Quantitation and kinetic properties of hepatic microsomal and recombinant flavin-containing monooxygenases 3 and 5 from humans

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

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

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

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

Molecular genetics of nicotine metabolism

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

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

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

The participation of human hepatic P450 isoforms, flavin-containing monooxygenases and aldehyde oxidase in the biotransformation of the insecticide fenthion

Although fenthion (FEN) is widely used as a broad spectrum insecticide on various crops in many countries, very scant data are available on its biotransformation in humans. In this study the in vitro human hepatic FEN biotransformation was characterized, identifying the relative contributions of cytochrome P450 (CYPs) and/or flavin-containing monooxygenase (FMOs) by using single c-DNA expressed human enzymes, human liver microsomes and cytosol and CYP/FMO-specific inhibitors. Two major metabolites, FEN-sulfoxide and FEN-oxon (FOX), are formed by some CYPs although at very different levels, depending on the relative CYP hepatic content. Formation of further oxidation products and the reduction of FEN-sulfoxide back to FEN by the cytosolic aldehyde oxidase enzyme were ruled out. Comparing intrinsic clearance values, FOX formation seemed to be favored and at low FEN concentrations CYP2B6 and 1A2 are mainly involved in its formation. At higher levels, a more widespread CYP involvement was evident, as in the case of FEN-sulfoxide, although a higher efficiency of CYP2C family was suggested. Hepatic FMOs were able to catalyze only sulfoxide formation, but at low FEN concentrations hepatic FEN sulfoxidation is predominantly P450-driven. Indeed, the contribution of the hepatic isoforms FMO(3) and FMO(5) was generally negligible, although at high FEN concentrations FMO's showed activities comparable to the active CYPs, accounting for up to 30% of total sulfoxidation. Recombinant FMO(1) showed the highest efficiency with respect to CYPs and the other FMOs, but it is not expressed in the adult human liver. This suggests that FMO(1)-catalysed sulfoxidation may represent the major extra-hepatic pathway of FEN biotransformation.

Differential regulation of human hepatic flavin containing monooxygenase 3 (FMO3) by CCAAT/enhancer-binding protein beta (C/EBPbeta) liver inhibitory and liver activating proteins

Flavin-containing monooxygenase 3 (FMO3) is important for oxidative xenobiotic metabolism, but regulation of the FMO3 gene remains poorly understood. FMO3 is not expressed in HepG2 cells, a commonly employed model for hepatic gene regulation studies. Transcription factor transient expression and treatment with histone deacetylase or DNA methylase inhibitors identified decreased hepatic nuclear factor (HNF) 4alpha levels and DNA hypermethylation as mechanisms suppressing HepG2 FMO3 expression. The absence of major deficiencies in transcriptional machinery suggested that within limits, the HepG2 model is suitable for the study of FMO3 regulation. DNA-protein binding studies with HepG2 cell and hepatic tissue nuclear protein extracts and reporter construct transient expression experiments were performed to characterize FMO3 sequences from position -494 to -439 (domain I), previously demonstrated to significantly impact promoter function. Although both HNF3beta and CCAAT enhancer-binding protein (C/EBP) were observed to specifically interact with this element using HepG2 cell nuclear proteins, only C/EBP DNA-protein interactions were observed using adult liver nuclear proteins. No specific DNA/protein interactions were observed using fetal liver nuclear proteins. Mutation of a putative HNF3beta element had no effect on FMO3 promoter activity, while mutagenesis of a distinct, but overlapping C/EBP element resulted in a 55% reduction in activity. Furthermore, promoter activity was regulated as a function of defined C/EBPbeta liver activating protein:liver inhibitory protein ratios through this same element. Chromatin immunoprecipitation demonstrated C/EBPbeta binding to the FMO3 domain I element in intact cells and adult liver tissue. These results are consistent with C/EBPbeta being important for regulating hepatic FMO3 expression.

Ontogeny of human hepatic cytochromes P450

Significant changes in drug-metabolizing enzyme (DME) expression occur during ontogeny. Such changes can have a profound effect on therapeutic efficacy in the fetus and child, as well as the risk for adverse drug reactions. To gain a better understanding of DME ontogeny, enzyme contents for six key cytochromes P450 were measured in 240 human liver samples representing ages from 8 weeks gestation to 18 years. Where possible, both quantitative western blotting and activity assays with probe substrates were performed. Although oversimplified, the DME can be grouped into one of three categories. As typified by CYP3A7, some enzymes are expressed at their highest level during the first trimester and either remain at high concentrations or decrease during gestation and are silenced or expressed at low levels within 1-2 years after birth. These data cause one to query whether these enzymes have an important endogenous function. Representatives of a second group, CYP3A5 and CYP2C19, are expressed at relatively constant levels throughout gestation. Postnatal increases in CYP2C19 are observed within the first year, but not for CYP3A5. CYP2C9, 2E1, and 3A4 are more typical of a third group of enzymes that are not expressed or are expressed at low levels in the fetus with the onset of expression generally in either the second or third trimester. Substantial increases in expression are observed within the first 1-2 years after birth; however, considerable interindividual variability is observed in the immediate postnatal (1-6 months) onset or increase in expression of these enzymes, often resulting in a window of hypervariability.

Functional activity of the mouse flavin-containing monooxygenase forms 1, 3, and 5

Three functional mouse flavin-containing monooxygenases (mFMOs) (i.e., mFMO1, mFMO3, and mFMO5) have been reported to be the major FMOs present in mouse liver. To examine the biochemical features of these enzymes, recombinant enzymes were expressed as maltose-binding protein fusion proteins (i.e., MBP-mFMO1, MBP-mFMO3, and MBP-mFMO5) in Escherichia coli and isolated and purified with affinity chromatography. The substrate specificity of these three mouse hepatic FMO enzymes were examined using a variety of substrates, including mercaptoimidazole, trimethylamine, S-methyl esonarimod, and an analog thereof, and a series of 10-(N,N-dimethylaminoalkyl)-2-(trifluoromethyl)phenothiazine analogs. The kinetic parameters of the three mouse FMOs for these substrates were compared in an attempt to explore substrate structure--function relationships specific for each mFMO. Utilizing a common phenothiazine substrate for all three enzymes, we compared the pH dependence for the recombinant enzymes under similar conditions. In addition, thermal stability for mFMO1, mFMO3, and mFMO5 enzymes was examined in the presence and absence of NADPH. The results revealed unique features for mFMO5, suggesting possible impact on the functional significance of this abundantly expressed FMO5 isoform in both human and mouse liver.

Mechanisms regulating human FMO3 transcription

Flavin-containing monooxygenases (FMOs) are important oxidative drug metabolizing enzymes. FMO3 is the primary human adult liver FMO enzyme, but is developmentally regulated. FMO3 promoter characterization using in vitro DNA binding assays with HepG2 cell and fetal and adult liver nuclear protein, as well as FMO3/reporter construct transient expression in HepG2 cells, provided evidence for specific mechanisms contributing to both developmental and constitutive adult regulation. NFY, USF1, an unidentified GC box binding protein, and YY1 appear to play major roles regulating constitutive FMO3 transcription, while Pbx(2) as a heterodimer with an unidentified Hox isoform also may contribute to FMO3 developmental expression.

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

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

Molecular evolution and balancing selection in the flavin-containing monooxygenase 3 gene (FMO3)

OBJECTIVES

Flavin-containing monooxygenase 3 (FMO3) is involved in the metabolism of foreign chemicals, including therapeutic drugs, and thus mediates interactions between humans and their chemical environment. Loss-of-function mutations in the gene cause the inherited disorder trimethylaminuria, or fish-odour syndrome. The objective was to gain insights into the evolutionary history of FMO3.

METHODS

Genetic diversity within FMO3 was characterized by sequencing 6.3 kb of genomic DNA, encompassing the entire coding sequence, some intronic and 3'-untranslated region, and 3.4 kb of 5'-flanking sequence, in 23 potential trimethylaminuric Japanese, and the same 3.4 kb 5'-flanking region in 45 unaffected Japanese. Mutational relationships among haplotypes were inferred from a reduced-median network. The time depth of the variation and ages of individual mutations were estimated by maximum-likelihood coalescent analysis. Test statistics were used to investigate whether the variation is compatible with neutral evolution.

RESULTS

Sixteen single-nucleotide polymorphisms (SNPs) were identified, which segregated as seven distinct haplotypes. Estimated ages of the mutations indicate that almost all predated migration out of Africa. Analysis of the heterozygosity of FMO3 SNPs indicates that genetic differentiation among continental populations is low (FST=0.050). Test statistics, based on allele-frequency spectrum, number and diversity of haplotypes, linkage disequilibrium and interspecific sequence comparisons, showed a significant departure from neutral expectations, because of an excess of intermediate-frequency SNPs and haplotypes, a ragged pairwise mismatch distribution and an excess of replacement polymorphisms.

CONCLUSION

The results provide evidence that FMO3 has been the subject of balancing selection. Finally, we identify mutations that are potential targets for selection.

Cyclic Conversion of the Novel Src Kinase Inhibitor [7-(2,6-Dichloro-phenyl)-5-methyl-benzo[1,2,4]triazin-3-yl]-[4-(2-pyrrolidin-1-yl-ethoxy)-phenyl]-amine (TG100435) and Its N-Oxide Metabolite by Flavin-Containing Monoxygenases and Cytochrome P450 Reductase

[7-(2,6-Dichloro-phenyl)-5-methyl-benzo[1,2,4]triazin-3-yl]-[4-(2-pyrrolidin-1-yl-ethoxy)-phenyl]-amine (TG100435) is a novel multi-targeted Src family kinase inhibitor with demonstrated anticancer activity in preclinical species. Potent kinase inhibition is associated with TG100435 and its major N-oxide metabolite [7-(2,6-dichlorophenyl)-5-methyl-benzo[1,2,4]triazin-3-yl]-{4-[2-(1-oxy-pyrrolidin-1-yl)-ethoxy]-phenyl}-amine (TG100855). The objectives of the current study were to identify the hepatic enzyme(s) responsible for 1) the total metabolic flux of TG100435, 2) the formation of TG100855, and 3) the subsequent metabolism of TG100855. Flavin-containing monooxygenases (FMO) and cytochrome P450 monooxygenases (P450s) contribute to TG100435 total metabolic flux. TG100435 metabolic flux was completely inhibited by methimazole and ketoconazole, suggesting only FMO- and CYP3A4-mediated metabolism. TG100855 formation was markedly inhibited (~90%) by methimazole or heat inactivation (>99%). FMO3 was the primary enzyme responsible for TG100855 formation. In addition, an enzyme mediated retroreduction of TG100855 back to TG100435 was observed. The N-oxidation reaction was approximately 15 times faster than the retroreduction reaction. Interestingly, the retroreduction of TG100855 to TG100435 in recombinant P450 or liver microsomes lacked inhibition by the P450 inhibitors. TG100435 formation in the human liver microsomes or recombinant P450 increased as a function of cytochrome P450 reductase activity, suggesting potential involvement of cytochrome P450 reductase. The results of this in vitro study demonstrate the potential of TG100435 and TG100855 to be interconverted metabolically. FMO seem to be the major N-oxidizing enzymes, whereas cytochrome P450 reductase seems to be responsible for the retroreduction reaction.

Genetic polymorphisms of human flavin-containing monooxygenase 3: implications for drug metabolism and clinical perspectives

Flavin-containing monooxygenase 3 (FMO3) is a hepatic microsomal enzyme that oxidizes a host of drugs, xenobiotics and other chemicals. Numerous variants in the gene encoding FMO3 have been identified, some of which result in altered enzymatic activity and, consequently, altered substrate metabolism. Studies also implicate individual and ethnic differences in the frequency of FMO3 polymorphisms. In addition, new variants continue to be identified with potentially important clinical implications. For example, the role of FMO3 variants in the pathophysiology of gastrointestinal diseases is an evolving area of research. Two commonly occurring polymorphisms of FMO3, E158K and E308G, have been associated with a reduction in polyp burden in patients with familial adenomatous polyposis who were treated with sulindac sulfide, an FMO3 substrate. These findings suggest a potential role for prospective genotyping of common FMO3 polymorphisms in the treatment of disease states that involve the use of drugs metabolized by FMO3. This review summarizes the current state of research on the genetic polymorphisms of FMO3, with a focus on their clinical implications in gastrointestinal diseases.

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.

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.

Identification and Functional Analysis of Common Human Flavin-Containing Monooxygenase 3 Genetic Variants

Flavin-containing monooxygenases (FMOs) are important for the disposition of many therapeutics, environmental toxicants, and nutrients. FMO3, the major adult hepatic FMO enzyme, exhibits significant interindividual variation. Eighteen FMO3 single-nucleotide polymorphism (SNP) frequencies were determined in 202 Hispanics (Mexican descent), 201 African Americans, and 200 non-Latino whites. Using expressed recombinant enzyme with methimazole, trimethylamine, sulindac, and ethylenethiourea, the novel structural variants FMO3 E24D and K416N were shown to cause modest changes in catalytic efficiency, whereas a third novel variant, FMO3 N61K, was essentially devoid of activity. The latter variant was present at an allelic frequency of 5.2% in non-Latino whites and 3.5% in African Americans, but it was absent in Hispanics. Inferring haplotypes using PHASE, version 2.1, the greatest haplotype diversity was observed in African Americans followed by non-Latino whites and Hispanics. Haplotype 2A and 2B, consisting of a hypermorphic promoter SNP cluster (-2650C>G, -2543T>A, and -2177G>C) in linkage with synonymous structural variants was inferred at a frequency of 27% in the Hispanic population, but only 5% in non-Latino whites and African Americans. This same promoter SNP cluster in linkage with one or more hypomorphic structural variant also was inferred in multiple haplotypes at a total frequency of 5.6% in the African-American study group but less than 1% in the other two groups. The sum frequencies of the hypomorphic haplotypes H3 [15,167G>A (E158K)], H5B [-2650C>G, 15,167G>A (E158K), 21,375C>T (N285N), 21,443A>G (E308G)], and H6 [15,167G>A (E158K), 21,375C>T (N285N)] was 28% in Hispanics, 23% in non-Latino whites, and 24% in African Americans.

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.

Developmental and tissue-specific expression of human flavin-containing monooxygenases 1 and 3

Substantial changes occur in drug and toxicant disposition during early life stages that can impact therapeutic efficacy and adverse reactions to drugs and toxicants. Of the many parameters involved, alterations in drug metabolism are of major importance. Although the cytochrome P450-dependent monooxygenases are accepted as playing a substantial role in drug and toxicant metabolism, the flavin-containing monooxygenases (FMOs) also have an important role. Apparently unique to the human, FMO3 is the most abundant FMO family member in the adult human liver, whereas FMO1 dominates in most animal models. However, early studies documented that FMO1 is the most abundant FMO enzyme in the human fetal liver, whereas FMO3 is essentially absent. This review focuses on recent studies characterising human FMO ontogeny and, in particular, the 'switch' in hepatic FMO enzyme expression. Because it is so closely related, tissue-specific expression patterns also are examined. Finally, a summary of what is known in animal models is presented as a point of comparison.

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.

Quantitative analysis of FMO gene mRNA levels in human tissues

The developmentally and tissue-specific expression of flavin-containing monooxygenase (FMO) enzymes has been previously characterized in a number of animal species, including humans, mice, rats, and rabbits. In this study, we used sensitive real-time reverse transcription-polymerase chain reaction methodology to systematically quantify the steady-state mRNA levels of FMO1, 2, 3, 4, and 5 in human tissues. We examined the developmental regulation of these enzymes in brain tissue. FMO1 was found to be down-regulated in human adult brain. The amount of other FMO mRNAs in human brains in different age groups was not significantly different. The study also provided a systematic quantitative comparison of the steady-state mRNA levels of FMO1 to 5 in several major human organs (i.e., liver, lung, kidney, small intestine, and brain). The nature of the quantitative analysis allowed a comprehensive comparison of each FMO mRNA in different tissues as well as among FMO isoforms in the same tissue. A comparison between fetal liver and adult liver showed that FMO1 was the only FMO that was down-regulated; all other FMOs had greater amounts of mRNA in adult liver. FMO5 was the most prominent FMO form detected in fetal liver. The FMO5 mRNA level was nearly as abundant as FMO3 in adult liver. Whereas other FMOs displayed a significant, dominant tissue-specific mRNA profile (i.e., FMO1 in kidney, FMO2 in lung, FMO3 and FMO5 in adult liver), FMO4 mRNA was observed more broadly at relatively comparable levels in liver, kidney, lung, and small intestine.

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.

The fmo genes of Caenorhabditis elegans and C. briggsae: characterisation, gene expression and comparative genomic analysis

The flavin-containing monooxygenase (FMO) gene family is conserved and ancient with representatives present in almost all phyla so far examined. The genes encode FAD-, NADP- and O(2)-dependent enzymes that catalyse oxygenation of soft-nucleophilic heteroatom centres in a range of substrates. Although usually classified as xenobiotic-metabolising enzymes, examples of FMOs exist that have evolved to metabolise specific endogenous substrates as part of a discrete physiological process. The genome of Caenorhabditis elegans contains five predicted genes encoding putative homologs of mammalian FMOs, K08C7.2, K08C7.5, Y39A1A.19, F53F4.5 and H24K24.5, which we have named fmo and numbered fmo-1 to fmo-5, respectively. As a first step towards determining their functional role(s), we have experimentally characterised these C. elegans fmo genes including analysing reporter gene expression patterns and RNAi phenotypes. Two major gene expression patterns were observed, either intestinal or hypodermal, but no gross RNAi phenotypes were found possibly due to functional redundancy. The internal structures of fmo-2, fmo-3 and fmo-4 have been compared with orthologs identified in the related nematode C. briggsae. For each orthologous pair, a global comparison of the paired upstream intergenic regions was performed and a number of conserved noncoding sequences, which may represent potential cis-regulatory elements, identified. Phylogenetic analysis reveals that several of the fmo homologs are the result of gene duplication along the lineage leading to the nematodes.

EXTRAHEPATIC METABOLISM OF CARBAMATE AND ORGANOPHOSPHATE THIOETHER COMPOUNDS BY THE FLAVIN-CONTAINING MONOOXYGENASE AND CYTOCHROME P450 SYSTEMS

The cytochrome P450 (P450) and flavin-containing monooxygenase (FMO) enzymes are the major oxidative enzymes in phase I metabolism. Many organophosphate and carbamate thioether compounds are excellent substrates for these enzymes. Stereoselective sulfoxidation of fenthion and methiocarb by human liver, kidney, and microsomes was investigated. A high level of stereoselectivity in the formation of fenthion +-sulfoxide was observed in kidney and intestinal microsomes. This activity was not inhibited by the P450 inhibitor 1-aminobenzotriazole but was dramatically reduced following mild heat treatment. In liver, fenthion was metabolized to its sulfoxide in a nonstereoselective manner, and the activity was sensitive to both 1-aminobenzotriazole and heat treatment. The carbamate pesticide methiocarb also was sulfoxidated with a high degree of stereoselectivity in human kidney microsomes. Human liver microsomes formed both stereoisomers in equal amounts. Sulfoxide formation in kidney was not inhibited by 1-aminobenzotriazole but was abolished in liver microsomes. Formation of methiocarb sulfoxides was not observed in intestinal microsomes. The relative contribution of FMO1 and FMO3 to the sulfoxidation of carbophenothion, demeton-O, ethiofencarb, fonofos, and methiocarb also was investigated by using baculovirus-expressed recombinant proteins. FMO1 showed the highest catalytic activity for all pesticides. This study indicates that FMO1 may have a bigger role in extrahepatic metabolism than previously thought.

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

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

In vitro sulfoxidation of thioether compounds by human cytochrome P450 and flavin-containing monooxygenase isoforms with particular reference to the CYP2C subfamily

Cytochrome P450 (P450) and flavin-containing monooxygenase (FMO) enzymes are major catalysts involved in the metabolism of xenobiotics. The sulfoxidation of the thioether pesticides, phorate, disulfoton, sulprofos, and methiocarb, was investigated. Using pooled human liver microsomes (HLMs), thioether compounds displayed similar affinities; however, phorate and disulfoton displayed higher intrinsic clearance rates than either sulprofos or methiocarb. The sulfoxidation of thioethers by HLMs was found to be predominantly P450-driven (85-90%) compared with FMO (10-15%). Among 16 cDNA-expressed human P450 isoforms and 3 human FMO isoforms examined, the following isoforms and their polymorphisms had the highest rates for sulfoxidation, as follows: phorate, CYP1A2, 3A4, 2B6, 2C9*1, 2C18, 2C19, 2D6*1, and FMO1; disulfoton, CYP1A2, 3A4, 2B6, 2C9*1, 2C9*2, 2C18, 2C19, 2D6*1, and FMO1; sulprofos, CYP1A1, 1A2, 3A4, 2C9*1, 2C9*2, 2C9*3, 2C18, 2C19, 2D6*1, and FMO1; methiocarb, CYP1A1, 1A2, 3A4, 2B6, 2C9*1, 2C19, 2D6*1, and FMO1. Among these isoforms, members of the CYP2C subfamily often had the highest affinities and clearance rates. Moreover, sulfaphenazole, a CYP2C9 competitive inhibitor, inhibited disulfoton sulfoxidation by CYP2C9 (IC50 0.84 microM) as well as in HLMs. Ticlopidine, a CYP2C19 mechanism-based inhibitor, inhibited disulfoton sulfoxidation by CYP2C19 (IC50 after coincubation, 43.5 microM; IC50 after preincubation, 4.3 microM) and also in HLMs. Our results indicate that current models of the substrate binding site of the CYP2C subfamily would not effectively predict thioether pesticide metabolism. Thus, the substrate specificity of CYP2Cs is more extensive than is currently believed, and some reevaluation of structure-activity relationships may be required.

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

Use of the human flavin-containing monooxygenases (FMOs) in drug design and discovery could represent a paradigm shift in drug development and basic research. Although FMOs have been previously viewed as minor contributors to drug metabolism, the advantages associated with using FMOs to diversify the metabolism of a drug are now being recognized. Because FMOs typically oxygenate a wide variety of nucleophilic compounds to polar, benign metabolites, and because drugs do not induce expression of FMOs or inhibit their activity, potential drug-drug interactions are minimized. Interindividual variation for this class of enzyme is largely dependent on genetic variation. Examples of FMO allelic variation and splicing variants suggest that these genetic mutations could contribute to the interindividual and interethnic variability of FMO-mediated metabolism.

Species differential stereoselective oxidation of a methylsulfide metabolite of MK-0767 [(+/-)-5-[(2,4-dioxothiazolidin-5-yl)methyl]-2-methoxy-N-[[(4-trifluoromethyl)phenyl]methyl]benzamide], a peroxisome proliferator-activated receptor dual agonist

MK-0767 [(+/-)-5-[(2,4-dioxothiazolidin-5-yl)methyl]-2-methoxy-N-[[(4-trifluoromethyl)phenyl]methyl]benzamide], a thiazolidinedione (TZD)-containing peroxisome proliferator-activated receptor agonist, is a rapidly interconverting racemate that possesses a chiral center at the five position of the TZD ring. M25 is a methyl sulfide metabolite generated from MK-0767 following CYP3A4-mediated TZD ring opening and subsequent methylation of the sulfide intermediate M22. M25, a major in vitro and in vivo metabolite, was further metabolized in liver microsomes to the methyl sulfoxide amide (M16) with two chiral centers and the methyl sulfone amide (M20) with one chiral center. Previous studies demonstrated that both CYP3A4 and flavin monooxygenase-3 (FMO3) catalyzed the formation of M16, whereas M20 was formed exclusively by CYP3A4. The relative contribution of CYP3A4 and FMO3 in the formation of M16 in human and preclinical species was evaluated by chiral analysis using supercritical fluid chromatography. No stereoselectivity was observed in incubations of M25 with human and rhesus liver and recombinant CYP3A4 microsomes, whereas a high degree of stereoselectivity (63 to >99% enantiomeric excess) was observed in rat and dog liver and human recombinant FMO3 microsomes. Also, polyclonal anti-rat CYP3A2 antibody and cytochrome P450 (P450) chemical inhibitors did not inhibit the oxidation of M25 in rat liver microsomes. Furthermore, M25 oxidation was more sensitive to heat inactivation at pH 8 and 8.7 in rat and dog liver microsomes than in human and monkey liver microsomes, consistent with the species difference in involvement of FMOs. Collectively, these results indicated that S-oxidation of M25 was catalyzed primarily by P450 enzymes in human and monkey liver microsomes and by FMO enzymes in rat and dog liver microsomes.

Alternative processing events in human FMO genes

In humans, flavin-containing monooxygenase (FMO) functional diversity is determined by the expression of five FMO genes, named FMO1 to FMO5, and their variants. In this study, we systematically analyzed transcripts of FMO1 to FMO5 in different human tissues by reverse-transcription-polymerase chain reaction and identified a large number of splice variants. Exon skipping was the major splicing event observed. Normally spliced transcripts were generally the prominent transcript detected. For FMO1, FMO2, and FMO3, two to three different splice variants were identified, and their corresponding expression was always low in the tissues examined. For FMO5 and particularly for FMO4, more complex alternative splice patterns were observed, with five and seven splice variants detected, respectively. Most identified FMO splice variants either caused a frame-shift or lacked essential functional sites. The corresponding transcripts were therefore incapable of encoding a functional enzyme and were not analyzed further in this study. However, a common in-frame exon 3- (exon 4 for FMO4) deleted variant, leading to the deletion of 63 amino acids, was identified for FMO1, FMO3, FMO4, and FMO5. To examine the functional importance of exon 3-deletion, FMO1, FMO3, FMO4 and the corresponding exon-deleted proteins were expressed as fusion proteins. Activity studies were done with two selective functional FMO substrates, methimazole, and 10-(N,N-dimethylaminopentyl)-2-(trifluoromethyl)phenothiazine and exon 3- (exon 4 for FMO4) deleted FMOs were not able to catalyze the S- and N-oxygenation of these substrates, respectively. It is not clear whether these FMO splice variants can oxygenate other substrates, including those that remain to be discovered.

Genetic variability at the human FMO1 locus: significance of a basal promoter yin yang 1 element polymorphism (FMO1*6)

The flavin-containing monooxygenases (FMOs) are important for the disposition of a variety of toxicants, therapeutics, and dietary components. Although FMO1 is the dominant isoform in fetal liver and adult kidney and intestine and despite up to a 10-fold intersubject variation in expression, a paucity of information is available on FMO1 genetic variability. To address this issue, 24 samples from the Coriell DNA Polymorphism Discovery Resource Panel were sequenced revealing 10 common single nucleotide polymorphisms (SNPs): four located upstream of the structural gene; three within exonic sequences; one within the intron 1 splice donor site; and two with the 3'-untranslated region. Six of these variants are novel. Compared with other FMO loci within the chromosome 1q23-25 cluster, FMO1 seems more highly conserved. Of the identified FMO1 SNPs, only a C>A transversion 9536 base pairs upstream of the exon 2 ATG start codon (g.-9536C>A) would likely affect function, because it lies within the conserved core binding sequence for the yin yang 1 (YY1) transcription factor. Electrophoretic mobility shift assays demonstrated that the g.-9536C>A transversion eliminated YY1 binding. Furthermore, data from transient expression assays in HepG2 cells suggested this SNP could account for a 2- to 3-fold loss of FMO1 promoter activity. Genotype analysis revealed a g.-9,536A allele (FMO1*6) frequency of 13 and 11% in African- and northern European-Americans, respectively, but a significantly higher frequency of 30% in Hispanic-Americans. Thus, the FMO1*6 variant may account for some of the observed interindividual variation in FMO1 expression.

Methimazole-induced damage in the olfactory mucosa: effects on ultrastructure and glutathione levels

Methimazole is an antithyroid drug that can induce loss of smell and taste in humans. It is also an olfactory toxicant in rodents. The aim of the present study was to examine involvement of glutathione in methimazole-induced damage of the olfactory mucosa (OM) of mice, and to study early onset of this damage using transmission electron microscopy (TEM). We found that an intraperitoneal dose of methimazole induced a dose-dependent decrease of nonprotein sulfhydryl groups (NP-SH; mainly glutathione) in the OM. Hepatic NP-SH was not decreased. One hour after administration (50 mg/kg), TEM demonstrated an extensive damage to acinar and intraepithelial excretory duct cells of Bowman's glands (BG) including dilatation of the endoplasmic reticulum and mitochondrial swelling. Furthermore, large vacuoles were noted in basal intraepithelial duct cells. After 2 hours there were ruptures of secretory granule membranes in BG and mitochondrial swelling and degeneration of sustentacular cells. The basal cells were less damaged. After four hours the neuroepithelium was disorganized although the columnar organization of neurons was largely intact. The acinar organization of the BG was frequently lost. The subsequent detachment of the neuroepithelium is suggested to be secondary to extensive damage of BG excretory ducts and sustentacular cells.

Identification of novel variants of the flavin-containing monooxygenase gene family in African Americans

Sequence polymorphisms in enzymes involved in drug metabolism have been widely implicated in the differences observed in the sensitivity to various xenobiotics. The flavin-containing monooxygenase (FMO) gene family in humans catalyzes the monooxygenation of numerous N-, P- and S-containing drugs, pesticides, and environmental toxicants. Six genes (FMO1-6) have been identified so far, but the major alleles of FMO2 and FMO6 encode nonfunctional proteins due to a nonsense mutation and splice-site abnormalities, respectively. Data on structural variants exist for human FMO2 and 3, whereas very little is known about the other FMO genes. FMO1-6 were scanned in 50 individuals of African-American descent using the method, detection of virtually all mutations-single-strand conformational polymorphism. A total of 49 sequence variants were identified in a total 1.35 megabases of scanned sequence, of which 29 were variants affecting protein structure or expression. Some of these are expected to affect the activity of the protein, including a nonsense mutation in FMO1 (R502X) and missense mutations in FMO1 (I303T), FMO4 (E339Q), and FMO5 (P457L) that occur in highly conserved amino acids. Additional deleterious substitutions in FMO2 (del337G) and FMO6 (Q105X) were also identified. Multiple structural variants in the FMO gene family were observed in this African-American sample. Some of the substitutions identified in this study might be useful markers in future association studies assessing sensitivity to environmental toxicants and common disease.

Human kidney flavin-containing monooxygenases and their potential roles in cysteine s-conjugate metabolism and nephrotoxicity

The potential roles of human hepatic and renal flavin-containing monooxygenases (FMOs) in the metabolism of the cysteine S-conjugates S-allyl cysteine (SAC) and S-(1,2-dichlorovinyl)-L-cysteine (DCVC) were investigated. Incubations of human cDNA-expressed FMO1, FMO3, FMO4, and FMO5 with SAC resulted in detection of SAC sulfoxide, with FMO3 exhibiting approximately 3-, 4-, and 10-fold higher activity than FMO1, FMO4, and FMO5, respectively. DCVC sulfoxide formation was only detected with FMO3 and was 59-fold lower than SAC sulfoxide formation. Incubations of human liver microsomes with SAC or DCVC resulted in detection of the corresponding sulfoxides and provided evidence for the involvement of FMO3. Incubations of SAC or DCVC with human kidney microsomes, however, led only to the detection of SAC sulfoxide. Immunoblots with monospecific antibodies to FMO1, FMO3, and FMO5 in kidney microsomes from 26 humans showed that the average expression levels for FMO1, FMO3, and FMO5 were 5.8 +/- 2.3, 0.5 +/- 0.4, and 2.4 +/- 1.4 pmol/mg (means +/- S.D.), respectively. Interestingly, African-American kidney samples (n = 8) exhibited significantly higher FMO1 levels than Caucasian samples (n = 17), whereas no difference in expression level between males and females was observed with any of the examined FMO isoforms. Collectively, the results provide evidence for the expression of three FMO isoforms in the human kidney and show that the contribution of renal FMOs in cysteine S-conjugate metabolism is likely to vary depending upon the cysteine S-conjugate and the relative expression levels of the active FMOs.

Interindividual differences of human flavin-containing monooxygenase 3: genetic polymorphisms and functional variation

The human flavin-containing monooxygenase (form 3) (FMO3) participates in the oxygenation of nucleophilic heteroatom-containing drugs, xenobiotics, and endogenous materials. Currently, six forms of the FMO gene are known, but it is FMO3 that is the major form in adult human liver that is likely responsible for the majority of FMO-mediated metabolism. The substrate structural feature requirements for human FMO3 is beginning to become known to a greater extent and a few chemicals extensively metabolized by FMO3 have been reported. Expression of FMO3 is species- and tissue-specific, but unlike human cytochrome p450, mammalian FMO3 does not appear to be inducible. Interindividual variation in FMO3-dependent metabolism of drugs, chemicals, and endogenous material is therefore more likely due to genetic effects and not environmental ones. Examples of such interindividual variation come from the study of very rare mutations of the human FMO3 gene that have been associated with deficient N-oxygenation of dietary trimethylamine. Defective trimethylamine N-oxygenation causes trimethylaminuria or "fish-like odor syndrome". Information on human FMO3 mutations from individuals suffering from the condition of trimethylaminuria has provided knowledge about the underlying molecular mechanism(s) for trimethylaminuria. A number of common variants of human FMO3 have been reported. Diversification of the FMO3 gene may have led to selective advantages and new functions. As more examples of human FMO3-mediated metabolism of drugs or new chemical entities are discovered in the future, it is possible that FMO3 allelic variation may be shown to contribute to interindividual and interethnic variability of FMO-mediated metabolism. Human FMO3 may be another example of an environmental gene that participates in a protective mechanism to help humans ward off potentially toxic exposure of chemicals.

Alternative processing of the human FMO6 gene renders transcripts incapable of encoding a functional flavin-containing monooxygenase

The flavin-containing monooxygenases (FMOs) are a family of five microsomal enzymes important for the oxidative metabolism of environmental toxicants, natural products, and therapeutics. With the exception of FMO5, the FMO are encoded within a single gene cluster on human chromosome 1q23-25. As part of the human genome effort, an FMO-like gene, FMO6, was identified between FMO3 and FMO2 (GenBank accession no. AL021026). Sequence analysis of this putative FMO family member revealed nothing that would a priori argue against a functional gene and encoded protein. When FMO6 expression was examined by reverse transcriptase coupled polymerase chain reaction DNA amplification, transcripts were identified in 8 of 11 human liver samples, but 0 of 4 kidney biopsy samples. However, in all cases, the observed transcripts were shorter than predicted. Sequence analysis revealed skipping of exon 4, exons 3 and 4, and/or the use of alternative splice donor or acceptor sites in introns 3, 4, 6, and 8, resulting in nine unique transcripts. Based on an analysis of possible open reading frames, none of these transcripts would encode a functional FMO enzyme. Taking advantage of the high sequence identity between FMO3 and FMO6, it is posited that the loss of binding sites for the serine-arginine-rich splicing factor protein family within exons 3 and 4 contributes to the exon skipping events, although the most commonly observed alternative splice event results from a 21-bp insertion immediately 3' to the predicted FMO6 intron 8 splice acceptor site, diminishing the efficiency of this site.

Human flavin-containing monooxygenase (form 3): polymorphisms and variations in chemical metabolism

The human flavin-containing monooxygenases catalyze the oxygenation of nucleophilic heteroatom-containing drugs, xenobiotics and endogenous materials. Evidence for six forms of the FMO gene exist but it is FMO form 3 (FMO3) that is the prominent form in adult human liver that is likely to be associated with the bulk of FMO-mediated metabolism. An understanding of the substrate specificity of human FMO3 is beginning to emerge and several examples of drugs and chemicals extensively metabolized by FMO3 have been reported. Expression of FMO3 is species- and tissue-specific, but unlike human cytochrome P450 (CYP450), mammalian FMO3 does not appear to be inducible. Interindividual variation in FMO3-dependent metabolism of drugs, chemicals and endogenous materials is therefore more likely to be due to genetic and not environmental effects. Certain mutations of the human FMO3 gene have been associated with abnormal N-oxygenation of trimethylamine. Deficient N-oxygenation of trimethylamine results in a condition called trimethylaminuria. Some treatment strategies for this inborn error of metabolism are discussed. Other common variants of the FMO3 gene including E158K, V257M and E308G have been observed. It is possible that allelic variation of human FMO3 causes abnormal metabolism of chemicals and has clinical implications for human drug metabolism, but this is an understudied area. Human FMO3 allelic variation may eventually be shown to contribute to interindividual and interethnic variability in FMO3-mediated metabolism. Human FMO3 may be another example of an environmental gene that participates in a protective mechanism to help shield humans from potentially toxic exposure to chemicals. Heterogeneity in the relative frequencies of single and multiple site alleles, haplotypes and genotypes of the human FMO3 amongst various ethnic groups suggests population differences.

Human hepatic flavin-containing monooxygenases 1 (FMO1) and 3 (FMO3) developmental expression

The flavin-containing monooxygenases (FMOs) are important for the metabolism of numerous therapeutics and toxicants. Six mammalian FMO genes (FMO1-6) have been identified, each exhibiting developmental and tissue- and species-specific expression patterns. Previous studies demonstrated that human hepatic FMO1 is restricted to the fetus whereas FMO3 is the major adult isoform. These studies failed to describe temporal expression patterns, the precise timing of the FMO1/FMO3 switch, or potential control mechanisms. To address these questions, FMO1 and FMO3 were quantified in microsomal fractions from 240 human liver samples representing ages from 8 wk gestation to 18 y using Western blotting. FMO1 expression was highest in the embryo (8-15 wk gestation; 7.8 +/- 5.3 pmol/mg protein). Low levels of FMO3 expression also were detectable in the embryo, but not in the fetus. FMO1 suppression occurred within 3 d postpartum in a process tightly coupled to birth, but not gestational age. The onset of FMO3 expression was highly variable, with most individuals failing to express this isoform during the neonatal period. FMO3 was detectable in most individuals by 1-2 y of age and was expressed at intermediate levels until 11 y (12.7 +/- 8.0 pmol/mg protein). These data suggest that birth is necessary, but not sufficient for the onset of FMO3 expression. A gender-independent increase in FMO3 expression was observed from 11 to 18 y of age (26.9 +/- 8.6 pmol/mg protein). Finally, 2- to 20-fold interindividual variation in FMO1 and FMO3 protein levels were observed, depending on the age bracket.

Identification of active flavin-containing monooxygenase isoform 2 in human lung and characterization of expressed protein

Full-length human (hFMO2.1) and monkey (mFMO2) flavin-containing monooxygenase proteins, which share 97% sequence identity, were produced by baculovirus-mediated expression in insect cells and assayed for S-oxygenation under conditions known to affect FMO activity. Both enzymes demonstrated maximal activity at pH 9.5; but hFMO2.1 retained significantly more activity than mFMO2 did at pH 9.0 and higher. hFMO2.1 also retained significantly more activity than mFMO2 did in the presence of magnesium and all detergents tested. Although hFMO2.1 had more residual activity after heating at 45 degrees C than mFMO2, under some conditions, both had less than 10% of control activity, whereas expressed rabbit FMO2 retained over 50% activity. Screening for NADPH-oxygenation by hFMO2.1, indicated that substituted thioureas with a small cross-sectional area (2.4-4.3 A) are good substrates, whereas 1,3-diphenylthiourea (11.2 A) was not oxygenated. We confirmed the presence of hFMO2.1 in lung tissue from a heterozygous individual (hFMO2*1/hFMO2*2A) by Western analysis and confirmed activity by S-oxygenation. These microsomes also demonstrated a heat-associated loss of activity similar to expressed hFMO2.1. The heat sensitivity of hFMO2.1 may partially explain why activity in post mortem human lung samples has previously been unreported. Individuals that have the FMO2*1 allele-encoding full-length hFMO2.1 may exhibit altered drug metabolism in the lung.

Regulation of flavin-containing monooxygenase 1 expression by ying yang 1 and hepatic nuclear factors 1 and 4

The flavin-containing monooxygenases (FMOs) are important for the oxidation of a variety of environmental toxicants, natural products, and therapeutics. Consisting of six family members (FMO1-5), these enzymes exhibit distinct but broad and overlapping substrate specificity and are expressed in a highly tissue- and species-selective manner. Corresponding to previously identified regulatory domains, a YY1 binding site was identified at the major rabbit FMO1 promoter, position -8 to -2, two overlapping HNF1alpha sites, position -132 to -105, and two HNF4alpha sites, position -467 to -454 and -195 to -182. Cotransfection studies with HNF1alpha and HNF4alpha expression vectors demonstrated a major role for each of these factors in enhancing FMO1 promoter activity. In contrast, YY1 was shown by site-directed mutagenesis to be dispensable for basal promoter activity but suppressed the ability of the upstream domains to enhance transcription. Finally, comparisons between rabbit and human FMO1 demonstrated conservation of each of these regulatory elements. With the exception of the most distal HNF4alpha site, each of the orthologous human sequences also was able to compete with rabbit FMO1 cis-elements for specific protein binding. These data are consistent with these same elements being important for regulating human FMO1 developmental- and tissue-specific expression.

Quantification and cellular localization of expression in human skin of genes encoding flavin-containing monooxygenases and cytochromes P450

The expression, in adult human skin, of genes encoding flavin-containing monooxygenases (FMOs) 1, 3, 4, and 5 and cytochromes P450 (CYPs) 2A6, 2B6, and 3A4 was determined by RNase protection. Each FMO and CYP exhibits inter-individual variation in expression in this organ. Of the individuals analysed, all contained CYP2B6 mRNA in their skin, 90% contained FMO5 mRNA and about half contained mRNAs encoding FMOs 1, 3, and 4, and CYPs 2A6 and 3A4. The amount of each of the FMO and CYP mRNAs in skin is much lower than in the organ in which it is most highly expressed, namely the kidney (for FMO1) and the liver (for the others). In contrast to the latter organs, in the skin FMO mRNAs are present in amounts similar to, or greater than, CYP mRNAs. Only the mRNA encoding CYP2B6 decreased in abundance in skin with increasing age of the individual. All of the mRNAs were substantially less abundant in cultures of keratinocytes than in samples of skin from which the cells were derived. In contrast, an immortalized human keratinocyte cell line, HaCaT, expressed FMO3, FMO5, and CYP2B6 mRNAs in amounts that fall within the range detected in the whole skin samples analysed. FMO1, CYP2A6, and CYP3A4 mRNAs were not detected in HaCaT cells, whereas FMO4 expression was markedly increased in this cell line compared to whole skin. In situ hybridization showed that the expression of each of the FMOs and CYPs analysed was localized to the epidermis, sebaceous glands and hair follicles.

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

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

Characterization of moclobemide N-oxidation in human liver microsomes

1. Moclobemide underdergoes morpholine ring N-oxidation to form a major metabolite in plasma Rol2-5637. 2. The kinetics of moclobemide N-oxidation in human liver microsomes (HLM) (n = 6) have been investigated and the mixed-function oxidase enzymes catalysing this reaction have been identified using inhibition, enzyme correlation, altered pH and heat pretreatment experiments. 3. N-oxidation followed single enzyme Michealis-Menten kinetics (0.02-4.0 mm). Km app and Vmax ranged from 0.48 to 1.35 mM (mean +/- SD) 0.77 +/- 0.34 mM) and 0.22 to 2.15 nmol mg(-1) min(-1) (1.39 +/- 0.80 nmol mg(-1) respectively. 4. The N-oxidation of moclobemide strongly correlated with benzydamine N-oxidation a probe reaction for flavin-containing monoxygenase (FMO) activity (0.1 mM moclobemide, rs = 0.81, p < 0.005; 4 mM moclobemide, rs = 0.94, p = 0.0001). Correlations were observed between moclobemide N-oxidation and specific cytochromre P450 (CYP) activities at both moclobemide concentrations (0.1 mM moclobemide, CYP2C19 0.66, p < 0.05; 4 mM moclobemide, CYP2E1 rs = 0.56, p < 0.05). 5. The general P450 inhibitor, N-benzylimidazole, did not affect the rate of Rol2-5637 formation (0% inhibition versus control) (at 1.3 mM moclobemide. Furthermore, the rate of Ro12-5637 formation in HLM was unaffected by inhibitors Or substrates of specific P450s (< 10% inhibition versus control). 6. Heat pretreatment of HLM in the absence of NADPH (inactivating FMOs) resulted in 97% inhibition of Ro12-5637 formation. N-oxidation activity was greatest when incubated at pH 8.5. These results ilre consistent with the reaction being FMO medialtetd . 7. In conclusion, moclobemide N-oxidation activity has been observed in HLM in vitro and the reaction is predominantly catalysed by FMOs with a potentially small contribution from cytochrome P450 isoforms.

Benzydamine N-oxidation as an index reaction reflecting FMO activity in human liver microsomes and impact of FMO3 polymorphisms on enzyme activity

AIMS

The role of flavin containing monooxygenases (FMO) on the disposition of many drugs has been insufficiently explored. In vitro and in vivo tests are required to study FMO activity in humans. Benzydamine (BZD) N-oxidation was evaluated as an index reaction for FMO as was the impact of genetic polymorphisms of FMO3 on activity.

METHODS

BZD was incubated with human liver microsomes (HLM) and recombinant enzymes. Human liver samples were genotyped using PCR-RFLP.

RESULTS

BZD N-oxide formation rates in HLM followed Michaelis-Menten kinetics (mean Km = 64.0 microM, mean Vmax = 6.9 nmol mg-1 protein min-1; n = 35). N-benzylimidazole, a nonspecific CYP inhibitor, and various CYP isoform selective inhibitors did not affect BZD N-oxidation. In contrast, formation of BZD N-oxide was almost abolished by heat treatment of microsomes in the absence of NADPH and strongly inhibited by methimazole, a competitive FMO inhibitor. Recombinant FMO3 and FMO1 (which is not expressed in human liver), but not FMO5, showed BZD N-oxidase activity. Respective Km values for FMO3 and FMO1 were 40.4 microM and 23.6 microM, and respective Vmax values for FMO3 and FMO1 were 29.1 and 40.8 nmol mg-1 protein min-1. Human liver samples (n = 35) were analysed for six known FMO3 polymorphisms. The variants I66M, P135L and E305X were not detected. Samples homozygous for the K158 variant showed significantly reduced Vmax values (median 2.7 nmol mg-1 protein min-1) compared to the carriers of at least one wild type allele (median 6.2 nmol mg-1 protein min-1) (P < 0.05, Mann-Whitney-U-test). The V257M and E308G substitutions had no effect on enzyme activity.

CONCLUSIONS

BZD N-oxidation in human liver is mainly catalysed by FMO3 and enzyme activity is affected by FMO3 genotype. BZD may be used as a model substrate for human liver FMO3 activity in vitro and may be further developed as an in vivo probe reflecting FMO3 activity.