The implications of polymorphisms in mammalian flavin-containing monooxygenases in drug discovery and development

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.

Effect of 5-fluorouracil on mRNA expression of drug metabolizing enzyme and transporter genes in human hepatoma cell lines

Fluoropyrimidines such as 5-fluorouracil (5-FU) are well known to have drug-drug interactions with anticoagulant medications such as warfarin. This study investigated the mRNA expression of pharmacokinetic (PK)-related genes in response to 5-FU using the hepatocarcinoma cell lines after examining relevant gene expression via RNA sequencing. We used HepaRG cells for 5-FU treatment analysis because these cells displayed PK-related gene expression. 5-FU exposure significantly reduced cytochrome P450 3A4 (CYP3A4) mRNA expression. Additionally, the mRNA expression of nuclear receptor subfamily 1 group I member 2 (also known as pregnane X receptor), a nuclear receptor transcription factor that promotes the expression of many CYP genes, was also decreased in HepaRG cells following 5-FU treatment. The mRNA expressions of the CYP2B6 and ATP-binding cassette transporter genes were decreased after 5-FU treatment. This study revealed that 5-FU treatment reduced PK-related gene expression in HepaRG cells. These findings should be useful for further drug-drug interaction research.

N- and S-oxygenation activity of truncated human flavin-containing monooxygenase 3 and its common polymorphic variants

Human flavin-containing monooxygenase 3 (FMO3) is a membrane-bound, phase I drug metabolizing enzyme. It is highly polymorphic with some of its variants demonstrating differences in rates of turnover of its substrates: xenobiotics including drugs as well as dietary compounds. In order to measure its in vitro activity and compare any differences between the wild type enzyme and its polymorphic variants, we undertook a systematic study using different engineered proteins, heterologously expressed in bacteria, purified and catalytically characterized with 3 different substrates. These included the full-length as well as the more soluble C-terminal truncated versions of the common polymorphic variants (E158K, V257M and E308G) of FMO3 in addition to the full-length and truncated wild-type proteins. In vitro activity assays were performed with benzydamine, tamoxifen and sulindac sulfide, whose products were measured by HPLC. Differences in catalytic properties between the wild-type FMO3 and its common polymorphic variants were similar to those observed with the truncated, more soluble versions of the enzymes. Interestingly, the truncated enzymes were better catalysts than the full-length proteins. The data obtained point to the feasibility of using the more soluble forms of this enzyme for in vitro drug assays as well as future biotechnological applications possibly in high throughput systems such as bioelectrochemical platforms and biosensors.

Ligand stabilization and effect on unfolding by polymorphism in human flavin-containing monooxygenase 3

Pharmacogenomics is a powerful tool to prevent adverse reactions caused by different response of individuals to drug administration. Single nucleotide polymorphisms (SNPs) represent up to 90% of genetic variations among individuals. Drug metabolizing enzymes are highly polymorphic therefore the kinetic parameters of their catalytic reactions can be significantly influenced. This work reports on the unfolding process of a phase I drug metabolizing enzyme, human flavin-containing monooxygenase 3 (hFMO3) and its single nucleotide polymorphic variants (SNPs) V257M, E158K and E308G. Differential scanning calorimetry (DSC) indicates that the thermal denaturation of the enzyme is irreversible. The melting temperature (T_m) for the (Wild Type) WT and its polymorphic variants is found to be in a range from 46 °C to 50 °C. Also the activation energies of unfolding (E_a) show no significant differences among all proteins investigated (290-328 KJ/mol), except for the E308G variant that showed a significantly higher E_a of 412 KJ/mol. The presence of the bound NADP⁺ cofactor is found to stabilize all the variants by shifting the main T_m by 4-5 °C for all the proteins, exception made for E308G where no changes are observed. Isothermal titration calorimetry (ITC) was used to characterize the interaction of the protein with NADP⁺ in terms of dissociation constant (K_d), enthalpy (ΔH) and entropy (ΔS). K_d values of 1.6 and 0.7 μM, ΔH of -13.9 Kcal/mol and -16.8 Kcal/mol, ΔS of -20.5 cal/mol/deg, and -28.5 cal/mol/deg were found for V257M and E158K respectively. E308G was found to be unable to bind the NADP⁺ cofactor, a result that is in line with the T_m results. Circular dichroism also confirmed an overall lower stability of E308G, while NADP⁺ was found to give a strong positive shift of the T_m stabilizing the structure of E158K (46.2 to 50.6 °C). Previous data highlighted significant differences in terms of activity among the SNPs of hFMO3. In this work a minor impact of the SNPs was found on the stability of the enzyme in the ligand free form, except for E308G, whereas the binding of NADP⁺ reveals major differences among WT and polymorphic variants that are all measurable in terms of heat capacity, enthalpy and secondary structure content. These data provide the first direct evidence of ligand stabilization effects on hFMO3 that can explain the differences observed in catalytic efficiencies and serve as the starting point for the development of inhibitors of this enzyme.

Electrochemical Biosensors Based on Membrane-Bound Enzymes in Biomimetic Configurations

In nature, many enzymes are attached or inserted into the cell membrane, having hydrophobic subunits or lipid chains for this purpose. Their reconstitution on electrodes maintaining their natural structural characteristics allows for optimizing their electrocatalytic properties and stability. Different biomimetic strategies have been developed for modifying electrodes surfaces to accommodate membrane-bound enzymes, including the formation of self-assembled monolayers of hydrophobic compounds, lipid bilayers, or liposomes deposition. An overview of the different strategies used for the formation of biomimetic membranes, the reconstitution of membrane enzymes on electrodes, and their applications as biosensors is presented.

N-Oxygenation of Oxycodone and Retro-reduction of Oxycodone N-Oxide

Oxycodone is used as a potent analgesic medication. Oxycodone is extensively metabolized. To fully describe its metabolism, the oxygenation of oxycodone to oxycodone N-oxide was investigated in hepatic preparations. The hypothesis tested was that oxycodone N-oxygenation was enzymatic and the amount of N-oxide detected was a consequence of both oxygenation and retro-reduction. Methods for testing the hypothesis included both in vitro and in vivo studies. Results indicated that oxycodone was N-oxygenated by the flavin-containing monooxygenase. Oxycodone N-oxide is chemically quite stable but in the presence of hepatic preparations and NADPH was retro-reduced to its parent compound oxycodone. Subsequently, oxycodone was metabolized to other metabolites including noroxycodone, noroxymorphone, and oxymorphone via cytochrome P-450. Retro-reduction of oxycodone N-oxide to oxycodone was facilitated by quinone reductase, aldehyde oxidase, and hemoglobin but not to a great extent by cytochrome P-450 or the flavin-containing monooxygenase. To confirm the in vitro observations, oxycodone was administered to rats and humans. In good agreement with in vitro results, substantial oxycodone N-oxide was observed in urine after oxycodone administration to rats and humans. Administration of oxycodone N-oxide to rats showed substantial amount of recovered oxycodone N-oxide. In vivo, noroxycodone was formed as a major rat urinary metabolite from oxycodone N-oxide presumably after retro-reduction to oxycodone and oxidative N-demethylation. To a lesser extent, oxycodone, noroxymorphone, and oxymorphone were observed as urinary metabolites. SIGNIFICANCE STATEMENT: This manuscript describes the N-oxygenation of oxycodone in vitro as well as in small animals and humans. A new metabolite was quantified as oxycodone N-oxide. Oxycodone N-oxide undergoes extensive retro-reduction to oxycodone. This re-establishes the metabolic profile of oxycodone and introduces new concepts about a metabolic futile cycle related to oxycodone metabolism.

Endovascular Closure Resolves Trimethylaminuria Caused by Congenital Portosystemic Shunts

This study aimed to report three new cases of an association between two rare conditions, congenital portosystemic shunts (CPSS) and trimethylaminuria (TMAU), and the efficacy of endovascular closure of the CPSS for resolving TMAU. Between November 2014 and April 2017, 15 patients with CPSS were enrolled in this prospective study to assess the efficacy of percutaneous endovascular shunt closure. Three patients presented with clinical symptoms of TMAU that were confirmed by urine analysis of trimethylamine (TMA) and TMA n-oxide. One year after endovascular closure of the congenital portosystemic shunt, the same parameters were evaluated were obtained and the values were compared to the pretreatment values. The results indicated the disappearance of clinical symptoms of TMAU and normalization of the urine test parameters in two patients and no changes in one patient, who developed new portosystemic communications.

Fishy Odor and TMA Content Levels in Duck Egg Yolks

The differences between the trimethylamine (TMA) content levels in duck and chicken egg yolks under normal dietary conditions were compared. Moreover, the association between the polymorphisms of the duck FMO3 gene and TMA content levels in duck egg yolks was analyzed. Then, to detect the mutations associated with the fish-flavor trait, duck populations were selected for a high-choline diet experiment, which was followed by full-length sequencing of the FMO3 exons. The results showed that the TMA content levels in duck eggs (3.60 μg/g) were significantly higher than those in chicken eggs (2.35 μg/g) under normal dietary conditions (P < 0.01). With regard to the high-choline diet, the average TMA content levels in duck egg yolks (9.21 μg/g; P < 0.01) increased significantly. Furthermore, 5 SNPs reported in Ensembl database were detected in duck FMO3 exons. However, no mutation loci were found to be significantly associated with the TMA content levels in duck egg yolks. Besides, duck liver FMO3 mRNA expression levels were not associated with the TMA content levels. The results indicated that excessive TMA deposition in duck eggs is one of main factors causing the fishy odor in duck eggs, and the addition of choline in the ducks' diets was responsible for inducing an increase in the TMA content levels in duck eggs.

PRACTICAL APPLICATION

Our study can help to diminish the fishy taste in duck eggs by reducing the amount of supplemented choline. Furthermore, this study laid a solid foundation for revealing the genetic factors involved in the fishy odor in duck eggs.

Determinants of the Magnitude of Interaction Between Tacrolimus and Voriconazole/Posaconazole in Solid Organ Recipients

Administration of azole antifungals to tacrolimus-treated solid organ recipients results in a major drug-drug interaction characterized by increased exposure to tacrolimus. The magnitude of this interaction is highly variable but cannot currently be predicted. We performed a retrospective analysis of 126 solid organ recipients (95 lung, 31 kidney) co-treated with tacrolimus and voriconazole (n = 100) or posaconazole (n = 26). Predictors of the change in tacrolimus dose-corrected trough concentrations (C/D) between baseline and tacrolimus-azole co-therapy were assessed using linear mixed modeling. Patients were genotyped for relevant polymorphisms in CYP3A4, CYP3A5, MDR1, CYP2C19, POR, and UGT1A4. Tacrolimus C/D increased by a factor 5.0 ± 2.7 (range 1.0-20.2) for voriconazole and 4.4 ± 2.6 (range 0.9-18.0) for posaconazole, suggesting that a 66% dose reduction is insufficient for the majority of patients. Change in C/D was blunted in CYP3A5 expressors (estimated effect: -43%, p = 0.017) and affected by hematocrit (+8% per %, p = 0.004), baseline C/D (-14% per 100% increase, p < 0.001), and age (+1%, p = 0.008). However, the final model explained only 22% of interindividual variability in C/D change. In conclusion, CYP3A5 genotype and several clinical variables were identified as modulators of the tacrolimus-azole interaction, but these did not permit accurate predictions in individual patients.

Predicting the Metabolic Sites by Flavin-Containing Monooxygenase on Drug Molecules Using SVM Classification on Computed Quantum Mechanics and Circular Fingerprints Molecular Descriptors

As an important enzyme in Phase I drug metabolism, the flavin-containing monooxygenase (FMO) also metabolizes some xenobiotics with soft nucleophiles. The site of metabolism (SOM) on a molecule is the site where the metabolic reaction is exerted by an enzyme. Accurate prediction of SOMs on drug molecules will assist the search for drug leads during the optimization process. Here, some quantum mechanics features such as the condensed Fukui function and attributes from circular fingerprints (called Molprint2D) are computed and classified using the support vector machine (SVM) for predicting some potential SOMs on a series of drugs that can be metabolized by FMO enzymes. The condensed Fukui function fA- representing the nucleophilicity of central atom A and the attributes from circular fingerprints accounting the influence of neighbors on the central atom. The total number of FMO substrates and non-substrates collected in the study is 85 and they are equally divided into the training and test sets with each carrying roughly the same number of potential SOMs. However, only N-oxidation and S-oxidation features were considered in the prediction since the available C-oxidation data was scarce. In the training process, the LibSVM package of WEKA package and the option of 10-fold cross validation are employed. The prediction performance on the test set evaluated by accuracy, Matthews correlation coefficient and area under ROC curve computed are 0.829, 0.659, and 0.877 respectively. This work reveals that the SVM model built can accurately predict the potential SOMs for drug molecules that are metabolizable by the FMO enzymes.

Drug metabolism by flavin-containing monooxygenases of human and mouse

INTRODUCTION

Flavin-containing monooxygenases (FMOs) play an important role in drug metabolism. Areas covered: We focus on the role of FMOs in the metabolism of drugs in human and mouse. We describe FMO genes and proteins of human and mouse; the catalytic mechanism of FMOs and their significance for drug metabolism; differences between FMOs and CYPs; factors contributing to potential underestimation of the contribution of FMOs to drug metabolism; the developmental and tissue-specific expression of FMO genes and differences between human and mouse; and factors that induce or inhibit FMOs. We discuss the contribution of FMOs of human and mouse to the metabolism of drugs and how genetic variation of FMOs affects drug metabolism. Finally, we discuss the utility of animal models for FMO-mediated drug metabolism in humans. Expert opinion: The contribution of FMOs to drug metabolism may be underestimated. As FMOs are not readily induced or inhibited and their reactions are generally detoxifications, the design of drugs that are metabolized predominantly by FMOs offers clinical advantages. Fmo1^(-/-),Fmo2^(-/-),Fmo4^(-/-) mice provide a good animal model for FMO-mediated drug metabolism in humans. Identification of roles for FMO1 and FMO5 in endogenous metabolism has implications for drug therapy and initiates an exciting area of research.

Variants in the flavin-containing monooxygenase 3 (FMO3) gene responsible for trimethylaminuria in a Japanese population

Loss-of-function mutations of flavin-containing monooxygenase 3 (FMO3), the enzyme responsible for trimethylamine N-oxygenation, cause the inherited disorder trimethylaminuria, or fish odor syndrome. The aim of this study was to further investigate the inter-individual variations of FMO3 activity in a Japanese cohort that we had studied previously. The subjects were 640 Japanese volunteers with self-reported trimethylaminuria; genomic DNA was sequenced in those that had 10-70% FMO3 metabolic capacity in urine tests. A heterozygote for the novel single nucleotide substitution p.Ile441Thr (proband 1) and a heterozygote for the novel single nucleotide substitution p.Ser195Leu (proband 2) were identified. The biological parents of probands 1 and 2 were heterozygous and had >90% trimethylamine N-oxygenation metabolic capacity. In addition, single nucleotide substitutions p.Val58Ile, p.Pro70Leu, and p.Gly421Val in FMO3 were found in probands 3-7. In the course of DNA sequencing, another FMO3 variant, p.Thr488Ala, was found in two unrelated heterozygous subjects. Variant FMO3 proteins recombinantly expressed in Escherichia coli membranes exhibited decreased activity toward typical FMO3 substrates. Although the allele frequencies of these six novel variants were low (<1%), the present results suggest that individuals homozygous or heterozygous for any of the six novel missense FMO3 variants or known nonsense mutations such as p.Cys197stop or p.Arg500stop may possess abnormal trimethylamine N-oxygenation.

Flavin-containing monooxygenase 3 gene polymorphisms in Turkish population

Flavin-containing monooxygenases (FMOs) represent the second most important human monooxygenase system, after cytochrome P450s (CYPs) and catalyze the oxygenation of many chemicals containing nitrogen-, sulphur-, phosphorous-, selenium- and other nucleophilic heteroatoms. FMO3 is the prominent FMO form in adult human liver. For FMO3, both interindividual variability within a single ethnic group and variability between ethnic groups have been reported. In our study, three prevalent functional FMO3 variants (E158K, V257M, and E308G) were genotyped in healthy Turkish people by polymerase chain reaction (PCR)-restriction fragment length polymorphism (RFLP) methods. The frequencies of alleles and haplotypes were compared with those obtained from different populations. It was found that FMO3 158K, 257M and 308G alleles, demonstrate impaired metabolism toward many FMO3 substrates, were observed frequently in Turkish population similar to the other populations. Also, the frequencies of haplotypes were determined based on individual allelic frequencies and it was observed that the most common haplotypes were haplotip EVE and KVE (E158K/V257M/E308G), which together accounted for 80% of all haplotypes. The obtained data from the present study could be useful for further studies assessing sensitivity to therapeutic drugs, environmental toxicants and common disease.

Trimethylaminuria: an under-recognised and socially debilitating metabolic disorder

Primary flavin mono-oxygenase 3 deficiency, an inborn error of choline metabolism, leads to an accumulation of trimethylamine, which because of its associated pungent odour of rotting fish, is a socially crippling disorder. Although it often has its onset in early childhood, it may take years or even decades before the diagnosis is established. In this review the clinical biochemical and genetic features of the disorder are reported. The principles of therapy will also be covered, including dietary, pharmacological approaches, as well as techniques used to manipulate the gastrointestinal environment as a strategy to reduce the gastrointestinal load of trimethylamine.

OnpA, an unusual flavin-dependent monooxygenase containing a cytochrome b(5) domain

ortho-Nitrophenol 2-monooxygenase (EC 1.14.13.31) from Alcaligenes sp. strain NyZ215 catalyzes monooxygenation of ortho-nitrophenol to form catechol via ortho-benzoquinone. Sequence analysis of this onpA-encoded enzyme revealed that it contained a flavin-binding monooxygenase domain and a heme-binding cytochrome b(5) domain. OnpA was purified to homogeneity as a His-tagged protein and was considered a monomer, as determined by gel filtration. FAD and heme were identified by high-performance liquid chromatography (HPLC) and HPLC-mass spectrometry (HPLC-MS) as cofactors in this enzyme, and quantitative analysis indicated that 1 mol of the purified recombinant OnpA contained 0.66 mol of FAD and 0.20 mol of heme. However, the enzyme activity of OnpA was increased by 60% and 450% after addition of FAD and hemin, respectively, suggesting that the optimal stoichiometry was 1:1:1. In addition, site-directed mutagenesis experiments confirmed that two highly conserved histidines located in the cytochrome b(5) domain were associated with binding of the heme, and the cytochrome b(5) domain was involved in the OnpA activity. These results indicate that OnpA is an unusual FAD-dependent monooxygenase containing a fused cytochrome b(5) domain that is essential for its activity. Therefore, we here demonstrate a link between cytochrome b(5) and flavin-dependent monooxygenases.

Drug interactions in African herbal remedies

Herbal usage remains popular as an alternative or complementary form of treatment, especially in Africa. However, the misconception that herbal remedies are safe due to their "natural" origins jeopardizes human safety, as many different interactions can occur with concomitant use with other pharmaceuticals on top of potential inherent toxicity. Cytochrome P450 enzymes are highly polymorphic, and pose a problem for pharmaceutical drug tailoring to meet an individual's specific metabolic activity. The influence of herbal remedies further complicates this. The plants included in this review have been mainly researched for determining their effect on cytochrome P450 enzymes and P-glycoprotein drug transporters. Usage of herbal remedies, such as Hypoxis hemerocallidea, Sutherlandia frutescens and Harpagophytum procumbensis popular in Africa. The literature suggests that there is a potential for drug-herb interactions, which could occur through alterations in metabolism and transportation of drugs. Research has primarily been conducted in vitro, whereas in vivo data are lacking. Research concerning the effect of African herbals on drug metabolism should also be approached, as specific plants are especially popular in conjunction with certain treatments. Although these interactions can be beneficial, the harm they pose is just as great.

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.

Revealing the moonlighting role of NADP in the structure of a flavin-containing monooxygenase

Flavin-containing monooxygenases (FMOs) are, after cytochromes P450, the most important monooxygenase system in humans and are involved in xenobiotics metabolism and variability in drug response. The x-ray structure of a soluble prokaryotic FMO from Methylophaga sp. strain SK1 has been solved at 2.6-A resolution and is now the protein of known structure with the highest sequence similarity to human FMOs. The structure possesses a two-domain architecture, with both FAD and NADP(+) well defined by the electron density maps. Biochemical analysis shows that the prokaryotic enzyme shares many functional properties with mammalian FMOs, including substrate specificity and the ability to stabilize the hydroperoxyflavin intermediate that is crucial in substrate oxygenation. On the basis of their location in the structure, the nicotinamide ring and the adjacent ribose of NADP(+) turn out to be an integral part of the catalytic site being actively engaged in the stabilization of the oxygenating intermediate. This feature suggests that NADP(H) has a moonlighting role, in that it adopts two binding modes that allow it to function in both flavin reduction and oxygen reactivity modulation, respectively. We hypothesize that a relative domain rotation is needed to bring NADP(H) to these distinct positions inside the active site. Localization of mutations in human FMO3 that are known to cause trimethylaminuria (fish-odor syndrome) in the elucidated FMO structure provides a structural explanation for their biological effects.

Osmotic regulation of a novel flavin-containing monooxygenase in primary cultured cells from rainbow trout (Oncorhynchus mykiss)

A cDNA encoding a hepatic isoform of flavin-containing monooxygenase (hFMO) (EF063736) containing an open reading frame of 1792 base pairs (bp) and encoding 554 amino acids was cloned and sequenced from liver mRNA of rainbow trout (Oncorhynchus mykiss). The genomic sequence of hFMO was also characterized and was 4.379 kilobases, possessing 10 exons and 9 introns (EU519462). Structural analysis of the promoter region showed several cis-acting elements including putative glucocorticoid and osmoregulatory response elements, which have been reported to be functionally related to induction of flavin-containing monooxygenase (FMO) proteins in vertebrates. The amino acid sequence showed 74% identity to a putative FMO gene from fugu (Takifugu rubripes; Q6ZZY9), 52 to 55% to zebrafish (Brachydanio rerio; Q5RGM6, Q5RGM3, Q6TLD2, Q7T1D7) FMO5, and 54 and 50% to human FMO1 (Q01740), FMO3 (P49326), and FMO5 (P49326). Southern blot analysis using a 180-bp fragment of the hFMO cDNA indicated at least seven potential genes. Treatment of primary trout hepatocytes with cortisol and sodium chloride for 24 h enhanced hFMO expression. Expression of hFMO was not detected in untreated or solute-treated primary cultures of gill epithelial cells, suggesting tissue-specific expression of hFMO. Induction of hFMO is consistent with the occurrence of cis-osmoregulatory and glucocorticoid response elements identified in the 5'-upstream sequence, indicating regulation of hFMO in response to hypersaline conditions and the osmoregulatory hormone cortisol.

Flavin-containing monooxygenase 3 and human disease

This review summarizes information concerning the association of the human flavin-containing monooxygenase 3 (FMO3) and human diseases. Human FMO3 oxygenates a wide variety of nucleophilic heteroatom-containing xenobiotics, including endogenous substrates and various clinically important drugs. In this article, the authors discuss the association of FMO3 with human disease, including: i) direct association of FMO3 genetic mutations to human genetic disease; ii) association of FMO3 genetic polymorphism to altered drug metabolism and, therefore, indirect association of FMO3 with drug therapeutic efficacy of human disease; and iii) the potential impact and/or effect of FMO3 transcriptional regulation during disease states. Even though many studies discussed for the latter two points are at a preliminary stage and require much more research to bring to a definite conclusion, the authors include these studies to stimulate general interest and invite further discussion.

The biochemistry of drug metabolism--an introduction: Part 2. Redox reactions and their enzymes

This review continues a general presentation of the metabolism of drugs and other xenobiotics started in a recent issue of Chemistry & Biodiversity. This Part 2 presents the numerous oxidoreductases involved, their nomenclature, relevant biochemical properties, catalytic mechanisms, and the very diverse reactions they catalyze. Many medicinally, environmentally, and toxicologically relevant examples are presented and discussed. Cytochromes P450 occupy a majority of the pages of Part 2, but a large number of relevant oxidoreductases are also considered, e.g., flavin-containing monooxygenases, amine oxidases, molybdenum hydroxylases, peroxidases, and the innumerable dehydrogenases/reductases.

Metabolism and Disposition of a Selective α7 Nicotinic Acetylcholine Receptor Agonist in Humans

The metabolism and disposition of N-(3R)-1-azabicyclo[2.2.2]oct-3-ylfuro[2,3-c]pyridine-5-carboxamide (1), an alpha(7) nicotinic acetylcholinergic receptor agonist, were elucidated in humans (4 female, 4 male; all white) after an oral dose of [(3)H]1. Overall, 1 was well tolerated, with >94% of administered radioactivity excreted renally by 48 h postdose; lyophilization of all urine and plasma samples confirmed (3)H stability within [(3)H]1. Across genders, 1 underwent low-to-moderate oral clearance comprising both renal (67%) and metabolic (33%) components, with the biotransformation of 1 occurring predominantly via oxidation of its furanopyridine moiety to carboxylic acid 2, and minimally by modification of its quinuclidine nitrogen to N-oxide 4 or N-glucuronide M5. Experiments using human in vitro systems were undertaken to better understand the enzyme(s) involved in the phase 1 biotransformation pathways. The formation of 2 was found to be mediated by CYP2D6, a polymorphically expressed enzyme absent in 5 to 10% of white people, whereas the generation of 4 was catalyzed by CYP2D6, FAD-containing monooxygenase 1 (FMO1), and FMO3. It is of interest that, although no overall gender-related differences in excretory routes, mass recoveries, pharmacokinetics, or metabolite profiles of 1 were evident, the observation of one of eight subjects (13%) showing disparate (relative to all other volunteers) systemic exposures to 1, and urinary and plasma quantitative profiles nearly devoid of 2 with the highest levels of 1, seem consistent with both the identification of CYP2D6 as the only major recombinant cytochrome P450 transforming 1 to 2 and the demographics of white CYP2D6 poor metabolizers. Data also reported herein suggest that 4 is generated predominantly by renal FMO1 in humans.

Pharmacogenetics and diseases of the colon

PURPOSE OF REVIEW

The deciphering of the human genome sequence has enabled the identification of genetic polymorphisms that are responsible for inter-individual variation in the response to drug therapy. This field is referred to as pharmacogenetics. We review the impact of pharmacogenetics on therapy in diseases of the colon using three common variant enzyme systems as examples.

RECENT FINDINGS

Many enzyme systems impact the treatment of diseases of the colon. Examples include thiopurine S-methyltransferase, dihydropyrimidine dehydrogenase and flavin monooxygenase 3. They affect the management of inflammatory bowel disease, colorectal cancer and the chemoprevention of colorectal adenoma by influencing the metabolism of their respective substrates, azathioprine/6-mercaptopurine, 5-fluorouracil and sulindac. Recent studies have implicated the significance of genetic polymorphisms in each of the three drug-metabolizing enzymes, which impacts on the therapeutic outcome of the stated diseases. These studies highlight the potential role of pharmacogenetics in the design of a therapeutic plan which would increase efficacy and limit toxicity.

SUMMARY

Pharmacogenetics of drug-metabolizing systems continues to gain significance in the drug therapy of a variety of disease states including those of the gastrointestinal tract.

Allele and genotype frequencies of polymorphicFMO3 gene in two genetically distinct populations

The aims of this study were to analyze flavin-containing monooxygenase 3 (FMO3) polymorphisms and allele and genotype frequencies in 256 Han Chinese and 50 African-American individuals, to compare the allele and genotype frequencies of these populations with those of other world populations. For Han Chinese, genotyping of three common single nucleotide polymorphisms, E158K, V257M and E308G was performed by polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP). For African-Americans, genotyping of all coding exons was performed by modified PCR-single strand conformational polymorphism (SSCP). Evolutionary rates of FMO3 were estimated computationally. We found that there were significant differences in allele and genotype frequencies among Han Chinese, African-Americans and other world populations. In Han Chinese, the minor allele frequencies (MAFs) were 0.229 (E158K), 0.203 (V257M) and 0.148 (E308G), respectively. In African-Americans, MAFs were 0.48 (E158K), 0.05 (V257M) and 0 (E308G), respectively. There was rapid evolution during the divergence of primate FMO3. This is the first report comparing FMO alleles and genotypes between Han Chinese and African-Americans. A Han Chinese population database has been established for three gene polymorphisms. The data presented here justify further pharmacogenetic studies for potentially optimizing recommended drug dosages and evaluating relationships with disease processes.

Stop codon mutations in the flavin-containing monooxygenase 3 (FMO3) gene responsible for trimethylaminuria in a Japanese population

The reduced capacity of flavin-containing monooxygenase 3 (FMO3) to N-oxidize trimethylamine (TMA) is believed to cause a metabolic disorder. The aim of this study was to investigate the inter-individual variations of FMO3. Genomic DNA of case subjects that showed only 10-20% of FMO3 metabolic capacity among self-reported trimethylaminuria Japanese volunteers was sequenced. Functional analysis of recombinant FMO3 proteins was also performed. One homozygote for a novel single nucleotide substitution causing a stop codon at Arg500 was observed. The biological parents of this Proband A were heterozygous and showed >90% TMA N-oxygenation metabolic capacity. Another Proband B had the Arg500Stop and Cys197Stop codons. The TMA N-oxygenation metabolic capacities of the father and brother of this Proband B were apparently observed by possessing Arg205Cys mutant that coded for decreased TMA N-oxygenase. Recombinant Arg500Stop FMO3 cDNA expressed in Escherichia coli membranes and a series of highly purified truncation mutants at different positions of the C-terminus of FMO3 showed no detectable functional activity toward typical FMO3 substrates. The results suggest that individuals homozygous for either of the nonsense mutations, Arg500Stop and/or Cys197Stop alleles, in the FMO3 gene can possess abnormal TMA N-oxygenation.

Effect of Genetic Variants of the Human Flavin-Containing Monooxygenase 3 on N- and S-Oxygenation Activities

The decreased capacity of the flavin-containing monooxygenase 3 (FMO3) to oxygenate xenobiotics including trimethylamine is believed to contribute to metabolic disorders. The aim of this study was to functionally characterize FMO3 variants recently found in a Japanese population and compare them with selective functional activity of other FMO3 variants. Recombinant Glu158Lys and Glu158Lys-Glu308Gly FMO3 expressed in Escherichia coli membranes showed slightly decreased N-oxygenation of benzydamine and trimethylamine. Selective functional S-oxygenation of these variants by methyl p-tolyl sulfide or sulindac sulfide was comparable to that of wild-type FMO3. The Glu158Lys-Thr201Lys-Glu308Gly and Val257Met-Met260Val variants showed significantly decreased oxygenation of typical FMO3 substrates (i.e., approximately one-tenth of the V(max)/K(m) values). Val257Met FMO3 had a lower catalytic efficiency for methyl p-tolyl sulfide and sulindac sulfide S-oxygenation. However, compared with wild-type FMO3, Val257Met FMO3 showed a similar catalytic efficiency for N-oxygenation of benzydamine and trimethylamine. The catalytic efficiency for benzydamine and trimethylamine N-oxygenation by Arg205Cys FMO3 was only moderately decreased, but it possessed decreased sulindac sulfide S-oxygenation activity. Kinetic analysis showed that Arg205Cys FMO3 was inhibited by sulindac in a substrate-dependent manner, presumably because of selective interaction between the variant enzyme and the substrate. The results suggest that the effects of genetic variation of human FMO3 could operate at the functional level for N- and S-oxygenation for typical FMO3 substrates. Genetic polymorphism in the human FMO3 gene might lead to unexpected changes of catalytic efficiency for N- and S-oxygenation of xenobiotics and endogenous materials.

Haplotype Frequency Distribution and Linkage Disequilibrium Analysis of Single Nucleotide Polymorphisms at the Human FMO3 Gene Locus

We analyzed flavin-containing monooxygenase 3 (FMO3) polymorphisms, haplotype structure, and linkage disequilibrium (LD) in 256 Han Chinese and 50 African-American individuals to compare their haplotype frequencies and LD with other world populations. For the Han Chinese, genotyping of three haplotype tag single nucleotide polymorphisms (E158K, V257M, and E308G) was performed by polymerase chain reaction (PCR)-restriction fragment length polymorphism. For the African-Americans, genotyping of all coding exons was performed by modified PCR-single strand conformational polymorphism. Haplotype frequencies, LD, and evolutionary rates were inferred and estimated computationally. There were significant differences in haplotype frequency distribution and LD pattern among Han Chinese, African-Americans, and other world populations. Four major haplotypes of Han Chinese were EVE, KVE, EME, and EVG. Two major haplotypes of African-Americans were EVE and KVE. We found that sites 158 and 257 are in significant LD in both populations. This is the first report comparing FMO haplotypes and LD of Han Chinese with African-Americans. The data presented here justify further pharmacogenetic studies for potentially optimizing recommended drug dosages and evaluating relationships with disease processes.

A role for a flavin-containing mono-oxygenase in resistance against microbial pathogens in Arabidopsis

Using activation tagging in the Arabidopsis Col-0 rps2-101C background, we identified a mutant (FMO1-3D) that showed virtually no symptoms after inoculation with virulent Pseudomonas syringae pv. tomato DC3000 bacteria. The dominant, gain-of-function phenotype of the FMO1-3D mutant is due to over-expression of a class 3 flavin-containing mono-oxygenase (FMO). We recapitulated the FMO1-3D mutant phenotype in independent transgenic Col-0 lines over-expressing the FMO1 cDNA under the control of the 35S CaMV promoter. The increased basal resistance observed in the FMO1-3D mutant was also effective against the taxonomically unrelated downy mildew-causing pathogen Hyaloperonospora parasitica. By investigating the progeny from crosses of the FMO1-3D mutant with the NahG transgenic line, we showed that the enhanced basal resistance phenotype was dependent on the accumulation of salicylic acid. FMO1-3D plants showed wild-type resistant reactions after inoculation with avirulent bacteria, indicating that the R-gene-mediated defence physiology was not compromised by FMO1 over-expression. Transcripts of the class 3 FMO1 gene accumulated within 6 h after inoculation of wild-type Col-0 plants with avirulent Pst + avrRpt2 cells. Moreover, a T-DNA insertion into the FMO1 gene resulted in enhanced susceptibility to virulent Pseudomonas and Hyaloperonospora parasitica, suggesting that expression of the FMO1 gene is a hitherto undescribed component of the plant's resistance repertoire. We discuss the possibility that the FMO may participate in the detoxification of virulence factors produced by pathogens.

Mechanism of action of a flavin-containing monooxygenase

Elimination of nonnutritional and insoluble compounds is a critical task for any living organism. Flavin-containing monooxygenases (FMOs) attach an oxygen atom to the insoluble nucleophilic compounds to increase solubility and thereby increase excretion. Here we analyze the functional mechanism of FMO from Schizosaccharomyces pombe using the crystal structures of the wild type and protein-cofactor and protein-substrate complexes. The structure of the wild-type FMO revealed that the prosthetic group FAD is an integral part of the protein. FMO needs NADPH as a cofactor in addition to the prosthetic group for its catalytic activity. Structures of the protein-cofactor and protein-substrate complexes provide insights into mechanism of action. We propose that FMOs exist in the cell as a complex with a reduced form of the prosthetic group and NADPH cofactor, readying them to act on substrates. The 4alpha-hydroperoxyflavin form of the prosthetic group represents a transient intermediate of the monooxygenation process. The oxygenated and reduced forms of the prosthetic group help stabilize interactions with cofactor and substrate alternately to permit continuous enzyme turnover.

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.

Insulin in flavin-containing monooxygenase regulation. Flavin-containing monooxygenase and cytochrome P450 activities in experimental diabetes

Microsomal monooxygenases - cytochrome P450 (CYP, EC 1.14.14.1) and flavin-containing monooxygenase (FMO, EC 1.14.13.8) - have profound roles in drug metabolism. While the induction of some metabolic enzymes such as hepatic FMO and intestinal CYP1A, CYP2B is recognized in experimental diabetes, the effect of insulin treatment on FMO and intestinal CYP3A in diabetic animals has not been reported before. Changes in abundance and activity of hepatic and intestinal microsomal CYPs and FMO were studied in streptozotocin-induced diabetic rats either treated or not with insulin. Hepatic FMO1 activity increased in diabetic rats, but it was restored to control value on insulin treatment. Insulin itself had no effect on FMO1 activity in non-diabetic animals. A remarkable increase of total CYP content was accompanied by a reduced CYP3A specific enzyme activity in the small intestine of diabetic animals. The extent of these changes decreased on insulin treatment. Both, hepatic FMO1 and intestinal CYP3A activity correlated with average blood glucose concentration in untreated diabetic rats. These results indicate that insulin is involved in the regulation of hepatic FMO1 and intestinal CYP3A in rats. Blood glucose level is a good marker for FMO induction. The marked reduction of intestinal CYP3A capacity suggests that diabetes exerts a substantial effect on the activity of most determining intestinal CYP enzyme.

Genetic polymorphisms of flavin monooxygenase 3 in sulindac-induced regression of colorectal adenomas in familial adenomatous polyposis

Sulindac is a nonsteroidal antiinflammatory drug with a chemopreventive effect in patients with familial adenomatous polyposis (FAP). In vivo, the active form of sulindac is sulindac sulfide, which is inactivated by the hepatic microsomal enzyme, flavin monooxygenase 3 (FMO3). In humans, numerous polymorphisms exist in FMO3, which alter enzymatic activity and subsequent substrate metabolism. We recently showed that certain polymorphic forms of FMO3 with reduced activity were associated with a more favorable response to sulindac in preventing the formation of adenomas in patients with FAP without polyps at baseline. Here, we determined whether these FMO3 polymorphisms correlated with the ability of sulindac to regress polyposis in patients with FAP who had polyps prior to treatment. Nineteen patients were treated with 150 mg sulindac twice a day for 6 months. The size and number of polyps in each patient was assessed at baseline (prior to the administration of sulindac), and at 3 and 6 months. Genotyping was done on seven established FMO3 polymorphisms with functional significance-M66I, E158K, P153L, V257M, E305X, E308G, and R492W. Statistical analyses were done with Wilcoxon rank sum test. Of the loci examined, only E158K and E308G showed polymorphic changes. Six patients exhibited polymorphisms in both E158K and E308G loci and were designated as genotype combination 1. The remaining patients were designated as genotype combination 2. Over the course of treatment, patients with genotype combination 1 had a greater reduction in both the size and number of polyps than those with genotype combination 2. These results suggest that combined polymorphic changes in the E158K and E308G alleles may protect against polyposis in patients with FAP treated with sulindac.

Dynamics involved in catalysis by single-component and two-component flavin-dependent aromatic hydroxylases

Flavoprotein monooxygenases are involved in a wide variety of biological processes including drug detoxification, biodegradation of aromatic compounds in the environment, biosynthesis of antibiotics and siderophores, and many others. The reactions use NAD(P)H and O2 as co-substrates and insert one atom of oxygen into the substrate. The flavin-dependent monooxygenases utilize a general cycle in which NAD(P)H reduces the flavin, and the reduced flavin reacts with O2 to form a C4a-(hydro)peroxyflavin intermediate, which is the oxygenating agent. This complicated catalytic process has diverse requirements that are difficult to be satisfied by a single site. Two general strategies have evolved to satisfy these requirements. para-Hydroxybenzoate hydroxylase, the paradigm for the single-component flavoprotein monooxygenases, is one of the most thoroughly studied of all enzymes. This enzyme undergoes significant protein and flavin dynamics during catalysis. There is an open conformation that gives access of substrate and product to solvent, and a closed or in conformation for the reaction with oxygen and the hydroxylation to occur. This closed form prevents solvent from destabilizing the hydroperoxyflavin intermediate. Finally, there is an out conformation achieved by movement of the isoalloxazine toward the solvent, which exposes its N5 for hydride delivery from NAD(P)H. The protein coordinates these dynamic events during catalysis. The second strategy uses a reductase to catalyze the reduction of the flavin and an oxygenase that uses the reduced flavin as a substrate to react with oxygen and hydroxylate the organic substrate. These two-component systems must be able to transfer reduced flavin from the reductase to the oxygenase and stabilize a C4a-peroxyflavin until a substrate binds to be hydroxylated, all before flavin oxidation and release of H2O2. Again, protein dynamics are important for catalytic success.

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.

Discovery of novel flavin-containing monooxygenase 3 (FMO3) single nucleotide polymorphisms and functional analysis of upstream haplotype variants

The flavin-containing monooxygenases (FMOs) are important for xenobiotic metabolism. FMO3, the predominant FMO enzyme in human adult liver, exhibits significant interindividual variation that is poorly understood. This study was designed to identify common FMO3 genetic variants and determine their potential for contributing to interindividual differences in FMO3 expression. FMO3 single nucleotide polymorphism (SNP) discovery was accomplished by resequencing DNA samples from the Coriell Polymorphism Discovery Resource. Population-specific SNP frequencies were determined by multiplexed, single-base extension using DNA from 201 Hispanic American (Mexican descent), 201 African American, and 200 White (northern European descent) subjects. Haplotypes were inferred and population frequencies estimated using PHASE version 2.1. Multiple site-directed mutagenesis was used to introduce inferred upstream haplotypes into an FMO3/luciferase construct for functional analysis in HepG2 cells. Sequence analysis revealed seven FMO3 upstream SNPs, 11 exon SNPs, and 22 intron SNPs. Five of the latter fell within consensus splice sites. A g.72G>T variant (E24D) is predicted to impact the structure of the Rossmann fold involved in FAD binding, whereas a g.11177C>A variant (N61K) is predicted to disrupt the secondary structure of a conserved membrane interaction domain. Seven common (>1%) promoter region haplotypes were inferred in one or more of the study populations that differed in estimated frequency among the groups. Haplotype 2 resulted in an 8-fold increase in promoter activity, whereas haplotypes 8 and 15 exhibited a near complete loss of activity. In conclusion, FMO3 promoter haplotype variants modulate gene function and probably contribute to interindividual differences in FMO3 expression.