Microsomal Monooxygenase Activities Research Articles

Retinoids Modulate Thioacetamide-Induced Acute Hepatotoxicity

The literature indicates that retinoids can influence the metabolism and actions of xenobiotics and conversely that xenobiotics can influence the metabolism and actions of retinoids. We were interested in understanding the degree to which hepatic retinoid stores, accumulated over a lifetime, affect xenobiotic metabolism, and actions. To investigate this, we induced liver injury through administration of the hepatotoxin thioacetamide (TAA) to chow fed wild type (WT) mice and lecithin:retinol acyltransferase-deficient (Lrat(-/-)) mice that are genetically unable to accumulate hepatic retinoid stores. Within 48 h of TAA-treatment, WT mice develop liver injury as evidenced by focal necrotic areas and increases in serum ALT activity and myeloperoxidase activity in hepatic parenchyma. Simultaneously, features of hepatic encephalopathy develop, as evidenced by a 25% increase in blood ammonia and a threefold reduction of blood glucose levels. This is accompanied by reduced hepatic glutathione, and increased thiobarbituric acid reactive substances, protein carbonyl and sulfhydryl groups, and increased cytochrome P450-catalyzed hydroxylation activity and flavin-containing monooxygenase activity in microsomes prepared from WT liver. Strikingly, none of these TAA-induced effects were observed for matched Lrat(-/-) mice. To confirm that TAA hepatotoxicity depends on retinoid availability, we administered, over 48 h, four oral doses of 3000 IU retinyl acetate each to the mice. This led to the development of hepatotoxicity in Lrat(-/-) mice that was similar in extent to that observed in WT mice. Our findings establish that endogenous hepatic retinoid stores can modulate the toxicity of TAA in mice.

Enzymes mediating resistance to lambda-cyhalothrin in Eriopis connexa (Coleoptera: Coccinellidae)

Resistance to widely used insecticide, lambda-cyhalothrin, was recently reported in the predatory lady beetle Eriopis connexa (Germar) (Coleoptera: Coccinellidae). However, to understand whether metabolic mechanisms underlie such resistance, synergism bioassays and in vitro studies were carried out by using inhibitors and model substrates for enzymatic assays, respectively. The LD50s estimated for susceptible and resistant populations ηg of lambda-cyhalothrin/insect, and thus, a 22-fold difference in resistance ratio. Synergism ratios for the susceptible population with piperonyl butoxide (PBO), diethyl maleate (DEM), triphenyl phosphate (TPP), and S,S,S-tributylphosphorotrithioate (DEF) were respectively 33.8-, 0.24-, 0.35-, and 4.25-fold, while for the resistant population, they were 1463.0-, 0.79-, 0.85-, and 282.6-fold, respectively. The synergized resistance ratios were 0.50-, 2.00-, 6.75-, and 8.77-fold with PBO, DEF, DEM, and TPP, respectively, while resistance was virtually suppressed with DEF. The esterase exhibited 4.16-, 4.03-, and 5.38-fold greater activity towards formation of α-naphthol, β-naphthol, and 4-nitrophenol in the resistant population of E. connexa than in the susceptible population. The activity of esterase depended on concentrations of DEF applied, either using α-naphthol or β-naphthol, which completely inhibited the activity at 636ηM. The PBO inhibited the β-naphthol formation in approximately 50%, suggesting it as inhibitor of esterases. The activities of glutathione-S-transferase were similar and corresponded to 0.36–0.47ηmol−1min−1μg of protein, for S and R populations, respectively. Similarly, the activities of cytochrome P450-dependent microsomal monooxygenases were 0.04 and 0.05ηmol−1min−1μg of protein. The native gel indicated that the formation of β-naphthol was completely inhibited by methyl-paraoxon, but only partially inhibited by eserine, TPP, and PBO. Although other studies with DEF and PBO have demonstrated strong inhibition of type B carboxylesterase associated with insecticide resistance, the results reported here do not rule out metabolism by cytochrome P450-dependent microsomal monooxygenases as a factor conferring E. connexa resistance to lambda-cyhalothrin and confirmed that PBO may also act by inhibiting esterases of insects.

Effects of flumequine on cytochrome P450 enzymes in allogynogenetic silver crucian carp, Carassius auratus gibelio

The effects of flumequine on activities of several microsomal P450 monooxygenases of allogynogenetic silver crucian carp (Carassius auratus gibelio) were determined and the induction of fluemquine on CYP1A was further explored at the level of protein expression and mRNA transcription.After 24 h following a single intraperitoneal injection at a dose of 35 mg/kg,the ethoxyresorufin O-deethylase (EROD) activity in the liver of the Crucian carp was 54.33 pmol/(mg·min),which was significantly higher than that of the control carps [34.00 pmol/(mg·min)](P0.01).It was revealed that flumequine could significantly induce EROD activity(P0.01).However,the other P450 monooxygenases,such as erythromycin N-demethylase (ERND) [177.98 pmol/(mg·min)],aminopyrine N-demethylase [934.40 pmol/(mg·min)] and ethoxycoumarin O-deethylase (ECOD) [9.84pmol/(mg·min)]were not significantly affected by flumequine,comparing with these activities in control [140.90 pmol/(mg·min),850.71 pmol/(mg·min) and 8.93 pmol/(mg·min)],respectively.Except that the ERND activity in kidney tissue was higher than that in liver,the activities of APD,EROD and ECOD were the highest in liver tissue.An immunoblot analysis with rabbit anti-rat P4501A polyclonal antibody showed that CYP 1A protein expression in flumequine-treated carps was markedly higher than that in control carps,which was consistent with the trend in EROD activities.These findings suggest that P450 1A isoform,constitutively expressed in crucian carp,was particularly susceptible to activation by flumequine.Semiquantitative RT-PCR indicated that CYP1A mRNA was not affected by flumequine.In vitro assay,microsomes incubating with various concentrations of flumequine demonstrated that this fluoroquinolone antibiotic had no induction or inhibition of CYP1A.On the basis of the above,liver CYP1A induction of flumequine on carp occurs at the post-translation level,which may enhance protein stability.

The effect of berberine administration of evaluation of the functional state of rat liver after ligation of common bile duct

On the eighth day after ligation of the common bile duct in rats a significant increase in the serum content of total lipids, cholesterol bilirubin and ALT, alkaline phosphatase, and gamma-glutamyltransferase was observed. In the microsomal fraction there was a marked decrease in the content and activity of microsomal monooxygenases. Introperitoneal injection of berberine (10 mg/kg) for 6 days caused a partial normalization of permeability of hepatocytes plasma membranes and activity microsomal flavin-containing monooxygenases. It is suggested that berberine is a substrate and inducer of flavin-containing monooxygenases. Membrane-stabilizing effect of berberine is probably realized at the level of inhibition of prooxidant status of liver cells.

The effect of berberine administration to rats on the functional state of liver after common bile duct ligation

On day 8 after ligation of the common bile duct in rats a significant increase in the serum content of total lipids, cholesterol, bilirubin and ALT, alkaline phosphatase, and gamma-glutamyltransferase was observed. In the hepatic microsomal fraction there was a marked decrease in the content and activity of microsomal monooxygenases. Introperitoneal injections of berberine (10 mg/kg) for 6 days caused a partial normalization of hepatocyte plasma permeability and activity of microsomal flavin-containing monooxygenases. It is suggested that berberine is a substrate and inducer of flavin-containing monooxygenases. The membrane-stabilizing effect of berberine is probably realized at the level of inhibition of the prooxidant status of liver cells.

Inhibition of hepatic microsomal monooxygenase activity by cinchocaine: mechanistic studies and effects of ionization.

The quinoline-based local anaesthetic cinchocaine (dibucaine) was found to be a mixed-type inhibitor of microsomal aminopyrine N-demethylase and 7-ethoxycoumarin O-deethylase activities from control and phenobarbitone-induced rat liver in-vitro. Cinchocaine also elicited a characteristic type I optical difference spectrum in oxidized liver microsomes (Ks = 24 microM; delta Amax = 3.4 X 10(-3) absorbance units (nmol cytochrome P450)-1) but did not appear to bind to the reduced form of the cytochrome. Additional studies indicated that cinchocaine competitively inhibited the type I spectral binding of substrate (aminopyrine) to ferric cytochrome P450. Studies of monooxygenase inhibition by cinchocaine over a relatively narrow pH range (6.5-8.5) indicated that, as might be expected, the un-ionized form of the drug is associated with inhibitory potency superior to that of the ionized form. Thus 40% inhibition of aminopyrine N-demethylase activity was observed with 100 microM cinchocaine at pH 8.0 and 8.5 (24% and 50% un-ionized drug, respectively), whereas only 16% inhibition was observed at pH 6.5 (1% un-ionized drug). These findings suggest that the inhibitory action of cinchocaine is mediated exclusively via an interaction with ferric cytochrome P450 and that the extent of ionization is a determinant of mixed function oxidase inhibition.

Effects of Cu 2+ and Pb 2+ on different fish species: Liver cytochrome P450-dependent monooxygenase activities and FTIR spectra

The effects of Cu 2+-sulfate and Pb 2+-acetate on carp ( Cyprinus carpio L.), silver carp ( Hypopthalmichtys molitrix V.) and wels ( Silurus glanis L.) were studied. The liver microsomal Cyt P450 content, the EROD, ECOD and APND monooxygenase activities were measured. In vivo treatment with 1 mg L − 1 Cu 2+ significantly elevated the activities of these enzymes and Cyt P450 content in silver carp livers. The high-dose Cu 2+ treatment (10 mg L − 1 ) on silver carp caused two-fold higher induction in the P450 dependent monooxygenase isoensymes than in wels. Although the 2 mg kg − 1 treatment with Pb 2+ in carp elevated significantly the P450 content, the EROD isoenzyme activities were significantly decreased after 1 day, showing the destructive effect of metal ion on the enzyme system. In vitro, Cu 2+ and Pb 2+ decreased the Cyt P450 content in the carp liver microsomes and the absorption peak shifted to higher wavelength. Fourier Transform Infrared (FTIR) spectroscopy was used to detect the damaging effects of the heavy metals. According to the inhibitory potency to Cu 2+, the most sensitive isoenzyme was the EROD in wels, the least was the silver carp's isoenzyme. The investigated fish P450 isoenzymes showed, that the Cu 2+ was a stronger inhibitor than Pb 2+.

Effect of glutathione on homo- and heterotropic cooperativity in cytochrome P450 3A4

Glutathione (GSH) exerted a profound effect on the oxidation of 7-benzyloxy-4-(trifluoromethyl)coumarin (BFC) and 7-benzyloxyquinoline (BQ) by human liver microsomes as well as by CYP3A4-containing insect cell microsomes (Baculosomes). The cooperativity in O-debenzylation of both substrates is eliminated in the presence of 1–4 mM GSH. Addition of GSH also increased the amplitude of the 1-PB induced spin shift with purified CYP3A4 and abolished the cooperativity of 1-PB or BFC binding. Changes in fluorescence of 6-bromoacetyl-2-dimethylaminonaphthalene attached to the cysteine-depleted mutant CYP3A4(C58,C64) suggest a GSH-induced conformational changes in proximity of α-helix A. Importantly, the K S value for formation of the GSH complex and the concentrations in which GSH decreases CYP3A4 cooperativity are consistent with the physiological concentrations of GSH in hepatocytes. Therefore, the allosteric effect of GSH on CYP3A4 may play an important role in regulation of microsomal monooxygenase activity in vivo.

Acute toxicity of polychlorinated naphthalenes and their effect on cytochrome P450

Polychlorinated naphthalenes (PCNs) are able to induce cytochrome P-450-dependent microsomal mono-oxygenase activities in vivo and in vitro. The aim of this study was to investigate the toxicity of a PCN mixture, and its effect on the levels of cytochrome P-450 in rats. The animals were intragastrically administered a mixture of PCNs in single doses of 250, 500 and 1000 mg/kg b.w. Dissection of animals was performed 24, 72 and 240 hours after administration. After PCN administration (all doses) the body weight loss (up to 30% in comparison with the control group, 240 hours after administration) and an increase of relative liver mass (about 126-153% of controls, 72 hours after administration) were observed. The exposure to PCN evoked an increase in the level of total cytochrome P-450 as well as the activity of CYP 1A (mediated 7-ethoxyresorufin O-deethylation) at all time points. The maximum activity of CYP 1A (about 12- to 15-fold increase in comparison with the control group) was observed 72 hours after dosing. Malondialdehyde (MDA), determined in the liver, showed a high increase and 240 hours after administration, the level of MDA was about one order of magnitude greater in comparison with control.

Communicated by A.A. Maradudin on 15 August 2005

Antioxidants inhibit the propagation of free radical reactions. Because of their widespread use as preservatives in processed foods, their biochemical effects have been investigated. Early feeding studies indicated that butylated hydroxytoluene (BHT) increased liver weight, induced proliferation of smooth endoplasmic reticulum, and elevated several hepatic microsomal mono-oxygenase activities typical of phase 1 metabolism. In addition to phase1, antioxidants also induce phase2 xenobiotic-metabolizing enzymes. Phase2 enzymes detoxify activated electrophilic metabolites of xenobiotics via conjugation of endogenous substrates such as glutathione (GSH). Antioxidants act to directly terminate the propagation of free radical reactions, and increase the activity of enzymes those readily metabolize and aid in the elimination of potential cytotoxic chemicals. Dietary administration of antioxidants induced phase 2 xenobiotic-metabolizing enzymes. Induction of phase 2 enzymes by antioxidants was found in many organs and tissues, such as liver, lung, kidney, small intestine, colon, and spleen, thereby affording protection at many anatomical sites .This chapter presents evidence for several newly identified antioxidant-inducible genes, describes the proposed mechanisms of antioxidant signal transduction leading to enhanced expression of these enzymes, and complements the information presented in related reviews concerning the mechanisms and consequences of induction of phase 2 xenobiotic-metabolizing enzymes by antioxidants. The chapter discusses the genes induced by antioxidants—cytochrome P450s, Glutathione, S-transferases, NAD(P)H: quinone reductase, UDP-glucuronosyltransferases, microsomal epoxide hydrolase, aflatoxin BI-aldehyde reductase, dihydrodiol dehydrogenases, aldehyde dehydrogenases, enzymes of glutathione and reduced nicotinamide metabolism, and other proteins and enzymes. The chapter delves into the mechanisms of gene induction by antioxidants—the antioxidant-response element (ARE), proteins binding and signal transduction through the ARE—and the consequences of antioxidant gene induction. The regulation of antioxidant-inducible genes helps understand the signals leading to cellular transformation and carcinogenesis. Antioxidants can be used to decipher the encrypted signal transduction pathways that prevent cell transformation and carcinogenesis.

Characterization and modulation by drugs of sheep liver microsomal flavin monooxygenase activity.

The flavin monooxygenases (FMO) catalyse the NADPH and oxygen-dependent oxidation of a wide range of nucleophilic nitrogen-, sulfur-, phosphorus-, and selenium heteroatom-containing chemicals, drugs, and agricultural agents. In the present study, sheep liver microsomal FMO activity was determined by measuring the S-oxidation rate of methimazole and the average specific activity obtained from different microsomal preparations was found to be 3.8 +/- 1.5 nmol methimazole oxidized min(-1) mg(-1) microsomal protein (mean +/- SE, n = 7). The presence of 0.1% Triton X-100 in the reaction mixture caused an increase of specific sheep liver microsomal FMO activity towards methimazole to 6.1 +/- 1.4 nmol methimazole oxidized min(-1) mg(-1) microsomal protein (mean +/- SE, n = 6). Metabolism of imipramine and chlorpromazine was measured by following the oxidation of cofactor NADPH spectrophotometrically at 340 nm. Sheep liver microsomal FMO activity towards imipramine and chlorpromazine was found to be 10.7 and 12.3 nmol NADPH oxidized min(-1) mg(-1) microsomal protein, respectively. Characterization of sheep liver enzyme was carried out using methimazole as substrate and the maximum FMO enzyme activity was detected at 37 degrees C and at pH 8.0. The apparent K(m) value of sheep liver microsomal FMO for methimazole was 0.118 mM. Effects of the detergents Triton X-100, Cholate, and Emulgen 913, on FMO activity were determined and FMO activity was found to increase with the addition of detergents to the reaction medium. Sheep liver microsomal FMO-catalysed methimazole oxidation was inhibited by imipramine and chlorpromazine when these drugs were used at high concentrations. Western blot-immunochemical analysis revealed the presence of FMO3 in sheep liver microsomes.

Effects of dioxane on cytochrome P450 enzymes in liver, kidney, lung and nasal mucosa of rat

The effect of acute and chronic dioxane administration on hepatic, renal, pulmonary and nasal mucosa P450 enzymes and liver toxicity were investigated in male rats. The acute treatment consisted of two doses (2 g/kg) of dioxane given for 2 days by gavage, whereas the chronic treatment consisted of 1.5% of dioxane in drinking water for 10 days. Both the acute and chronic dioxane treatments induced cytochrome P450 2B1/2- and P450 2E1-dependent microsomal monooxygenase activities (pentoxyresorufin O-depentylase and p-nitrophenol hydroxylase) in the liver, whereas in the kidney and nasal mucosa, only the 2E1 marker activities were enhanced. In addition in the liver, an induction of 2alpha-testosterone hydroxylase (associated with the constitutive and hormone-dependent P450 2C11) was also revealed, whereas the hepatic P450 4A-dependent omega-lauric acid hydroxylase was not enhanced by any dioxane treatment. These inductions were mostly confirmed by western blot analysis of liver, kidney and nasal mucosa microsomes. In the lung, no alteration of P450 activities was observed. To assess the mechanism of 2E1 induction, the hepatic, renal and nasal mucosa 2E1 mRNA levels were also examined. Following two kinds of dioxane administration, in the liver the 2E1 induction was not accompanied by a significant alteration of 2E1 mRNA levels, while both in the kidney and nasal mucosa the 2E1 mRNA increased about 2- to 3-fold, indicating an organ-specific regulation of this P450 isoform. Furthermore, dioxane was unable to alter the plasma alanine aminotransferase activity and hepatic glutathione (GSH) content, examined as an index of toxicity, when it was administered into rats with P450 2B1/2 and 2E1 preinduced by phenobarbital or fasting pretreatment. These results support the lack of or a poor formation of reactive and toxic intermediates during the biotrasformation of this solvent, even when its metabolism was enhanced by P450 inducers. The chronic administration of dioxane was also unable to induce the palmitoyl CoA oxidase, a marker of peroxisome proliferation, excluding this as a way to explain its toxicity. Thus, although the mechanism of dioxane carcinogenicity remains unclear, the present results suggest that the induction of 2E1 following a prolonged administration of dioxane might provide oxygen radical species, and thereby contribute to its organ-specific toxicity.

Atrazine induction of a family 4 cytochrome P450 gene in Chironomus tentans (Diptera: Chironomidae)

Cytochrome P450-dependent microsomal monooxygenase (P450) activity was measured in control and atrazine-exposed third instar midge larvae, Chironomus tentans. Significantly elevated O-demethylase activity was observed in gut homogenates taken from midges exposed to atrazine concentrations from 1 to 10 ppm for 90 h. No significant induction was observed at atrazine concentrations below 1 ppm. A region of a cytochrome P450 family 4 gene was amplified and sequenced from C. tentans larvae. Alignments of inferred amino acid sequences with other insect CYP4 gene homologues indicate a high degree of similarity. Northern blot analysis employing the CYP4 gene fragment as a probe showed an over-expression in C. tentans exposed to atrazine. The results support the previously identified inducibility of cytochrome P450-dependent activity and provide insight into the potential consequences of atrazine exposure to aquatic organisms.

Depletion of Kupffer cell function by gadolinium chloride attenuates thioacetamide-induced hepatotoxicity: Expression of metallothionein and HSP70

Kupffer cell function plays an important role in drug-induced liver injury. Thus, gadolinium chloride (GD), by selectively inactivating Kupffer cells, can alleviate drug-induced hepatotoxicity. The effect of GD was studied in reference to metallothionein and heat shock proteins expression in an in vivo model of liver necrosis induced by thioacetamide. Rats, pre-treated or not with GD (0.1 mmol/kg), were intraperitoneally injected with thioacetamide (6.6 mmol/kg), and samples of blood and liver were obtained at 0, 12, 24, 48, 72 and 96 hr. Parameters related to liver damage, Kupffer cell function, microsomal FAD monooxygenase activity, oxidative stress, and the expression of metallothionein and HSP70 were determined. GD significantly reduced serum myeloperoxidase activity and serum concentration of TNFα and IL-6, increased by thioacetamide. The extent of necrosis, the degree of oxidative stress and lipoperoxidation and microsomal FAD monooxygenase activity were significantly diminished by GD. The effect of GD induced noticeable changes in the expression of both metallothionein and HSP70, compared to those induced by thioacetamide. We conclude that GD pre-treatment reduces thioacetamide-induced liver injury and enhances the expression of metallothionein and HSP70. This effect, parallel to reduced levels of serum cytokines and myeloperoxidase activity, demonstrates that Kupffer cells are involved in thioacetamide-induced liver injury, the degree of contribution being approximately 50%.

Trifluoroacetylated proteins in liver and plasma of guinea pigs treated with HCFC-123 and halothane

Trifluoroacetylated (TFA)-protein adducts were investigated by immunoblotting in liver and plasma of guinea pigs treated with the hepatotoxic anaesthetic halothane or the chlorofluorocarbon replacement 1,1-dichloro-2,2,2-trifluoroethane (HCFC-123). Male outbred Hartley guinea pigs (320–400 g) were administered HCFC-123 (1, 5, 10 and 15 mmol/kg b.w.) or halothane (positive control, 10 mmol/kg b.w.) i.p. in corn oil. Blood and liver samples were collected 24 h after administration of HCFC-123 or halothane. Immunoreactive bands were demonstrated in liver microsomes at all HCFC-123 and halothane concentrations, and in plasma of animals treated with 10 and 15 mmol/kg b.w. HCFC-123 and 10 mmol/kg b.w. halothane, while no alteration of microsomal P450 content or monooxygenase activities markers of the P450 2A, 2E1 and 2B isoforms was observed. Instead, when HCFC-123 was administered at doses of 1 and 5 mmol/kg b.w., the 2E1-dependent p-nitrophenol hydroxylase activity was enhanced. The presence of TFA-proteins in plasma was always associated with hepatic damage. However, mild liver damage in some animals treated with 1 or 5 mmol/kg b.w. HCFC-123 was not associated with the presence of TFA-proteins in plasma. This indicates a lower threshold dose for the appearance of TFA-proteins or damage in the liver (1 mmol/kg b.w.) than for the presence of TFA-proteins in plasma (10 mmol/kg b.w.), thus suggesting that the presence of TFA-proteins in plasma may be the result of liver damage.

Effects of the mosquito larvicide GB-1111 on red-winged blackbird embryos

Golden Bear Oil (GB-1111; legal trade name for GB-1313) is a petroleum distillate that is used in the United States and other countries as a larvicide for mosquito suppression. As part of a multi-species evaluation of the potential effects of GB-1111 on birds, red-winged blackbird eggs were collected, artificially incubated, and treated with one of five amounts of GB-1111 varying from 0 to 10 times the expected exposure from a spray application of the maximum recommended amount ( X=47 l/ha, 5 gal/ac). The application of 10 X caused a significant reduction in hatching success. A dose-related reduction of hepatic microsomal monooxygenase activity (EROD) was detected. Among body weights, skeletal measurements, and age at death, only crownrump length was different among experimental groups. Overall, the potential hazard to embryos of a representative wetland passerine appears minimal until the application rate exceeds 3 X.

The CYP P450 arachidonate monooxygenases: enzymatic relays for the control of kidney function and blood pressure.

During P450-dependent metabolism, arachidonic acid (AA) is: a) hydroxylated at its ω or ω-1 carbons to 19- and 20-hydroxyeicosatetraenoic acids (HETEs)(AA ω/ω-1 hydroxylase), or b) epoxidized at any one of its four olefins to 5,6-, 8,9-, 11,12-, or 14,15-epoxyeicosatrienoic acids (EETs)(AA epoxygenase). The demonstration that many of these P450- derived eicosanoids, were formed endogenously by several rat, rabbit, and human tissues, and that they were present in human and rat plasma and/or urine, established the P450 AA monooxygenase as a formal metabolic pathway, and stimulated an extensive functional characterization of its metabolites [1, 2, 3, 4, 5, 6, 7]. During the last 15 years, the list of biological activities attributed to the P450-eicosanoids has increased at a rapid pace, and led to an extensive in vitro characterization of their vasoactive properties, roles in cell and organ ion transport, and of their participation in hormone signaling cascades [1, 2, 3, 4, 5, 6, 7]. In male Sprague-Dawley rats, AA ω/ω-1 hydroxylation and epoxidation account for 70–80, and 20–30% of the total kidney microsomal monooxygenase activity, respectively, and 20-HETE and 11(R), 12(S)-EET (88% optical purity) are recovered as the major ω-hydroxylase and epoxygenase metabolites, respectively [[3]]. The high degree of structural homology displayed by the members of a P450 gene family [8] has complicated the identification of the P450 isoforms responsible for the metabolism of endogenous AA pools [[3],[5],[9]–[11]]. Nevertheless, extensive enzymatic, biochemical, immunological, and functional evidence indicates that CYPs of the 2 and 4 gene families are responsible for AA metabolism, and that, in rat kidney, CYPs 2C and 4A are the predominant and functionally significant AA epoxygenases and ω/ω-1 hydroxylases, respectively [[3],[9]–[19]]. For example, CYPs, 2C23, 2C24, and 2C11 are expressed in the rat kidney in decreasing order of abundance [[17]] but, of these, only CYP 2C23 metabolizes AA with a regio- and enantioselectivity that matches that of the microsomal enzymes [[16],[17]]. Furthermore, as shown in Figure 1,antibodies raised against CYP 2C23 inhibited better than 95% of the microsomal epoxygenase activity present in rat kidney microsomes, and had no effect on microsomal AA ω/ω-1 hydroxylation. The recognition of CYPs 2C and 4A as endogenous AA epoxygenases and ω/ hydroxylases, as well the early proposal of a role for these isoforms in the pathophysiology of experimental hypertension [1,[4]–[7]], open the way to the introduction of molecular approaches for the study of the functional roles of the AA monooxygenases, and the characterization of P450-isoform/gene-dependent phenotypes of renal dysfunction.

Flavin-containing monooxygenase activity in hepatocytes and microsomes: in vitro characterization and in vivo scaling of benzydamine clearance.

Liver microsomes, and more recently cryopreserved hepatocytes, are commonly used in the in vitro characterization of the metabolism of new xenobiotics. The flavin-containing monooxygenases (FMO) are a major non p450 oxidase present in liver microsomes and hepatocytes. Since FMO is known to be thermally labile, and this enzyme may be involved in the metabolic clearance of some drugs, we sought to more completely characterize the metabolic competency of this enzyme in cryopreserved hepatocytes and in liver microsomes preincubated under various conditions using benzydamine as an in vitro and in vivo probe. The metabolism of benzydamine to its major metabolite, the N-oxide, is mediated by FMO3 in humans. We found that the in vitro microsomal t(1/2) was 70% longer when incubations were prewarmed at 37 degrees C in the absence of NADPH compared with prewarming in the presence of an NADPH-regenerating system, and N-oxide formation was inhibited >99%. Interestingly, the in vivo clearance predicted from these incubations and from human hepatocytes overpredicted the observed clearance of benzydamine in humans (>10.5 versus 2.4 ml/min/kg). In contrast, rat hepatocytes successfully predicted rat in vivo benzydamine clearance to within approximately 30% (>68 versus 48 ml/min/kg). Benzydamine N-oxidation in liver microsomes from all common preclinical species demonstrated heat sensitivity. This information should be considered when extrapolating metabolism data of xenobiotics from these in vitro systems.

Multiple mechanisms in indomethacin-induced impairment of hepatic cytochrome p450 enzymes in rats

Indomethacin impairs liver microsomal monooxygenase activities mediated by cytochrome P450 (CYP). We investigated the inhibition mechanism and the isoform selectivity in vitro and in vivo. In an in vitro study, liver microsomes from male Wistar rats were preincubated with indomethacin and a reduced nicotinamide adenine dinucleotide phosphate-generating system, followed by assay of monooxygenase activities indicative of several CYP isoforms. In an in vivo study, rats were intraperitoneally treated with indomethacin, followed by preparation of microsomes and the enzyme assays. The preincubation of microsomes with indomethacin and reduced nicotinamide adenine dinucleotide phosphate decreased CYP3A2 activity but not any other isoforms. Kinetic analysis showed the mechanism-based inactivation of CYP3A2. The metabolism of [14C]indomethacin resulted in covalent binding to microsomal protein, which was diminished by inhibiting CYP3A enzyme. Administration of indomethacin caused impairment of not only CYP3A2 but also other CYP isoforms. Rats were protected from the impairment of the CYP enzymes except CYP3A2 by depleting macrophages and inhibiting inducible nitric oxide synthase. Metabolism of indomethacin causes inactivation of CYP3A2, which is the result of the covalent binding of its metabolite, whereas partially selective in vivo impairment of CYP isoforms is suggested to be indirect inhibition by inflammatory mediators probably released from Kupffer cells.

Propranolol as an inhibitor of some cytochrome P450-dependent monooxygenase activities in native and induced rat liver microsomes.

The in vitro effect of propranolol (10(-3) M and 10(-4) M), a nonselective and extensively metabolized beta-adrenergic blocking agent, on rat liver drug metabolism in native and induced (with phenobarbital and beta-naphthoflavone [beta-NF]) microsomes was studied. The type of inhibition and the inhibitory constants of some cytochrome P450-dependent microsomal enzyme reactions (hexobarbital oxidation [HBO], ethylmorphine-N-demethylation [EMND], aniline hydroxylation [AH], ethoxycoumarin-O-deethylation [ECOD], ethoxyresorufin-O-dealkylation [EROD] and penthoxyresorufin-O-dealkylation [PROD]) were estimated. The results showed that propranolol competitively inhibited AH activity in native microsomes. The type of inhibition was changed from competitive to noncompetitive in all other enzyme activities studied. This inhibition was more pronounced after phenobarbital induction in PROD (Ki = 0.11 +/- 0.01 mM), ECOD (Ki = 0.40 +/- 0.09 mM) and EMND (Ki = 0.59 +/- 0.1 mM), and after beta-NF induction in AH (Ki = 0.28 +/- 0.05 mM) and in HBO (Ki = 0.35 +/- 0.1 mM) in native microsomes. It was assumed that the noncompetitive type of inhibition is due to the covalent binding of reactive metabolites derived from propranolol to hepatic microsomal proteins. The competitive type of inhibition of AH suggested a common P450 isoenzyme in the metabolism of propranolol and aniline. Thus, in this study, propranolol has been found to be not only a selective inhibitor of CYP2D6 isoenzyme-dependent reactions, but also a nonspecific inhibitor of other cytochrome P450 isoenzymes.