Recognition and oxidative metabolism of cyclodipeptides by hepatic cytochrome P450

Possible recognition of peptide derivatives by hepatic cytochrome P450 3A has been suggested by binding and metabolism of numerous pseudopeptidic compounds such as ergot derivatives and cyclosporin. Natural linear or cyclic dipeptides containing hydrophobic amino acids produced by microorganisms and present in mammals are able to interact with the P450 active site through either iron-amine interactions (Type II) or hydrophobic Type I interactions. P450 3A from dexamethasone-treated rats or yeast-expressed P450 human 3A4 are the most potent in such interactions, which are particularly strong with peptides containing a histidyl residue. Some cyclodipeptides are rapidly transformed by rat cytochrome P450 3A to mono- or dihydroxylated metabolites, with turnovers around 3 nmoles min(-1) P450(-1). Linear peptides are poorly transformed in these conditions. This metabolism of cyclodipeptides occurs in 8 species including man. Such interactions and metabolism have only minor consequences in terms of P450 3A binding and metabolism of classical P450 3A substrates. These data reinforce the concept that, in addition to their effect on the regulation of P450 neosynthesis, naturally occurring endogenous peptides are also substrates of P450 3A. The physiological activities of these peptides may be modulated by their metabolism.

Inhibitory metabolite complex formation of methylenedioxymethamphetamine with rat and human cytochrome P450. Particular involvement of CYP 2D

Methylenedioxymethamphetamine (MDMA or ecstasy) is a common recreational drug used at rave parties. Unfortunately, MDMA may have neurological effects and in some cases causes hepatotoxicity. MDMA binds to cytochrome P450 in rat and human hepatic microsomal preparations. Upon metabolic transformation of either the methylenedioxy or the methylamino function, it forms an inhibitory P450-metabolite complex. This inhibitory complex is formed predominantly with the P450 2D isozymes. This complex formation may account for the clinical toxicity observed upon ingestion of MDMA, particularly with other compounds normally metabolized by P450 2D6.

Ornithine alpha-ketoglutarate counteracts the decrease of liver cytochrome P-450 content in burned rats

The effect of ornithine alpha-ketoglutarate (OKG) on cytochrome P-450 enzyme activities was studied in a well-defined model of injury (burn followed by fasting then subsequent hypocaloric diet) administered to young rats for 3 d. Hepatic microsomes were prepared by ultracentrifugation and levels of cytochromes P-450 were determined spectrophotometrically. The activities of ethoxy-resorufin-O-deethylase (EROD), benzyloxy-resorufin-O-dealkylase (BROD), and erythromycin demethylase were measured as markers of P-450 1A, 2A, and 3A isotypes respectively. The level of total hepatic microsomal proteins (8 mg/mL) remained constant. The level of cytochrome P-450 (1.14+/-0.08 nmol/mg microsomal proteins) was decreased by a hypocaloric diet (23%, P = 0.003) and burn further enhanced this phenomenon (15%, P = 0.03). Both healthy and burned rats receiving OKG showed the same level of cytochrome P-450 as the rats fed ad libitum. OKG supplementation counteracted the enhancement (40%) of EROD activity induced by hypocaloric diet but did not influence BROD and erythromycin demethylase activities. OKG sustained cytochrome P-450 levels in rats fed a hypocaloric diet, even after burning. These findings indicate that OKG may favor drug metabolism in this injured population.

The cyclo-oxygenase-dependent regulation of rabbit vein contraction: evidence for a prostaglandin E2-mediated relaxation

1. Arachidonic acid (0.01-1 microM) induced relaxation of precontracted rings of rabbit saphenous vein, which was counteracted by contraction at concentrations higher than 1 microM. Concentrations higher than 1 microM were required to induce dose-dependent contraction of vena cava and thoracic aorta from the same animals. 2. Pretreatment with a TP receptor antagonist (GR32191B or SQ29548, 3 microM) potentiated the relaxant effect in the saphenous vein, revealed a vasorelaxant component in the vena cava response and did not affect the response of the aorta. 3. Removal of the endothelium from the venous rings, caused a 10 fold rightward shift in the concentration-relaxation curves to arachidonic acid. Whether or not the endothelium was present, the arachidonic acid-induced relaxations were prevented by indomethacin (10 microM) pretreatment. 4. In the saphenous vein, PGE2 was respectively a 50 and 100 fold more potent relaxant prostaglandin than PGI2 and PGD2. Pretreatment with the EP4 receptor antagonist, AH23848B, shifted the concentration-relaxation curves of this tissue to arachidonic acid in a dose-dependent manner. 5. In the presence of 1 microM arachidonic acid, venous rings produced 8-10 fold more PGE2 than did aorta whereas 6keto-PGF1alpha and TXB2 productions remained comparable. 6. Intact rings of saphenous vein relaxed in response to A23187. Pretreatment with L-NAME (100 microM) or indomethacin (10 microM) reduced this response by 50% whereas concomitant pretreatment totally suppressed it. After endothelium removal, the remaining relaxing response to A23187 was prevented by indomethacin but not affected by L-NAME. 7. We conclude that stimulation of the cyclo-oxygenase pathway by arachidonic acid induced endothelium-dependent, PGE2/EP4 mediated relaxation of the rabbit saphenous vein. This process might participate in the A23187-induced relaxation of the saphenous vein and account for a relaxing component in the response of the vena cava to arachidonic acid. It was not observed in thoracic aorta because of the lack of a vasodilatory receptor and/or the poorer ability of this tissue than veins to produce PGE2.

Importance of metabolism in pharmacological studies: possible in vitro predictability

Metabolic transformation of drug leads to the formation of a large number of secondary compounds. These metabolites may (a) participate to the elimination of the patent drug, (b) have similar or different therapeutic effects compared to the parent drug (c) exert toxic effects. Cytochromes P450 are the main enzymes involved in the biotransformation of exogenous drugs, leading to oxidized, reduced or peroxidized metabolites. Different isozymes of P450 are present in already all the organs and differ by their affinity for substrate families. P450 3A is the most abundant P450 protein in the adult human liver and is able to transform hundreds of substrates into either drugs or endogenous compounds such as testosterone. Its catalytic activities are regulated either by induction or by inhibition. Attempts to predict metabolic transformation of a given drug are based on the amount of P450 expressed in heterologous systems, induction, and inhibition experiments and by comparison to classical P450 substrates. Erythromycin metabolism and its P450 effects are used to illustrate the complexity and the consequences of metabolic transformation of a given drug.

Conformational analysis of ketolide, conformations of RU 004 in solution and bound to bacterial ribosomes

A new structurally distinct class of 14-membered-ring macrolides is characterized by a keto-function instead of the cladinose sugar, well-known for its fragility even in weakly acidic media. This new class called ketolides is endowed with remarkable antibacterial activity against macrolide-resistant strains. A complete assignment of the 1H and 13C NMR spectra of RU 004 in deuteriochloroform, methanol-d4 and D2O has been made using different two-dimensional (2D) chemical-shift correlation methods. The study of ketolide-ribosome interaction has been investigated using 2D transferred nuclear Overhauser effect spectroscopy (TRNOESY). A comparison of the conformations in solution and bound to ribosomes was made with those of previous macrolides. This study can highlight some of the significant differences between RU 004 and other antibiotics.

Solution conformation of methylated macrolide antibiotics roxithromycin and erythromycin using NMR and molecular modelling. Ribosome-bound conformation determined by TRNOE and formation of cytochrome P450-metabolite complex

Conformational study of methylated derivatives of macrolide antibiotics roxithromycin (6-OMe-roxithromycin and 6,11-OMe-roxithromycin) has been achieved by NMR in solution and molecular dynamics (MD) simulations and compared to 6-OMe-erythromycin (clarithromycin). A complete conformational study by NMR has been led by determination of homonuclear coupling constants and NOEs. Heteronuclear 1H-13C coupling constants were also measured to investigate the orientation of the sugar moieties with respect to the erythronolide. MD simulations were performed using the crystallographic coordinates as the starting conformation. For each compound, experimental results were compared to calculated conformations in order to identify eventual conformational equilibrium in solution. It is shown that the effect of the methylation is opposite for roxithromycin compared to erythromycin especially on motional properties as the roxithromycin derivatives gain in mobility while the erythromycin derivatives behaves as a more restrained molecule. The study of macrolide-ribosome interactions has been investigated using transferred NOESY 1H NMR experiments and the conformations weakly bound to bacterial ribosomes were determined. Biological interactions of these compounds with membranar liver protein cytochrome P450 was also discussed with regard to their structural properties.

Transferred nuclear Overhauser effect study of macrolide-ribosome interactions: correlation between antibiotic activities and bound conformations

The study of macrolide-ribosome interactions has been investigated using two-dimensional transferred nuclear Overhauser effect spectroscopy (TRNOESY). A new medically important macrolide antibiotic, roxithromycin, with the replacement of the 9-keto group in erythromycin by a 9-oxime chain, was studied in the complex state with the bacterial ribosome. Analysis of transferred nuclear Overhauser effect (TRNOE) experiment resulted in a set of constraints for all protons pairs. These constraints were used in structure determination procedures based on molecular modelling to obtain a bound structure compatible with the experimental NMR data. The results compared with the conformational analysis of the substrate in solution indicate that only one specific conformation is preferred in the bound state while in the free state the sugar ring moities were relatively disordered. The bioactive macrolide antibiotics studied roxithromycin and erythromycin which displayed a strong NMR response, are metabolized in RU39001 and erythralosamine respectively which do not retain antimicrobial activity. The inactive major metabolites were used to define if TRNOEs observation may be characteristic of a biological activity. These control experiments gave essentially blank TRNOESY spectra. This study shows that Mg2+ does not play a direct role for the low affinity binding site studied by TRNOE what is in agreement with an hypothesis of two distinct binding levels, with a low affinity binding level necessary for the tight binding one.

Oxidation of tetrahydro-beta-carboline by cytochrome P-450cam--determination and rationalisation of product distribution

Cytochrome P-450cam oxidises 1,2,3,4-tetrahydro-beta-carboline, an indolic alkaloid. We report here measurements of the product distribution of this oxidation. To rationalise the experimental results, ab initio quantum-chemistry calculations of the product stabilities and molecular-dynamics calculations of the substrate-binding mode in the active site were performed. The calculations suggest that the product distribution is influenced by both the relative intrinsic gas-phase stabilities of the monohydroxy products and by conformational rearrangement of the active site on substrate binding.

Strong inhibition of neuronal nitric oxide synthase by the calmodulin antagonist and anti-estrogen drug tamoxifen

The anti-estrogen drug tamoxifen (TMX) was found to act as a strong inhibitor of purified neuronal nitric oxide synthase (nNOS) (IC50 = 2 +/- 0.5 microM), whereas it was inactive toward inducible macrophage NOS (IC50 > 100 microM). TMX affected the activation of NOS by calmodulin, as it not only inhibited L-arginine oxidation to nitric oxide and L-citrulline but also NADPH oxidation and calmodulin-dependent cytochrome c reduction catalyzed by nNOS. These results suggest that TMX could exert some of its biological effects by interfering with constitutive NOS-dependent formation of nitric oxide and/or superoxide ion in various tissues.

Metabolism of tentoxin by hepatic cytochrome P-450 3A isozymes

The interaction between rat and human liver cytochrome P-450 with tentoxin, a natural phytotoxic cyclotetrapeptide having chlorotic properties, was studied by difference ultraviolet visible spectroscopy. Tentoxin interacted with rat liver microsomes and the difference spectrum was characteristic of binding to a protein site close to the heme. The intensity of this spectrum was clearly dependent on the amounts of P-450 3A in the microsomes and was optimal in dexamethasone-treated rat microsomes. Tentoxin exhibited a high affinity for P-450 3A (Ks approximately 10 microM). Similar results were observed with human P-450 isozymes expressed in yeast. Only P-450 3A4 and 3A5 were able to give spectral interactions with tentoxin. Liver microsomes from rats pretreated with dexamethasone, a specific inducer of P-450 3A, were found to be particularly active for the oxidation of tentoxin, which occurs mainly on its Ala(Me) function leading to demethylation. Yeast-expressed P-450 3A also exhibited high activity to metabolize tentoxin. The metabolites were identified by their ultraviolet and mass spectra in fast atom bombardment and collision-activated dissociation modes. In addition to the major N-demethylated metabolite, other hydroxylated metabolites were formed. Preliminary analysis showed that as tentoxin, some metabolites were still efficient chloroplast ATPase inhibitors, while at least one of them exhibited even at low concentration stimulatory effects.

Metabolism of 14C-labelled 5-nitro-1,2,4-triazol-3-one by rat liver microsomes--evidence for the participation of cytochrome P-450

In the present study, we synthesized 14C-labelled 5-nitro-1,2,4-triazol-3-one (NTO) and investigated its hepatic metabolism by dexamethasone-induced murine hepatic microsomes. Under the nitrogen atmosphere, 5-amino-1,2,4-triazol-3-one was the only detected metabolite of NTO. The microsomal nitroreductase activity was dependent on NADPH, totally inhibited by carbon monoxide and partially inhibited by oxygen. In aerobic conditions, beside a low amount of amine, the major metabolite formed is the 5-hydroxy-triazolone, urazole. This compound resulted from the oxidative denitrification of NTO, which produced equivalent amount of nitrite. This reaction, like the nitroreductase activity, was dependent on NADPH and totally inhibited by carbon monoxide. Both nitroreduction and oxidative denitrification were inhibited by imidazole-related inhibitors: miconazole and methimazole, and to a less extent by N-octylamine. The microsomal denitrification was induced by the treatment of rats with dexamethasone and phenobarbital. The microsomal reductase activity is present in untreated rat microsomes, and recovered with various inducers. The results of this study indicate the role played by cytochrome P-450 in the metabolism of NTO, supported by its transformation with reconstituted cytochrome P-450 systems.

Metabolite involvement in bromocriptine-induced prolactin inhibition in rats

Bromocriptine (BCT) is a dopamine D2 receptor agonist used for the treatment of Parkinson's disease and hyperprolactinemic disorders. After oral administration, BCT is metabolized into mono- or dihydroxylated metabolites. To study how these metabolites influence parent drug pharmacodynamics, we administered BCT to rats intravenously (1 mg/kg i.v.) and orally (10 mg/kg p.o.) and measured the inhibition of prolactin secretion. Despite similar areas under the curve for BCT, the duration of the effect was 36 h after oral and only 18 h after intravenous administration. Pharmacokinetic/pharmacodynamic models were used to correlate the concentration of BCT in the effect compartment with the lowering of prolactin. One of these models (effect compartment model) showed that the effective concentration (EC50) at the site of action was much lower after oral (0.56 nM) than after intravenous administration (3.68 nM). In contrast, the EC50 values based on BCT metabolite data were in the same range for both administrations. These observations suggested the activity of one or more BCT metabolites. To confirm this hypothesis, hydroxylated metabolites of BCT (produced in vitro by rat liver microsomes) were administered i.v. (100 microg/kg) in rats. We found that monohydroxylated BCT was able to lower prolactin secretion like BCT. Dihydroxylated metabolites, as well as monohydroxylated metabolites, were effective in reducing in vitro prolactin secretion. Because we demonstrated that the concentration of hydroxylated metabolites after oral administration is 55-fold that of BCT, it can be concluded that BCT activity in the pituitary after oral administration is mediated by its metabolites.

Bromocriptine is a strong inhibitor of brain nitric oxide synthase: possible consequences for the origin of its therapeutic effects

The ergot alkaloid bromocriptine (BKT) was found to act as a strong inhibitor of purified neuronal nitric oxide synthase (NOS) (IC50 = 10 +/- 2 microM) whereas it was poorly active towards inducible macrophage NOS (IC50 > 100 microM). BKT affects the activation of NOS by calmodulin, as it not only inhibits L-arginine oxidation to NO and L-citrulline but also NADPH oxidation and calmodulin-dependent cytochrome c reduction catalyzed by neuronal NOS. These results suggest that BKT could exert some of its therapeutic effects by interfering with the NOS-dependent formation of nitric oxide and/or superoxide ion in various tissues.

Kinetic evaluation of nitric oxide production in pleural exudate after induction of two inflammatory reactions in the rat

NO generation in the course of two acute, non immune, inflammatory reactions (pleurisy induced by rat isologous serum and carrageenan) was assessed by means of nitrite measurement in pleural exudate from 0.5 to 24 h. NO release varied time-dependently, similarly for the two inflammatory reactions. A first, but transient, peak was reached in 30 min while a second peak, more sustained, began at the fourth hour and was maximum at the tenth. Kinetic evolution of NO release was consistent with activation, in a first step, of a constitutive NO synthase probably from endothelial origin (inhibited by 2-Methyl-2-Thiopseudourea sulfate but not by dexamethasone) and with activation, in a second wave, of inducible NOS from endothelial and exudative cells. NO release was potentiated by administration per os of L-Arginine and seems to be involved in the evolution of acute inflammatory reactions and oxygen metabolite production.

Enzyme immunoassays for bromocriptine and its metabolites

We have developed two bromocriptine enzyme immunoassays with different specificities for applications in human and animal pharmacokinetic studies. The first assay uses antibodies directed against the cyclopeptide structure of bromocriptine, and is specific for untransformed bromocriptine. The second assay uses antibodies directed against the bromolysergic part of the molecule and allows the measurement of both bromocriptine and its metabolites. Enzymatic tracers were obtained by covalent coupling of bromocriptine analogs to acetylcholinesterase from the electric eel Electrophorus electricus. Both assays have a limit of detection of 10 pg/ml and a limit of quantification of 50 pg/ml. The specificity of the assays was determined following fractionation by high-performance liquid chromatography of rat samples obtained after administration of bromocriptine.

In vitro hydroxylation and epoxidation of some isomeric lauric acid analogs by rat liver microsomes. Identification of metabolites and effects of clofibrate or phenobarbital pretreatment

The primary metabolites of a series of unsaturated lauric acid analogs (8-, 9-, 10-, and 11-dodecenoic acids) used as radiolabeled substrates for rat liver microsomes were quantitated by TLC and reverse phase-HPLC analysis, and identified by chemical derivation and GC/MS. Isomeric epoxidodecanoic acids and omega- and (omega-1)-monohydroxydodecenoic acids were essentially the only products formed from the incubations of the unsaturated fatty acids. Rat liver microsomes predominantly oxidized the terminal carbons of all substrates, leading to omega- and (omega-1)-hydroxylated metabolites, with the exception of 11-dodecenoic acid, which was efficiently converted to the epoxide. The E and Z isomers of dodecenoic acids were metabolized with the same efficiency and gave rise to the same pattern of hydroxylated vs. epoxidized products. The hydroxylation/epoxidation ratio was directly related to the position, but not to the geometry of the double bond in the aliphatic chain. Clofibrate pretreatment of the animals resulted in a strong induction of omega-oxidation, with a decrease in the ability to catalyze epoxidation of internal olefins, whereas phenobarbital pretreatment only stimulated (omega-1)-hydroxylation without any effect on epoxidation. In contrast to higher plants in which carbon 9 is the major target, rat liver cytochromes P450 selectively carried out hydroxylation (or epoxidation) at carbons 12 and 11 of lauric acid, as well as its unsaturated isomeric analogs.

Immunostimulating lipopeptide, LtriP (RP 56142): comparison of the effect on hepatic cytochrome P 450 modulation and radioprotection in male and female of three mouse strains

The sex-dependent effect of lauroyl-L-Ala-D-gamma-Glu-L,L-A2pmNH2 (LtriP, RP 56142) on hepatic microsomal cytochromes P 450 (cyt P 450) was studied in three mouse strains NMRI, C3H/OuJ and C3H/HeJ. In NMRI and C3H/OuJ, strains which are responsive to bacterial lipopolysaccharides (LPS-responsive), regardless of the sex of the mouse, significant decrease in the amount of cyt P 450 was observed after LtriP treatment, with a concomitant reduction in ethoxyresorufin-O-deethylase (cyt P 450 1A-dependent) and 7-ethoxycoumarin-O-deethylase activities. This was not seen in C3H/HeJ (LPS-hyporesponsive) mice. These effects may be related to LtriP-dependent cytokine induction, since neither LtriP nor LPS stimulated interleukin-1 (IL-1) secretion by C3H/HeJ macrophages. 11- and 12-hydroxylations (11- and 12-OH) of lauric acid were compared in C3H/OuJ and C3H/HeJ mice. LtriP depressed the total enzymatic conversion of lauric acid in the two strains without modification of the 11/12-OH ratio for C3H/OuJ or male C3H/HeJ mice. However, in females C3H/HeJ mice this decrease was particularly significant and concerned especially the 12-OH activity (a marker of cyt P450 4A family). Although males of the three strains were more sensitive to irradiation than females, LtriP exerted a sex-independent radioprotection on NMRI and C3H/OuJ mice. Its radioprotective effect was illustrated by the preservation of all the enzymatic activities studied in treated NMRI mice, contrary to irradiated control animals. In contrast, for the C3H/HeJ strain, males were not protected by LtriP treatment and, furthermore, females showed a marked sensitization to irradiation. The effects in CH3/HeJ strain implicate LtriP in the control of cyt P 450 induction and of sensitivity to irradiation independently of IL-1 induction.

Nitric oxide formation during the cytochrome P-450-dependent reductive metabolism of 18-nitro-oxyandrostenedione

18-Nitro-oxyandrostenedione (18-ONO2A), a potential mechanism-based inhibitor of the last steps of aldosterone biosynthesis, is well recognized by different cytochrome P-450s, which are able to metabolize it reductively into nitric oxide (NO) and 18-hydroxyandrostenedione. Rat liver microsomal P-450s are able to carry out this reaction with increased efficiency under anaerobic conditions. P-450 3A isozymes induced upon treatment of rats with dexamethasone or troleandomycin were best able to bind and metabolize 18-ONO2A. This reaction was shown to occur in the presence of dioxygen as well, suggesting that it may be of physiological relevance. The formation of NO was detected as a transient P-450-Fe(II)NO complex by UV-visible and EPR spectroscopy. In addition, steroidogenic tissues containing cytochrome P-450s such as bovine adrenal mitochondria or human placental microsomes also were capable of binding and metabolizing 18-ONO2A as judged by the formation of an Fe(II)NO complex. This recognition of a steroid nitrate, a potential antialdosterone and its subsequent metabolism under reductive conditions to generate NO both in hepatic and steroidogenic tissues, can be of pharmacological interest, because NO has been demonstrated to modulate steroidogenesis in addition to other processes such as vascular relaxation, neurotransmission or cytostasis. A nitrate derivative of a steroid could perhaps act as a vectorized NO precursor in which the steroid moiety is targeted specifically to steroid receptors or steroidogenic tissues, thus leading to localized NO liberation.

Conformational change due to esterification of hydroxy groups in erythromycin A and its major metabolite: analysis of these derivatives with different biological properties using NMR and molecular dynamics (MD) data

A conformational study is performed on the acylated erythromycin and erythralosamine derivatives from comparison between experimental results (NMR) and theoretical calculations by Molecular Dynamics (MD) in attempts to correlate their conformations with their abilities to generate cytochrome P450-nitroso metabolite complexes in vitro. As the 3'-dimethyl-amino function of the desosamine is metabolized and responsible for the interaction with cytochrome P450, its position, mobility and steric hindrance in the proximity of this functional group are related to its biological properties. The major conformations of the lactone ring were termed A (A1, A2, A3) and B (B1, B2), and this macrocycle flexibility induced five different orientations a, b, c, d and e for the desosamine sugar. Conformations A and B differ in many ways but the major change is the inward folding of the C(3) fragment in B. Conformer a exhibits an orientation of the desosamine nearly perpendicular to the macrocycle whereas the two units are in the same plane in conformations c and e. For conformation b, the cladinose unit lifts up above the macrocycle. Conformation d exhibits a turned-back cladinose. In the erythromycin derivatives esterification at the beta position to the N(CH3)2 group of the desosamine reduces the degree of freedom of the macrocyclic lactone ring which corresponds to conformation A only. The desosamine sugar was found to be perpendicular to the macrocycle (a conformer) and both sugar groups are parallel to reduce the steric energy. In the erythralosamine derivatives, the macrocycle is always present as conformation B with the two conformations b and c of the sugar rings. The steric parameters favour the b conformers in which the amino group is tilted up, while in 3,2'-dibenzoylated stacking aromatic attraction stabilizes the planar c conformer. Both isomers are thus shown to adopt well-defined conformations and to be well-adapted for a comparative structure-activity correlation studies. There is a significant relationship between the conformation b and the formation of cytochrome P450-nitroso metabolite complexes.