Genetic polymorphisms of flavin-containing monooxygenase (FMO)

Flavin containing monooxygenase 2 regulates renal tubular cell fibrosis and paracrine secretion via SMURF2 in AKI‑CKD transformation

In the follow‑up of hospitalized patients with acute kidney injury (AKI), it has been observed that 15‑30% of these patients progress to develop chronic kidney disease (CKD). Impaired adaptive repair of the kidneys following AKI is a fundamental pathophysiological mechanism underlying renal fibrosis and the progression to CKD. Deficient repair of proximal tubular epithelial cells is a key factor in the progression from AKI to CKD. However, the molecular mechanisms involved in the regulation of fibrotic factor paracrine secretion by injured tubular cells remain incompletely understood. Transcriptome analysis and an ischemia‑reperfusion injury (IRI) model were used to identify the contribution of flavin‑containing monooxygenase 2 (FMO2) in AKI‑CKD. Lentivirus‑mediated overexpression of FMO2 was performed in mice. Functional experiments were conducted using TGF‑β‑induced tubular cell fibrogenesis and paracrine pro‑fibrotic factor secretion. Expression of FMO2 attenuated kidney injury induced by renal IRI, renal fibrosis, and immune cell infiltration into the kidneys. Overexpression of FMO2 not only effectively blocked TGF secretion in tubular cell fibrogenesis but also inhibited aberrant paracrine activation of pro‑fibrotic factors present in fibroblasts. FMO2 negatively regulated TGF‑β‑mediated SMAD2/3 activation by promoting the expression of SMAD ubiquitination regulatory factor 2 (SMURF2) and its nuclear translocation. During the transition from AKI to CKD, FMO2 modulated tubular cell fibrogenesis and paracrine secretion through SMURF2, thereby affecting the outcome of the disease.

Roles of selected non-P450 human oxidoreductase enzymes in protective and toxic effects of chemicals: review and compilation of reactions

This is an overview of the metabolic reactions of drugs, natural products, physiological compounds, and other (general) chemicals catalyzed by flavin monooxygenase (FMO), monoamine oxidase (MAO), NAD(P)H quinone oxidoreductase (NQO), and molybdenum hydroxylase enzymes (aldehyde oxidase (AOX) and xanthine oxidoreductase (XOR)), including roles as substrates, inducers, and inhibitors of the enzymes. The metabolism and bioactivation of selected examples of each group (i.e., drugs, “general chemicals,” natural products, and physiological compounds) are discussed. We identified a higher fraction of bioactivation reactions for FMO enzymes compared to other enzymes, predominately involving drugs and general chemicals. With MAO enzymes, physiological compounds predominate as substrates, and some products lead to unwanted side effects or illness. AOX and XOR enzymes are molybdenum hydroxylases that catalyze the oxidation of various heteroaromatic rings and aldehydes and the reduction of a number of different functional groups. While neither of these two enzymes contributes substantially to the metabolism of currently marketed drugs, AOX has become a frequently encountered route of metabolism among drug discovery programs in the past 10–15 years. XOR has even less of a role in the metabolism of clinical drugs and preclinical drug candidates than AOX, likely due to narrower substrate specificity.

Adaptive Modelling of Mutated FMO3 Enzyme Could Unveil Unexplored Scenarios Linking Variant Haplotypes to TMAU Phenotypes

Background: Trimethylaminuria (TMAU) is a rare genetic disease characterized by the accumulation of trimethylamine (TMA) and its subsequent excretion trough main body fluids, determining the characteristic fish odour in affected patients. We realized an experimental study to investigate the role of several coding variants in the causative gene FMO3, that were only considered as polymorphic or benign, even if the available literature on them did not functionally explain their ineffectiveness on the encoded enzyme. Methods: Mutational analysis of 26 TMAU patients was realized by Sanger sequencing. Detected variants were, subsequently, deeply statistically and in silico characterized to determine their possible effects on the enzyme activity. To achieve this goal, a docking prediction for TMA/FMO3 and an unbinding pathway study were performed. Finally, a TMAO/TMA urine quantification by 1H-NMR spectroscopy was performed to support modelling results. Results: The FMO3 screening of all patients highlighted the presence of 17 variants distributed in 26 different haplotypes. Both non-sense and missense considered variants might impair the enzymatic kinetics of FMO3, probably reducing the interaction time between the protein catalytic site and TMA, or losing the wild-type binding site. Conclusions: Even if further functional assays will confirm our predictive results, considering the possible role of FMO3 variants with still uncertain effects, might be a relevant step towards the detection of novel scenarios in TMAU etiopathogenesis.

Non-P450 Drug-Metabolizing Enzymes: Contribution to Drug Disposition, Toxicity, and Development

Most clinically used drugs are metabolized in the body via oxidation, reduction, or hydrolysis reactions, which are considered phase I reactions. Cytochrome P450 (P450) enzymes, which primarily catalyze oxidation reactions, contribute to the metabolism of over 50% of clinically used drugs. In the last few decades, the function and regulation of P450s have been extensively studied, whereas the characterization of non-P450 phase I enzymes is still incomplete. Recent studies suggest that approximately 30% of drug metabolism is carried out by non-P450 enzymes. This review summarizes current knowledge of non-P450 phase I enzymes, focusing on their roles in controlling drug efficacy and adverse reactions as an important aspect of drug development. Expected final online publication date for the Annual Review of Pharmacology and Toxicology, Volume 62 is January 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

How Can Drug Metabolism and Transporter Genetics Inform Psychotropic Prescribing?

Many genetic variants in drug metabolizing enzymes and transporters have been shown to be relevant for treating psychiatric disorders. Associations are strong enough to feature on drug labels and for prescribing guidelines based on such data. A range of commercial tests are available; however, there is variability in included genetic variants, methodology, and interpretation. We herein provide relevant background for understanding clinical associations with specific variants, other factors that are relevant to consider when interpreting such data (such as age, gender, drug-drug interactions), and summarize the data relevant to clinical utility of pharmacogenetic testing in psychiatry and the available prescribing guidelines. We also highlight areas for future research focus in this field.

Ancestral-sequence reconstruction unveils the structural basis of function in mammalian FMOs

Flavin-containing monooxygenases (FMOs) are ubiquitous in all domains of life and metabolize a myriad of xenobiotics, including toxins, pesticides and drugs. However, despite their pharmacological importance, structural information remains bereft. To further our understanding behind their biochemistry and diversity, we used ancestral-sequence reconstruction, kinetic and crystallographic techniques to scrutinize three ancient mammalian FMOs: AncFMO2, AncFMO3-6 and AncFMO5. Remarkably, all AncFMOs could be crystallized and were structurally resolved between 2.7- and 3.2-Å resolution. These crystal structures depict the unprecedented topology of mammalian FMOs. Each employs extensive membrane-binding features and intricate substrate-profiling tunnel networks through a conspicuous membrane-adhering insertion. Furthermore, a glutamate-histidine switch is speculated to induce the distinctive Baeyer-Villiger oxidation activity of FMO5. The AncFMOs exhibited catalysis akin to human FMOs and, with sequence identities between 82% and 92%, represent excellent models. Our study demonstrates the power of ancestral-sequence reconstruction as a strategy for the crystallization of proteins.

Flavin Containing Monooxygenases and Metabolism of Xenobiotics

This review summarizes recent information concerning the pharmacological and toxicological significance of the flavin-containing monooxygenases (FMOs). FMOs are a family of microsomal enzymes involving in the oxygenation of certain xenobiotics and drugs containing nucleophilic heteroatoms. The activities of FMOs in drug metabolism and their relationships with diseases are the areas of research requiring further exploration. Future studies on FMOs may provide considerable information about the pathophysiology of diseases and the information related to this enzyme family may be important for drug designs in future.

Regulation of flavin-containing mono-oxygenase (Fmo3) gene expression by steroids in mice and humans

Abstract Flavin-containing mono-oxygenases (FMOs) are a family of microsomal chemical- and drug-metabolizing enzymes. FMO3 is a major FMO form in adult mouse and human liver. FMO3 mutations have been associated with the incidence and severity of trimethylaminuria (TMAU), a metabolic disorder characterized by the inability of the affected individual to metabolize the odorous trimethylamine to its non-odorous N-oxide. In addition to this primary genetic form, there are other forms of TMAU that support the hypothesis that FMO3 activity may be modulated by steroid hormones. To understand the molecular mechanism involved in the regulation of Fmo3 gene expression by steroid hormones, we performed this study in an in vitro cellular system, mouse liver cells, and on the human FMO3 gene. Dexamethasone, 5α-dihydrotestosterone, thyroid hormone, and progesterone had no effect on the accumulation of Fmo3 mRNA. The use of increased concentration of theophylline inhibited estrogen receptor α (ERα)-mediated transcription of Fmo3 mRNA. 17β-Estradiol inhibited Fmo3 mRNA accumulation. The use of ICI 164,384 abolished the inhibitory effect induced by estrogen. Gel-shift analyses showed a binding in the 5' region of the Fmo3 gene. This binding was abrogated by an excess of a cDNA containing an estrogen-responsive element. An estrogen-binding site was also present in the first intron of the human gene, as demonstrated by the gel-shift assay. Supershift experiments confirmed the binding of ERα in both mouse and human samples. Furthermore, chromatin immunoprecipitation assay confirmed the binding of ERα in the promoter region of mouse Fmo3 and in the first intron of the human FMO3 gene. Thus, 17β-estradiol plays a fundamental role in the regulation of Fmo3 gene transcription.

Flavin-containing monooxygenase S-oxygenation of a series of thioureas and thiones

Mammalian flavin-containing monooxygenase (FMO) is active towards many drugs with a heteroatom having the properties of a soft nucleophile. Thiocarbamides and thiones are S-oxygenated to the sulfenic acid which can either react with glutathione and initiate a redox-cycle or be oxygenated a second time to the unstable sulfinic acid. In this study, we utilized LC-MS/MS to demonstrate that the oxygenation by hFMO of the thioureas under test terminated at the sulfenic acid. With thiones, hFMO catalyzed the second reaction and the sulfinic acid rapidly lost sulfite to form the corresponding imidazole. Thioureas are often pulmonary toxicants in mammals and, as previously reported by our laboratory, are excellent substrates for hFMO2. This isoform is expressed at high levels in the lung of most mammals, including non-human primates. Genotyping to date indicates that individuals of African (up to 49%) or Hispanic (2-7%) ancestry have at least one allele for functional hFMO2 in lung, but not Caucasians nor Asians. In this study the major metabolite formed by hFMO2 with thioureas from Allergan, Inc. was the sulfenic acid that reacted with glutathione. The majority of thiones were poor substrates for hFMO3, the major form in adult human liver. However, hFMO1, the major isoform expressed in infant and neonatal liver and adult kidney and intestine, readily S-oxygenated thiones under test, with Kms ranging from 7 to 160 μM and turnover numbers of 30-40 min(-1). The product formed was identified by LC-MS/MS as the imidazole. The activities of the mouse and human FMO1 and FMO3 orthologs were in good agreement with the exception of some thiones for which activity was much greater with hFMO1 than mFMO1.

Effects upon in-vivo nicotine metabolism reveal functional variation in FMO3 associated with cigarette consumption

BACKGROUND

Flavin-containing monooxygenases (FMO) catalyze the metabolism of nucleophilic heteroatom-containing drugs and xenobiotics, including nicotine. Rare mutations in FMO3 are responsible for defective N-oxidation of dietary trimethylamine leading to trimethylaminuria, and common genetic variation in FMO3 has been linked to interindividual variability in metabolic function that may be substrate specific.

METHODS

A genetic model of CYP2A6 function is used as a covariate to reveal functional polymorphism in FMO3 that indirectly influences the ratio of deuterated nicotine metabolized to cotinine following oral administration. The association is tested between FMO3 haplotype and cigarette consumption in a set of nicotine-dependent smokers.

RESULTS

FMO3 haplotype, based on all common coding variants in Europeans, significantly predicts nicotine metabolism and accounts for ∼2% of variance in the apparent percent of nicotine metabolized to cotinine. The metabolic ratio is not associated with FMO2 haplotype or an FMO1 expression quantitative trait locus. Cross-validation demonstrates calculated FMO3 haplotype parameters to be robust and significantly improve the predictive nicotine metabolism model over CYP2A6 genotype alone. Functional classes of FMO3 haplotypes, as determined by their influence on nicotine metabolism to cotinine, are also significantly associated with cigarettes per day in nicotine-dependent European Americans (n=1025, P=0.04), and significantly interact (P=0.016) with CYP2A6 genotype to predict cigarettes per day.

CONCLUSION

These findings suggest that common polymorphisms in FMO3 influence nicotine clearance and that these genetic variants in turn influence cigarette consumption.

Utility of DPX2 cells for predicting CYP3A induction-mediated drug-drug interactions and associated structure-activity relationships

The increase in cytochrome P450 (P450) enzyme activity noted upon exposure to therapeutics can elicit marked drug-drug interactions (DDIs) that may ultimately result in poor clinical outcome or adverse drug effects. As such, in vitro model systems that can rapidly and accurately determine whether potential therapeutics activate the human pregnane X receptor (PXR) and thus induce CYP3A P450 levels are highly sought after tools for drug discovery. To that end, we assessed whether DPX2 cells, a HepG2-derived cell line stably integrated with a PXR expression vector plus a luciferase reporter, could detect agents that not only cause PXR activation/CYP3A induction but also elicit clinical DDIs. All 20 clinical inducers and 9 of 15 clinical noninducers examined activated PXR in DPX2 cells (E(max) > 8-fold), although activation parameters obtained with the noninducers were not predictive of DDI. The relative induction score, calculated by combining PXR activation parameters (EC(50) and E(max)) in DPX2 cells for seven inducers plus four noninducers with their efficacious total plasma concentrations, strongly correlated (R(2) = 0.90) with the magnitude of induction of midazolam clearance. Thus, the DPX cell-based PXR activation system is not only capable of distinguishing potential inducers in a high-throughput manner but can also differentiate among compounds in predicting the magnitude of induction-mediated DDIs, providing a means for structure-activity relationship screening during discovery and development.

Genetic variation in metabolizing enzyme and transporter genes: comprehensive assessment in 3 major East Asian subpopulations with comparison to Caucasians and Africans

The advent of high-throughput technologies has proven valuable in the assessment of genetic differences and their effects on drug activation, metabolism, disposition, and transport. However, most studies to date have focused on a small number of genes or few alleles, some of which are rare and therefore observed infrequently or lacked rigorous ethnic characterization, thus reducing the ability to extrapolate within and among populations. In this study, the authors comprehensively assessed the allele frequencies of 165 variants comprising 27 drug-metabolizing enzyme and transporter (DMET) genes from 2188 participants across 3 major ethnic populations: Caucasians, Africans, and East Asians. This sample size was sufficiently large to demonstrate genetic differences among these major ethnic groups while concomitantly confirming similarities among East Asian subpopulations (Korean, Han Chinese, and Japanese). A comprehensive presentation of allele and genotype frequencies is included in the online supplement, and 3 of the most widely studied cytochrome P450 (CYP) genes, CYP2D6, CYP2C19, and CYP2C9; 2 non-CYP enzymes, NAT1 and TMPT; and 2 transporter genes, SLCO1B1 and SLCO2B1, are presented herein according to ethnic classification.

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.

The Role of Flavin-Containing Monooxygenase (FMO) in the Metabolism of Tamoxifen and Other Tertiary Amines

Tamoxifen is utilized in breast cancer therapy and in chemoprevention. Tamoxifen may enhance risk for other neoplasias, especially endometrial cancer. The risk:benefit depends on the rate of metabolic activation versus detoxication. Cytochrome P450-dependent alpha-hydroxylation, followed by sulfonation, represents a metabolic activation pathway, producing products capable of covalent DNA adduction. In contrast, tamoxifen N-oxygenation represents a detoxication pathway, with the caveat that N-oxides can be reduced back to the parent amines. The N-oxygenation pathway will be the focus for this review. Dr. David Kupfer pioneered studies on cytochrome P450 and flavin-containing monooxygenase (FMO) tamoxifen metabolism. We collaborated with Dr. Kupfer's laboratory and recently determined that the low level of tamoxifen N-oxide production in human liver microsomes may be explained by the kinetics of FMO1 versus FMO3.

Metabolism of the anti-tuberculosis drug ethionamide by mouse and human FMO1, FMO2 and FMO3 and mouse and human lung microsomes

Tuberculosis (TB) results from infection with Mycobacterium tuberculosis and remains endemic throughout the world with one-third of the world's population infected. The prevalence of multi-drug resistant strains necessitates the use of more toxic second-line drugs such as ethionamide (ETA), a pro-drug requiring bioactivation to exert toxicity. M. tuberculosis possesses a flavin monooxygenase (EtaA) that oxygenates ETA first to the sulfoxide and then to 2-ethyl-4-amidopyridine, presumably through a second oxygenation involving sulfinic acid. ETA is also a substrate for mammalian flavin-containing monooxygenases (FMOs). We examined activity of expressed human and mouse FMOs toward ETA, as well as liver and lung microsomes. All FMOs converted ETA to the S-oxide (ETASO), the first step in bioactivation. Compared to M. tuberculosis, the second S-oxygenation to the sulfinic acid is slow. Mouse liver and lung microsomes, as well as human lung microsomes from an individual expressing active FMO, oxygenated ETA in the same manner as expressed FMOs, confirming this reaction functions in the major target organs for therapeutics (lung) and toxicity (liver). Inhibition by thiourea, and lack of inhibition by SKF-525A, confirm ETASO formation is primarily via FMO, particularly in lung. ETASO production was attenuated in a concentration-dependent manner by glutathione. FMO3 in human liver may contribute to the toxicity and/or affect efficacy of ETA administration. Additionally, there may be therapeutic implications of efficacy and toxicity in human lung based on the FMO2 genetic polymorphism, though further studies are needed to confirm that suggestion.

Characterization of mouse flavin-containing monooxygenase transcript levels in lung and liver, and activity of expressed isoforms

The significance of active versus inactive flavin-containing monooxygenase 2 (FMO2) for human drug and xenobiotic metabolism and sensitivity is unknown, but the underlying ethnic polymorphism is well documented. We used quantitative real-time PCR to measure message levels of Fmo1, Fmo2, Fmo3 and Fmo5 in lung and liver from eight strains of 8 week old female mice to determine if a strain could be identified that predominately expressed Fmo2 in lung, recapitulating the human FMO expression profile and being the ideal strain for Fmo2 knockout studies. We also characterized enzyme activity of baculovirus expressed mouse Fmo1, Fmo2 and Fmo3 to identify a substrate or incubation conditions capable of discriminating Fmo2 from Fmo mixtures. Fmo transcript expression patterns were similar for all strains. In lung, 59% of total FMO message was Fmo2, but Fmo1 levels were also high, averaging 34%, whereas Fmo3 and Fmo5 levels were 2 and 5%, respectively. In liver, Fmo1, Fmo2, Fmo3 and Fmo5 contributed 16, 1, 7 and 76% respectively, of detected message. Peak activity varied by isoform and was pH- and substrate-dependent. Fmo3 oxidation of methyl p-tolyl sulfide was negligible at pH 9.5, but Fmo3 oxidation of methimazole was comparable to Fmo1 and Fmo2. Heating microsomes at 50 degrees C for 10min eliminated most Fmo1 and Fmo3 activity, while 94% of Fmo2 activity remained. Measurement of activity in heated and unheated lung and liver microsomes verified relative transcript abundance. Our results show that dual Fmo1/2 knockouts will be required to model the human lung FMO profile.

Involvement of enzymes other than CYPs in the oxidative metabolism 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. The (major) oxidative enzymes, other than CYPs, involved in the metabolism of drugs and other xenobiotics are: the flavin-containing monooxygenases, the molybdenum hydroxylases (aldehyde oxidase and xanthine oxidase), the prostaglandin H synthase, the lipoxygenases, the amine oxidases (monoamine, polyamine, diamine and semicarbazide-sensitive amine oxidases) and the alcohol and aldehyde dehydrogenases. 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, and, consequently, interactions between drugs and endogenous compounds might occur when they are involved in drug metabolism. In general, most non-CYP oxidative enzymes appear to be noninducible or much less inducible than the CYP system, although some of them may be as inducible as some CYPs. Some of these oxidative enzymes exhibit polymorphic expression, as do some CYPs. 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 in discovery and lead optimisation are performed using in vitro test systems optimised for CYP activity.

Increased incidence of FMO1 gene single nucleotide polymorphisms in sporadic amyotrophic lateral sclerosis

Flavin-containing monooxygenases (FMO) represent a gene family involved in the oxidative metabolism of a variety of xenobiotics, pesticides and drugs. A new function for FMO proteins has been recently uncovered: yeast FMO has been demonstrated to take part in maintaining the redox balance, catalysing the oxidation of reduced glutathione (GSH) to glutathione disulfide (GSSG). The GSSG/GSH balance is an important buffering system for reactive oxygen species and its involvement has been documented in ALS and other neurodegenerative disorders. Human FMO genes present different mutations, which may be related to ethnicity, altered metabolic activity and, in some cases, specific diseases. The human FMO1 gene presents 20 single nucleotide polymorphisms (SNPs) located in coding regions, intronic sequences and untranslated regions. The FMO1 gene has also recently been found underexpressed in spinal cord of ALS patients. Using SSCP and direct sequencing, we studied the allelic and genotypic frequency of two 3'UTR SNPs of the FMO1 gene in sporadic ALS patients compared to a healthy control population. We found a significantly higher frequency of these two polymorphisms, exclusive of the female population, in SALS patients compared to controls (p<0.01), suggesting that specific allelic variants of the FMO1 gene might be associated to susceptibility to develop ALS.

Human flavin-containing monooxygenases

This review summarizes recent information concerning the pharmacological and toxicological significance of the human flavin-containing monooxygenase (FMO, EC 1.14.13.8). The human FMO oxygenates nucleophilic heteroatom-containing chemicals and drugs and generally converts them into harmless, polar, readily excreted metabolites. Sometimes, however, FMO bioactivates chemicals into reactive materials that can cause toxicity. Most of the interindividual differences of FMO are due to genetic variability and allelic variation, and splicing variants may contribute to interindividual and interethnic variability observed for FMO-mediated metabolism. In contrast to cytochrome P450 (CYP), FMO is not easily induced nor readily inhibited, and potential adverse drug-drug interactions are minimized for drugs prominently metabolized by FMO. These properties may provide advantages in drug design and discovery, and by incorporating FMO detoxication pathways into drug candidates, more drug-like materials may be forthcoming. Although exhaustive examples are not available, physiological factors can influence FMO function, and this may have implications for the clinical significance of FMO and a role in human disease.

Flavin-containing monooxygenase genetic polymorphism: impact on chemical metabolism and drug development

The flavin-containing monooxygenases (FMOs) metabolize a broad range of therapeutics. Consisting of five gene products in humans (FMO1–5), the different FMO family members exhibit pronounced tissue- and temporal-specific expression patterns. Substantial interindividual differences are also observed, and the inability to modulate with exogenous agents is consistent with an important role for genetic variation. Several rare FMO3 alleles causative for trimethylaminuria have been well characterized. However, the identification and characterization of functional FMO1–5 polymorphisms has been more recent. Although none of these polymorphisms has been associated with an adverse drug reaction, the continued broadening of our therapeutic armamentarium makes such an event likely in the future. Furthermore, at least one example has been reported for a direct association between FMO3 polymorphism and therapeutic efficacy. Thus, it is anticipated that knowledge regarding functionally-relevant FMO genetic variability will become increasingly important for making drug development and patient therapeutic choices.

Some distinctions between flavin-containing and cytochrome P450 monooxygenases

This minireview summarizes information concerning the differences and similarities of the human flavin-containing- (FMO, E.C. 1.14.13.8) and the cytochrome P450-monooxygenases (CYP, E.C. 1.14.14.1). Human FMO oxygenates soft nucleophiles. CYP mainly catalyzes C-H abstraction but also oxidizes nitrogen- and sulfur-containing compounds. Both FMO and CYP generally convert lipophilic compounds into more hydrophilic metabolites. The mechanism by which each monooxygenase operates is quite distinct. Sometimes, CYP or FMO bioactivate chemicals to reactive metabolites but to date, drug toxicity thus far observed in the clinic is mainly the result of CYP-dependent oxidation. Both FMO and CYP possess genetic variability that may contribute to inter-individual variability observed for drug metabolism. In contrast to CYP, FMO is not induced or readily inhibited and potential adverse drug-drug interactions are minimized for drugs prominently metabolized by FMO. These properties may provide advantages in drug design, and by incorporating FMO detoxication pathways into drug candidates, more drug-like materials may emerge.

Haplotype and functional analysis of four flavin-containing monooxygenase isoform 2 (FMO2) polymorphisms in Hispanics

OBJECTIVES

Previous work defined two flavin-containing monooxygenase 2 (FMO2) alleles. The major allele, FMO2*2 (g.23,238C>T), encodes truncated inactive protein (p.X472) whereas the minor allele, FMO2*1, present in African- and Hispanic-American populations, encodes active protein (p.Q472). Recently, four common (27 to 51% incidence) FMO2 single nucleotide polymorphisms (SNPs) were detected in African-Americans (N=50); they encode the following protein variants: p.71Ddup, p.V113fs, p.S195L and p.N413 K. Our objectives were to: (1) determine the incidence of these SNPs in 29 Hispanic individuals previously genotyped as g.23,238C (p.Q472) and 124 previously genotyped as homozygous g.23,238 T (p.X472); (2) determine FMO2 haplotypes in this population; and (3) assess the functional impact of SNPs in expressed proteins.

METHODS

SNPs were detected via allele-specific oligonucleotide amplification coupled with real-time or electrophoretic product detection, or single strand conformation polymorphism.

RESULTS

The g.7,700_7,702dupGAC SNP (p.71Ddup) was absent. The remaining SNPs were present but, except for g.13,732C>T (p.S195L), were less common in the current Hispanic study population versus the previously described African-Americans. Only expressed p.N413 K was as active as p.Q472, as determined by methimazole- and ethylenethiourea-dependent oxidation. Haplotype determination demonstrated that the g.10,951delG (p.V113fs), g.13,732C>T (p.S195L) and g.22,060T>G (p.N413 K) variants segregated with g.23,238C>T (p.X472).

CONCLUSIONS

SNPs would not alter FMO2 activity in individuals possessing at least one FMO2*1 allele. It is likely that these SNPs will segregate similarly in African-American populations. Therefore, estimates that 26% of African-Americans and 2-7% of Hispanic-Americans have at least one FMO2*1 allele should closely reflect the percentages producing active FMO2 protein.

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.

Genetic Polymorphisms of Human Flavin Monooxygenase 3 in Sulindac-Mediated Primary Chemoprevention of Familial Adenomatous Polyposis

PURPOSE

Sulindac is a nonsteroidal anti-inflammatory drug (NSAID) effective in regressing adenomas in patients with familial adenomatous polyposis (FAP). However, a recent randomized trial showed that sulindac, when compared with placebo, failed to prevent the development of adenomatous polyps in genotypically positive but phenotypically negative FAP patients. The present study determined whether polymorphisms in the gene encoding flavin monooxygenase 3 (FMO3), a hepatic microsomal enzyme that inactivates sulindac, played a role in determining the efficacy of sulindac in preventing polyposis in this cohort of FAP patients.

EXPERIMENTAL DESIGN

Genotyping was performed on seven established FMO3 polymorphisms previously shown to have functional relevance-M66I, P153L, E158K, V257M, E305X, E308G, and R492W-in 21 and 20 FAP patients, who received sulindac and placebo, respectively.

RESULTS

None of the 41 patients exhibited heterozygous or homozygous M66I and R492W variant alleles, or homozygous P153L, V257M, and E305X variant alleles. Among sulindac-treated patients who did not develop adenomas ("responders"), 4 (33%) were homozygous for E158K and 2 (17%) were homozygous for E308G variant alleles. In contrast, none of the patients on sulindac who developed adenomas ("nonresponders") exhibited homozygosity for either of the two variant alleles. In addition, polymorphisms in the E158K or E308G allele were associated with a significant reduction in mucosal prostanoid levels in patients treated with sulindac.

CONCLUSIONS

Polymorphisms in FMO3, particularly at the E158K and E308G loci, may reduce activity in catabolizing sulindac and result in an increased efficacy to prevent polyposis in FAP.

Flavin-containing monooxygenase activity in camel tissues: comparison with rat and human liver enzymes

We previously reported the occurrence of multiple forms of drug metabolizing enzymes in camel tissues. In this study, we demonstrated for the first time, flavin-containing monooxygenase (FMO)-dependent metabolism of two model substrates methimazole (MEM) and N,N'-dimethylaniline (DMA) by camel liver, kidney, brain and intestine. FMO-catalyzed metabolism in the microsomes of camel tissues was independent of cytochrome P450 (CYP) activity and exhibited a pH and temperature dependence characteristic of FMO enzymes. Use of inhibitors of CYP activities, SKF525A, octylamine or antibody against NADPH-P450 reductase, did not significantly alter the FMO-dependent substrate metabolism. Using MEM as a model substrate for FMO activity, we show that camel liver has an activity similar to that in rat and human livers. MEM metabolism in extrahepatic tissues in camels was significantly lower (60%-80%) than that in liver. Our results suggest occurrence of FMO in camel tissues, with catalytic properties similar to those in rat and human livers. These results may help in better understanding the effects of pharmacologically and toxicologically active compounds administered to camels.

Differences in FMO2*1 allelic frequency between Hispanics of Puerto Rican and Mexican descent

A polymorphism for the phase I drug-metabolizing enzyme, flavin-containing monooxygenase isoform 2 (FMO2), encoding either truncated inactive protein, FMO2X472 (FMO2.2A), or full-length active enzyme, FMO2Q472 (FMO2.1), is known and exhibits significant interethnic differences in allelic frequency. FMO2 is the major or sole FMO isoform expressed in the lung of most mammals, including nonhuman primates. To date, FMO2.1 has been found only in African-American and Hispanic populations, rendering individuals with this allele subject to drug metabolism that is potentially different from that of the general population. Approximately 26% of African-Americans (n = 180) possess the FMO2*1 allele. In preliminary studies, we initially estimated that 5% of Hispanics (n = 40) have the FMO2*1 allele, but access to large cohorts of individuals of defined national origin has allowed us to determine the occurrence among Mexican-American and Puerto Rican-American groups. We used allele-specific genotyping to detect FMO2*1 from 632 Hispanic individuals, including 280 individuals of Mexican origin and 327 individuals of Puerto Rican origin. Statistical analysis indicated that results from Mexican (five sample sources) and Puerto Rican (three sample sources) samples were consistent with the hypothesis of homogeneity within each group from different sources. Data were subsequently pooled across sources to test for evidence of a difference in occurrence of FMO2*1 between ethnic groups. There was strong evidence (p = 0.0066) that FMO2*1 is more common among Puerto Ricans (7%) than among individuals of Mexican descent (2%). The overall occurrence of FMO2*1 among Hispanics of all origins is estimated to be between 2 and 7%.

S-Oxygenation of the thioether organophosphate insecticides phorate and disulfoton by human lung flavin-containing monooxygenase 2

Phorate and disulfoton are organophosphate insecticides containing three oxidizable sulfurs, including a thioether. Previous studies have shown that only the thioether is oxygenated by flavin-containing monooxygenase (FMO) and the sole product is the sulfoxide with no oxygenation to the sulfone. The major FMO in lung of most mammals, including non-human primates, is FMO2. The FMO2*2 allele, found in all Caucasians and Asians genotyped to date, codes for a truncated, non-functional, protein (FMO2.2A). Twenty-six percent of individuals of African descent and 5% of Hispanics have the FMO2*1 allele, coding for full-length, functional protein (FMO2.1). We have here demonstrated that the thioether-containing organophosphate insecticides, phorate and disulfoton, are substrates for expressed human FMO2.1 with Km of 57 and 32 microM, respectively. LC/MS confirmed the addition of oxygen and formation of a single polar metabolite for each chemical. MS/MS analysis confirmed the metabolites to be the respective sulfoxides. Co-incubations with glutathione did not reduce yield, suggesting they are not highly electrophilic. As the sulfoxide of phorate is a markedly less effective acetylcholinesterase inhibitor than the cytochrome P450 metabolites (oxon, oxon sulfoxide or oxon sulfone), humans possessing the FMO2*1 allele may be more resistant to organophosphate-mediated toxicity when pulmonary metabolism is an important route of exposure or disposition.