Rat liver flavin-containing monooxygenase (FMO): cDNA cloning and expression in yeast

Molecular and functional characterization of flavin-containing monooxygenases (FMO1-6) in tree shrews

Flavin-containing monooxygenases (FMOs) are a family of important drug oxygenation enzymes that, in humans, consist of five functional enzymes (FMO1-5) and a pseudogene (FMO6P). The tree shrew is a non-rodent primate-like species that is used in various biomedical studies, but its usefulness in drug metabolism research has not yet been investigated. In this study, tree shrew FMO1-6 cDNAs were isolated and characterized by sequence analysis, tissue expression, and metabolic function. Compared with human FMOs, tree shrew FMOs showed sequence identities of 85-90 % and 81-89 %, respectively, for cDNA and amino acids. Phylogenetic analysis showed that each tree shrew and human FMO were closely clustered. The genomic and genetic structures of the FMO genes were conserved in tree shrews and humans. Among the five tissue types analyzed (lung, heart, kidney, small intestine, and liver), FMO3 and FMO1 mRNAs were most abundant in liver and kidney, respectively. Recombinant tree shrew FMO1-6 proteins expressed in bacterial membranes all mediated benzydamine and trimethylamine N-oxygenations and methyl p-tolyl sulfide S-oxygenation. The selective human FMO3 substrate trimethylamine was predominantly metabolized by tree shrew FMO3. Additionally, tree shrew FMO6 was active toward trimethylamine, as is cynomolgus macaque FMO6, in contrast with the absence of activity of the human FMO6P pseudogene product. Tree shrew FMO1-6, which are orthologous to human FMOs (FMO1-5 and FMO6P) were identified, and tree shrew FMO3 has functional and molecular features generally comparable to those of human FMO3 as the predominant FMO in liver.

Molecular and functional characterization of flavin-containing monooxygenases in pigs, dogs, and cats

Flavin-containing monooxygenases (FMOs) are drug-oxygenating enzymes that are present in the human genome as FMO1-5 and FMO6P. Among pig, dog, and cat FMOs, pig and dog FMO1 and FMO3 have been partly characterized, but other FMOs have not been systematically identified. In this study, orthologous FMO cDNAs were isolated from pig, dog, and cat livers and evaluated by sequence and phylogenetic analyses, tissue expression, and catalytic function. The amino acid sequences of pig, dog, and cat FMO1-5 shared high sequence identities (83-89%) with human FMO1-5 and were closely clustered in a phylogenetic tree. The gene structure and genomic organization of FMO1-5 were conserved across these species. Dog and pig FMO6P contained insertions of 1 and 83 bases, respectively, and are possibly pseudogenes similar to human FMO6P. Among the tissue types analyzed, pig FMO1 mRNA was abundant in liver, kidney, and lung; dog FMO3, FMO2, and FMO5 mRNAs were abundant in liver, lung, and kidney, respectively; cat FMO1 and FMO3 mRNAs were abundant in kidney and liver, respectively. Recombinant pig and dog FMO1-5 and cat FMO1-6 all mediated benzydamine and trimethylamine N-oxygenations and methyl p-tolyl sulfoxide S-oxygenation. The selective human FMO3 substrate trimethylamine was predominantly metabolized by pig FMO1, dog FMO3, and cat FMO3. Cat FMO6 was also active toward trimethylamine. These results suggest some similarities in the drug-metabolizing capabilities of FMO3 in dogs, cats, and humans and that dog and cat FMO3 generally have molecular and functional characteristics similar to human FMO3, being the major FMO in human liver.

Effective Application of Metabolite Profiling in Drug Design and Discovery

At one time, biotransformation was a descriptive activity in pharmaceutical development, viewed simply as structural elucidation of drug metabolites, completed only once compounds entered clinical development. Herein, we present our strategic approach using structural elucidation to enable chemistry design/SAR development. The approach considers four questions that often present themselves to medicinal chemists optimizing their compounds for candidate selection: (1) What are the important clearance mechanisms that mediate the disposition of my molecule? (2) Can metabolic liabilities be modulated in a favorable way? (3) Does my compound undergo bioactivation to a reactive metabolite? (4) Do any of the metabolites possess activity, either on- or off-target? An additional question necessary to support compound development relates to metabolites in safety testing (MIST) and our approach also addresses this question. The value in structural elucidation is derived from its application to better design molecules, guide their clinical development, and underwrite patient safety.

Cytochrome P450 and flavin-containing monooxygenase enzymes are responsible for differential oxidation of the anti-thyroid-cancer drug vandetanib by human and rat hepatic microsomal systems

We studied the in vitro metabolism of the anti-thyroid-cancer drug vandetanib in a rat animal model and demonstrated that N-desmethylvandetanib and vandetanib N-oxide are formed by NADPH- or NADH-mediated reactions catalyzed by rat hepatic microsomes and pure biotransformation enzymes. In addition to the structural characterization of vandetanib metabolites, individual rat enzymes [cytochrome P450 (CYP) and flavin-containing monooxygenase (FMO)] capable of oxidizing vandetanib were identified. Generation of N-desmethylvandetanib, but not that of vandetanib N-oxide, was attenuated by CYP3A and 2C inhibitors while inhibition of FMO decreased formation of vandetanib N-oxide. These results indicate that liver microsomal CYP2C/3A and FMO1 are major enzymes participating in the formation of N-desmethylvandetanib and vandetanib N-oxide, respectively. Rat recombinant CYP2C11 > >3A1 > 3A2 > 1A1 > 1A2 > 2D1 > 2D2 were effective in catalyzing the formation of N-desmethylvandetanib. Results of the present study explain differences between the CYP- and FMO-catalyzed vandetanib oxidation in rat and human liver reported previously and the enzymatic mechanisms underlying this phenomenon.

Identification of Human Enzymes Oxidizing the Anti-Thyroid-Cancer Drug Vandetanib and Explanation of the High Efficiency of Cytochrome P450 3A4 in its Oxidation

The metabolism of vandetanib, a tyrosine kinase inhibitor used for treatment of symptomatic/progressive medullary thyroid cancer, was studied using human hepatic microsomes, recombinant cytochromes P450 (CYPs) and flavin-containing monooxygenases (FMOs). The role of CYPs and FMOs in the microsomal metabolism of vandetanib to N-desmethylvandetanib and vandetanib-N-oxide was investigated by examining the effects of CYP/FMO inhibitors and by correlating CYP-/FMO-catalytic activities in each microsomal sample with the amounts of N-desmethylvandetanib/vandetanib-N-oxide formed by these samples. CYP3A4/FMO-activities significantly correlated with the formation of N-desmethylvandetanib/ vandetanib-N-oxide. Based on these studies, most of the vandetanib metabolism was attributed to N-desmethylvandetanib/vandetanib-N-oxide to CYP3A4/FMO3. Recombinant CYP3A4 was most efficient to form N-desmethylvandetanib, while FMO1/FMO3 generated N-oxide. Cytochrome b₅ stimulated the CYP3A4-catalyzed formation of N-desmethylvandetanib, which is of great importance because CYP3A4 is not only most efficient in generating N-desmethylvandetanib, but also most significant due to its high expression in human liver. Molecular modeling indicated that binding of more than one molecule of vandetanib into the CYP3A4-active center can be responsible for the high efficiency of CYP3A4 N-demethylating vandetanib. Indeed, the CYP3A4-mediated reaction exhibits kinetics of positive cooperativity and this corresponded to the in silico model, where two vandetanib molecules were found in CYP3A4-active center.

Expression and metabolic activity of flavin-containing monooxygenase 1 in cynomolgus macaque kidney

Flavin-containing monooxygenase 1 (FMO1) largely remains to be characterized in cynomolgus macaque kidney. Immunoblotting showed expression of cynomolgus FMO1 in kidneys where activities of FMO1 (benzydamine N-oxygenation) were detected. No sex differences were observed in their contents or activities. These results suggest the functional role of cynomolgus FMO1 in kidney.

Functional assessment of rat pulmonary flavin-containing monooxygenase activity

The expression of flavin-containing monooxygenase (FMO) varies extensively between human and commonly used preclinical species such as rat and mouse. The aim of this study was to investigate the pulmonary FMO activity in rat using benzydamine. Furthermore, the contribution of rat lung to the clearance of benzydamine was investigated using an in vivo pulmonary extraction model. Benzydamine N-oxygenation was observed in lung microsomes and lung slices. Thermal inactivation of FMO and CYP inhibition suggested that rat pulmonary N-oxygenation is predominantly FMO mediated while any contribution from CYPs is negligible. The predicted lung clearance (CL_lung) estimated from microsomes and slices was 16 ± 0.6 and 2.1 ± 0.3 mL/min/kg, respectively. The results from in vivo pulmonary extraction indicated no pulmonary extraction following intravenous and intra-arterial dosing to rats. Interestingly, the predicted CL_lung using rat lung microsomes corresponded to approximately 35% of rat CL_liver suggesting that the lung makes a smaller contribution to the whole body clearance of benzydamine. Although benzydamine clearance in rat appears to be predominantly mediated by hepatic metabolism, the data suggest that the lung may also make a smaller contribution to its whole body clearance.

Species Differences in the Oxidative Desulfurization of a Thiouracil-Based Irreversible Myeloperoxidase Inactivator by Flavin-Containing Monooxygenase Enzymes

N1-Substituted-6-arylthiouracils, represented by compound 1 [6-(2,4-dimethoxyphenyl)-1-(2-hydroxyethyl)-2-thioxo-2,3-dihydropyrimidin-4(1H)-one], are a novel class of selective irreversible inhibitors of human myeloperoxidase. The present account is a summary of our in vitro studies on the facile oxidative desulfurization in compound 1 to a cyclic ether metabolite M1 [5-(2,4-dimethoxyphenyl)-2,3-dihydro-7H-oxazolo[3,2-a]pyrimidin-7-one] in NADPH-supplemented rats (t1/2 [half-life = mean ± S.D.] = 8.6 ± 0.4 minutes) and dog liver microsomes (t1/2 = 11.2 ± 0.4 minutes), but not in human liver microsomes (t1/2 > 120 minutes). The in vitro metabolic instability also manifested in moderate-to-high plasma clearances of the parent compound in rats and dogs with significant concentrations of M1 detected in circulation. Mild heat deactivation of liver microsomes or coincubation with the flavin-containing monooxygenase (FMO) inhibitor imipramine significantly diminished M1 formation. In contrast, oxidative metabolism of compound 1 to M1 was not inhibited by the pan cytochrome P450 inactivator 1-aminobenzotriazole. Incubations with recombinant FMO isoforms (FMO1, FMO3, and FMO5) revealed that FMO1 principally catalyzed the conversion of compound 1 to M1. FMO1 is not expressed in adult human liver, which rationalizes the species difference in oxidative desulfurization. Oxidation by FMO1 followed Michaelis-Menten kinetics with Michaelis-Menten constant, maximum rate of oxidative desulfurization, and intrinsic clearance values of 209 μM, 20.4 nmol/min/mg protein, and 82.7 μl/min/mg protein, respectively. Addition of excess glutathione essentially eliminated the conversion of compound 1 to M1 in NADPH-supplemented rat and dog liver microsomes, which suggests that the initial FMO1-mediated S-oxygenation of compound 1 yields a sulfenic acid intermediate capable of redox cycling to the parent compound in a glutathione-dependent fashion or undergoing further oxidation to a more electrophilic sulfinic acid species that is trapped intramolecularly by the pendant alcohol motif in compound 1.

In vitro oxidative metabolism of xenobiotics in the lancet fluke (Dicrocoelium dendriticum) and the effects of albendazole and albendazole sulphoxide ex vivo

Dicrocoeliosis, a parasitic infection caused by Dicrocoelium dendriticum (lancet fluke), is often treated by the anthelmintic drug albendazole (ABZ). In the lancet fluke, ABZ metabolism via enzymatic sulphoxidation was found, but no information about ABZ oxidases has been available. The aim of our project was to find out which enzyme of the lancet fluke is responsible for ABZ sulphoxidation, as well as to assay the activities of oxidation enzymes. We also studied whether ex vivo 24-h exposures of flukes to ABZ or its sulphoxide (ABZ.SO) would alter ABZ sulphoxidation rate and the activities of tested enzymes. In subcellular fractions from flukes, marked activities of peroxidase (Px), glutathione Px (GPx), catalase (CAT), superoxide dismutase, and thioredoxin glutathione reductase were found. Using specific inhibitors, the participation of flavine monooxygenases in ABZ-oxidation was found. The ex vivo exposition of flukes to ABZ or ABZ.SO did not change the rate of ABZ sulphoxidation in vitro, but the ex vivo exposure of flukes to anthelmintics increased Px, CAT, and GPx activity. The modulation of these enzyme activities after ABZ or ABZ.SO exposition may be characteristic of the parasite’s protective mechanism against oxidative stress caused by drug treatment.

Alternative promoters and repetitive DNA elements define the species-dependent tissue-specific expression of the FMO1 genes of human and mouse

In humans, expression of the FMO1 (flavin-containing mono-oxygenase 1) gene is silenced postnatally in liver, but not kidney. In adult mouse, however, the gene is active in both tissues. We investigated the basis of this species-dependent tissue-specific transcription of FMO1. Our results indicate the use of three alternative promoters. Transcription of the gene in fetal human and adult mouse liver is exclusively from the P0 promoter, whereas in extra-hepatic tissues of both species, P1 and P2 are active. Reporter gene assays showed that the proximal P0 promoters of human (hFMO1) and mouse (mFmo1) genes are equally effective. However, sequences upstream (-2955 to -506) of the proximal P0 of mFmo1 increased reporter gene activity 3-fold, whereas hFMO1 upstream sequences (-3027 to -541) decreased reporter gene activity by 75%. Replacement of the upstream sequence of human P0 with the upstream sequence of mouse P0 increased activity of the human proximal P0 8-fold. Species-specific repetitive elements are present immediately upstream of the proximal P0 promoters. The human gene contains five LINE (long-interspersed nuclear element)-1-like elements, whereas the mouse gene contains a poly A region, an 80-bp direct repeat, an LTR (long terminal repeat), a SINE (short-interspersed nuclear element) and a poly T tract. The rat and rabbit FMO1 genes, which are expressed in adult liver, lack some (rat) or all (rabbit) of the elements upstream of mouse P0. Thus silencing of FMO1 in adult human liver is due apparently to the presence upstream of the proximal P0 of L1 (LINE-1) elements rather than the absence of retrotransposons similar to those found in the mouse gene.

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.

Hepatic flavin-containing monooxygenase activity attenuated by cGMP-independent nitric oxide-mediated mRNA destabilization

To identify the novel mechanism by which nitric oxide (NO) suppresses flavin-containing monooxygenase (FMO) activity in endotoxemic rat livers, NO-overproducing conditions were induced in primary cultured rat hepatocytes by treatment with a mixture (LCM) of lipopolysaccharide and proinflammatory cytokines (IL-1beta, TNF-alpha, and IFN-gamma), or by the addition of a pure NO donor, spermine-NONOate. mRNA levels of the major hepatic form, FMO1, decreased via a cGMP-independent destabilizing effect of NO rather than by decreased transcription. The decrease in the mRNA levels caused by LCM-induced inducible NO synthase (iNOS) was completely blocked by co-treatment with aminoguanidine, a selective iNOS inhibitor. Furthermore, spermine-NONOate, but not the cGMP analog, 8-bromo-cGMP, dose- and time-dependently attenuated FMO1 mRNA stability in actinomycin-D-pretreated cells, resulting in decreases in protein levels and biochemical activity. These results suggest that NO acts directly in a cGMP-independent mechanism by decreasing the half-life of FMO1 mRNA, thereby inducing impairment of FMO-related functions in endotoxemia.

Flavin-containing monooxygenase activity can be inhibited by nitric oxide-mediated S-nitrosylation

Nitric oxide (NO) modifies the functions of a variety of proteins containing cysteine thiols or transition-metal centers, particularly by S-nitrosylation. In inflamed liver, NO is overproduced and hepatic drug-metabolizing enzymes, the flavin-containing monooxygenases (FMOs) and cytochrome P450s (CYPs), are suppressed. However, the NO-related mechanisms underlying the loss of these activities are not well understood, particularly for FMOs. In this study, we suggest that FMO3, the major FMO in human liver, is modified post-translationally by NO. This hypothesis is based on the imbalance observed between the decrease in FMO3 expression (40.7% of controls) and FMO3-specific ranitidine N-oxidation activity (15.1%), and on the partial or complete reversibility of FMO inhibition by sulfhydryl-reducing regents such as DTT (effective on both S-S and S-NO adducts) and ascorbate (effective on S-NO only). Furthermore, NO donors (SNP, SNAP, and Sin-1), including the pure NO donor DEA/NO, directly suppressed in vitro FMO activity (N- or S-oxidation of ranitidine, trimethylamine, and thiobenzamide) in human liver microsomal proteins and recombinant human FMO3. These activities were restored completely after treatment with DTT or ascorbate. These results suggest that NO-mediated S-nitrosylation is involved in the rigorous inhibition of FMO activity in vitro and in vivo, resulting in the suppression of FMO-based drug metabolism or detoxification.

Cloning, sequencing and tissue distribution of rat flavin-containing monooxygenase 4: two different forms are produced by tissue-specific alternative splicing

The nucleotide sequence of rat flavin-containing monooxygenase 4 (FMO4) mRNA was obtained by reverse transcription-polymerase chain reaction (RT-PCR) and 5'/3' terminal extension. Complete cDNA was amplified, cloned, and sequenced from the mRNA obtained from rat kidney and brain. Two different transcripts (short and long) stemming from the splicing of an internal region of 189 bases pair, corresponding to exon 4 were identified. This alternative splicing seems to be specific of the brain. The long cDNA encodes a protein of 560 amino acids with a predicted molecular mass of 63,395 Da. The short cDNA encodes a protein of 497 amino acids with a predicted molecular mass of 55,871 Da. Both of these encoded sequences contain the NADPH- and FAD-binding sites and a hydrophilic carboxyl terminus. These sequences are 80 and 79% identical to the sequences of human and rabbit FMO4. By Northern blotting and/or RT-PCR, the long-form FMO4 mRNA was detected in the rat kidney, intestine, and liver and the short form particularly in the brain. For the first time, the expression of FMO4 protein was demonstrated. By Western blotting using the two different forms of FMO4 antibodies, a long FMO4 protein was detected in the rat kidney, whereas in the rat brain, only the short form of FMO4 was observed.

Change in the gene expression of hepatic tamoxifen-metabolizing enzymes during the process of tamoxifen-induced hepatocarcinogenesis in female rats

Altered gene expression of the enzymes responsible for tamoxifen metabolism during the process of tamoxifen-induced hepatocarcinogenesis in female Sprague-Dawley rats was examined by the RT-PCR method. Treatment of rats with tamoxifen (20 mg/kg body/day) for 52 weeks, but not the 1 day, 2 or 12 week treatments, resulted in the formation of the liver hyperplastic nodules. The gene expression of CYP3A subfamily enzymes, especially CYP3A1, responsible for not only detoxification (N-demethylation) but also activation (alpha-hydroxylation) of tamoxifen, was increased by the tamoxifen treatments for 2 and 12 weeks, whereas after the 52 week treatment, the expression in the induced nodules returned to the control level. The gene expression of SULT2A subfamily sulfotransferases, especially HSTa, responsible for metabolic activation of alpha-hydroxytamoxifen was decreased to a level <20% of the control in the nodules, although no significant change in the expression was observed in the liver of rats treated with tamoxifen for 1 day, 2 or 12 weeks. On the other hand, the gene expression of CYP3A2 and flavin-containing monooxygenase 1 (FMO1), responsible for the N-demethylation and N-oxidation, respectively, of tamoxifen was increased in a time-dependent fashion up to the 52 week treatment. Although the gene expression of UDP-glucuronosyltransferase(s), which might be responsible for detoxification of tamoxifen, was also increased by the tamoxifen treatment for 2 or 12 weeks, it decreased to the control level in the nodules after the 52 week treatment. The present findings demonstrate that in the early stage of the formation of the liver hyperplastic nodules by tamoxifen, the genes of the enzymes responsible for not only detoxification but also activation of tamoxifen were activated, whereas in the later stage (in the nodules), the genes of the detoxification enzymes, CYP3A2 and FMO1, remained active, but those of the activation enzymes such as CYP3A1 and HSTa were suppressed.

A Mutation in the Flavin-containing Monooxygenase 3 Gene and its Effects on Catalytic Activity for N-oxidation of Trimethylamine In Vitro

To clarify the mutation of the flavin-containing monooxygenase (FMO) 3 gene causing fish-odor syndrome, we analyzed the FMO3 gene of a Thai subject who possibly suffered from fish-odor syndrome. A novel mutation, a single-base substitution from G to A at the position of 265 (G265A), was identified in exon 3. The mutation caused an amino acid substitution from valine to isoleucine at residue 58 (V58I). The mutated FMO3 protein with V58I exhibited the reduced trimethylamine N-oxidase activity when it was expressed in E. coli. The V(max)/K(m) value for the activity of the mutant-type FMO3 was about 5 times lower than that for the wild-type FMO3.

Cloning, sequencing, tissue distribution, and heterologous expression of rat flavin-containing monooxygenase 3

The sequence of rat FMO3 was obtained by RT-PCR and 5'/3' terminal extension. Complete cDNA was amplified, cloned, and sequenced. The cDNA encodes a protein of 531 amino acids which contains the NADPH- and FAD-binding sites and a hydrophobic carboxyl terminus characteristic of FMOs. This sequence is 81, 81, and 91% identical to sequences of human, rabbit, and mouse FMO3, respectively, and 60% identical to rat FMO1. Rat FMO3 was expressed in Escherichia coli. The recombinant protein and the native protein purified from rat liver microsomes migrated with the same mobility (56 kDa) as determined in sodium dodecyl sulfate-polyacrylamide gel electrophoresis and immunoblotting. Recombinant rat FMO3 showed activities of methimazole S-oxidation, and NADPH oxidation associated with the N- or S-oxidation of trimethylamine and thioacetamide, in good concordance with those reported for human FMO3. When probed with rat FMO3 cDNA (bases 201 to 768), a strong signal corresponding to the 2.3-kb FMO3 transcript was detected in RNA samples from rat liver and kidney while a weak signal was observed with lung RNA samples. In contrast, the probe did not hybridize with any RNA from brain, adipose tissue, or muscle.

Restitutive response of Mini rat liver to injury induced by a single oral administration of thioacetamide

Mini rats are a transgenic rat strain carrying antisense gene for rat growth hormone (GH), resulting in retarded growth and a lower blood GH level (136 +/- 42.0 ng/mL) compared with that of age-matched parental strain Wistar rats (329 +/- 337 ng/mL). Mini rats have been used by several investigators as a GH deficiency model. In this work, we gave a single oral administration of thioacetamide (TAA), a hepatotoxicant, to both Mini rats and Wistar rats to ascertain the influence of GH deficiency on liver response to chemically induced injury and subsequent regeneration. TAA administration caused liver injury in both strains, with a greater extent of injury in Mini rats. Proliferation of bile epithelial cells and so-called oval cells was prominent at Day 3 in Mini rats only, and this change correlated well with serum total bilirubin concentrations. Antibody against Ki-67 antigen revealed that cellular proliferation after TAA-induced liver injury was suppressed but prolonged in the Mini rat liver. Although hepatic stellate cells and Kupffer cells/macrophages were more abundant in the livers of TAA-treated Mini rats, the hepatic expression patterns of hepatocyte growth factor and transforming growth factor beta 1 were comparable to those of Wistar rats. Insulin-like growth factor-I gene expression was significantly reduced in the Mini rat liver. Our results imply that a lower GH level may exacerbate chemically induced liver injury, enhance infiltration/proliferation of non-parenchymal cells, suppress regeneration of hepatocytes, and induce proliferation of bile epithelial cells and oval cells when the liver is injured by TAA.

The house musk shrew (Suncus murinus): a unique animal with extremely low level of expression of mRNAs for CYP3A and flavin-containing monooxygenase

Expression of drug-metabolizing enzymes including cytochrome P450 (CYP) and flavin-containing monooxygenase (FMO) in various tissues of Suncus murinus (Suncus) were examined. Northern blot analysis showed that mRNAs hybridizable with cDNAs for rat CYP1A2, human CYP2A6, rat CYP2B1, human CYP2C8, human CYP2D6, rat CYP2E1, human CYP3A4 and rat CYP4A1 were expressed in various tissues from Suncus. The mRNA level of CYP2A in the Suncus lung was very high. Furthermore, it was found that the level of CYP2A mRNA in the Suncus lung was higher compared to the Suncus liver. The expression level of mRNA hybridizable with cDNA for human CYP3A4 was very low. The presence of CYP3A gene in Suncus was proven by the induction of the CYP with dexamethasone. Very low expression levels of mRNAs hybridizable with cDNAs for rat FMO1, rat FMO2, rat FMO3 and rat FMO5 were also seen in Suncus liver. No apparent hybridization band appeared when human FMO4 cDNA was used as a probe. The hepatic expression of mRNAs hybridizable with cDNAs for UDP-glucuronosyltransferase 1*6, aryl sulfotransferase, glutathione S-transferase 1, carboxyesterase and microsomal epoxide hydrolase in the Suncus were observed. These results indicate that the Suncus is a unique animal species in that mRNAs for CYP3A and FMO are expressed at very low levels.

Two novel mutations of the FMO3 gene in a proband with trimethylaminuria

The mammalian flavin-containing monooxygenases catalyze the NADPH-dependent N-oxygenation of nucleophilic nitrogen-, sulfur-, and phosphorus-containing chemicals, drugs, and xenobiotics, including trimethylamine. The FMO3 gene encodes the dominant catalytically active isoform present in human liver. We have identified two missense mutations in the coding region of the gene in a proband with trimethylaminuria (TMA): M66I and R492W. Whereas two mutations (P153L, E305X) accounted for TMA in our eight unrelated previously documented Australian families of British origin, the present report is the first evidence of compound heterozygosity for two rare mutations in a proband with this disorder. This suggests that other rarer alleles, also causing TMA, will be found in the same populations.

Sensitive assay of trimethylamine N-oxide in liver microsomes by headspace gas chromatography with flame thermionic detection

To compare the trimethylamine N-oxygenase activity of liver microsomes from house musk shrew (Suncus murinus) and rat, a sensitive method for the quantitation of trimethylamine (TMA) N-oxide was developed using gas chromatography with flame thermionic detection. The limit of quantification was 0.5 microM and the calibration curve was linear at least up to 5 microM in incubations containing liver microsomal preparations from Suncus. The intra-day RSD values ranged from 10.4 to 12.8 at 0.5 microM and from 3.5 to 6.7 at 5 microM. The inter-day RSD values were 11.6 and 6.5 at 0.5 and 5 microM, respectively. This method provides a sensitive assay for TMA N-oxygenase activity in liver microsomes. Using this method we found that Suncus was capable of N-oxidizing trimethylamine at a very slow rate.

Trimethylaminuria is caused by mutations of the FMO3 gene in a North American cohort

Trimethylaminuria (TMAuria) (McKusick 602079) first described in 1970 is an autosomal recessive condition caused by a partial or total incapacity to catalyze the N-oxygenation of the odorous compound trimethylamine (TMA). The result is a severe body odor and associated psychosocial conditions. This inborn error of metabolism, previously thought to be rare, is now being increasingly detected in severe and milder presentations. Mutations of a phase 1 detoxicating gene, flavin-containing monooxygenase 3 (FMO3), have been shown to cause TMAuria. Herein we describe a cohort of individuals ascertained in North America with severe TMAuria, defined by a reduction of TMA oxidation below 50% of normal with genotype-phenotype correlations. We detected four new FMO3 mutations; two were missense (A52T and R387L), one was nonsense (E314X). The fourth allele is apparently composed of two relatively common polymorphisms (K158-G308) found in the general population. On the basis of this study we conclude that one common mutation and an increasing number of private mutations in individuals of different ethnic origins cause TMAuria in this cohort.

Suppression of flavin-containing monooxygenase by overproduced nitric oxide in rat liver

Effects of excessive nitric oxide (NO) produced in vivo by an i.p. injection of bacterial lipopolysaccharide (LPS) on hepatic microsomal drug oxidation catalyzed by flavin-containing monooxygenase (FMO) were determined. At 6 and 24 h after the LPS injection, liver microsomes were isolated and FMO activities were determined by using FMO substrates like thiobenzamide, trimethylamine, N,N-dimethylaniline, and imipramine. Liver microsomal FMO activities of LPS-treated rats were decreased significantly for all these substrates. Microsomal content of FMO1 (the major form in rat liver) in LPS-treated rats as determined by immunoblotting, was severely decreased as well. In support of this, hepatic content of FMO1 mRNA was decreased by 43.6 to 67.3%. However, the hepatic content of inducible NO synthase (iNOS) mRNA was increased by 2.6- to 5.4-fold and the plasma nitrite/nitrate concentration was increased by about 30-fold in the LPS-treated rats. When this overproduction of NO in the LPS-treated rats was inhibited in vivo by a single or repeat doses of either a general NOS inhibitor N(G)-nitro-L-arginine or a specific iNOS inhibitor aminoguanidine, the FMO1 mRNA levels were not severely depressed (70-85% of the control level). Attendant with the reduction of plasma nitrite/nitrate concentration by single and repeated doses of NOS inhibitors, activity and content of FMO1 in liver microsomes isolated from these NOS inhibitor cotreated rats were restored partially (in single-dose inhibitors) or completely (in repeat doses). In contrast to these NO-mediated in vivo suppressive effects on the mRNA and enzyme contents of FMO1 as well as the FMO activity, the NO generated in vitro from sodium nitroprusside did not inhibit the FMO activities present in microsomes of rat and rabbit liver as well as those present in rabbit kidney and lung. Combined, the excessive NO produced in vivo (caused by the LPS-dependent induction of iNOS) suppresses the FMO1 mRNA and enzyme contents as well as the FMO activities without any direct in vitro effect on the activities of premade FMO enzyme. These findings suggest that NO is an important mediator involved in the suppression of FMO1 activity in vivo. Thus, together with the previously reported suppression on the cytochrome P-450 activities, the overproduced NO in the liver caused by induction of iNOS under conditions of endotoxemia or sepsis suppresses FMO and appears to be responsible for the decreased drug oxidation function observed generally under conditions of systemic bacterial or viral infections.

Stereoselective S-oxidation and reduction of flosequinan in rat

1. The stereoselective S-oxidation and reduction pathways of flosequinan [(+/-)-7-fluoro-1-methyl-3-methylsulphinyl-4-quinolone] in rat were investigated in vitro. 2. Cytosol from both the liver and kidney catalysed the reduction of R(+)-flosequinan (R-FSO) and S(-)-flosequinan (S-FSO) to flosequinan sulphide (FS, 7-fluoro-1-methyl-3-methylthio-4-quinolone). Flosequinan sulphone (FSO2, 7-fluoro-1-methyl-3-methylsulphonyl-4-quinolone) was not reduced to R-FSO or S-FSO. 3. Liver microsomes catalysed four different S-oxidation pathways in the presence of NADPH, namely oxidation of FS to R-FSO and S-FSO and from R-FSO and S-FSO to FSO2. The formation of R-FSO and S-FSO from FS each exhibited a biphasic kinetic pattern, indicating that at least two distinct enzymes were involved. The pathway from FS to R-FSO appeared mainly catalysed by flavin-containing monooxygenases (FMO). 4. S-oxidation of FS to R-FSO was more rapid than that of FS to S-FSO. S-oxidation of FS to either R-FSO or S-FSO in liver microsomes was more rapid than that of either R-FSO or S-FSO to FSO2. 5. Microsomes from both the kidney and lung catalysed the stereoselective S-oxidation of FS to R-FSO, and FMO was likely to have participated in these reactions.

Estimation of flavin-containing monooxygenase activity in intact hepatocyte monolayers of rat, hamster, rabbit, dog and human by using N-oxidation of benzydamine

The flavin-containing monooxygenase (FMO)-dependent N-oxidation of benzydamine has been assessed as a method for monitoring the activity of FMOs in monolayer cultures of hepatocytes from rat, dog, rabbit, hamster and human. The advantage of this substrate is that benzydamine N-oxide formation can be measured directly in extracts of cellular incubations without an intensive work-up procedure. Benzydamine and its N-oxide are readily separated by HPLC with fluorometric detection. This assay proved sensitive enough to monitor FMOs activity in intact monolayer of cultured hepatocytes. The formation of benzydamine N-oxide was inhibited when hepatocytes were coincubated with methimazole (another FMO substrate) in a dose-dependent manner, whereas N-octylamine (an inhibitor of cytochrome P450) had no inhibitory effect. In contrast to cytochrome P450, FMO activity assessed by benzydamine N-oxidation was relatively stable for all species studied during 72-h cultures.

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.

Diastereospecific kinetics of nicotine N'-oxidation in rat liver microsomes

1. In kinetic studies, both Eadie-Hofstee plots for cis- and trans-nicotine-1'-N-oxide formation from nicotine in rat liver microsomes were linear. For the formation of cis- and trans-nicotine-1'-N-oxide, the apparent K(m) were 0.240 +/- 0.069 and 1.524 +/- 0.951 mM respectively. Corresponding Vmax were 1.52 +/- 0.48 and 1.19 +/- 0.74 nmol/mg/min respectively. 2. The formation of cis-nicotine-1'-N-oxide was greater than the formation of trans-nicotine-1'-N-oxide in rat liver microsomes and the intrinsic clearance of cis-nicotine-1'-N-oxide formation was 8.1-fold greater than that of trans-nicotine-1'-N-oxide formation. 3. The formation of both cis- and trans-nicotine-1'-N-oxide in rat liver microsomes was inhibited by the addition of 1-(1-naphthyl)-2-thiourea or by heat-treatment of microsomes. 2-Diethylaminoethyl-2, 2-diphenylvalerate (SKF525A) and carbon monoxide did not affect these activities even at high concentrations. 4. Formations of cis- and trans-nicotine-1'-N-oxide correlated significantly with each other (r = 0.862, p < 0.01). These results suggested that the same flavin-containing monooxygenase (FMO) isoform is responsible for the formation of cis- and trans-nicotine-1'-N-oxide in rat liver.

Determination of FAD-binding domain in flavin-containing monooxygenase 1 (FMO1)

The flavin-containing monooxygenases (FMOs) are a family of flavoenzymes and contain one molecule of FAD per monomer. In order to demonstrate where FMO interacts with FAD, four mutants for the rat liver FMO1 protein were expressed in yeast and characterized. All four mutants were immunochemically similar to the unmodified form, although the contents of FAD in all four mutants were much lower than that in the unmodified form. Interestingly, the mutant generated by changing the first glycine of the proposed FAD-binding domain (GxGxxG) to alanine revealed catalytic activities, but was lower than those seen with the unmodified form. The conversion of the first glycine to alanine markedly increased and decreased the Km and Vmax values for imipramine N-oxidation, respectively. The other three mutants (RFMOm2, RFMOm3, and RFMOm4) were catalytically inactive. Our results suggest that three glycines, especially the second and third glycines, in the proposed FAD-binding domain were necessary for FMO to show catalytic activities. Using RFMOm1 and the unmodified form, the effects of n-octylamine on the activity of FMO1 were investigated. The activities of both wild-type and RFMOm1 enzymes for all of the compounds examined were enhanced by n-octylamine. The Km and Vmax values of both RFMOm1 and the unmodified form for imipramine N-oxidation were lowered and raised by n-octylamine, respectively.

Expression and characterization of a modified flavin-containing monooxygenase 4 from humans

The inability to obtain flavin-containing monooxygenase 4 (FMO4) in heterologous systems has hampered efforts to characterize this isoform of the FMO gene family. Neither the human nor the rabbit ortholog of FMO4, each of which has been cloned and sequenced, has been expressed. Attempts to achieve expression of FMO4 have been made with Escherichia coli, baculovirus, yeast, and COS systems. The cDNAs encoding FMO4 have extended coding regions compared with those encoding other FMO isoforms. The derived amino acid sequences of FMO1, -2, -3, and -5 from all species examined contain about the same number of residues (531-535 residues), whereas the derived sequences of human and rabbit FMO4 contain 558 and 555 residues, respectively. We have investigated whether the elongation of the FMO4 coding region is related to the inability to achieve expression. The cDNA encoding human FMO4 has been modified by a single base change that introduces a stop codon at the consensus position. This modification allows for expression in E. coli. Lack of expression of intact FMO4 is caused by a problem that occurs following transcription, a problem that is overcome completely by relocation of the stop codon 81 bases to 5' of its normal position. Truncated FMO4 is expressed as an active enzyme with characteristics typical of an FMO isoform. Possible functional changes resulting from altering the 3'-end of an FMO were investigated with human FMO3. Elongation of the coding region of the FMO3 cDNA to the next available stop codon (FMO3*) resulted in the expression of an enzyme with properties very similar to those of unmodified FMO3. Elongation of FMO3 lowered the level of expression in E. coli but did not eliminate it. As with FMO4, the difference in expression levels between FMO3 and elongated FMO3 (FMO3*) appears to be related to translation rather than transcription. The functional characteristics of FMO3 and FMO3* are not significantly different.

Xenobiotic biotransforming enzymes in the central nervous system: an isoform of flavin-containing monooxygenase (FMO4) is expressed in rabbit brain

The flavin-containing monooxygenase (FMO, EC 1.14.13.8) is involved in the metabolism of a number of important xenobiotics including many which affect the central nervous system (CNS). Recently, reports in the literature concerning the amount, activity, location, and isozyme characteristics of this enzyme in the brain have presented conflicting evidence. In order to resolve some of the controversy surrounding FMO in the brain, a highly sensitive method for the detection of flavin-containing monooxygenase (FMO) mRNA in whole brain was employed. A poorly conserved region of FMO transcripts was used to design five sets of oligonucleotide primers. Each primer set was specific for one of the five currently known isoforms of FMO. Four and five isoforms, respectively, are expressed in rabbit liver and kidney, as determined by reverse transcription-polymerase chain reaction. However, only one set of primers amplified a specific rabbit brain cDNA fragment. The sequence of the amplification produced affirmed its identity as a segment of FMO4 cDNA. Thus, the FMO of rabbit brain may consist of a single, as yet uncharacterized isozyme and, contrary to several recent reports, is likely to be expressed at low levels.

S-oxidation of (+)-cis-3,5-dimethyl-2-(3-pyridyl)-thiazolidin-4-one hydrochloride by rat hepatic flavin-containing monooxygenase 1 expressed in yeast

1. Rat hepatic flavin-containing monooxygenase 1 (FMO1) expressed in yeast catalyzed the S-oxidation of (+)-cis-3,5-dimethyl-2-(3-pyridyl)thiazolidin-4-one hydrochloride (SM-12502) in vitro. 2. S-oxidation was inhibited by 1-(1-naphthyl)-2-thiourea and thiobenzamide, known inhibitors of FMO, but was not enhanced by n-octylamine, a known enhancer of FMO. 3. The rate of S-oxide formation from SM-12502 was about four-fold lower than that from (+/-)-trans-3,5-dimethyl-2-(3-pyridyl)thiazolidin-4-one hydrochloride (SM-9979) and enantioselectivity and diastereoselectivity of the S-oxidation reaction were observed. 4. The ability of the recombinant yeast to produce the S-oxide from SM-12502 was maintained for long periods and exemplifies the recombinant yeast as a bioreactor to produce a large amount of the S-oxide.

Stereoselective sulfoxidation of the pesticide methiocarb by flavin-containing monooxygenase and cytochrome P450-dependent monooxygenases of rat liver microsomes. Anticholinesterase activity of the two sulfoxide enantiomers

Evidence based on thermal lability and enzyme inhibition data suggests that the sulfoxidation of methiocarb (an N-methylcarbamate insecticide) by rat liver microsomes is catalyzed by flavin-containing monooxygenase(s) (FMO) and by cytochrome(s) P450 (P450). In control rats, the relative proportion is ca. 50% P450:50% FMO. Stereoselective formation of methiocarb sulfoxide from the corresponding sulfide has also been examined to compare the enantioselectivity of the two different enzyme systems. Only the FMO-dependent sulfoxidation presents a high stereoselectivity with an enantiomeric excess of 88% in favor of the (A)-enantiomer. Pretreatment of rats with different P450 inducers such as phenobarbital, 3-methylcholanthrene, dexamethasone, and pyrazole did not affect, or decreased, the rate of methiocarb sulfoxidation. Stereoselectivity of the reaction was modified, mainly because of changes in the relative involvement of FMO and P450 in sulfoxidase activity in pretreated animals. The acetylcholinesterase inhibition properties of methiocarb and its main metabolites were also investigated. Racemic methiocarb sulfoxide was slightly less inhibitory (Ki = 0.216 microM-1.min-1) than methiocarb, but a 10-fold difference was observed between the bimolecular rate constants found for the two sulfoxides produced (0.054 and 0.502 microM-1.min-1 for the (A) and (B) enantiomers, respectively).

N-oxygenation of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine by the rat liver flavin-containing monooxygenase expressed in yeast cells

N-oxygenation of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), a dopaminergic neurotoxin, was studied using recombinant rat liver flavin-containing monooxygenase (FMO), FMO1A1, expressed in yeast cells. The mean (+/- S.D.) kinetic parameters of MPTP N-oxygenation were: Km = 1.8 +/- 0.5 microM, Vmax = 9.5 +/- 1.6 nmol/mg per min, and Vmax/Km = 4.6 +/- 0.5 ml/mg per min. n-Octylamine, an activator of FMO, enhanced the MPTP N-oxygenation activity by 51%, while methimazole, thiobenzamide and alpha-naphthylthiourea, alternate substrates of FMO, inhibited it by 27.4, 68.0 and 59.2%, respectively. The results indicate that MPTP is efficiently N-oxygenated by the recombinant rat liver FMO1A1, and that the responses to the modulators of FMO activity found in the recombinant rat liver FMO1A1 resemble those of mouse and rat liver microsomes as reported previously. The findings suggest that the recombinant FMO expressed in yeast cells is considered as a useful tool to study an involvement of FMO in the metabolism of environmental toxins or chemicals. In addition, FMO1A1 appears to be one of the predominant enzymes responsible for the N-oxygenation of MPTP at least in rat liver.

The molecular biology of the flavin-containing monooxygenases of man

cDNA clones encoding five distinct members of the FMO family of man (FMOs 1, 2, 3, 4 and 5) were isolated by a combination of library screening and reverse transcription-polymerase chain reaction techniques. The deduced amino acid sequences of the human FMOs have 82-87% identity with their known orthologues in other mammal but only 51-57% similarity to each other. The hydropathy profiles of the proteins are very similar. From the calculated rate of evolution of FMOs (a 1% change in sequence per 6 million years) it would appear that individual members of the FMO gene family arose by duplication of a common ancestral gene some 250-300 million years ago. Each of the FMO genes was mapped by the polymerase chain reaction to the long arm of human chromosome 1. The localization of the FMO1 gene was further refined to 1q23-q25 by in situ hybridization of human metaphase chromosomes. RNase protection assays demonstrated that in man each FMO gene displays a distinct developmental and tissue-specific pattern of expression. In the adult, FMO1 is expressed in kidney but not in liver, whereas in the foetus its mRNA is abundant in both organs. FMO3 expression is essentially restricted to the liver in the adult and the mRNA is either absent, or present in low amounts, in foetal tissues. FMO4 is expressed more constitutively. Human FMO1 and FMO3 cDNAs were functionally expressed in prokaryotic and eukaryotic cells. FMO1 and FMO3, expressed in either system, displayed product stereoselectivity in their catalysis of the N-oxidation of the pro-chiral tertiary amines, N-ethyl-N-methylaniline (EMA) and pargyline. Both enzymes were stereoselective with respect to the production of the (-)-S-enantiomer of EMA N-oxide. But in the case of pargyline, the enzymes displayed opposite stereoselectivity, FMO1 producing solely the (+)-enantiomer and FMO3 predominantly the (-)-enantiomer of the N-oxide.

Stereoselective S-oxidation of flosequinan sulfide by rat hepatic flavin-containing monooxygenase 1A1 expressed in yeast

Rat hepatic flavin-containing monooxygenase (FMO) 1A1 expressed in yeast catalysed the S-oxidation of flosequinan sulfide (7-fluoro-1-methyl-3-methylthio-4-quinolone) to R(+)-flosequinan (sulfoxide form, R(+)-7-fluoro-1-methyl-3-methylsulfinyl-4-quinolone) but not to S(-)-flosequinan, and did not catalyse the oxidation of R(+)- and S(-)-flosequinan to flosequinan sulfone. The Km and Vmax for the stereoselective S-oxidation were 33 microM and 6.2 nmol per min per mg of microsomal protein, respectively. The S-oxidation was inhibited by 1-(1-naphthyl)-2-thiourea and thiobenzamide. n-Octylamine activated the S-oxidation with little change in stereoselectivity. The ability of the recombinant yeast to produce R(+)-flosequinan from flosequinan sulfide could be maintained for at least 2 days and exemplifies the value of a recombinant yeast expressing FMO as a stereoselective bioreactor.