Mammalian FMO: Structure/function, genetic polymorphism and role in drug metabolism

Department of Environmental and Molecular Toxicology and The Linus Pauling Institute, Oregon State University, USA.
Pharmacology [?] Therapeutics (Impact Factor: 9.72). 07/2005; 106(3):357-87. DOI: 10.1016/j.pharmthera.2005.01.001
Source: PubMed


Flavin-containing monooxygenase (FMO) oxygenates drugs and xenobiotics containing a "soft-nucleophile", usually nitrogen or sulfur. FMO, like cytochrome P450 (CYP), is a monooxygenase, utilizing the reducing equivalents of NADPH to reduce 1 atom of molecular oxygen to water, while the other atom is used to oxidize the substrate. FMO and CYP also exhibit similar tissue and cellular location, molecular weight, substrate specificity, and exist as multiple enzymes under developmental control. The human FMO functional gene family is much smaller (5 families each with a single member) than CYP. FMO does not require a reductase to transfer electrons from NADPH and the catalytic cycle of the 2 monooxygenases is strikingly different. Another distinction is the lack of induction of FMOs by xenobiotics. In general, CYP is the major contributor to oxidative xenobiotic metabolism. However, FMO activity may be of significance in a number of cases and should not be overlooked. FMO and CYP have overlapping substrate specificities, but often yield distinct metabolites with potentially significant toxicological/pharmacological consequences. The physiological function(s) of FMO are poorly understood. Three of the 5 expressed human FMO genes, FMO1, FMO2 and FMO3, exhibit genetic polymorphisms. The most studied of these is FMO3 (adult human liver) in which mutant alleles contribute to the disease known as trimethylaminuria. The consequences of these FMO genetic polymorphisms in drug metabolism and human health are areas of research requiring further exploration.

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    • "In human liver, transcription factors regulating constitutive FMO3 expression as well as those involved in developmental expression pattern have been extensively studied (Klick and Hines, 2007; Klick et al., 2008; Shimizu et al., 2008). Because the mammalian FMOs were considered non-inducible by xenobiotics (Cashman and Zhang, 2002; Krueger and Williams, 2005), the transcriptional regulation of FMO involving stressactivated transcription factors or receptors that bind ligands and interact with DNA was not studied as other forms of regulation. Thus, little is known about the transcriptional regulation of Fmo3 in response to toxicant exposure. "
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    ABSTRACT: Flavin-containing monooxygenase-3 (FMO3) catalyzes metabolic reactions similar to cytochrome P450 monooxygenase, however, most metabolites of FMO3 are considered non-toxic. Recent findings in our laboratory demonstrated Fmo3 gene induction following toxic acetaminophen (APAP) treatment in mice. The goal of this study was to evaluate Fmo3 gene expression in other diverse mouse models of hepatic oxidative stress and injury. Fmo3 gene regulation by Nrf2 was also investigated using Nrf2 knockout (Nrf2 KO) mice. In our studies, male C57BL/6J mice were treated with toxic doses of hepatotoxicants or underwent bile duct ligation (BDL, 10 days). Hepatotoxicants included APAP (400 mg/kg, 24–72 h), alpha-naphthyl isothiocyanate (ANIT; 50 mg/kg, 2–48 h), carbon tetrachloride (CCl4; 10 or 30 μL/kg, 24 and 48 h) and allyl alcohol (AlOH; 30 or 60 mg/kg, 6 and 24 h). Because oxidative stress activates nuclear factor (erythroid-derived 2)-like 2 (Nrf2), additional studies investigated Fmo3 gene regulation by Nrf2 using Nrf2 knockout (Nrf2 KO) mice. At appropriate time-points, blood and liver samples were collected for assessment of plasma alanine aminotransferase (ALT) activity, plasma and hepatic bile acid levels, as well as liver Fmo3 mRNA and protein expression. Fmo3 mRNA expression increased significantly by 43-fold at 12 h after ANIT treatment, and this increase translates to a 4-fold change in protein levels. BDL also increased Fmo3 mRNA expression by 1899-fold, but with no change in protein levels. Treatment of mice with CCl4 decreased liver Fmo3 gene expression, while no change in expression was detected with AlOH treatment. Nrf2 KO mice are more susceptible to APAP (400 mg/kg, 72 h) treatment compared to their wild-type (WT) counterparts, which is evidenced by greater plasma ALT activity. The Fmo3 mRNA and protein expression increased in Nrf2 KO mice after APAP treatment. Collectively, not all hepatotoxicants that produce oxidative stress alter Fmo3 gene expression. Along with APAP, toxic ANIT treatment in mice markedly increased Fmo3 gene expression. While BDL increased the Fmo3 mRNA expression, the protein level did not change. The discrepancy with Fmo3 induction in cholestatic models, ANIT and BDL, is not entirely clear. Results from Nrf2 KO mice with APAP suggest that the transcriptional regulation of Fmo3 during liver injury may not involve Nrf2.
    Toxicology 11/2014; 325. DOI:10.1016/j.tox.2014.08.013 · 3.62 Impact Factor
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    • "Oxidative stress resulting from the enhanced production of reactive oxygen species (ROS) and decreased activity of antioxidant defense enzymes has been implicated in pathogenesis of essential hypertension (EH) [1] [2]. The flavin-containing monooxygenase 3 (FMO3) is a microsomal antioxidant defense enzyme involving the NADPHdependent oxygenation of a variety of nucleophilic xenobiotics possessing oxidant capacity [3]. Mutations in the FMO3 gene have been found to contribute to the disease trimethylaminuria (TMAuria), an inborn error of metabolism resulting from diminished oxidation of the tertiary amine trimethylamine to trimethylamine N-oxide resulting in a severe body odour in affected individuals [4]. "
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    ABSTRACT: Gene encoding flavin-containing monooxygenase 3 (FMO3), a microsomal antioxidant defense enzyme, has been suggested to contribute to essential hypertension (EH). The present study was designed to investigate whether common functional polymorphism E158K (rs2266782) of the FMO3 gene is associated with EH susceptibility in a Russian population. A total of 2 995 unrelated subjects from Kursk (1 362 EH patients and 843 healthy controls) and Belgorod (357 EH patients and 422 population controls) regions of Central Russia were recruited for this study. DNA samples from all study participants were genotyped for the FMO3 gene polymorphism through PCR followed by RFLP analysis. We found that the polymorphism E158K is associated with increased risk of essential hypertension in both discovery population from Kursk region (OR 1.36 95% CI 1.09-1.69, P = 0.01) and replication population from Belgorod region (OR 1.54 95% CI 1.07-1.89, P = 0.02) after adjustment for gender and age using logistic regression analysis. Further analysis showed that the increased hypertension risk in carriers of genotype 158KK gene occurred in cigarette smokers, whereas nonsmoker carriers of this genotype did not show the disease risk. This is the first study reporting the association of the FMO3 gene polymorphism and the risk of essential hypertension.
    International Journal of Hypertension 08/2014; 2014:712169. DOI:10.1155/2014/712169
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    • "The observed interspecies variation of enniatin B biotransformation may either reflect diverging levels of the relevant cytochrome P450 isoforms or the fact that the same compound can be substrate to different monooxygenase families, resulting in the production of different metabolites with potentially significant toxicological consequences (Gupta and Abou-Donia, 1998; Krueger and Williams, 2005). FMO is known to be co-expressed in liver microsomes with haeme-containing cytochrome P450 monooxygenases (CYPs) (Chung et al., 1998; Iyer and Sinz, 1999). "
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    ABSTRACT: Enniatins, a large group of cyclodepsipeptides, are widely distributed contaminants of different crops intended for human and animal consumption. Enniatin B is one of the principal analogues in species of the genus Fusarium, known to have ionophoric, antibiotic, and insecticidal activity. Regardless of considerable cytotoxic effects observed in vitro, enniatins have been characterised as compounds with low acute toxicity in vivo. The biotransformation of enniatin B has previously been elucidated in liver microsomes, and 12 different metabolites (M1 to M12) have been reported. In order to provide a better basis for understanding the potential toxic effects in humans and animals, different samples (eggs, livers, plasma) from two different feeding studies have been analysed for the presence of enniatin B and its hepatic metabolites. The earlier reported metabolite M11, and a novel metabolite (designated M13), were dominant in liver samples from enniatin B exposed broilers. The peak area corresponding to the sodiated molecular ion of M1 1 was approximately 2.5 times larger than that of parent enniatin B in liver samples collected after one week of exposure. The same metabolites were also present in serum samples. In egg samples, only metabolites M13 and M4 were detected. The comparison of mass spectrometric data of M13 and enniatin B suggested that M13 is a monohydroxylated metabolite. The hepatic biotransformation of enniatin B. was also investigated in vitro in chicken microsomes demonstrating good correlation with the metabolite profiles in the chicken samples. The results of the present study demonstrated an extensive biotransformation of enniatin B in vivo confirming previously reported in vitro data.
    World Mycotoxin Journal 01/2014; 7(2):167. DOI:10.3920/WMJ2013.1609 · 2.16 Impact Factor
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