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

Biosensing of reactive intermediates produced by the photocatalytic activities of titanium dioxide nanoparticles

The development of an enzyme based biosensing method is described for evaluating the toxicity of solutions treated by titanium dioxide photocatalysis. The method is based on the potential of rat liver microsomal glutathione transferase ability (mGST) to get enhanced in the conditions of chemical and oxidative toxicity. Phenol is taken as a model pollutant due to its toxicity and prevalence in industrial processes. Chemical analysis of the parent compound, products and acute toxicity assays using the mGST activity, were conducted during and after the various photocatalytic treatments. The maximum mGST activity was observed from 60 and 120 min treated samples. This post-treatment toxicity might be due to toxic phenolic products, which may include p-benzoquinone, hydroquinone, benzenetriol and other intermediates. The enzymatic activity pattern observed after photocatalytic treatment corresponded well with the chemical degradation data obtained by HPLC-UV. The mGST assay seems to be an easy to use and promising approach for evaluating the effectiveness of wastewater treatment processes.

Compound lipophilicity as a descriptor to predict binding affinity (1/K(m)) in mammals

In bioaccumulation models, biotransformation is one of the processes decreasing the concentration of chemicals in an organism. In order to be metabolized, a compound needs to bind to an enzyme. In this study, we derived relationships between binding affinity and lipophilicity, expressed as Log (1/K(m)) and Log K(ow), respectively. We focused on oxidations in mammals catalyzed by alcohol dehydrogenase (ADH), aldehyde dehydrogenase (ALDH), flavin-containing monooxygenase (FMO), and cytochrome P450 (CYP) enzymes. For all regressions, 1/K(m) increased with compound K(ow), which can be understood from the tendency to biotransform lipophilic compounds into more polar, thus more easily excretable metabolites. Lipophilicity was relevant to the binding of most of the substrate classes of ADH, ALDH, and CYP. The resulting slopes had 95% Confidence Intervals covering the value of 0.63, typically noted in protein-water distribution (Log K(pw)) and Log K(ow) regressions. A reduced slope (0.2-0.3) was found for FMO: this may be due to a different reaction mechanism involving a nucleophilic attack. The general patterns of metabolism were mechanistically interpreted in terms of partitioning theory. Information on the overall principles determining biotransformation may be helpful in predicting metabolic rates.

Metabolism of xenobiotics of human environments

Xenobiotics have been defined as chemicals to which an organism is exposed that are extrinsic to the normal metabolism of that organism. Without metabolism, many xenobiotics would reach toxic concentrations. Most metabolic activity inside the cell requires energy, cofactors, and enzymes in order to occur. Xenobiotic-metabolizing enzymes can be divided into phase I, phase II, and transporter enzymes. Lipophilic xenobiotics are often first metabolized by phase I enzymes, which function to make xenobiotics more polar and provide sites for conjugation reactions. Phase II enzymes are conjugating enzymes and can directly interact with xenobiotics but more commonly interact with metabolites produced by phase I enzymes. Through both passive and active transport, these more polar metabolites are eliminated. Most xenobiotics are cleared through multiple enzymes and pathways. The relationship between chemical concentrations, enzyme affinity and quantity, and cofactor availability often determine which metabolic reactions dominate in a given individual.

Metabolism and pharmacokinetics of the anti-tuberculosis drug ethionamide in a flavin-containing monooxygenase null mouse

Multiple drug resistance (MDR) in Mycobacterium tuberculosis (mTB), the causative agent for tuberculosis (TB), has led to increased use of second-line drugs, including ethionamide (ETA). ETA is a prodrug bioactivated by mycobacterial and mammalian flavin-containing monooxygenases (FMOs). FMO2 is the major isoform in the lungs of most mammals, including primates. In humans a polymorphism exists in the expression of FMO2. FMO2.2 (truncated, inactive) protein is produced by the common allele, while the ancestral allele, encoding active FMO2.1, has been documented only in individuals of African and Hispanic origin, at an incidence of up to 50% and 7%, respectively. We hypothesized that FMO2 variability in TB-infected individuals would yield differences in concentrations and ratios of ETA prodrug and metabolites. In this study we assessed the impact of the FMO2 genetic polymorphism on the pharmacokinetics of ETA after administration of a single oral dose of ETA (125 mg/kg) to wild type and triple Fmo1/2/4-null mice, measuring levels of prodrug vs. metabolites in plasma collected from 0 to 3.5 h post-gavage. All mice metabolized ETA to ETA S-oxide (ETASO) and 2-ethyl-4-amidopyridine (ETAA). Wild type mice had higher plasma concentrations of metabolites than of parent compound (p = 0.001). In contrast, Fmo1/2/4-null mice had higher plasma concentrations of parent compound than of metabolites (p = 0.0001). Thus, the human FMO2 genotype could impact the therapeutic efficacy and/or toxicity of ETA.

Flavin-containing monooxygenases from Phanerochaete chrysosporium responsible for fungal metabolism of phenolic compounds

We investigated the cellular responses of the white-rot basidiomycete Phanerochaete chrysosporium against vanillin. Based upon a proteomic survey, it was demonstrated that two flavin-containing monooxygenases (PcFMO1 and PcFMO2) are translationally up-regulated in response to exogenous addition of vanillin. To elucidate their catalytic functions, we cloned cDNAs and heterologously expressed them in Escherichia coli. The recombinant PcFMO1 showed catalytic activities against monocyclic phenols such as phenol, hydroquinone, and 4-chlorophenol. In addition, the product from hydroquinone was identified as 1,2,4-trihydroxybenzene, an important intermediate in a metabolic pathway of aromatic compounds in which the aromatic ring of 1,2,4-trihydroxybenzene can be further cleaved by fungal dioxygenases for mineralization. Thus, the ortho-cleavage pathway of phenolic compounds would presumably be associated with PcFMO1.

Two structures of an N-hydroxylating flavoprotein monooxygenase: ornithine hydroxylase from Pseudomonas aeruginosa

The ornithine hydroxylase from Pseudomonas aeruginosa (PvdA) catalyzes the FAD-dependent hydroxylation of the side chain amine of ornithine, which is subsequently formylated to generate the iron-chelating hydroxamates of the siderophore pyoverdin. PvdA belongs to the class B flavoprotein monooxygenases, which catalyze the oxidation of substrates using NADPH as the electron donor and molecular oxygen. Class B enzymes include the well studied flavin-containing monooxygenases and Baeyer-Villiger monooxygenases. The first two structures of a class B N-hydroxylating monooxygenase were determined with FAD in oxidized (1.9 Å resolution) and reduced (3.03 Å resolution) states. PvdA has the two expected Rossmann-like dinucleotide-binding domains for FAD and NADPH and also a substrate-binding domain, with the active site at the interface between the three domains. The structures have NADP(H) and (hydroxy)ornithine bound in a solvent-exposed active site, providing structural evidence for substrate and co-substrate specificity and the inability of PvdA to bind FAD tightly. Structural and biochemical evidence indicates that NADP(+) remains bound throughout the oxidative half-reaction, which is proposed to shelter the flavin intermediates from solvent and thereby prevent uncoupling of NADPH oxidation from hydroxylated product formation.

Thioredoxin mediates oxidation-dependent phosphorylation of CRMP2 and growth cone collapse

Semaphorin3A (Sema3A) is a repulsive guidance molecule for axons, which acts by inducing growth cone collapse through phosphorylation of CRMP2 (collapsin response mediator protein 2). Here, we show a role for CRMP2 oxidation and thioredoxin (TRX) in the regulation of CRMP2 phosphorylation and growth cone collapse. Sema3A stimulation generated hydrogen peroxide (H2O2) through MICAL (molecule interacting with CasL) and oxidized CRMP2, enabling it to form a disulfide-linked homodimer through cysteine-504. Oxidized CRMP2 then formed a transient disulfide-linked complex with TRX, which stimulated CRMP2 phosphorylation by glycogen synthase kinase-3, leading to growth cone collapse. We also reconstituted oxidation-dependent phosphorylation of CRMP2 in vitro, using a limited set of purified proteins. Our results not only clarify the importance of H2O2 and CRMP2 oxidation in Sema3A-induced growth cone collapse but also indicate an unappreciated role for TRX in linking CRMP2 oxidation to phosphorylation.

Structural and functional analysis of bacterial flavin-containing monooxygenase reveals its ping-pong-type reaction mechanism

A bacterial flavin-containing monooxygenase (bFMO) catalyses the oxygenation of indole to produce indigoid compounds. In the reductive half of the indole oxygenation reaction, NADPH acts as a reducing agent, and NADP(+) remains at the active site, protecting bFMO from reoxidation. Here, the crystal structures of bFMO and bFMO in complex with NADP(+), and a mutant bFMO(Y207S), which lacks indole oxygenation activity, with and without indole are reported. The crystal structures revealed overlapping binding sites for NADP(+) and indole, suggestive of a double-displacement reaction mechanism for bFMO. In biochemical assays, indole inhibited NADPH oxidase activity, and NADPH in turn inhibited the binding of indole and decreased indoxyl production. Comparison of the structures of bFMO with and without bound NADP(+) revealed that NADPH induces conformational changes in two active site motifs. One of the motifs contained Arg-229, which participates in interactions with the phosphate group of NADPH and appears be a determinant of the preferential binding of bFMO to NADPH rather than NADH. The second motif contained Tyr-207. The mutant bFMO(Y207S) exhibited very little indoxyl producing activity; however, the NADPH oxidase activity of the mutant was higher than the wild-type enzyme. It suggests a role for Y207, in the protection of hydroperoxyFAD. We describe an indole oxygenation reaction mechanism for bFMO that involves a ping-pong-like interaction of NADPH and indole.

Benzydamine as a useful substrate of hepatic flavin-containing monooxygenase activity in veterinary species

Benzydamine (BZ), a weak base and an indazole derivative with analgesic and antipyretic properties used in human and veterinary medicine, is metabolized in human, rat, cattle and rabbit to a wide range of metabolites. One of the main metabolites, BZ N-oxide (BZ-NO), is produced in the liver and brain by flavin-containing monooxygenases (FMOs), by liver and brain enzymes. To evaluate the suitability of BZ as an FMO probe in veterinary species, BZ metabolism was studied in vitro using liver microsomes from bovine, rabbit and swine. Kinetic parameters, K(m) and V(max), of BZ-NO production, were evaluated to corroborate the pivotal role of FMOs. Inhibition studies were carried out by heat inactivation and by specific FMO chemical inhibitors: trimethylamine and methimazole. The results confirmed the presence of FMO activity in the liver and the role of BZ as a suitable marker of FMO enzyme activities for the veterinary species considered.

Allelic analyses of the Arabidopsis YUC1 locus reveal residues and domains essential for the functions of YUC family of flavin monooxygenases

Flavin monooxygenases (FMOs) play critical roles in plant growth and development by synthesizing auxin and other signaling molecules. However, the structure and function relationship within plant FMOs is not understood. Here we defined the important residues and domains of the Arabidopsis YUC1 FMO, a key enzyme in auxin biosynthesis. We previously showed that simultaneous inactivation of YUC1 and its homologue YUC4 caused severe defects in vascular and floral development. We mutagenized the yuc4 mutant and screened for mutants with phenotypes similar to those of yuc1 yuc4 double mutants. Among the isolated mutants, five of them contained mutations in the YUC1 gene. Interestingly, the mutations identified in the new yuc1 alleles were concentrated in the two GXGXXG motifs that are highly conserved among the plant FMOs. One such motif presumably binds to flavin adenine dinucleotide (FAD) cofactor and the other binds to nicotinamide adenine dinucleotide phosphate (NADPH). We also identified the Ser(139) to Phe conversion in yuc1, a mutation that is located between the two nucleotide-binding sites. By analyzing a series of yuc1 mutants, we identified key residues and motifs essential for the functions of YUC1 FMO.

FAD-dependent enzymes involved in the metabolic oxidation of xenobiotics

Although the majority of oxidative metabolic reactions are mediated by the CYP superfamily of enzymes, non-CYP-mediated oxidative reactions can play an important role in the metabolism of xenobiotics. Among the major oxidative enzymes, other than CYPs, involved in the oxidative metabolism of drugs and other xenobiotics, the flavin-containing monooxygenases (FMOs), the molybdenum hydroxylases [aldehyde oxidase (AO) and xanthine oxidase (XO)] and the FAD-dependent amine oxidases [monoamine oxidases (MAOs) and polyamine oxidases (PAOs)] are discussed in this minireview. In a similar manner to CYPs, these oxidative enzymes can also produce therapeutically active metabolites and reactive/toxic metabolites, modulate the efficacy of therapeutically active drugs or contribute to detoxification. Many of them have been shown to be important in endobiotic metabolism (e.g. XO, MAOs), and, consequently, interactions between drugs and endogenous compounds might occur when they are involved in drug metabolism. In general, most non-CYP oxidative enzymes (e.g. FMOs, MAOs) appear to be noninducible or much less inducible than the CYP system. Some of these oxidative enzymes exhibit polymorphic expression, as do some CYPs (e.g. FMO3). It is possible that the contribution of non-CYP oxidative enzymes to the overall metabolism of xenobiotics is underestimated, as most investigations of drug metabolism have been performed using experimental conditions optimised for CYP activity, although in some cases the involvement of non-CYP oxidative enzymes in xenobiotic metabolism has been inferred from not sufficient experimental evidence.

Joint functions of protein residues and NADP(H) in oxygen activation by flavin-containing monooxygenase

The reactivity of flavoenzymes with dioxygen is at the heart of a number of biochemical reactions with far reaching implications for cell physiology and pathology. Flavin-containing monooxygenases are an attractive model system to study flavin-mediated oxygenation. In these enzymes, the NADP(H) cofactor is essential for stabilizing the flavin intermediate, which activates dioxygen and makes it ready to react with the substrate undergoing oxygenation. Our studies combine site-directed mutagenesis with the usage of NADP(+) analogues to dissect the specific roles of the cofactors and surrounding protein matrix. The highlight of this "double-engineering" approach is that subtle alterations in the hydrogen bonding and stereochemical environment can drastically alter the efficiency and outcome of the reaction with oxygen. This is illustrated by the seemingly marginal replacement of an Asn to Ser in the oxygen-reacting site, which inactivates the enzyme by effectively converting it into an oxidase. These data rationalize the effect of mutations that cause enzyme deficiency in patients affected by the fish odor syndrome. A crucial role of NADP(+) in the oxygenation reaction is to shield the reacting flavin N5 atom by H-bond interactions. A Tyr residue functions as backdoor that stabilizes this crucial binding conformation of the nicotinamide cofactor. A general concept emerging from this analysis is that the two alternative pathways of flavoprotein-oxygen reactivity (oxidation versus monooxygenation) are predicted to have very similar activation barriers. The necessity of fine tuning the hydrogen-bonding, electrostatics, and accessibility of the flavin will represent a challenge for the design and development of oxidases and monoxygenases for biotechnological applications.

Bioactivation of antituberculosis thioamide and thiourea prodrugs by bacterial and mammalian flavin monooxygenases

The thioamide and thiourea class of antituberculosis agents encompasses prodrugs that are oxidatively converted to their active forms by the flavin monooxygenase EtaA of Mycobacterium tuberculosis. Reactive intermediates produced in the EtaA-catalyzed transformations of ethionamide and prothionamide result in NAD(+)/NADH adducts that inhibit the enoyl CoA reductase InhA, the ultimate target of these drugs. In the case of thiacetazone and isoxyl, EtaA produces electrophilic metabolites that mediate the antibacterial activity of these agents. The oxidation of the thioamide/thiourea drugs by the human flavin monooxygenases yields similar reactive metabolites that contribute to the toxicities associated with these second line antituberculosis drugs.

Characterization of human flavin-containing monooxygenase (FMO) 3 and FMO5 expressed as maltose-binding protein fusions

The flavin-containing monooxygenase (FMO) family of enzymes oxygenates nucleophilic xenobiotics and endogenous substances. Human FMO3 and FMO5 are the predominant FMO forms in adult liver. These enzymes are naturally membrane-bound, and recombinant proteins are commercially available as microsomal preparations from insect cells (i.e., Supersome FMO). As an alternative, FMO3 has previously been expressed as a soluble protein, through use of an N-terminal maltose-binding protein (MBP) fusion. In the current study, MBP fusions of both human FMO3 and FMO5 were prepared to >90% purity in the presence of detergent and characterized for biochemical and kinetic parameters, and the parameters were compared with those of Supersome FMO samples. Although MBP-FMO enzymes afforded lower rates of turnover than the corresponding Supersome FMOs, both types of FMO showed identical substrate dependencies and similar responses to changes in assay conditions. Of interest, the FMO3 enzymes showed a 2-fold activation of k(cat)/K(m) in the presence of Triton X-100. Oligomeric analysis of MBP-FMO3 also showed disassociation from a high-order oligomeric form to a monomeric status in the presence of Triton X-100. This report serves as the first direct comparison between Supersome FMOs and the corresponding MBP fusions and the first report of a detergent-based activation of k(cat)/K(m) that corresponds to changes in oligomerization.

Flavin-containing monooxygenase-3: induction by 3-methylcholanthrene and complex regulation by xenobiotic chemicals in hepatoma cells and mouse liver

Flavin-containing monooxygenases often are thought not to be inducible but we recently demonstrated aryl hydrocarbon receptor (AHR)-dependent induction of FMO mRNAs in mouse liver by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) (Celius et al., Drug Metab Dispos 36:2499, 2008). We now evaluated FMO induction by other AHR ligands and xenobiotic chemicals in vivo and in mouse Hepa1c1c7 hepatoma cells (Hepa-1). In mouse liver, 3-methylcholanthrene (3MC) induced FMO3 mRNA 8-fold. In Hepa-1 cells, 3MC and benzo[a]pyrene (BaP) induced FMO3 mRNA >30-fold. Induction by 3MC and BaP was AHR dependent but, surprisingly, the potent AHR agonist, TCDD, did not induce FMO3 mRNA in Hepa-1 cells nor did chromatin immunoprecipitation assays detect recruitment of AHR or ARNT to Fmo3 regulatory elements after exposure to 3MC in liver or in Hepa-1 cells. However, in Hepa-1, 3MC and BaP (but not TCDD) caused recruitment of p53 protein to a p53 response element in the 5'-flanking region of the Fmo3 gene. We tested the possibility that FMO3 induction in Hepa-1 cells might be mediated by Nrf2/anti-oxidant response pathways, but agents known to activate Nrf2 or to induce oxidative stress did not affect FMO3 mRNA levels. The protein synthesis inhibitor, cycloheximide (which causes "superinduction" of CYP1A1 mRNA in TCDD-treated cells), by itself caused dramatic upregulation (>300-fold) of FMO3 mRNA in Hepa-1 suggesting that cycloheximide prevents synthesis of a labile protein that suppresses FMO3 expression. Although FMO3 mRNA is highly induced by 3MC or TCDD in mouse liver and in Hepa-1 cells, FMO protein levels and FMO catalytic function showed only modest elevation.

A common FMO3 polymorphism may amplify the effect of nicotine exposure in sudden infant death syndrome (SIDS)

Smoking during pregnancy has been identified as one of the major modifiable risk factors of sudden infant death syndrome (SIDS). It has been demonstrated that the risk of SIDS increases with increasing cigarette consumption. A variety of hypotheses have been proposed for explanation, including a genetic predisposition. The flavin-monooxygenase 3 (FMO3) is one of the enzymes metabolising nicotine, and several polymorphisms have already been described in this gene. Here, we studied variations in the exons and introns of the FMO3 gene by direct sequencing analysis and minisequencing in 159 SIDS cases and 170 controls. The three common variants G472A (E158K), G769A (V257M) and A923G (E308G) in the exons of the FMO3 gene were identified. The homozygote 472AA genotype occurred more frequently in SIDS cases than in controls (p = 0.0054) and was more frequent in those SIDS cases for which the mothers reported heavy smoking (p = 0.0084). This study is the first to demonstrate a gene-environment interaction in SIDS. The findings suggest that the common polymorphism G472A of FMO3 could act as an additional genetic SIDS risk factor in children whose mothers smoke. Parents who could pass on the 472A allele should be informed of the increased risk associated with smoking. Smoking mothers should be strongly advised to give up smoking during pregnancy and for at least the first year of the child's life.

Modulation of stress genes expression profile by nitric oxide-releasing aspirin in Jurkat T leukemia cells

NO-donating aspirin (NO-ASA, para isomer) has been reported to exhibit strong growth inhibitory effect in Jurkat T-acute lymphoblastic leukemia (T-ALL) cells mediated in part by beta-catenin degradation and caspase activation, but the mechanism(s) still remains unclear. In this study, DNA oligoarrays with 263 genes were used to examine the gene expression profiles relating to stress and drug metabolism, and characterize the stress responses at IC(50) and subIC(50) concentrations of p-NO-ASA (20 and 10microM, respectively) in Jurkat T cells. A total of 22 genes related to heat shock response, apoptosis signaling, detoxifiers and Phase II enzymes, and regulators of cell growth were altered in expression by array analysis based on the expression fold change criteria of > or =1.5-fold or < or =0.65-fold. Real time quantitative RT-PCR confirmed that 20microM p-NO-ASA strongly upregulated the mRNA levels of two heat shock genes HSPA1A (41.5+/-7.01-fold) and HSPA6 (100.4+/-8.11-fold), and FOS (16.2+/-3.2-fold), moderately upregulated HSPH1 (1.71+/-0.43-fold), FMO4 (4.5+/-1.67-fold), CASP9 (1.77+/-0.03-fold), DDIT3 (5.6+/-0.51-fold), and downregulated NF-kappaB1 (0.54+/-0.01-fold) and CCND1 (0.69+/-0.06-fold). Protein levels of Hsp70, the product of HSPA1A, and fos were increased in p-NO-ASA-treated Jurkat T and HT-29 colon cancer cells in a dose-dependent manner. Silencing of Hsp70 enhanced the growth inhibitory effect of p-NO-ASA at low concentrations. The altered gene expression patterns by NO-ASA in Jurkat T cells suggest mechanisms for carcinogen metabolism, anti-proliferative activity and possible chemoprotective activity in T-ALL.

Expression of contactin associated protein-like 2 in a subset of hepatic progenitor cell compartment identified by gene expression profiling in hepatitis B virus-positive cirrhosis

BACKGROUND

Hepatic progenitor cells (HPC), a cell compartment capable of differentiating into hepatocytic and biliary lineages, may give rise to the formation of intermediate hepatobiliary cells (IHBC) or ductular reactions (DR).

AIMS

The aim of this study was to analyse the gene expression profiles of DR in cirrhosis and further investigate novel proteins expressed by HPC and their intermediate progeny.

METHODS

DR in hepatitis B virus (HBV)-positive cirrhotic liver tissues adjacent to hepatocellular carcinoma and interlobular bile ducts (ILBDs) in normal liver tissues were isolated by laser capture microdissection and then subjected to microarray analysis. Differential gene expression patterns were verified by quantitative reverse transcriptase-polymerase chain reaction and immunohistochemistry on serial sections. HPC and their intermediate progeny were recognized by immunostaining with hepatocytic and biliary markers [HepPar1, cytokeratin (CK)7, CK19, neural cell adhesion molecule (NCAM), epithelial cell adhesion molecule (EpCAM)].

RESULTS

A total of 88 genes showed upregulation in DR compared with ILBDs. Gene ontology analyses revealed that these upregulated genes were mostly associated with cell adhesion, immune response and the metabolic process. Contactin associated protein-like 2 (CNTNAP2) was first confirmed to be a novel protein expressed in a subpopulation of DR that was positive for CK7, NCAM or EpCAM. In addition, immunoreactivity for CNTNAP2 was also noted in a subset of isolated CK7-positive HPC as well as some ductular IHBC positive for CK19 and HepPar1 in DR.

CONCLUSION

CNTNAP2 is specifically associated with the emergence of ductular populations and may be identified as a novel protein for defining a subset of HPC and their intermediate progeny in cirrhosis.

Drug interactions

Drugs for allergy are often taken in combination with other drugs, either to treat allergy or other conditions. In common with many pharmaceuticals, most such drugs are subject to metabolism by P450 enzymes and to transmembrane transport. This gives rise to considerable potential for drug-drug interactions, to which must be added consideration of drug-diet interactions. The potential for metabolism-based drug interactions is increasingly being taken into account during drug development, using a variety of in silico and in vitro approaches. Prediction of transporter-based interactions is not as advanced. The clinical importance of a drug interaction will depend upon a number of factors, and it is important to address concerns quantitatively, taking into account the therapeutic index of the compound.

Isoform distinct time-, dose-, and castration-dependent alterations in flavin-containing monooxygenase expression in mouse liver after 2,3,7,8-tetrachlorodibenzo-p-dioxin treatment

Flavin-containing monooxygenase (FMO) expression in male mouse liver is altered after 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) exposure or castration. Because TCDD is slowly eliminated from the body, we examined hepatic Fmo mRNA alterations for up to 32 days following 10 or 64 microg/kg TCDD exposure by oral gavage in male C57BL/6J mice. Fmo2 mRNA was significantly induced at 1, 4, and 8 days whereas Fmo3 mRNA was also induced at 32 days relative to controls. Fmo3 mRNA levels exhibited a dose-dependent increase at 4, 8, and 32 days after exposure; Fmo1, Fmo4, and Fmo5 mRNA did not exhibit clear trends. Because castration alone also increased Fmo2, Fmo3, and Fmo4 mRNA we examined the combined effects of castration and TCDD treatment on FMO expression. A greater than additive effect was observed with Fmo2 and Fmo3 mRNA expression. Fmo2 mRNA exhibited a 3-5-fold increase after castration or 10 microg/kg TCDD exposure by oral gavage, whereas an approximately 20-fold increase was observed between the sham-castrated control and castrated TCDD-treated mice. Similarly, treatment with 10 microg/kg TCDD alone increased Fmo3 mRNA 130- and 180-fold in the sham-castrated and castrated mice compared to their controls respectively, whereas, Fmo3 mRNA increased approximately 1900-fold between the sham control and castrated TCDD-treated mice. An increase in hepatic Fmo3 protein in TCDD-treated mice was observed by immunoblotting and assaying methionine S-oxidase activity. Collectively, these results provide evidence for isoform distinct time-, dose-, and castration-dependent effects of TCDD on FMO expression and suggest cross-talk between TCDD and testosterone signal transduction pathways.

[Individual differences of drug-metabolizing enzymes as determinants for the metabolic fate of chemicals--a study of trimethylamine and flavin-containing monooxygenase 3-]

Individual differences of drug-metabolizing enzymes are important determinants for the metabolic fate of chemicals. This article focuses on polymorphic human flavin-containing monooxygenase 3 (FMO3) and dietary-derived trimethylamine. Malodorous trimethylamine is generally converted to odorless trimethylamine N-oxide by liver microsomal FMO3. Trimethylaminuria is caused by functional disorder of FMO3. In this study mutations of the FMO3 gene were examined in self-reported Japanese trimethylaminuria subjects that showed low FMO3 metabolic capacity in urine tests. Nine novel polymorphisms in the FMO3 gene were discovered in self-reported Japanese volunteers. Functional analyses of recombinant FMO3 proteins suggested that these FMO3 gene mutations were one of the causal factors for decreased FMO3 function resulting in trimethylaminuria. Inter-individual variations of FMO3-mediated microsomal oxygenation activities, levels of FMO3 protein and FMO3 mRNA, and its modification in liver microsomes from Japanese samples were observed. Both genetic polymorphisms in the 5'-upstream of the FMO3 gene and some hormonal changes related to menstruation may be causal factors for inter- and/or intra- individual expression levels of FMO3. To assess the palliative cares, it was found that absorbed levels of trimethylamine in vivo would be possibly controlled by selection of precursor foods like fish containing a variety of trimethylamine amounts. These lines of evidence suggest that individual differences of FMO3 are important determinants for the metabolic fate of dietary-derived trimethylamine.

Flavin-containing monooxygenase 3 polymorphisms in 13 ethnic populations from Europe, East Asia and sub-Saharan Africa: frequency and linkage analysis

Aims: To investigate intra- and inter-ethnic differences in three widespread (E158K, V257M and E308G) and two African-specific (D132H and L360P) flavin-containing monooxygenase 3 (FMO3) polymorphisms. Materials & methods: Allele frequencies were determined by TaqMan® allelic discrimination assay in 2152 healthy volunteers from Europe (Swedes, Italians and Turks), East Asia (Japanese) and sub-Saharan Africa (nine ethnic groups covering eastern, southern and western regions), followed by haplotype and linkage analysis. Results: Significant subpopulation differences (p < 0.001) in allele frequencies were found for E158K, V257M and E308G in Europeans and regional differences (p < 0.01) for D132H among Africans. No carrier of P360 was identified. Cis-linkage between G308 and K158 was confirmed with the compound variant (K158/G308) being found in a high proportion (12.0–38.3%) of non-African subjects, but rarely (1.3%) among Africans. Conclusions: Distribution of functionally relevant FMO3 polymorphisms varies not only between ethnicities but also within. The K158/G308 variant may have potential clinical importance primarily in non-African populations due to its low prevalence in Africa.

Xenobiotic metabolizing enzymes and metabolism of anthelminthics in helminths

Anthelminthics remain the only accessible means in the struggle against helminth parasites, which cause significant morbidity and mortality in man and farm animals. The treatment of helminthic infections has become problematic because of frequent drug resistance of helminth parasites. The development of drug resistance can be facilitated by the action of xenobiotic metabolizing enzymes (XMEs). In all organisms, XMEs serve as an efficient defense against the potential negative action of xenobiotics. The activities of XMEs determine both desired and undesired effects of drugs, and the knowledge of drug metabolism is necessary for safe, effective pharmacotherapy. While human and mammalian XMEs have been intensively studied for many years, XMEs of helminth parasites have undergone relatively little investigation, so far. However, many types of XMEs, including oxidases, reductases, hydrolases, transferases, and transporters, have been described in several helminth species. XMEs of helminth parasites may protect these organisms from the toxic effects of anthelminthics. In case of certain anthelminthics, metabolic deactivation was reported in helminth larvae and/or adults. Moreover, if a helminth is in the repeated contact with an anthelminthic, it defends itself against the chemical stress by the induction of biotransformation enzymes or transporters. This induction can represent an advantageous defense strategy of the parasites and may facilitate the drug-resistance development.

Enantioselective in-line and off-line CE methods for the kinetic study on cimetidine and its chiral metabolites with reference to flavin-containing monooxygenase genetic isoforms

An in-line screening and an off-line chiral CE method were developed to determine the stereoselectivity of flavin-containing monooxygenase (FMO) isoforms using cimetidine (CIM) as a substrate. The S-oxygenation of CIM was investigated using achiral chemical oxidants and (human supersomes) enzymatic metabolism procedures. In the off-line setup, the chiral selector sulfobutylether-beta-CD was chosen to separate the CIM S-oxide (CSO) metabolites. The electrophoretic migration order of CSO was confirmed to be (+) before (-) through the use of single enantiomers obtained by preparative chromatography. For the electrophoretically mediated microanalysis method, the in-line enzymatic reaction was performed in 100 mM phosphate reaction buffer (pH 8.3), whereas 50 mM phosphate buffer with 30 mM chiral selector (pH 2.5) was used as a BGE. During the screening of FMO isoenzymes by the electrophoretically mediated microanalysis method, formation of the new chiral center on the CIM sulfur was found to be stereoselective. FMO1 produces more (-)-CSO-enantiomer, while FMO3 generates mainly (+)-CSO-enantiomer. On the other hand, FMO5 shows no activity. The kinetic constants of FMO1 and FMO3 were measured by the off-line method. A K(m)=4.31 mM for the formation of the (+)-CSO-enantiomer and a K(m)=4.56 mM for the (-)-CSO-enantiomer are reported for the first time for FMO1.

Molecular phylogeny, long-term evolution, and functional divergence of flavin-containing monooxygenases

Flavin-containing monooxygenases (FMOs) metabolize xenobiotic compounds, many of which are clinically important, as well as endogenous substrates as part of a discrete physiological process. The FMO gene family is conserved and ancient with representatives present in all phyla so far examined. However, there is a lack of information regarding the long-term evolution and functional divergence of these proteins. This study represents the first attempt to characterize the long-term evolution followed by the members of this family. Our analysis shows that there is extensive silent divergence at the nucleotide level suggesting that this family has been subject to strong purifying selection at the protein level. Invertebrate FMOs have a polyphyletic origin. The functional divergence of FMOs 1-5 started before the split between amphibians and mammals. The vertebrate FMO5 is more ancestral than other four FMOs. Moreover, the existence of higher levels of codon bias was detected at the N-terminal ends, which can be ascribed to the critical role played by the FAD binding motif in this region. Finally, critical amino acid residues for FMO functional divergence (type I & II) after gene duplication were detected and characterized.

Characterization of sulfoxygenation and structural implications of human flavin-containing monooxygenase isoform 2 (FMO2.1) variants S195L and N413K

Catalytically active human flavin-containing monooxygenase isoform 2 (FMO2.1) is encoded by an allele detected only in individuals of African or Hispanic origin. Genotyping and haplotyping studies indicate that S195L and N413K occasionally occur secondary to the functional FMO2*1 allele encoding reference protein Gln472. Sulfoxygenation under a range of conditions reveals the role these alterations may play in individuals expressing active FMO2 and provides insight into FMO structure. Expressed S195L lost rather than gained activity as pH was increased or when cholate was present. The activity of S195L was mostly eliminated after heating at 45 degrees C for 5 min in the absence of NADPH, but activity was preserved if NADPH was present. By contrast, Gln472 was less sensitive to heat, a response not affected by NADPH. A major consequence of the S195L mutation was a mean 12-fold increase in K(m) for NADPH compared with Gln472. Modeling an S213L substitution, the equivalent site, in the structural model of FMO from the Methylophaga bacterium leads to disruption of interactions with NADP(+). N413K had the same pattern of activity as Gln472 in response to pH, cholate, and magnesium, but product formation was always elevated by comparison. N413K also lost more activity when heated than Gln472; however, NADPH attenuated this loss. The major effects of N413K were increases in velocity and k(cat) compared with Gln472. Although these allelic variants are expected to occur infrequently as mutations to the FMO2*1 allele, they contribute to our overall understanding of mammalian FMO structure and function.

Molecular genetics of nicotine metabolism

The molecular genetics of nicotine metabolism involves multiple polymorphic catalytic enzymes. Variation in metabolic pathways results in nicotine disposition kinetics that differ between individuals and ethnic groups. Twin studies indicate that a large part of this variance is genetic in origin, although environmental influences also contribute. The primary aim of this chapter is to review the current knowledge regarding the genetic variability in the enzymes that metabolize nicotine in humans. The focus is on describing the genetic polymorphisms that exist in cytochromes P450 (CYPs), aldehyde oxidase 1 (AOX1), UDP-glucuronosyltransferases (UGTs), and flavin-containing monooxygenase 3 (FMO3). Genetic studies have demonstrated that polymorphisms in CYP2A6, the primary enzyme responsible for nicotine breakdown, make a sizable contribution to the wide range of nicotine metabolic capacity observed in humans. Thus, special attention will be given to CYP2A6, because slower nicotine metabolism requires less frequent self-administration, and accordingly influences smoking behaviors. In addition, the molecular genetics of nicotine metabolism in nonhuman primates, mice, and rats will be reviewed briefly.

Differential localization of flavin-containing monooxygenase (FMO) isoforms 1, 3, and 4 in rat liver and kidney and evidence for expression of FMO4 in mouse, rat, and human liver and kidney microsomes

Flavin-containing monooxygenases (FMOs) play significant roles in the metabolism of drugs and endogenous or foreign compounds. In this study, the regional distribution of FMO isoforms 1, 3, and 4 was investigated in male Sprague-Dawley rat liver and kidney using immunohistochemistry (IHC). Rabbit polyclonal antibodies to rat FMO1 and FMO4, developed using anti-peptide technology, and commercial anti-human FMO3 antibody were used; specificities of the antibodies were verified using Western blotting, immunoprecipitation, and IHC. In liver, the highest immunoreactivity for FMO1 and FMO3 was detected in the perivenous region, and immunoreactivity decreased in intensity toward the periportal region. In contrast, FMO4 immunoreactivity was detected with the opposite lobular distribution. In the kidney, the highest immunoreactivity for FMO1, -3, and -4 was detected in the distal tubules. FMO1 and FMO4 immunoreactivity was also detected in the proximal tubules with strong staining in the brush borders, whereas less FMO3 immunoreactivity was detected in the proximal tubules. Immunoreactivity for FMO3 and FMO4 was detected in the collecting tubules in the renal medulla and the glomerulus, whereas little FMO1 immunoreactivity was detected in these regions. The FMO1 antibody did not react with human liver or kidney microsomes. However, the FMO4 antibody reacted with male and female mouse and human tissues. These data provided a compelling visual demonstration of the isoform-specific localization patterns of FMO1, -3, and -4 in the rat liver and kidney and the first evidence for expression of FMO4 at the protein level in mouse and human liver and kidney microsomes.

Role of flavin-containing monooxygenase in drug development

This review summarizes some recent observations and information related to the role of the flavin-containing monooxygenase (FMO) in preclinical drug development. Flavin-containing monooxygenase is a complimentary enzyme system to the cytochrome P450 (CYP) family of enzymes and oxygenates several soft, highly polarizable nucleophilic heteroatom-containing chemicals and drugs. The products of FMO-mediated metabolism are generally benign and highly polar, readily excreted materials. There may be some advantages in designing drugs that are metabolized in part by FMO and not exclusively by CYP. In this review, I describe the practical aspects for the participation of FMO in drug and chemical metabolism including: i) the study of FMO using in vitro preparations; ii) some observations about metabolism of drugs and chemicals by FMO in vivo; and iii) the consequences of studying FMO-related metabolism in various small animal models. Some of the preclinical research and development areas related to FMO are not fully mature areas and there are certain gaps in our knowledge. However, I include discussion of these areas to stimulate further work and invite further discussion.

Effect of hyperosmotic conditions on flavin-containing monooxygenase activity, protein and mRNA expression in rat kidney

Flavin-containing monooxigenases (FMOs) are a polymorphic family of drug and pesticide metabolizing enzymes, found in the smooth endoplasmatic reticulum that catalyze the oxidation of soft nucleophilic heteroatom substances to their respective oxides. Previous studies in euryhaline fishes have indicated induction of FMO expression and activity in vivo under hyperosmotic conditions. In this study we evaluated the effect of hypersaline conditions in rat kidney. Male Sprague-Dawley rats were injected intraperitoneal with 3.5M NaCl at a doses ranging from 0.3cm(3)/100g to 0.6cm(3)/100g in two separate treatments. Three hours after injection, FMO activities and FMO1 protein was examined in the first experiment, and the expression of FMO1 mRNA was measured in the second experiment from kidneys after treatment with NaCl. A positive significant correlation was found between FMO1 protein expression and plasma osmolarity (p<0.05, r=0.6193). Methyl-p-tolyl sulfide oxidase showed a statistically significant increase in FMO activity, and a positive correlation was observed between plasma osmolarity and production of FMO1-derived (R)-methyl-p-tolyl sulfoxide (p<0.05, r=0.6736). Expression of FMO1 mRNA was also positively correlated with plasma osmolality (p<0.05, r=0.8428). Similar to studies in fish, these results suggest that expression and activities of FMOs may be influenced by hyperosmotic conditions in the kidney of rats.

The role of biotransformation and bioactivation in toxicity

Biotransformation is essential to convert lipophilic chemicals to water-soluble and readily excretable metabolites. Formally, biotransformation reactions are classified into phase I and phase II reactions. Phase I reactions represent the introduction of functional groups, whereas phase II reactions are conjugations of such functional groups with endogenous, polar products. Biotransformation also plays an essential role in the toxicity of many chemicals due to the metabolic formation of toxic metabolites. These may be classified as stable but toxic products, reactive electrophiles, radicals, and reactive oxygen metabolites. The interaction of toxic products formed by biotransformation reactions with cellular macromolecules initiates the sequences resulting in cellular damage, cell death and toxicity.

Subclade of flavin-monooxygenases involved in aliphatic glucosinolate biosynthesis

Glucosinolates (GSLs) are amino acid-derived secondary metabolites with diverse biological activities dependent on chemical modifications of the side chain. We previously identified the flavin-monooxygenase FMO(GS-OX1) as an enzyme in the biosynthesis of aliphatic GSLs in Arabidopsis (Arabidopsis thaliana) that catalyzes the S-oxygenation of methylthioalkyl to methylsulfinylalkyl GSLs. Here, we report the fine mapping of a quantitative trait locus for the S-oxygenating activity in Arabidopsis. In this region, there are three FMOs that, together with FMO(GS-OX1) and a fifth FMO, form what appears to be a crucifer-specific subclade. We report the identification of these four uncharacterized FMOs, designated FMO(GS-OX2) to FMO(GS-OX5). Biochemical characterization of the recombinant protein combined with the analysis of GSL content in knockout mutants and overexpression lines show that FMO(GS-OX2), FMO(GS-OX3), and FMO(GS-OX4) have broad substrate specificity and catalyze the conversion from methylthioalkyl GSL to the corresponding methylsulfinylalkyl GSL independent of chain length. In contrast, FMO(GS-OX5) shows substrate specificity toward the long-chain 8-methylthiooctyl GSL. Identification of the FMO(GS-OX) subclade will generate better understanding of the evolution of biosynthetic activities and specificities in secondary metabolism and provides an important tool for breeding plants with improved cancer prevention characteristics as provided by the methylsulfinylalkyl GSL.

Environmental sensing and response genes in cnidaria: the chemical defensome in the sea anemone Nematostella vectensis

The starlet sea anemone Nematostella vectensis has been recently established as a new model system for the study of the evolution of developmental processes, as cnidaria occupy a key evolutionary position at the base of the bilateria. Cnidaria play important roles in estuarine and reef communities, but are exposed to many environmental stressors. Here, I describe the genetic components of a "chemical defensome" in the genome of N. vectensis and review cnidarian molecular toxicology. Gene families that defend against chemical stressors and the transcription factors that regulate these genes have been termed a chemical defensome and include the cytochromes P450 and other oxidases, various conjugating enyzymes, the ATP-dependent efflux transporters, oxidative detoxification proteins, as well as various transcription factors. These genes account for about 1% (266/27,200) of the predicted genes in the sea anemone genome, similar to the proportion observed in tunicates and humans, but lower than that observed in sea urchins. While there are comparable numbers of stress-response genes, the stress sensor genes appear to be reduced in N. vectensis relative to many model protostomes and deuterostomes. Cnidarian toxicology is understudied, especially given the important ecological roles of many cnidarian species. New genomic resources should stimulate the study of chemical stress sensing and response mechanisms in cnidaria and allow us to further illuminate the evolution of chemical defense gene networks.

The potentially deleterious functional variant flavin-containing monooxygenase 2*1 is at high frequency throughout sub-Saharan Africa

BACKGROUND

The drug-metabolizing enzyme flavin-containing monooxygenase 2 (FMO2) is the predominant FMO isoform present in the lung of most mammals, including non-human primates. All Europeans and Asians tested have been shown to be homozygous for a non-functional variant, FMO2*2A, which contains a premature stop codon due to a single-nucleotide change in exon 9 (g.23238C>T). The ancestral allele, FMO2*1, encodes a functionally active protein and has been found in African-Americans (26%) and Hispanics (2% to 7%). Possessing this variant increases the risk of pulmonary toxicity when exposed to thioureas, a widely used class of industrial compounds. FMO2 may also be involved in the metabolism of drugs that are used to treat diseases that are prevalent in Africa.

RESULTS AND CONCLUSION

We conducted a survey of g.23238C>T variation across Africa that revealed that the distribution of this SNP is relatively homogeneous across sub-Saharan Africa, with approximately one third of individuals possessing at least one FMO2*1 allele, though in some populations the incidence of these individuals approached 50%. Thus many sub-Saharan Africans may be at substantially increased health risk when encountering thiourea-containing substrates of FMO2. Analysis of HapMap data with the Long-Range Haplotype test found no evidence for positive selection of either 23238C>T allele and maximum-likelihood coalescent analysis indicated that this mutation occurred some 500,000 years before present. This study demonstrates the value of performing genetic surveys in Africa, a continent in which human genetic diversity is thought to be greatest, but where studies of the distribution of this diversity are few.