Inhibitors of Cytochrome P-450s and their mechanism of action

Differential effects of cytochrome P-450 induction on ligand binding to sigma receptors

The identity of the sigma receptor as a form of cytochrome P-450 was investigated in rats treated with 3-methylcholanthrene or phenobarbital. The density of [3H]N,N'-di(o-tolyl)guanidine (DTG) binding to sigma 2 receptors in hepatic subcellular fractions increased following both treatments, while [3H](+)-pentazocine binding to sigma 1 receptors was unchanged. Furthermore, proadifen and piperonyl butoxide inhibited [3H](+)-pentazocine and [3H]DTG binding with low potency. The low affinity of cytochrome P-450 inhibitors for sigma receptors, the similar degree of enhancement of [3H]DTG binding by agents with disparate cytochrome P-450 induction profiles and the lack of change in [3H](+)-pentazocine binding are inconsistent with the identity of the sigma receptor as a cytochrome P-450.

H3 receptor antagonist, thioperamide, inhibits adrenal steroidogenesis and histamine binding to adrenocortical microsomes and binds to cytochrome P450

1. Thioperamide (TP), an imidazole and a highly potent, specific antagonist of the histamine H3 receptor, inhibited the secretion of cortisol from bovine isolated adrenocortical cells (IC50 0.20 microM) and in the rat (5 mg kg-1) prevented both basal and stress-induced secretion of corticosterone. 2. In adrenocortical microsomes, low affinity binding of [3H]-histamine (KD 27.7 microM) was potently inhibited by TP (Ki 0.33 microM). 3. In adrenocortical microsomal membranes, both histamine and TP yielded type II difference absorption spectra, characteristic of the interaction between imidazole and cytochrome P450 enzymes. Dissociation constants for binding to P450, calculated from spectral data, were 15.9 microM and 1.5 mM for histamine, and 0.3 microM and 3.7 microM for TP. 4. In view of previously reported evidence for an intracellular mediator role of histamine in platelets, the present findings suggest a physiological role for histamine in the modulation of adrenal P450 monooxygenases that generate adrenocortical steroids. 5. The results suggest that direct adrenocortical inhibition by thioperamide at a non-H3 intracellular site must be taken into account in studies designed to elucidate functional roles of H3 receptors.

Reductase and oxidase activity of rat liver cytochrome P450 with 2,3,5,6-tetramethylbenzoquinone as substrate

The main objective of the present study was to investigate the proposed role of cytochrome P450 in the reductive metabolism of quinones as well as in the formation of reduced oxygen species in liver microsomes from phenobarbital (PB-microsomes) and beta-naphthoflavone (beta NF-microsomes) pretreated rats. In the present study, 2,3,5,6-tetramethylbenzoquinone (TMQ) was chosen as a model quinone. Anaerobic one-electron reduction of TMQ by PB-microsomes showed relatively strong electron spin resonance (ESR) signals of the oxygen-centered semiquinone free radical (TMSQ), whereas these signals were hardly detectable with beta NF-microsomes. Under aerobic conditions TMSQ formation was diminished and concomitant reduction of molecular oxygen occurred in PB-microsomes. Interestingly, TMQ-induced superoxide anion radicals, measured by ESR (using the spin trap 5,5'-dimethyl-1-pyrroline-N-oxide), and hydrogen peroxide generation was found to occur with beta NF-microsomes as well. Furthermore, SK&F 525-A (a type I ligand inhibitor of cytochrome P450) inhibited TMQ-induced hydrogen peroxide formation in both PB- and beta NF-microsomes. However, metyrapone and imidazole (type II ligand inhibitors of cytochrome P450) inhibited molecular oxygen reduction in beta NF-microsomes and not in PB-microsomes. The present study indicates that cytochrome P450-mediated one-electron reduction of TMQ to TMSQ and subsequent redox cycling of TMSQ with molecular oxygen constitutes the major source for superoxide anion radical and hydrogen peroxide generation in PB-microsomes (i.e. from the reductase activity of cytochrome P450). However, most of the superoxide anion radical formed upon aerobic incubation of TMQ with beta NF-microsomes originates directly from the dioxyanion-ferri-cytochrome P450 complex (i.e. from the oxidase activity of cytochrome P450). In conclusion, both the one-electron reduction of TMQ and molecular oxygen were found to be cytochrome P450 dependent. Apparently, both the reductase and oxidase activities of cytochrome P450 may be involved in the reductive cytotoxicity of chemotherapeutic agents containing the quinoid moiety.

Suicide inhibitors of cytochrome P450 1A1 and P450 2B1

The inhibition of the P450 1A1 dependent de-ethylation of 7-ethoxyphenoxazone (7EPO) and the P450 2B1 dependent de-pentylation of 7-pentoxyphenoxazone (7PPO) by 1-ethynylnaphthalene (1EN), 2-ethynylnaphthalene (2EN), 1-ethynylanthracene (1EA), 2-ethynylanthracene (2EA), 9-ethynylanthracene (9EA), 2-ethynylphenathrene (2EPh), 3-ethynylphenanthrene (3EPh), 9-ethynylphenanthrene (9EPh), 1-ethynylpyrene (1EP) and 2-ethynylpyrene (2EP) was studied in hepatic microsomal preparations from rats. Although all of the polycyclic aromatic acetylenes studied inhibited the dealkylation of 7EPO or 7PPO, only some of the acetylenes produced a mechanism-based irreversible inactivation (suicide inhibition) of the P450 dependent dealkylation of 7EPO or 7PPO. Of the molecules tested, only 1EP, 1EN, 2EN, 2EPh and 3EPh were effective suicide inhibitors of the P450 1A1 dependent de-ethylation of 7EPO and only 1EN, 2EN, 1EA and 9EPh were effective suicide inhibitors of the P450 2B1 dependent de-pentylation of 7PPO. In addition to the size and shape of the polycyclic aromatic ring system, placement of the carbon--carbon triple bond on the ring system was critical for suicide inhibition. In contrast to 1EP, 2EP was not a mechanism-based inhibitor of P450 1A1; 9EPh, but not 2EPh or 3EPh, was a suicide inhibitor of P450 2B1. None of the aryl acetylenes tested produced heme destruction under assay conditions that produced the suicide inhibition of the P450 dependent 7EPO or 7PPO dealkylation activities. Because a precise orientation of the terminal acetylene is required to produce suicide inhibition without heme destruction, acetylenic suicide inhibitors can potentially be used to differentiate between P450 isozymes and to establish some distinguishing geometric features of the active site of these isozymes.

Hydroxylation of 5 alpha-androstane-3 beta,17 beta-diol by rat prostate microsomes: potent inhibition by imidazole-type antimycotic drugs and lack of inhibition by steroid 5 alpha-reductase inhibitors

5 alpha-Dihydrotestosterone, the principal androgen mediating prostate growth and function in the rat, is formed from testosterone by steroid 5 alpha-reductase. The inactivation of 5 alpha-dihydrotestosterone involves reversible reduction to 5 alpha-androstane-3 beta,17 beta-diol by 3 beta-hydroxysteroid oxidoreductase followed by 6 alpha-, 7 alpha-, or 7 beta-hydroxylation. 5 alpha-Androstane-3 beta,17 beta-diol hydroxylation represents the ultimate inactivation step of dihydrotestosterone in rat prostate and is apparently catalyzed by a single, high-affinity (Km approximately 0.5 microM) microsomal cytochrome P450 enzyme. The present studies were designed to determine if 5 alpha-androstane-3 beta,17 beta-diol hydroxylation by rat prostate microsomes is inhibited by agents that are known inhibitors of androgen-metabolizing enzymes. Inhibitors of steroid 5 alpha-reductase (4-azasteroid analogs; 10 microM) or inhibitors of 3 beta-hydroxysteroid oxidoreductase (trilostane, azastene, and cyanoketone; 10 microM) had no appreciable effect on the 6 alpha-, 7 alpha-, or 7 beta-hydroxylation of 5 alpha-androstane-3 beta,17 beta-diol (10 microM) by rat prostate microsomes. Imidazole-type antimycotic drugs (ketoconazole, clotrimazole, and miconazole; 0.1-10 microM) all markedly inhibited 5 alpha-androstane-3 beta,17 beta-diol hydroxylation in a concentration-dependent manner, whereas triazole-type antimycotic drugs (fluconazole and itraconazole; 0.1-10 microM) had no inhibitory effect. The rank order of inhibitory potency of the imidazole-type antimycotic drugs was miconazole greater than clotrimazole greater than ketoconazole. In the case of clotrimazole, the inhibition was shown to be competitive in nature, with a Ki of 0.03 microM. The imidazole-type antimycotic drugs inhibited all three pathways of 5 alpha-androstane-3 beta,17 beta-diol hydroxylation to the same extent, which provides further evidence that, in rat prostate microsomes, a single cytochrome P450 enzyme catalyzes the 6 alpha-, 7 alpha-, and 7 beta-hydroxylation of 5 alpha-androstane-3 beta,17 beta-diol. These studies demonstrate that certain imidazole-type compounds are potent, competitive inhibitors of 5 alpha-androstane-3 beta,17 beta-diol hydroxylation by rat prostate microsomes, which is consistent with the effect of these antimycotic drugs on cytochrome P450 enzymes involved in the metabolism of other androgens and steroids.

Cytochrome P450-mediated prostaglandin omega/omega-1 hydroxylase activities in porcine ciliary body epithelial cells

The ocular hypotensive effect of topically applied prostaglandins (PGs) is well documented. Although PGs introduced in the posterior chamber accumulate in the anterior tissues (e.g. iris/ciliary complex), little is known about the metabolism of PGs by these tissues. We have recently found that non-pigmented epithelial (NPE) cells and pigmented epithelial (PE) cells are readily separated from porcine ciliary body and cytochrome P450-dependent xenobiotic metabolism is considerably higher in NPE cells than in PE cells. We have therefore investigated in this study the cytochrome P450-mediated PG omega/omega-1 hydroxylase activities of porcine ciliary epithelial cells. The NPE cells show about three times higher activities than do PE cells; the NPE cells, in fact, demonstrate the highest PG omega/omega-1 hydroxylase activities among different ocular tissues. Both omega and omega-1 hydroxylases show broad substrate specificities and hydroxylate PGA1, A2, E1, E2, and lauric acid. The omega/omega-1 hydroxylase activities of NPE and PE, as determined with PGA2 and lauric acid as substrates, are enhanced or induced by treatment of primary cultures of the individual epithelial cells with clofibrate, both activities reaching maximum levels within 48 hr of induction. The induced activities are inhibited almost completely by cycloheximide and actinomycin D. The omega/omega-1 hydroxylase activities of both NPE and PE cells require NADPH and molecular oxygen, are associated with the microsomal fraction, respond to inducers such as clofibrate, and are inhibited by metyrapone and SKF525 A (inhibitors of P450 enzymes). These results support the suggestion that PG omega/omega-1 hydroxylations by NPE and PE are cytochrome P450-mediated reactions.(ABSTRACT TRUNCATED AT 250 WORDS)

The role of cytochrome P450 and cytochrome P450 reductase in the reductive bioactivation of the novel benzotriazine di-N-oxide hypoxic cytotoxin 3-amino-1,2,4-benzotriazine-1,4-dioxide (SR 4233, WIN 59075) by mouse liver

SR 4233 or WIN 59075 (3-amino-1,2,4-benzotriazine-1,4-dioxide) is a novel and highly selective hypoxic cell cytotoxin requiring reductive bioactivation for its impressive antitumour effects. Expression of appropriate reductases will contribute to therapeutic selectivity. Here we provide more detailed information on the role of cytochrome P450 and cytochrome P450 reductase in SR 4233 reduction by mouse liver microsomes. Reduction of SR 4233 to the mono-N-oxide SR 4317 (3-amino-1,2,4-benzotriazine-1-oxide) is NADPH, enzyme and hypoxia dependent. An inhibitory antibody to cytochrome P450 reductase decreased the microsomal SR 4233 reduction rate by around 20%. Moreover, studies with purified rat cytochrome P450 reductase showed unequivocally that this enzyme was able to catalyse SR 4233 reduction at a rate of 20-30% of that for microsomes with equivalent P450 reductase activity. Exposure to the specific cytochrome P450 inhibitor carbon monoxide (CO) inhibited microsomal reduction by around 70% and CO plus reductase antibody blocked essentially all activity. Additional confirmation of cytochrome P450 involvement was provided by the use of other P450 ligands: beta-diethylaminoethyl diphenylpropylacetate hydrochloride gave a slight stimulation while aminopyrine, n-octylamine and 2,4-dichloro-6-phenylphenoxyethylamine were inhibitory. Induction of SR 4233 reduction was seen with phenobarbitone, pregnenalone-16-alpha-carbonitrile and beta-napthoflavone, suggesting that cytochrome P450 subfamilies IIB, IIC and IIIA may be involved. Since cytochrome P450 and P450 reductase catalyse roughly 70 and 30%, of mouse liver microsomal SR 4233 reduction respectively, we propose that expression of these and other reductases in normal and tumour tissue is likely to be a major factor governing the toxicity and antitumour activity of the drug.

Modelling of interaction of basic lipophilic ligands with cytochrome P-450 reconstituted in liposomes. Determination of membrane partition coefficients of S-(-)-nicotine and N,N-diethylaniline from spectral binding studies and fluorescence quenching

The spectral interaction of N,N-diethylaniline and S-(-)-nicotine with cytochrome P450IIB4 reconstituted into large unilamellar vesicles could properly be described by a model for interaction of basic lipid-soluble ligands with membrane-bound acceptor sites in which linear partitioning of non-ionized ligand in the membrane is postulated. Apparent spectral dissociation constants Ksapp for type I binding of N,N-diethylaniline and for type II binding of S-(-)-nicotine increased linearly with increasing lipid volume fraction alpha L of the proteoliposomes. From plots of Ksapp vs. alpha L, the membrane partition coefficient of each ligand was calculated. The apparent affinity of cytochrome P450IIB4 for the ligands increased as the pH was raised from 6.0 to 8.5. However, effective dissociation constants were virtually independent of the pH, indicating that only the uncharged form of the basic ligands interact with cytochrome P450IIB4. For each compound, the apparent quenching rate constants kqapp derived from the Stern-Volmer plots for dynamic quenching of the fluorescence intensity of 8-(2-anthryl)octanoic acid in liposomes, decreased with increasing liposomal concentration. Plots of (kqapp)-1 vs. alpha L of the liposomes yielded the overall bimolecular quenching rate constant kq of each quencher. The kq value for S-(-)-nicotine was about three orders of magnitude less than that for N,N-diethylaniline. The values of the partition coefficient of N,N-diethylaniline, obtained from the binding studies and the fluorescence quenching measurements, were identical (on average, Kp amounted to 383). Analysis of the quenching data of N,N-diethylaniline with Scatchard plots likewise revealed that the association of the compound with liposomal membranes is a pure partition process.

The effects of cobalt chloride, SKF-525A, and N-(3,5-dichlorophenyl)succinimide on in vivo hepatic mixed function oxidase activity as determined by single-sample plasma clearances

1. Four drugs--antipyrine, theophylline, quinidine, and ethosuximide--were used as probes of in vivo hepatic mixed function oxidase (MFO) activity. Functional MFO activity was evaluated by estimating probe clearances subsequent to pretreatment of rats with either cobalt chloride, SKF-525A, or N-(3,5-dichlorophenyl) succiminide (NDPS). 2. Clearances of each probe were estimated from single plasma concentration measurements. Each pretreatment altered the clearances of this panel of probes in a different way. NDPS pretreatment increased theophylline clearance while slowing quinidine and ethosuximide clearances. SKF-525A slowed all probe clearances except for ethosuximide. Cobalt chloride slowed all probe clearances except for theophylline. 3. The use of multiple probes as substrates for the hepatic cytochrome P-450 system can provide some insight into the functional consequences of xenobiotic exposures on that system. Moreover, xenobiotic-induced functional changes on hepatic MFO when assessed in vivo appear to be modest relative to changes in in vitro activity or hepatic cytochrome P-450 content. This minimally invasive multiprobe method may be useful for assessing xenobiotic influences on human hepatic MFO in vivo.

In vitro formation of glutathione conjugates of the dimethylester of bilirubin

Rat hepatic microsomes catalyzed the formation of two distinct glutathione conjugates of bilirubin dimethylester (DMB). The two conjugates were identical to those isolated from the bile of Gunn rats infused with DMB. The microsomal reaction was dependent on NADPH, oxygen and glutathione and was inhibited by nitrogen and the cytochrome P450 inhibitors metyrapone, 1-benzyl-imidazole, and alpha-naphthoflavone. Conjugate formation was inducible with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) but not phenobarbital pretreatment. The rate of formation of conjugates was not affected by washings of the microsomal pellet or by the presence of superoxide dismutase and/or catalase. Cation fast atom bombardment mass spectrometry (FAB/MS) of the conjugates indicated a molecular ion of 937 atomic mass units (amu). Fragmentation revealed a loss of 307 amu, consistent with glutathione, and a residual mass of 629 amu suggesting a hydroxylated derivative of DMB (612 amu). Cation FAB/MS/MS of conjugates formed in vitro under an atmosphere of oxygen-16 and oxygen-18 demonstrated the incorporation of molecular oxygen by a difference of 2 amu in the respective molecular ions. Our results suggest that DMB is oxidized by the cytochrome P450 IA gene family to an epoxide intermediate which is then subsequently conjugated with glutathione.

The role of specific cytochromes P450 in the formation of 7,12-dimethylbenz(a)anthracene-protein adducts in rat liver microsomes in vitro

The role of specific cytochrome P450 (P450) isoforms in the formation of adducts of 7,12-dimethylbenz(a)anthracene metabolites and membrane proteins has been investigated in vitro with microsomal fractions prepared from rats pretreated with various isoenzyme selective inducers. The effects of isoenzyme selective inhibitors were also evaluated. Adduct formation was shown to be mediated by P450 catalysed reactions but was unaltered, relative to untreated animals, in membranes from pyrazole- and clofibrate-treated animals suggesting that CYP2E1 and CYP4A1 are not involved in this process. However, adduct formation was significantly increased in microsomes from Sudan III-, phenobarbital- and dexamethasone-treated rats, suggesting the involvement of the CYP1A, CYP2B and CYP3A subfamilies, respectively. These conclusions were further supported by the finding that adduct formation in these microsomes could be inhibited by the isoenzyme-selective inhibitors alpha-naphthoflavone, metyrapone and troleandomycin, respectively.

Comparative effects of medetomidine enantiomers on in vitro and in vivo microsomal drug metabolism

The effects of dexmedetomidine, a selective alpha 2-adrenoceptor agonist, and its levo enantiomer (MPV-1441), on in vitro microsomal P450-dependent drug-metabolizing activities as well as on in vivo aminopyrine elimination and hexobarbital sleeping time were studied. Both enantiomers inhibited the oxidative metabolism of several model substrates and testosterone in rat liver microsomal incubations. Microsomal activities derived from control animals or rats pretreated with phenobarbital were more sensitive to inhibitory effects of dexmedetomidine than those from rats treated with 3-methylcholanthrene. Enzyme activities in human liver microsomes were also inhibited by dexmedetomidine. Retardation of the elimination of aminopyrine was dose-dependent; elimination was marginally retarded with doses up to 100 micrograms/kg (from 17 to 23 min.; both enantiomers). Higher doses of the levo enantiomer prolonged aminopyrine half-life to 78 (1 mg/kg) and 162 min. (10 mg/kg). The hexobarbital sleeping time was prolonged by the dose of 1 mg/kg of the levo enantiomer (128 min. versus 20 min. in controls), while the dose of 0.1 mg/kg had no effect (23 versus 20 min.). These studies indicate that both enantiomers of medetomidine are inhibitors of microsomal drug metabolism in vitro, but significant effects on aminopyrine elimination or hexobarbital sleeping time are apparent only at doses, which do not allow the use of dexmedetomidine because of excessive sedative effect.

Effect of metyrapone on the timing of oviposition and ovarian steroidogenesis in the laying hen

1. The purpose of this study was to observe the effects of metyrapone on the time of oviposition and LH-stimulated steroidogenesis by granulosa cells and small yellow follicles. 2. In experiment 1, White Leghorn hens were injected for 11 d with 240 mg metyrapone 5 h before 'lights off'. Control hens were injected with 1 ml of vehicle (PEG-400). Metyrapone treatment resulted in a 28% decrease in the rate of lay and the modal frequency of the time of oviposition was phase-shifted by 15 h. 3. In experiment 2, hens were injected with 240 mg metyrapone 5 h before 'lights off' or at 'lights on'. While metyrapone treatment reduced the rate of lay, a clear phase-shift in the distribution of oviposition was not observed. Basal and LH-stimulated progesterone synthesis by the granulosa cells of the largest follicle and oestradiol synthesis by small yellow follicles was significantly reduced. 4. Metyrapone treatment significantly reduced basal, but not LH-stimulated output of androstenedione by whole small yellow follicles compared to that observed in control hens. 5. The addition of metyrapone in vitro to isolated granulosa cells from the three largest preovulatory follicles inhibited LH-stimulated progesterone production in a dose-specific manner. 6. The results of this study suggest that the ability of metyrapone to perturb the open-period is a pharmacological effect mediated through inhibition of ovarian and adrenal steroidogenesis.

Comparative studies on coumarin and testosterone metabolism in mouse and human livers. Differential inhibitions by the anti-P450Coh antibody and metyrapone

We have studied coumarin 7-hydroxylase (COH) and testosterone 15 alpha-hydroxylase (15 alpha OH) activities in human liver microsomes and compared them with corresponding activities catalysed by members of the P450IIA sub-family in DBA/2N mouse liver microsomes. Human liver contained low levels of 15 alpha OH (about 5-30 pmol/min/mg protein) when compared with control mouse liver microsomes (about 200 pmol/min/mg protein). The anti-P450Coh antibody efficiently inhibited mouse liver 15 alpha OH, also 7 alpha OH (which is a member of the P450IIA sub-family), but it did not inhibit human 15 alpha OH or other testosterone hydroxylases. In mouse liver microsomes, metyrapone preferentially inhibited 15 alpha OH, but in human liver microsomes it inhibited all testosterone hydroxylations measured, including 15 alpha OH (IC50 = 2.0-5.0 microM). Metyrapone clearly inhibited COH in mouse liver microsomes, but interestingly it had no effect on COH activity in human liver microsomes, although these two isozymes have earlier been shown to be immunologically similar. On the basis of available evidence human and mouse P450Coh isozymes seem to be orthologous enzymes whereas the present results indicate that the human 15 alpha OH is different from the mouse P45015 alpha.

Inducing effect of oxfendazole on cytochrome P450IA2 in rabbit liver. Consequences on cytochrome P450 dependent monooxygenases

Male New Zealand rabbits were dosed with either 0.9, 4.5 or 22.5 mg/kg/day of oxfendazole by gastric intubation for 10 days. Oxfendazole administered at the therapeutic dose (4.5 mg/kg) and at the highest dose (22.5 mg/kg) increased 1.54- and 2.36-fold the total liver microsomal cytochrome P450 and more particularly the isoenzyme P450IA2 (95 and 184% increases) as demonstrated by western blotting. Increases in ethoxyresorufin O-deethylation and hydroxylations of benzopyrene and acetanilide occurred in livers of the same animals without any change in N-demethylation of aminopyrine, benzphetamine or erythromycin. Because of the unchanged level of mRNA specific to cytochrome P450IA2, as shown by northern blot analysis of poly mRNA, an enzyme stabilization rather than a transcriptional activation of IA2 genes should be involved in the P450IA2 regulation mechanisms. Oxfendazole bound strongly to cytochrome P450, giving rise to a type II spectrum, and inhibited noncompetitively the ethoxyresorufin O-deethylase and acetanilide hydroxylase activities, this confirmed that oxfendazole interacts only with the P450IA2 family. On the basis of a comparison of the enzymatic activities induced by various imidazole drugs, it was concluded that oxfendazole, like omeprazole and albendazole, behaved as a 3-methylcholanthrene-type inducer. These three benzimidazoles did not all belong to the same category of cytochrome P450 inducers as the antifungal drugs miconazole, clotrimazole and ketoconazole.

Heterogeneous binding of sigma radioligands in the rat brain and liver: possible relationship to subforms of cytochrome P-450

The binding of four sigma receptor ligands, 3H-(+)-N-allyl-N-normetazocine (3H-(+)-SKF 10,047), 3H-(+)-3-(3-hydroxyphenyl)-N-(1-propyl)piperidine (3H-(+)-3-PPP), 3H-haloperidol and 3H-N,N'-di(o-totyl)guanidine (3H-DTG), and the cytochrome P450IID6 ligand and dopamine uptake inhibitor 3H-1-[2-(diphenylmethoxy)ethyl]-4-(3-phenylpropyl)piperazine (3H-GBR 12935) to membranal preparations of rat liver or whole rat brain was examined regarding kinetical properties and inhibition by various compounds with affinity for sigma binding sites or cytochrome P-450. In rat brain the density of binding sites was increased in order (+)-SKF 10,047 less than (+)-3-PPP less than DTG much less than GBR 12935. In liver the corresponding order was (+)-SKF 10,047 less than DTG less than haloperidol less than (+)-3-PPP less than GBR 12935. The inhibition pattern of each ligand was similar in brain and liver, indicating that the binding sites were similar in the two tissues. With the exception of 3H-(+)-SKF 10,047 which appears to bind to a homogeneous haloperidol-sensitive site, there were quite marked differences between the ligands studied, suggesting heterogeneous binding sites. For instance, (+)-SKF 10,047 and progesterone were potent inhibitors of the binding of 3H-(+)-SKF 10,047, 3H-(+)-3-PPP and 3H-haloperidol but inhibited only a minor fraction of the binding of 3H-DTG to the brain and liver preparations. Multiple binding sites were also indicated by the low Hill coefficients found for most of the compounds studied.(ABSTRACT TRUNCATED AT 250 WORDS)

In vitro biotransformation of 2-methylpropene (isobutene): epoxide formation in mice liver

Until now, no data are available concerning the biotransformation and toxicity of 2-methylpropene (or isobutene), a gaseous alkene widely used in industry (rubber, fuel additives, plastic polymers, adhesives, antioxidants). In this work, the biotransformation of 2-methylpropene (MP) has been studied, using total liver homogenates of mice, supplemented with a NADPH-generating system. In analogy to other olefins, 2-methylpropene is metabolized to its epoxide 2-methyl-1,2-epoxypropane (MEP), as proved by the identification by gas chromatography coupled with mass spectrometry. The epoxidation is cytochrome P-450 dependent, as shown by experiments in the absence of the NADPH-generating system and in the presence of various concentrations of metyrapone and SKF 525-A, two known inhibitors of the mono-oxygenases. A simple gas chromatographic headspace method has been developed for the quantitative determination of the epoxide formed. The formation of MEP is never linear in function of time and it reaches a maximum after 20 min. Thereafter is decreases continuously to undetectable levels. This observation can be explained by the immediate action of epoxide hydrolase and glutathione S-transferase, converting the epoxide to 2-methyl-1,2-propanediol and to the glutathione conjugate respectively. The involvement of both enzymes has been demonstrated by the addition of 3,3,3-trichloropropene oxide and indomethacin. These inhibitors of, respectively, epoxide hydrolase and glutathione S-transferase increase the epoxide formation in a significant way. The actual concentration of MEP is therefore not only dependent on its formation by cytochrome P-450 dependent mono-oxygenases, but also on its conversion by epoxide hydrolase and glutathione S-transferase, both very active in liver tissue.

Assay of metyrapone, metyrapol and the isomeric mono-N-oxides of metyrapone in biological fluids by high-performance liquid chromatography

A sensitive high-performance liquid chromatographic (HPLC) method for the analysis of metyrapone [2-methyl-1,2-di-(3-pyridyl)-1-propanone], its reduced metabolite metyrapol and metyrapone mono-N-oxide metabolites in biological fluids is reported. These components were extracted into dichloromethane (2 x 5 ml) from alkalinised microsomal incubates, urine and blood (final pH about 12.5), or from cytosolic incubates at pH 7.4 (final aqueous volume 2-4 ml). Recoveries were in the range 70-100% under these conditions. The intact drug and metabolites were separated by reversed-phase HPLC with ultraviolet detection at 261 nm. All calibration curves were linear (correlation coefficient greater than 0.997). For the analysis of hepatic microsomal or cytosolic incubates, the coefficient of variation was less than 10% for samples over the range 2.5-250 nmol/ml N-oxides and 10-250 nmol/ml metyrapol. Measurement of metyrapone and metyrapol in rat blood (0.25-ml sample volume) was linear in the ranges 4.4-265 and 26-263 nmol/ml, respectively, the lower concentration being the limit of detection. The coefficient of variation was less than 20% for samples over the ranges tested for both these compounds. The N-oxide metabolites were not detectable in blood using this assay, their concentrations being below the limit of detection.

SKF 525-A and cytochrome P-450 ligands inhibit with high affinity the binding of [3H]dextromethorphan and sigma ligands to guinea pig brain

The DM1/sigma 1 site binds dextromethorphan (DM) and sigma receptor ligands. The broad binding specificity of this site and its peculiar subcellular distribution prompted us to explore the possibility that this site is a member of the cytochrome P-450 superfamily of enzymes. We tested the effects of the liver microsomal monooxygenase inhibitor SKF 525-A (Proadifen), and other P-450 substrates on the binding of [3H]dextromethorphan, [3H]3-(-3-Hydroxyphenyl)-N-(1-propyl)piperidine and (+)-[3H]1,3-Di-o-tolyl-guanidine ([3H]DTG) to the guinea pig brain. SKF 525-A, l-lobeline and GBR-12909 inhibited the binding of the three labeled ligands with nM affinity. Each drug has identical nM Ki values for the high-affinity site labeled by the three ligands. This indicated that they displaced the labeled ligands from the common DM1/sigma 1 site. Debrisoquine and sparteine, prototypical substrates for liver debrisoquine 4-hydroxylase, displayed Ki values of 9-13 and 3-4 microM respectively against the three labeled ligands. These results, the broad specificity of the DM1/sigma 1 binding site, and its peculiar subcellular distribution, raises the possibility that this binding site is a member of the cytochrome P-450 superfamily of isozymes, rather than a neurotransmitter receptor. These findings may have important implications for the understanding of the therapeutic, side effects and toxicity of several neurotropic drugs.

Drug metabolizing enzymes in the brain and cerebral microvessels

Several families of brain parenchyma and microvessel endothelial cell enzymes can metabolize substrates of exogenous origin. This xenobiotic metabolism includes functionalization and conjugation reactions and results in detoxication, but also possibly in the formation of pharmacologically active or neurotoxic products. The brain is partially protected from chemical insults by the physical barrier formed by the cerebral microvasculature of endothelial cells, which prevents the influx of hydrophilic molecules. These cells provide also, as a result of their drug-metabolizing enzyme activities, a metabolic barrier against penetrating lipophilic substances. The involvement of these enzymatic activities in neurotoxic events, probably responsible for neuronal dysfunctioning and/or death, neurodegenerative diseases and normal aging, is discussed.

Modulation of carbon tetrachloride hepatotoxicity and xenobiotic-metabolizing enzymes by corticosterone pretreatment, adrenalectomy and sham surgery

These investigations sought to determine the role of physiological concentrations of natural glucocorticoids in modulating chemical toxicity, and to ascertain if effects on toxicity may be due to alterations of chemical metabolizing enzymes by glucocorticoids. The hepatotoxic response to carbon tetrachloride (CCl4) in adrenalectomized or naive Long Evans rats treated with corticosterone was assessed. Alterations of hepatic cytochrome P-450 concentration, mono-oxygenase activities, NADPH-cytochrome (P-450)c reductase activity, and glutathione S-transferase activity were examined. Adrenalectomy and to a lesser extent sham surgery were protective, but corticosterone administration increased CCl4 hepatotoxicity. Corticosterone administration to adrenalectomized or sham-operated rats reduced the protective effect of these treatments. Correlating with the in vivo response, mono-oxygenase activities decreased after adrenalectomy and sham surgery, but increased with glucocorticoid administration. These studies suggest that basal, stress-associated, and pharmacological concentrations of a natural glucocorticoid can modify chemical toxicity and alter hepatic enzymes important to chemical metabolism.

Species variation in the response of the cytochrome P-450-dependent monooxygenase system to inducers and inhibitors

1. In the safety evaluation of drugs and other chemicals it is important to evaluate their possible inducing and inhibitory effects on the enzymes of drug metabolism. 2. While many similarities exist between species in their response to inducers and inhibitors, there are also important differences. Possible mechanisms of such variation are considered, with particular reference to the cytochrome P-450 system. 3. Differences in inhibition may be due to differences in inhibitory site of the enzyme involved, which is not always the active site of the enzyme, in competing pathways or in the pharmacokinetics of the inhibitor. 4. Differences in induction could be due to differences in the nature of the induction mechanism, in the isoenzyme induced, in tissue- or age-dependent regulation, in competing pathways for the substrate or its products, or in the pharmacokinetics of the inducing agent. 5. Examples of each of these possible differences are considered, often from our own work on the P450 IA subfamily, and results in animals are compared with those in humans, where possible. 6. At present, the differences between species in their response to inducers and inhibitors make extrapolation to humans from the results of animal studies difficult, so that ultimately such effects should be studied in the species of interest, humans.

Mechanisms of inhibition of xenobiotic-metabolizing enzymes

1. Various molecular mechanisms underlie the action of inhibitors of xenobiotic-metabolizing enzymes acting upon the enzyme itself and not elsewhere. 2. The activity of directly acting inhibitors is due to the compounds themselves rather than to metabolic intermediates thereof. When the enzyme's ground state is the target, competitive or non-competitive inhibition can be seen, depending on a number of factors. 3. Some inhibitors are transition-state analogues and bind slowly but with high affinity to the enzyme. Inhibition may be due to the compound itself and/or a metabolic intermediate. 4. Mechanism-based inhibitors are those which act via an in situ generated metabolic intermediate that can bind reversibly or irreversibly to the enzyme. 5. These various mechanisms are exemplified and discussed in terms of selectivity and reversibility.

One-electron reduction of mitomycin c by rat liver: role of cytochrome P-450 and NADPH-cytochrome P-450 reductase

1. The role of cytochrome P-450 in the one-electron reduction of mitomycin c was studied in rat hepatic microsomal systems and in reconstituted systems of purified cytochrome P-450. Formation of H2O2 from redox cycling of the reduced mitomycin c in the presence of O2 and the alkylation of p-nitrobenzylpyridine (NBP) in the absence of O2 were taken as parameters. 2. With liver microsomes from both 3-methylcholanthrene (MC)- and phenobarbital (PB)-pretreated rats, reverse type I difference spectra were observed, indicative of a weak interaction between mitomycin c and the substrate binding site of cytochrome P-450. Mitomycin c inhibited the oxidative dealkylation of aminopyrine and ethoxyresorufin in both microsomal systems. 3. Under aerobic conditions the H2O2 production in the microsomal systems was dependent on NADPH, O2 and mitomycin c, and was inhibited by the cytochrome P-450 inhibitors, metyrapone and SKF-525A. 4. Although purified NADPH-cytochrome P-450 reductase was also effective in reduction of mitomycin c and the concomitant reduction of O2, complete microsomal systems and fully reconstituted systems of cytochrome P-450b or P-450c and the reductase were much more efficient. 5. Under anaerobic conditions in the microsomal systems both reduction of mitomycin c (measured as the rate of substrate disappearance) and the reductive alkylation of NBP were dependent on cytochrome P-450. 6. The relative rate of reduction of mitomycin c by purified NADPH-cytochrome P-450 reductase was lower than that by a complete microsomal system containing both cytochrome P-450 and a similar amount of NADPH-cytochrome P-450 reductase. 7. It is concluded that although NADPH-cytochrome P-450 reductase is active in the one-electron reduction of mitomycin c, the actual metabolic locus for the reduction of this compound in liver microsomes under a relatively low O2 tension is more likely the haem site of cytochrome P-450.

Inhibition of rat liver monooxygenase activities by 2-methyl-1,4-naphthoquinone (menadione)

In rat liver microsomes, 2-methyl-1,4-naphthoquinone (menadione) inhibits cytochrome P450 (cyt P450)-mediated aniline-p-hydroxylation and aminopyrine-N-demethylation with Ki values of 12 and 14.5 microM, respectively. The inhibitions of aniline-p-hydroxylation and aminopyrine-N-demethylation are mixed uncompetitive-noncompetitive and mixed competitive-noncompetitive, respectively. NADP antagonizes the inhibitory effect of menadione on aniline-p-hydroxylase activity but not that on aminopyrine-N-demethylase activity. Menadione does not give rise to any spectral change of cyt P450, but modifies the type I binding spectrum induced by aminopyrine. In contrast, menadione does not change the type II binding spectrum induced by aniline. These results indicate that menadione may inhibit aniline-p-hydroxylase activity by acting as a substrate for NADPH-cyt P450 reductase in the place of cyt P450 and inhibit aminopyrine-N-demethylase activity by impairing the binding of aminopyrine to cyt P450.

Studies on the cytochrome P-450 of avocado (Persea [corrected] americana) mesocarp microsomal fraction

1. Because of the low concentration of cytochrome P-450 in avocado fruit, microsomal fractions were prepared using polyethylene glycol aggregation and low-speed centrifugation, thus avoiding the need for high-speed centrifugation of large volumes of post-mitochondrial supernatant. Recoveries of cytochrome P-450 by this means (0.29 nmol/g tissue) were similar to those after the usual high-speed centrifugation preparation (0.26 nmol/g). The cytochrome P-450 content of tulip bulb (0.30 nmol/g) was similar to that of avocado, but both plant tissues had much lower P-450 contents than did rat liver (13.0 nmol/g). 2. Spectral studies indicate that cytochrome P-450 of avocado mesocarp microsomal fraction binds fewer substrates than does the rat liver enzyme system. Type I binding spectra are given by fatty acids (C7-C14), aryl hydrocarbons (C7-C12), p-chloro-N-methylaniline and N,N-dimethylaniline. Type II binding is seen with inhibitors of mammalian cytochrome P-450 such as metyrapone, and with the imidazole antifungal agents such as clotrimazole. 3. These binding spectra provide a rapid method for identifying possible substrates and inhibitors of avocado cytochrome P-450, and also provide information concerning the nature of the active site of avocado cytochrome P-450. 4. Avocado cytochrome P-450 catalysed the N-demethylation of N,N-dimethylaniline (17.1 nmol/min per nmol P-450) and p-chloro-N-methylaniline (13.1 nmol/min per nmol P-450), and the hydroxylation of lauric (dodecanoic) acid (1.1 nmol/min per nmol P-450).

Metabolism of fluperlapine by cytochrome P450-dependent and flavin-dependent monooxygenases in continuous cultures of rat and human cells

The metabolism of fluperlapine, a neuroleptic dibenzazepine derivative with a N-methyl-piperazinyl substituent, was investigated in continuous cultures of rat and human cells which express various cytochrome P450-dependent monooxygenase activities. The differentiated rat hepatoma cells H4IIEC3/G- and their variants 2sFou and FGC-5 metabolized fluperlapine predominantly by N-oxygenation and only to a minor degree by N-demethylation or glucuronidation of primary phenolic products. Total fluperlapine metabolism in dedifferentiated rat hepatoma cells H5 and partially differentiated human hepatoma cells HepG2 was much smaller than in the differentiated rat hepatoma lines. This was primarily attributable to their low capacity for N-oxygenation. Human lung adenocarcinoma lines NCI-H322 and NCI-H358 formed only trace amounts of fluperlapine N-oxide. Pretreatment of 2sFou cells with benz(a)anthracene, phenobarbital or dexamethasone markedly increased the formation of N-demethylated and glucuronidated products but did not affect the rate of N-oxide formation. Guanethidine and cysteamine, inhibitors of flavin-dependent monooxygenase activity, reduced fluperlapine N-oxidation more strongly than aldrin epoxidation, a marker for cytochrome P450 activity. In contrast, n-octylamine inhibited aldrin epoxidation but was without effect on fluperlapine N-oxygenation. The results suggest that certain cells in continuous culture are capable of expressing flavin-dependent monooxygenase(s) in addition to cytochrome P450-containing monooxygenases. Such cells may offer useful systems for studying the oxidative metabolism of a broad spectrum of xenobiotics and analysing the importance of the two oxygenation reactions for the biological effects of their substrates.

Further studies on the high-affinity binding of 3H-alaproclate to membranes from rat liver and some other tissues

The high affinity binding of 3H-alaproclate to membranes in liver homogenates from naive rats and those treated with phenobarbital sodium, 75 mg/kg intraperitoneally, alaproclate hydrochloride, 40 mg/kg intraperitoneally proadifen hydrochloride, 40 mg/kg intraperitoneally once daily for 7 days and killed 24 hr after the last injection was examined. The treatment increased the normal number of alaproclate binding sites (Bmax: 1.1 nmol/g tissue, KD: 0.6 nM) by factors of about 10, 4 and 6, respectively. About 80% of the binding was localized to the microsomal fraction in both normal and phenobarbital treated rats. Ninety % of the alaproclate displaceable binding in a microsomal preparation of the normal liver was inhibited by low (nM) concentrations of proadifen whereas only about 20% in the liver preparations from phenobarbital treated rats was inhibited by low concentrations of proadifen. Thus, the main part of the induced binding sites was insensitive to proadifen. The same was found for the alaproclate and proadifen-induced alaproclate binding sites. The stereoselectivity of alaproclate enantiomers for binding to the normal and the induced binding sites was different: the S-(-) form was 100 times more potent than the R-(+)- enantiomer in inhibiting binding of racemic alaproclate to the normal sites, whereas the latter form was 3 times more potent than the former in inhibiting the binding to the phenobarbital-induced proadifen insensitive binding sites.(ABSTRACT TRUNCATED AT 250 WORDS)

Hepatic microsomal N-hydroxylation of adenine to 6-N-hydroxylaminopurine

The enzymatic N-hydroxylation of the purine base adenine to the genotoxic and mutagenic compound 6-N-hydroxylaminopurine is reported for the first time. Adenine was N-oxygenated in vitro by aerobic incubations with 3-methylcholanthrene or isosafrole induced microsomal fractions of rat liver homogenates and NADPH. The formation of 6-N-hydroxylaminopurine in the incubation mixtures under widely differing conditions was assayed using newly-developed, high-performance liquid- and thin-layer chromatographic methods. Optimal reaction conditions and kinetic parameters were determined. Neither superoxide anion nor hydrogen peroxide was directly involved in the N-hydroxylation reaction. Oxidases like xanthine oxidase and peroxidase (in the presence of hydrogen peroxide) did not catalyse this N-hydroxylation. The involvement of cytochrome P-450 isoenzymes in this reaction is supported by the observation that the N-hydroxylation is only observed after pretreatment of the rats with 3-methylcholanthrene or isosafrole. Other inducers (phenobarbital, ethanol, 5-pregnen-3 beta ol-20-one-16 alpha-carbonitrile) were without effect. This is the first example of the microsomal transformation of an endogenous substance to a toxic derivative by usually foreign substances (xenobiotics) metabolizing cytochrome P-450 isoenzymes. The significance for the in vivo situation is discussed on the basis of the data obtained in this study.

Catechol estrogen formation in the mouse uterus and its role in implantation

Estradiol-2/4-hydroxylase (E-2/4-H) activity was determined in the mouse uterus during early pregnancy as well as in ovarian steroid hormone-treated ovariectomized uterus. Under the assay conditions used, E-4-H was the predominant catechol estrogen-forming monooxygenase enzyme. The inhibition of E-4-H activity by SKF-525A, metyrapone and alpha-naphthoflavone suggested involvement of cytochrome P450-dependent monooxygenases. A haloestrogen, 2-fluoroestradiol (2-FL-E2), also inhibited this activity. During the peri-implantation period, no change in uterine E-4-H activity was noted on the morning of days 2 through 5, but the activity significantly (P less than 0.01) increased in the afternoon of day 4 of pregnancy. A single injection of estradiol-17 beta (E2, 100 ng/mouse) to ovariectomized mice significantly (P less than 0.01) elevated the level of E-4-H activity at 24 h as did injections of progesterone (P4, 2 mg/mouse) for 2 days. When 2 days of P4 (2 mg/mouse) treatment was combined with a single injection of E2 (20 ng/mouse), E-4-H activity increased 1.3-fold (P less than 0.05) by 24 h above that of P4 treatment alone. Dexamethasone (200 micrograms/mouse) and cholesterol (2 mg/mouse) treatment for 2 days had no effect on E-4-H activity. Thus, the stimulatory effect of P4 and E2 on E-4-H activity appeared to be specific. The increased activity of uterine E-4-H prior to implantation on day 4 evening and the modulation of its activity by P4 and/or E2 suggest an involvement of 4-hydroxyestradiol in embryo implantation.(ABSTRACT TRUNCATED AT 250 WORDS)

Estradiol-15 alpha-hydroxylation: a new avenue of estrogen metabolism in peri-implantation pig blastocysts

Pig blastocysts have the capacity to convert estradiol into catechol estrogens. Our present study shows that they also have the capacity to hydroxylate estradiol in cycloaliphatic C-atom 15, and this aliphatic hydroxylation reaction is more predominate than the aromatic hydroxylations. The conversion of [4-14C]estradiol to [4-14C]15 alpha-hydroxyestradiol by mitochondrial-rich/microsomal fractions was examined by isolation of this product using reversed phase high-performance liquid chromatography (HPLC) attached to a radiometric flow detector, and its identification by gas chromatography-mass spectrometry. The enzyme kinetics for estrogen 15 alpha-hydroxylase were performed in the pig blastocyst obtained on Day 13 of pregnancy (Day 0 = first acceptance of the male). The enzyme follows classical Michaelis-Menten kinetics. The apparent Kms for estradiol were 2.47 and 1.85 microM, and the apparent Vmaxs were 0.25 and 0.197 nmol/mg/min in the mitochondrial-rich and microsomal fractions, respectively. The enzyme activity was inhibited by different steroidal compounds and non-steroidal estrogens, as well as by CO, SKF-525A, piperonyl butoxide and antibody to cytochrome P450 reductase. Ontogenesis of the blastocyst's estrogen 15 alpha-hydroxylase follows a similar pattern to that of estrogen-2/4-hydroxylase. Thus, highest activity was observed on Days 12 and 13 and lowest was on Day 15 of pregnancy. Furthermore, the enzyme is abundant primarily in the extraembryonic tissues rather than in the embryo proper. The abundance of the enzyme in the extraembryonic tissues, and its surge at a critical time of pregnancy recognition and just prior to implantation suggest that 15 alpha-hydroxylated estradiol could be involved in these processes.

Evidence for complexation of P-450 IIC6 by an orphenadrine metabolite

Removal of the orphenadrine metabolite from its complex with rat liver P-450 IIB1 is associated with a discrepancy in the reactivation of IIB1 activity. Two possible explanations are that either (1) NADPH-P-450-reductase is inaccessible to the restored IIB1, or (2) complexation of other P-450s may occur. Exogenous P-450 reductase increased all pathways of steroid hydroxylation (1.9 to 3.6-fold) but did not enhance reactivation of IIB1-dependent steroid 16 beta-hydroxylation. Instead, P-450 IIC6-dependent progesterone 21-hydroxylase activity was increased after dissociation to 122% of control. IIC6 activity was also inhibited in vitro in microsomes from phenobarbital-induced rats (ki = 151 microM). Thus, orphenadrine appears to complex P-450 IIC6 as well as IIB1 in rat liver.

Monoterpene biosynthesis: specificity of the hydroxylations of (-)-limonene by enzyme preparations from peppermint (Mentha piperita), spearmint (Mentha spicata), and perilla (Perilla frutescens) leaves

Microsomal preparations from the epidermal oil glands of Mentha piperita, Mentha spicata, and Perilla frutescens leaves catalyze the NADPH- and O2-dependent allylic hydroxylation of the monoterpene olefin (-)-limonene at C-3, C-6, and C-7, respectively, to produce the corresponding alcohols, (-)-trans-isopiperitenol, (-)-trans-carveol, and (-)-perillyl alcohol. These transformations are the key steps in the biosynthesis of oxygenated monoterpenes in the respective species, and the responsible enzyme systems meet most of the established criteria for cytochrome P450-dependent mixed function oxygenases. The reactions catalyzed are completely regiospecific and, while exhibiting only a modest degree of enantioselectivity, are highly specific for limonene as substrate. Of numerous monoterpene olefins tested, including several positional isomers of limonene, only the 8,9-dihydro analog served as an alternate substrate for ring (C-3 and C-6) hydroxylation, but not side chain (C-7) hydroxylation. In addition to the regiospecificity of the allylic hydroxylation, these enzymes are also readily distinguishable based on differential inhibition by substituted imidazoles.

Role of cytochrome P450 in the control of the production of erythropoietin

Effects of agents affecting cytochrome P450 were studied on the production of erythropoietin (Epo) in cultures of the human hepatoma cell line HepG2. Epo was measured by radioimmunoassay of the culture media after 24 h of incubation. The addition of phenobarbital or 3-methylcholanthrene, which induce cytochrome P450, significantly enhanced the formation of Epo. Likewise, the thyroid hormones T3 and T4 stimulated the rate of the production of Epo. On the other hand, the formation of Epo was lowered following the addition of diethyldithiocarbamate or cysteamine chloride, which inhibit cytochrome P450. These findings support the idea that O2 sensitive hemoproteins of the microsomal mixed-functional oxidases play a role in the control of the synthesis of Epo.

Chronic ethanol intake and reduction of lung tumours from urethane in strain A mice

Combinations of ethanol and urethane were added to the drinking-water of female strain A/Ph mice for 12 wk, at the end of which the animals were killed. Urethane concentrations were 0, 200, 500 and 1000 ppm and ethanol concentrations, 0, 5, 10 and 20% (v/v). All possible combinations of these urethane and ethanol concentrations were tested. Urethane induced primary lung adenomas in all treated mice in a dose-dependent manner. An average of 71 +/- 15 tumours/mouse were found, when the animals were killed, after treatment with 1000 ppm urethane for 12 wk. Ethanol alone did not alter the background incidence of tumours and produced only marginal hepatotoxicity. The tumour yields induced by urethane treatment were greatly reduced by simultaneous treatment with ethanol. The effect of ethanol was independent of urethane dose. When the concentrations of ethanol in the drinking-water were 20 and 10% the incidences of lung adenomas induced by urethane were reduced by about two-thirds and one-half, respectively. The effect of 5% ethanol, if any, was not statistically significant.

Direct modulation of Aplysia S-K+ channels by a 12-lipoxygenase metabolite of arachidonic acid

Lipoxygenase metabolites of arachidonic acid have recently been shown to modulate the activity of ion channels in nerve and muscle cells. The mechanism of action of these metabolites is, however, unknown. In sensory neurons of Aplysia, the S-K- channel is under the dual modulatory control of 5-hydroxytryptamine (5-HT), which decreases the number of active S channels through cyclic AMP-dependent phosphorylation, and the neuropeptide FMRFamide, which increases the probability of S-channel opening through the 12-lipoxygenase metabolite 12-hydroperoxyeicosatetraenoic acid (12-HPETE). Here we report that the increase in the probability of S-channel opening with FMRFamide is mimicked by application of 12-HPETE to cell-free membrane patches that lack ATP and GTP. Thus, 12-HPETE can act directly to modulate S-channel activity, independently of protein phosphorylation or dephosphorylation, G-protein activation or cyclic nucleotides.

Biochemical approaches to selective antifungal activity. Focus on azole antifungals

Azole antifungals (e.g. the imidazoles: miconazole, clotrimazole, bifonazole, imazalil, ketoconazole, and the triazoles: diniconazole, triadimenol, propiconazole, fluconazole and itraconazole) inhibit in fungal cells the 14 alpha-demethylation of lanosterol or 24-methylenedihydrolanosterol. The consequent inhibition of ergosterol synthesis originates from binding of the unsubstituted nitrogen (N-3 or N-4) of their imidazole or triazole moiety to the heme iron and from binding of their N-1 substituent to the apoprotein of a cytochrome P-450 (P-450(14)DM) of the endoplasmic reticulum. Great differences in both potency and selectivity are found between the different azole antifungals. For example, after 16h of growth of Candida albicans in medium supplemented with [14C]-acetate and increasing concentrations of itraconazole, 100% inhibition of ergosterol synthesis is achieved at 3 x 10(-8) M. Complete inhibition of this synthesis by fluconazole is obtained at 10(-5) M only. The agrochemical imidazole derivative, imazalil, shows high selectivity, it has almost 80 and 98 times more affinity for the Candida P-450(s) than for those of the piglet testes microsomes and bovine adrenal mitochondria, respectively. However, the topically active imidazole antifungal, bifonazole, has the highest affinity for P-450(s) of the testicular microsomes. The triazole antifungal itraconazole inhibits at 10(-5) M the P-450-dependent aromatase by 17.9, whereas 50% inhibition of this enzyme is obtained at about 7.5 x 10(-6)M of the bistriazole derivative fluconazole. The overall results show that both the affinity for the fungal P-450(14)DM and the selectivity are determined by the nitrogen heterocycle and the hydrophobic N-1 substituent of the azole antifungals. The latter has certainly a greater impact. The presence of a triazole and a long hypdrophobic nonligating portion form the basis for itraconazole's potency and selectivity.

Involvement of cytochrome P-450 in the synthesis of 5,16,androstadien-3 beta-ol from pregnenolone in pig testes microsomes

The conversion of pregnenolone to 5,16,androstadien-3 beta-ol, the first intermediate in the biosynthetic pathway of the androst-16-ene steroids, is catalysed by a microsomal enzyme system in the testes of the pig. This reaction is analogous to the conversion of pregnenolone to dehydroepiandrosterone in the biosynthesis of the androgens, since both systems involve the conversion of C21 steroids to C19 steroids by removal of the 2-carbon side chain. Cytochrome P-450SCCII catalyses the formation of the first C19 androgen intermediates, while the enzyme system that catalyzes the formation of the first androst-16-ene intermediates, so called andien-beta synthase, has not previously been well characterized. Andien-beta synthase and cytochrome P-450SCCII activities have been measured in an in vitro assay system with boar testes microsomes using [14C]pregnenolone as substrate. Both enzyme systems require NADPH and oxygen for maximal activity and are inhibited by carbon monoxide when oxygen levels are low. Classical inhibitors of cytochrome P-450 including SKF-525A, metyrapone and alpha-naphthoflavone inhibited both enzyme systems to a similar extent. In addition, inhibitory antibodies against NADPH cytochrome P-450 reductase also inhibited both enzyme activities in testes microsomes. It is concluded that the formation of 5,16,androstadien-3 beta-ol from pregnenolone in pig testes microsomes is catalyzed by cytochrome P-450.

Products of heme-catalyzed transformation of the arachidonate derivative 12-HPETE open S-type K+ channels in Aplysia

In Aplysia mechanosensory neurons, the neuropeptide FMRFamide increases the opening of the background S-K+ channel. This action is mediated by activation of arachidonic acid metabolism. Arachidonic acid in Aplysia nervous tissue is transformed through the 12-lipoxygenase pathway to 12-HPETE, which undergoes further metabolism. In intact sensory cells, 12-HPETE simulates the FMRFamide response, raising the question of whether 12-HPETE is the messenger molecule ultimately acting on the S-K+ channel. Here we show that in cell-free (inside-out) patches from sensory cells, 12-HPETE fails to modulate the S-K+ channel, but in the presence of hematin (which catalyzes 12-HPETE metabolism), it triggers sharp increases in the channel opening probability. We also found that SKF-525A, an inhibitor of the cytochrome P450, reduces the response to FMRFamide, arachidonic acid, and 12-HPETE in intact cells. We conclude that a heme-catalyzed transformation of 12-HPETE is necessary and sufficient to promote the opening of the S-K+ channel and a heme-containing enzyme such as cytochrome P450 might play this key role.

In vitro metabolism of azasqualene derivatives and their effects on aminopyrine N-demethylase activity in rat liver microsomes

The metabolism of squalene dimethylamine (I), a potent inhibitor of 2,3-oxidosqualene (SO) cyclase, and of sixteen other squalene derivatives was investigated in rat liver microsomes. N-oxidation was the only metabolic pathway observed, squalene dimethylamine N-oxide being the only metabolite isolated from incubation of I. The azasqualane and quaternary ammonium derivatives did not form N-oxides during their metabolism. The inhibition of aminopyrine N-demethylase activity was also studied and the IC50, for compound I, which shows weak competitive inhibition, was determined. At 1 mM concentration the other squalene derivatives showed a range of inhibition activity possibly due to their different lipophilicity.

Preferential inhibition of mouse hepatic coumarin 7-hydroxylase by inhibitors of steroid metabolizing monooxygenases

Etomidate, metomidate and metyrapone, all potent inhibitors of steroid metabolizing monooxygenases, inhibit preferentially coumarin 7-hydroxylase (COH) amongst several liver microsomal monooxygenase activities from control and pyrazole-treated D2 mice in vitro. SKF-525A, an inhibitor of phenobarbital-inducible monooxygenase activities has a much weaker effect on COH than the other three drugs, even though COH is a phenobarbital-inducible enzyme. Treatment of mice with eto- and metomidate decreases the microsomal COH also in vivo while the other activities remained unchanged (with the exception of 7-ethoxycoumarin O-deethylase (ECDE) in case of metomidate). Despite of the decrease in COH no parallel decrease in the amount of microsomal P450Coh (P450 isoenzyme highly active in the 7-hydroxylation of coumarin) could be found in dot immuno-binding analysis. These data suggest that among several liver microsomal P450 isoenzymes, metyrapone, eto- and metomidate interact preferentially with the P450Coh and that eto- and metomidate may alter selectively the catalytic properties of P450Coh leading to decreased enzyme activity. Two different Ks-values could be found for all three drug in their binding to microsomal cytochrome(s) P450. Based on substrate binding spectra, potassium ferricyanide treatment does not dissociate the complex between reduced P450 and metomidate and does it only partly for etomidate. Furthermore potassium ferricyanide treatment of microsomes does not increase COH after in vivo treatment of mice with eto- and metomidate. These data further suggest that the complex between P450Coh and eto- and metomidate may be particularly strong and independent from the redox state of the haem iron.