The assessment of flavin-containing monooxygenase activity in intact animals

Cytoprotective Effects of Organosulfur Compounds against Methimazole Induced Toxicity in Isolated Rat Hepatocytes

PURPOSE

Methimazole is a drug widely used in hyperthyroidism. However, life threatening hepatotoxicity has been associated with its clinical use. No protective agent has been found to be effective against methimazole induced hepatotoxicity yet. Hence, the capacity of organosulfur compounds to protect rat hepatocytes against cytotoxic effects of methimazole and its proposed toxic metabolite, N-methylthiourea was evaluated.

METHODS

Hepatocytes were prepared by the method of collagenase enzyme perfusion via portal vein. Cells were treated with different concentrations of methimazole, N methylthiourea, and organosulfur chemicals. Cell death, protein carbonylation, reactive oxygen species formation, lipid peroxidation, and mitochondrial depolarization were assessed as toxicity markers and the role of organosulfurs administration on them was investigated.

RESULTS

Methimazole caused a decrease in cellular glutathione content, mitochondrial membrane potential (ΔΨm) collapse, and protein carbonylation. In addition, an increase in reactive oxygen species (ROS) formation and lipid peroxidation was observed. Treating hepatocytes with N methylthiourea caused a reduction in hepatocytes glutathione reservoirs and an elevation in carbonylated proteins, but no significant ROS formation, lipid peroxidation, or mitochondrial depolarization was observed. N-acetyl cysteine, allylmercaptan, and diallyldisulfide attenuated cell death and prevented ROS formation and lipid peroxidation caused by methimazole. Furthermore, organosulfur compounds diminished methimazole induced mitochondrial damage and reduced the carbonylated proteins. In addition, these chemicals showed protective effects against cell death and protein carbonylation induced by methimazole metabolite.

CONCLUSION

Organosulfur chemicals extend their protective effects against methimazole-induced toxicity by attenuating oxidative stress caused by this drug and preventing the adverse effects of methimazole and/or its metabolite (s) on subcellular components such as mitochondria.

Mammalian flavin-containing monooxygenases: structure/function, genetic polymorphisms and role in drug metabolism

Flavin-containing monooxygenase (FMO) oxygenates drugs and xenobiotics containing a "soft-nucleophile", usually nitrogen or sulfur. FMO, like cytochrome P450 (CYP), is a monooxygenase, utilizing the reducing equivalents of NADPH to reduce 1 atom of molecular oxygen to water, while the other atom is used to oxidize the substrate. FMO and CYP also exhibit similar tissue and cellular location, molecular weight, substrate specificity, and exist as multiple enzymes under developmental control. The human FMO functional gene family is much smaller (5 families each with a single member) than CYP. FMO does not require a reductase to transfer electrons from NADPH and the catalytic cycle of the 2 monooxygenases is strikingly different. Another distinction is the lack of induction of FMOs by xenobiotics. In general, CYP is the major contributor to oxidative xenobiotic metabolism. However, FMO activity may be of significance in a number of cases and should not be overlooked. FMO and CYP have overlapping substrate specificities, but often yield distinct metabolites with potentially significant toxicological/pharmacological consequences. The physiological function(s) of FMO are poorly understood. Three of the 5 expressed human FMO genes, FMO1, FMO2 and FMO3, exhibit genetic polymorphisms. The most studied of these is FMO3 (adult human liver) in which mutant alleles contribute to the disease known as trimethylaminuria. The consequences of these FMO genetic polymorphisms in drug metabolism and human health are areas of research requiring further exploration.

Determination of the sulphoxides and sulphones of three simple sulphides in rat urine: effects of phenobarbitone,?-naphtho?avone and methimazole

In this investigation, the measurement and identification of the S-oxidation products of three simple sulphides-ethyl methyl sulphide (EMS), 4-chlorophenyl methyl sulphide (CPMS) and diphenyl sulphide (DPS)-in rat urine were carried out and a study of the effects of phenobarbitone (PB), beta-naphtho flavone (betaNF) and methimazole on the urinary levels of their metabolites was conducted. Male Wistar rats (n = 4) were pretreated with PB (80 mg/kg/day in saline, i.p.), betaNF (100 mg/kg/day in corn oil, i.p.), methimazole (50 mg/kg/day in saline, i.p.) or the vehicles alone (1 mL/kg) for three consecutive days. After pretreatment, EMS, CPMS or DPS (50 mg/kg in corn oil, 500 microL) was administered orally to the appropriate groups of rats. The animals were placed in metabolic cages and urine samples collected at 24 h intervals over 96 h. Chromatographic and spectroscopic techniques were used for the measurement and identification of the sulphoxides and sulphones of EMS, CPMS and DPS in rat urine. Although only a trace of ethyl methyl sulphoxide (EMSO) was present in rat urine after administration of EMS, ethyl methyl sulphone (EMSO(2)) accounted for about 16% of the administered dose in the urine of male rats given EMS. In addition, pretreatment of rats with methimazole significantly decreased the S-oxidation of EMS. 4-Chlorophenyl methyl sulphone (CPMSO(2)) was the main metabolite recovered in the urine of male rats treated with CPMS, accounting for about 10% of the dose. Pretreatment of rats with PB before administration of CPMS significantly increased the levels of CPMSO(2) excreted in the urine. Additionally, pretreatment of rats with methimazole significantly decreased the S-oxidation of CPMS in vivo. About 2.5% of diphenyl sulphoxide (DPSO) and 4% of diphenyl sulphone (DPSO(2)) were recovered in the urine of male rats given DPS. Pretreatment of rats with PB, betaNF or methimazole before administration of DPS decreased the levels of DPSO and DPSO(2) excreted in the urine, although this was not statistically significant. These results indicate that microsomal monooxygenases mediate the S-oxidation of EMS, CPMS and DPS to their corresponding sulphones via a transient sulphoxide in rats.

S-oxidation of S-methyl-esonarimod by flavin-containing monooxygenases in human liver microsomes

1. Studies using human liver microsomes and recombinant human cytochrome P450 (CYP) and flavin-containing monooxygenase (FMO) were performed to identify the enzymes responsible for the metabolism of S-methyl-esonarimod (M2), an active metabolite of esonarimod (KE-298, a novel antirheumatic drug). 2. S-oxidative activities of M2 significantly correlated with those of methyl p-tolyl sulfide, a specific substrate of FMOs, as tested using 10 different human liver microsomes (r(2) = 0.539, p<0.05). Thermal treatment of microsomes reduced the S-oxidative activity in the absence of the NADPH-generating system at 45 degrees C for 5 min. However, methimazole, a known competitive substrate of FMOs, was a weak inhibitor of the S-oxidation in liver microsomes. 3. Recombinant human FMO1 and FMO5 produced M3 in greater quantities than recombinant human FMO3. The S-oxidation of M2 by recombinant human FMO5 was not appreciably inhibited in the presence of methimazole. In contrast, methimazole was effective in suppressing the catalytic activity of recombinant human FMO1 and FMO3. 4. The apparent K(m) (K(m app)) for the S-oxidation of M2 in human recombinant FMO5 (2.71 microM) was similar to that obtained using human liver microsomes (2.43 microM). 5. The present results suggest that the S-oxidation of S-methyl esonarimod reflects FMO5 activity in the human liver because the recombinant FMO5 data match well with the human liver microsomal experiments.

Involvement of cytochrome P450 and the flavin-containing monooxygenase(s) in the sulphoxidation of simple sulphides in human liver microsomes

This study was conducted to examine the involvement of cytochrome P450 (CYP450) and the flavin-containing monooxygenase (FMO) in the sulphoxidation of ethyl methyl sulphide (EMS), 4-chlorophenyl methyl sulphide (CPMS) and diphenyl sulphide (DPS) in human liver microsomes from a phenotypic CYP2D6 extensive metabolizer. Human liver microsomes catalyzed the sulphoxidation of EMS, CPMS and DPS to their corresponding sulphoxides. Lineweaver-Burk plots for the sulphoxidation of EMS in human liver microsomes indicated that the apparent K(m) and V(max) were 1.53 +/- 0.07 mM and 1.11 +/- 0.25 nmoles/mg protein/min, respectively. The apparent K(m) and V(max) for the sulphoxidation of CPMS were 0.17 +/- 0.05 mM and 1.41 +/- 0.16 nmoles/mg protein/min, respectively. The apparent K(m) and V(max) for the sulphoxidation of DPS were 0.10 +/- 0.01 mM and 1.08 +/- 0.05 nmoles/mg protein/min, respectively. Methimazole noncompetitively inhibited the sulphoxidation of EMS, CPMS and DPS by human liver microsomes with K(i) values of 8.6 +/- 0.6, 5.7 +/- 0.4 and 6.6 +/- 0.5 mM, respectively. SKF525A noncompetitively inhibited the sulphoxidation of CPMS and DPS by human liver microsomes with K(i) values of 6.6 +/- 0.4 and 0.40 +/- 0.1 mM, respectively. The results suggest that FMO is involved in the sulphoxidation of EMS, CPMS and DPS while CYP450 is involved in the sulphoxidation of CPMS and DPS in human liver microsomes.

Sulphoxidation of ethyl methyl sulphide, 4-chlorophenyl methyl sulphide and diphenyl sulphide by purified pig liver flavin-containing monooxygenase

1. The biotransformation of ethyl methyl sulphide (EMS), 4-chlorophenyl methyl sulphide (CPMS) and diphenyl sulphide (DPS) to their corresponding sulphoxides by purified flavin-containing monooxygenase (FMO) is described. 2. Purified pig liver flavin-containing monooxygenase catalysed the sulphoxidation of EMS, CPMS and DPS to their corresponding sulphoxides and the reactions followed single enzyme Michelis-Menten kinetics. 3. The apparent K(m) and V(max) for the sulphoxidation of EMS were 1.38+/-0.05 mM and 78.74+/-3.9 nmoles mg(-1) protein min(-1), respectively. The apparent K(m) and V(max) for the sulphoxidation of CPMS were 0.185+/-0.03 mM and 103+/-5.0 nmoles mg(-1) protein min(-1), respectively. The apparent K(m) and V(max) for the sulphoxidation of DPS were 0.068+/-0.002 mM and 49.26+/-2.05 nmoles mg(-1) protein min(-1), respectively. 4. A significant reduction of the sulphoxidation of these simple sulphides was observed with addition of 1-naphthylthiourea in the incubation medium. On the other hand, incorporation of catalase and superoxide dismutase into the incubation media produced no appreciable inhibition of the observed sulphoxidation of the sulphides. 5. These results suggest that FMO is responsible, at least in part, for the sulphoxidation of nucleophilic sulphides as well as for the oxidation of sulphur atoms that reside within or adjacent to aromatic systems.

HPLC analysis of 4-chlorophenyl methyl sulphide and diphenyl sulphide and their corresponding sulphoxides and sulphones in rat liver microsomes

Simple high performance liquid chromatography (HPLC) methods for the analysis of 4-chlorophenyl methyl sulphide (CPMS), diphenyl sulphide (DPS) and their corresponding sulphoxide and sulphone metabolites in rat liver microsomes are described. The assay methods are based on a reversed phase HPLC column (Spherisorb(R) 5 ODS, 15 x 0.46 cm) using a mixture of water and tetrahydrofuran (THF) as mobile phase at a flow rate of 0.5 ml/min and ultraviolet detection at 260 nm. The compounds were extracted into diethyl ether (2 x 5 ml) from rat liver microsomal incubation mixture (2 ml) and the recoveries were more than 80%. The calibration curves for determining the sulphoxide and sulphone of CPMS or DPS were linear (r > or =0.995) in the range of 0-50 microg/ml and the assays were reproducible with low inter- and intra-assay variation of less than 13.5%. The lower limit of quantitation (LOQ) was 0.1 microg/ml for CPMSO and 0.025 microg/ml for CPMSO(2), diphenyl sulphoxide (DPSO) and diphenyl sulphone (DPSO(2)). The HPLC methods were successfully applied to measure enzymically formed CPMSO, CPMSO(2), DPSO and DPSO(2) in rat liver microsomes and to characterise the Michaelis-Menten kinetics associated with the metabolism of CPMS and DPS and their corresponding sulphoxides. About 20% of the initial CPMS (0.5 mM) concentration in the incubation was converted to the sulphoxide although the sulphone was not detected under these optimum incubation conditions. Similarly, about 15-20% of DPS was converted to the sulphoxide while less than 0.1% of DPS was converted to DPSO(2). Eadie-Hofstee plot of CPMS sulphoxidation was biphasic. This suggests that the sulphoxidation of CPMS is a consequence of at least two enzyme systems, one characterized by low affinity and high capacity (K(m)=0.1 mM; V(max)=2.1 nmoles/mg protein/min) and the other by high affinity and low capacity (K(m)=0.05 mM; V(max)=1.5 nmoles/mg protein/min). On the other hand, the Eadie-Hofstee plot of DPS sulphoxidation was monophasic with an apparent V(max) and K(m) of 1.8 nmoles/mg protein/min and 0.036 mM, respectively.

The effects of a synthetic diet on the pharmacokinetics of ethyl methyl sulphide and its sulphoxide and sulphone metabolites in rats

Ethyl methyl sulphide (EMS) is a simple dialkyl sulphide, which occurs naturally and forms part structures of more complex drug molecules. EMS is oxidized to the corresponding sulphoxide (EMSO) and sulphone (EMSO2) derivatives both in vitro and in vivo. Two distinct enzymatic pathways appear to be involved in this sulphoxidation process; the flavin-containing monooxygenase (FMO) is largely responsible for the S-oxidation of EMS to its sulphoxide while both cytochrome P450 and FMO are involved in the further oxidation of the sulphoxide to the sulphone. The pharmacokinetics of EMS and its sulphoxide and sulphone metabolites were examined in male wistar rats placed on normal rat chow and those placed on a synthetic diet. Blood levels of EMS were analysed by a sensitive headspace gas chromatographic assay. A separate gas chromatographic assay was developed to monitor the blood levels of EMSO and EMSO2. The pharmacokinetics of EMS in control rats were linear from 10 to 40 mg/kg dose range. The blood concentration-time profile of EMS declined monoexponentially. EMS was rapidly eliminated from rat blood with a terminal half-life of 0.14 h and was not dytectable 1 h after administration. Following intravenous administration of EMSO (5 mg/kg), the blood concentration-time profile of EMSO declined with a terminal half-life (t 1/2) of 1.46 h, about ten times longer than that of the parent sulphide. After administration of EMSO2 (15 mg/kg), the sulphone was metabolically stable and was eliminated very slowly from the blood. The in vivo disposition of EMS and EMSO were clearly altered in rats maintained on a synthetic diet following administration of EMS or EMSO. The pharmacokinetic data were consistent with a diminished drug oxidising capacity in rats placed on the synthetic diet and could serve as a useful probe for monitoring the regulation of FMO in animals.

Metabolism of ethyl methyl sulphide and sulphoxide in rat microsomal fractions

1. Gas chromatographic (GC) methods for the analysis of ethyl methyl sulphide (EMS) and its corresponding sulphoxide (EMSO) and sulphone (EMSO2) in rat microsomes and aspects of the in vitro metabolism of EMS and EMSO are described. 2. EMS and the internal standard (dimethyl sulphide) were extracted by a headspace procedure and separated satisfactorily using a column packed with 4% Carbowax 20 M/0.8% KOH on Carbopack B. EMSO, EMSO2 and the internal standard n-propyl sulphone were separated satisfactorily using a 2-m column packed with 10% Carbowax 20 M on Chromosorb W. 3. Under the optimum conditions (incubation of 10 min and microsomal protein content of approximately 4 mg/ml), 10% of the initial EMS concentration (2.5 mM) was converted to the corresponding sulphoxide in rat liver microsomal incubations. However, < 0.1% of the sulphone was detected when rat liver microsomes were incubated with EMS. Similarly, 2.5% of the initial EMSO concentration (2.5 mM) was converted to the corresponding sulphone by rat liver microsomes (approximately 4 mg/ml protein) during an incubation of 30 min. However, no EMS was detected after incubation with EMSO under these conditions. 4. The estimated apparent Vmax and Km for the sulphoxidation of EMS were 3.8+/-0.02 nmol/mg protein/min and 1.9+/-0.10 mM respectively. Vmax1, Vmax2 and Km1 and Km2 for the S-oxidation of EMSO were 0.5+/-0.01 and 0.2+/-0.01 nmol/mg protein/min and 0.7+/-0.02 and 0.1+/-0.00 mM respectively. 5. Studies with selective inducers and inhibitors of microsomal monooxygenases indicated that the sulphoxidation of EMS is mediated mainly by FMO, whereas both FMO and cytochrome P450 are involved in the S-oxidation of EMSO.

Hepatic cytochrome P450 and flavin-containing monooxygenase in male Nts:Mini rat, a transgenic rat carrying antisense RNA transgene for rat growth hormone

We investigated the characteristics of hepatic cytochrome P450s and flavin-containing monooxygenase 1 (FMO1) in male Nts:Mini rats, a Wistar/Jcl-derived transgenic rat strain showing less plasma GH concentration than the parental strain. The total hepatic P450 contents of Mini rats were significantly reduced. A suppression was observed in the activities and protein expression of male-specific P450s (CYP3A and CYP2C11) and was speculated to be a potential cause of the reduction in total P450 contents. The activity and protein expression of CYP2B1 were suppressed and those of CYP2E1 and CYP2B2 were enhanced. With the exception of our data on CYP2B1, these results largely agreed with previous reports concerning GH-depletion rat models (hypophysectomized rats, rats neonatally treated with glutamate, and dwarf rats), implying that the changes in Mini rats were caused by GH insufficiency. The liver FMO1 protein expression in Mini rats was higher than that in Wistar rats but the activity was comparable, suggesting that GH is not a positive regulator of FMO expression. With their insufficient but not depleted levels of plasma GH, Mini rats may thus become another candidate for use in the investigation of GH regulation of hepatic mixed-function monooxygenases.

Stereoselectivity in S- and N-oxygenation by the mammalian flavin-containing and cytochrome P-450 monooxygenases

In general, the use of stereoselectivity studies in examining the contribution of monooxygenases or other catalysts to the N- and S-oxidation of drugs, xenobiotics and endogenous substrates provides a useful method to distinguish enzymatic from nonenzymatic processes. Recent developments in this active area of research have been rapid, presumably due to advances in bioanalytical chemistry, chiral stationary-phase HPLC, and attendant breakthroughs in the instruments to measure centers of chirality. This research area has also been aided by the availability of enzymes and other catalysts. In light of the ever-increasing necessity for new single-isomer drugs, metabolites, and other chiral drug market materials, the demand for stereoselectivity information in the drug development business should continue to expand. In the future, demand for enantiomeric intermediates and metabolites to be studied in their own right for pharmacological activity will undoubtedly increase. Finally, technologies related to the creation or characterization of enantiomerically pure drugs or their metabolites presumably will grow because of the increased number of compounds entering the drug development pipeline due to combinatorial chemistry.

Flavin-containing monooxygenase mediated metabolism of benzydamine in perfused brain and liver

Benzydamine (BZY) N-oxidation mediated by flavin-containing monooxygenase (FMO) was evaluated in perfused brain and liver. Following 20 min of perfusion with modified Ringer solution, the infusion of BZY into brain or liver led to production of BZY N-oxide. BZY N-oxide, a metabolite of BZY oxidized exclusively by FMO, was mostly recovered in the effluent without undergoing further metabolism or reduction back to the parent substrate. The BZY N-oxide formation rate increased as the infusion concentration of BZY increased both in perfused brain and perfused liver. BZY N-oxidation activities in perfused rat brain and liver were 4.2 nmol/g brain/min and 50 nmol/g liver/min, respectively, although the BZY N-oxidation activity in brain homogenates was one 4000th that in liver homogenates. This is the first study of FMO activity in brain in situ.