Disease relevance of FMO1

• Oxidative activation of thiacetazone by the Mycobacterium tuberculosis flavin monooxygenase EtaA and human FMO1 and FMO3 [1].
• To determine the functional relevance of the coding FMO1 variants (H97Q, I303V, I303T, R502X), they were heterologously expressed using a baculovirus system [2].
• Cloning, sequencing, distribution, and expression in Escherichia coli of flavin-containing monooxygenase 1C1. Evidence for a third gene subfamily in rabbits [3].
• Thiourea (TU), phenylthiourea (PTU), and alpha-naphthylthiourea (ANTU) were toxic to FMO 3 cells but not to parental and FMO 1 clones; 50% toxicity was attained at 1x10(-4) M TU, 5x10(-6) M PTU, and 1x10(-6) M ANTU [4].
• These results suggested that the reduction of nicotine metabolism in cirrhosis was due to decreases in CYP and flavin-containing monooxygenase protein expression levels [5].

High impact information on FMO1

• Complementary DNA (cDNA) clones encoding the adult human liver flavin-containing monooxygenase (FMO; dimethylaniline N-oxidase, EC 1.14.13.8) were isolated from lambda gt10 and lambda gt11 libraries [6].
• Tamoxifen has been found to be metabolized by liver primarily into three metabolites, tamoxifen-N-oxide, formed by the flavin-containing monooxygenase, and N-desmethyl- and 4-hydroxytamoxifen, formed by cytochrome P450 [7].
• Exposure of phenobarbital-treated microsomes and control-microsomes to 50 degrees C for 90 s, which inactivates the flavin-containing monooxygenase (FMO), diminished binding and pH 8.6 enhanced binding [8].
• 2-[(2,4-Dichloro-6-phenyl)phenoxy]ethylamine, a potent cytochrome P-450 inhibitor, decreased microsomal N-OH-AF formation by 96, 83, 70, and 46% in the rat, dog, human, and pig, respectively; and further addition of methimazole, a high-affinity flavin-containing monooxygenase substrate, abolished the residual N-hydroxylating activity [9].
• Using the purified porcine flavin-containing monooxygenase, metabolic formation of N-OH-AF occurred at a rate of 4.9 nmol/min/nmol flavin adenine nucleotide and was insensitive to 2-[(2,4-dichloro-6-phenyl)phenoxy]ethylamine inhibitor [9].

Chemical compound and disease context of FMO1

• Expression in Escherichia coli of the flavin-containing monooxygenase D (form II) from adult human liver: determination of a distinct tertiary amine substrate specificity [10].
• Microsomes isolated from Spodoptera frugiperda (Sf)9 cells infected with human flavin-containing monooxygenase (FMO)1 recombinant baculovirus catalyzed the NADPH- and O2-dependent oxidation of methimazole, thiourea, and phenylthiourea [11].
• Characterization of two human flavin-containing monooxygenase (form 3) enzymes expressed in Escherichia coli as maltose binding protein fusions [12].

Biological context of FMO1

• Analysis of human metaphase chromosomes by in situ hybridization confirmed the mapping of FMO1 and localized this gene more precisely to 1q23-q25 [13].
• A previous single nucleotide polymorphism (SNP) analysis of FMO1 in African-Americans identified seven novel SNPs [2].
• The substrate specificity for FMO 3 is distinct from that of FMO1 [14].
• A nomenclature for the mammalian flavin-containing monooxygenase gene family based on amino acid sequence identities [15].
• FMO1 expression was highest in the embryo (8-15 wk gestation; 7.8 +/- 5.3 pmol/mg protein) [16].

Anatomical context of FMO1

• For comparison, the N- and S-oxygenation of the same chemicals and drugs were examined with adult human liver microsomes from a normal healthy female donor and FMO1 from pig liver and FMO2 from rabbit lung [17].
• Metabolism of methyl p-tolyl sulfide by detergent-solubilized hepatic microsomes from a wide variety of experimental animals yielded predominantly (R)- methyl p-tolyl sulfoxide, which, at least in rabbit liver, is indicative of catalysis dominated by FMO1 [18].
• Human FMO1 and FMO3 cDNAs were functionally expressed in prokaryotic and eukaryotic cells [19].

Associations of FMO1 with chemical compounds

• Although (S)-cotinine was observed to inhibit pig FMO1 (Ki = 675 microM), partially purified cDNA-expressed adult human liver FMO3 was not inhibited by (S)-cotinine [20].
• The I303V variant showed catalytic constants equal to wild-type FMO1 for methimazole and methyl p-tolyl sulfide [2].
• All the variants demonstrated the same stereoselectivity for methyl p-tolyl sulfide oxidation as wild-type FMO1 [2].
• CONCLUSIONS: FMO1 and FMO3 catalyse benzydamine N-oxygenation with the highest efficiency [21].
• FMO1 also metabolized the commonly used insecticide fenthion to its (+)-sulfoxide, with relatively high catalytic efficiency [2].

Regulatory relationships of FMO1

• The conclusion that FMO was predominantly responsible for trans oxime formation in human liver microsomes was based on the effect of incubation conditions on tyramine N-oxygenation and the observation that cDNA-expressed human FMO3 also N-oxygenated tyramine to give exclusively the trans oxime [22].
• 2. Thermal inactivation and antibodies to NADPH P450 reductase were used to selectively inactivate FMO and P450 respectively [23].

Other interactions of FMO1

• For FMO1, FMO2, and FMO3, two to three different splice variants were identified, and their corresponding expression was always low in the tissues examined [24].
• 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 [24].
• 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 [24].
• Localization of human flavin-containing monooxygenase genes FMO2 and FMO5 to chromosome 1q [25].
• FMO seems to be the major enzymatic oxidant, whereas several P450 enzymes and even reduced hemoglobin are capable of reducing TNO back to TAM [26].

Analytical, diagnostic and therapeutic context of FMO1

• Southern blot hybridization of human genomic DNA suggests that the protein is encoded by a single gene, which has been designated FMO1 and mapped to chromosome 1 [27].
• Northern blot analysis demonstrated that human FMO1 mRNA is more abundant in fetal than in adult liver, indicating that in man the enzyme is subject to developmental regulation [27].
• 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 [16].
• The amino acid substitutions were introduced into wild-type FMO3 or FMO1 cDNA by site-directed mutagenesis [28].
• 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 [29].

References