, partitioning of the ionized species is significant and is influenced by electrostatic interaction with the membranes. We have demonstrated that the pH-metric technique is an efficient and accurate way to determine the liposomal membrane-water partition coefficients of ionizable substances.

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.

Ligand dependence of cytochrome P450c17 protection against proteolytic inactivation: structural, methodological and functional implications

Rate constants for the subtilisin-catalyzed proteolytic inactivation of cytochrome P450c17 (CYP17), the endoplasmic reticulum membrane-bound limiting enzyme of gonadal androgen synthesis, have been determined in the absence and presence of various CYP17 ligands and correlated with fractional enzyme saturation (Y). Extrapolation to Y = 1 reveals 15.1-, 4.0- and 7.4-fold enzyme stabilization with progesterone (substrate-type ligand), testosterone (product-type ligand) and ketoconazole (imidazole-type inhibitory ligand), respectively. Structural features of ligand accommodation can therefore be monitored by the susceptibility of target enzymes to proteolysis. It is further proposed that specific protection of a membrane protein by ligand binding during proteolytic digestion may assist in the purification of that protein. Evidence is finally presented that the gonadotropin-induced rapid CYP17 down-regulation is not promoted by an elevation of steroid hormone levels.

Multiphasic modelling of ligand/acceptor interactions. The hydrophobicity-dependent binding of relatively small amphiphilic substances to acceptor proteins and the nature and facedness of acceptor sites

The modelling of multiphasic ligand/acceptor equilibrium binding systems proceeds at three logically distinct levels: (1) A suitable response quantity, e.g. the amount of acceptor-bound ligand nEL, is expressed as a function of the ligand concentrations [Li] (L = A,B,...) in the compartment i that contains the acceptor sites. One thus obtains a response function nEL = f1([Li]). In general, the equilibrium constants KL contained in such mathematical models are physically ill-defined. (2) Each local concentration [Li] is further expressed as a function of [Laq], the corresponding concentration in the aqueous phase, leading to nEL = f2([Laq]). In this way, the constants KL are transformed into effective constants K'L which (i) can be assessed experimentally and (ii) depend on ligand hydrophobicity in a way that is characteristic of the binding site. Formulation of the functions f1 and F2 only requires knowledge of the reactions in which the acceptor sites participate directly. (3) For each ligand, the experimentally accessible total ligand concentration Lt is expressed as a function of [Laq], leading to concentration balance equations Lt = Lt([Laq]). The latter transformation takes account of any reactions, distinct from ligand/acceptor interaction, in which the ligands are involved, e.g. binding to additional protein sites. As a result of steps 2 and 3, each binding system is described by a set of simultaneous equations dependent on the auxiliary variable [Laq]: (i) the response function f2([Laq]) and (ii) a concentration balance for each ligand Lt = Lt([Laq]). The formulae are rendered more conscise and their discussion and application to data fitting are simplified by introducing, for each ligand L, a function FL characterising the distribution of unbound monomeric ligand over the various partition compartments. When the acceptor acts on unbound ligand, the formulae are further expressed in terms of a new auxiliary variable i.e. the total concentration of unbound monomeric ligand microL. In contrast to data analysis as a function of local concentrations, analysis in terms of total ligand concentrations avoids losing sight of alternate hypotheses about the nature of the binding sites. The present formulation has also permitted clarification of several consequences of the multiphasic nature of the binding systems that, as yet, have been poorly recognised.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

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.