Purification of macaque liver flavin-containing monooxygenase: a form of the enzyme related immunochemically to an isozyme expressed selectively in adult human liver

Proteomics-Based Identification of Interaction Partners of the Xenobiotic Detoxification Enzyme FMO3 Reveals Involvement in Urea Cycle

The hepatic xenobiotic metabolizing enzyme flavin-containing monooxygenase 3 (FMO3) has been implicated in the development of cardiometabolic disease primarily due to its enzymatic product trimethylamine-N oxide (TMAO), which has recently been shown to be associated with multiple chronic diseases, including kidney and coronary artery diseases. Although TMAO may have causative roles as a pro-inflammatory mediator, the possibility for roles in metabolic disease for FMO3, irrespective of TMAO formation, does exist. We hypothesized that FMO3 may interact with other proteins known to be involved in cardiometabolic diseases and that modulating the expression of FMO3 may impact on these interaction partners. Here, we combine a co-immunoprecipitation strategy coupled to unbiased proteomic workflow to report a novel protein:protein interaction network for FMO3. We identified 51 FMO3 protein interaction partners, and through gene ontology analysis, have identified urea cycle as an enriched pathway. Using mice deficient in FMO3 on two separate backgrounds, we validated and further investigated expressional and functional associations between FMO3 and the identified urea cycle genes. FMO3-deficient mice showed hepatic overexpression of carbamoylphosphate synthetase (CPS1), the rate-limiting gene of urea cycle, and increased hepatic urea levels, especially in mice of FVB (Friend leukemia virus B strain) background. Finally, overexpression of FMO3 in murine AML12 hepatocytes led to downregulation of CPS1. Although there is past literature linking TMAO to urea cycle, this is the first published work showing that FMO3 and CPS1 may directly interact, implicating a role for FMO3 in chronic kidney disease irrespective of TMAO formation.

Flavin-containing monooxygenases in aging and disease: Emerging roles for ancient enzymes

Flavin-containing monooxygenases (FMOs) are primarily studied as xenobiotic metabolizing enzymes with a prominent role in drug metabolism. In contrast, endogenous functions and substrates of FMOs are less well understood. A growing body of recent evidence, however, implicates FMOs in aging, several diseases, and metabolic pathways. The evidence suggests an important role for these well-conserved proteins in multiple processes and raises questions about the endogenous substrate(s) and regulation of FMOs. Here, we present an overview of evidence for FMOs' involvement in aging and disease, discussing the biological context and arguing for increased investigation into the function of these enzymes.

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

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

Species differences in the biotransformation of an alpha 4 beta 2 nicotinic acetylcholine receptor partial agonist: the effects of distinct glucuronide metabolites on overall compound disposition

The metabolism and disposition of (1R,5S)-2,3,4,5-tetrahydro-7-(trifluoromethyl)-1,5-methano-1H-3-benzazepine (1), an alpha(4)beta(2) nicotinic acetylcholine receptor partial agonist, was investigated in Sprague-Dawley rats and cynomolgus monkeys receiving (1R,5S)-2,3,4,5-tetrahydro-7-(trifluoromethyl)-1,5-methano-1H-4[(14)C]-3- benzazepine hydrochloride ([(14)C]1) orally. Although both species chiefly (>or=62%) cleared 1 metabolically, species-specific dispositional profiles were observed for both 1 and total radioactivity. Radioactivity was excreted equally in the urine and feces of intact rats but largely (72%) in bile in bile duct-cannulated animals. In monkeys, radioactivity recoveries were 50-fold greater in urine than feces and minimal (<5%) in bile. Both species metabolized 1 similarly: four-electron oxidation to one of four amino acids or two lactams (minor) and glucuronide formation (major). In rats, the latter pathway predominantly formed an N-carbamoyl glucuronide (M6), exclusively present in bile (69% of dose), whereas in monkeys it afforded an N-O-glucuronide (M5), a minor biliary component (4%) but the major plasma (62%) and urinary (42%) entity. In rats, first-pass hepatic conversion of 1 to M6, which was confirmed in rat hepatocytes, and its biliary secretion resulted in the indirect enterohepatic cycling of 1 via M6 and manifested in double-humped plasma concentration-time curves and long t(1/2) for both 1 and total radioactivity. In monkeys, in which only M5 was formed, double-humped plasma concentration-time curves were absent, and moderate t(1/2) for both 1 and total radioactivity were observed. A seemingly subtle, yet critical, difference in the chemical structures of these two glucuronide metabolites considerably affected the overall disposition of 1 in rats versus monkeys.

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.

Purification and characterization of flavin-containing monooxygenase isoform 3 from rat kidney microsomes

Rats are a common animal model for metabolism and toxicity studies. Previously, the enzymatic properties of rat flavin-containing monooxygenase (FMO) 1 purified from hepatic and renal microsomes and that of FMO3 purified from hepatic microsomes were characterized. This study investigated the physical, immunological, and enzymatic properties of FMO3 purified from male rat kidney microsomes and compared the results with those obtained with isolated rat liver FMO3. Renal FMO3 was purified via affinity columns based on the elution of L-methionine (Met) S-oxidase activity and reactivity of the eluted proteins with human FMO3 antibody. In general, Met S-oxidase-specific activity was increased 100-fold through the purification steps. The resulting protein had similar mobility (approximately 56 kDa) as isolated rat liver FMO3 and cDNA-expressed human FMO3 by SDS-polyacrylamide gel electrophoresis. When the isolated kidney protein band was subjected to trypsin digestion and matrix-assisted laser desorption ionization/time of flight mass spectral analysis, 34% of the sequence of rat FMO3 was detected. The apparent K(m) and V(max) values for rat kidney FMO3 were determined using the known FMO substrates Met, seleno-L-methionine, S-allyl-L-cysteine (SAC), and methimazole (N-methyl-2-mercaptoimidazole). The stereoselectivity of the reactions with Met and SAC were also examined using high-performance liquid chromatography. The obtained kinetic and stereoselectivity results were similar to those we obtained in the present study, or those previously reported, for rat liver FMO3. Taken together, the results demonstrate many similar properties between rat hepatic and renal FMO3 forms and suggest that renal FMO3 may play an important role in kidney metabolism of xenobiotics containing sulfur and selenium atoms.

Homeostatic Signature of Anabolic Steroids in Cattle Using1H−13C HMBC NMR Metabonomics

We used metabonomics to discriminate the urinary signature of different anabolic steroid treatments in cattle having different physiological backgrounds (age, sex, and race). (1)H-(13)C heteronuclear multiple bonding connectivity NMR spectroscopy and multivariate statistical methods reveal that metabolites such as trimethylamine-N-oxide, dimethylamine, hippurate, creatine, creatinine, and citrate characterize the biological fingerprint of anabolic treatment. These urinary biomarkers suggest an overall homeostatic adaptation in nitrogen and energy metabolism. From results obtained in this study, it is now possible to consider metabonomics as a complementary method usable to improve doping control strategies to detect fraudulent anabolic treatment in cattle since the oriented global metabolic response provides helpful discrimination.

Mammalian flavin-containing monooxygenases: structure/function, genetic polymorphisms and role in drug metabolism

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

Potential roles of flavin-containing monooxygenases in sulfoxidation reactions of l-methionine, N-acetyl-l-methionine and peptides containing l-methionine

Flavin-containing monooxygenases (FMOs) are microsomal enzymes that catalyze the NADPH-dependent oxidation of a large number of sulfur-, selenium-, and nitrogen-containing compounds. Five active isoforms (FMO1-5) have been identified and shown to be differently expressed in various mammalian tissues. Previous work from this laboratory has shown l-methionine to be S-oxidized by rat, rabbit and human FMO1-4, with FMO3 exhibiting the highest stereoselectivity for the formation of the d-diastereomer of methionine sulfoxide. In this report, we describe new studies aimed at determining if N-acetyl-l-methionine and peptides containing l-methionine can be substrates for FMOs. Experiments were carried out using either rabbit liver microsomes or human cDNA-expressed FMOs. The results show that while N-acetyl-l-methionine and peptides with a modified methionine amino group may not function as substrates for FMOs, peptides containing a free N-terminal methionine may act as FMO substrates. With human cDNA-expressed FMO1, FMO3, and FMO5, both FMO1 and FMO3 exhibited activity with the active peptides whereas FMO5 was inactive. With FMO3, the activity measured with methionine was similar (1 mM) or higher (5 mM) than the activity measured with H-Met-Val-OH and H-Met-Phe-OH. With FMO1, H-Met-Phe-OH and methionine exhibited similar activities whereas activity with H-Met-Val-OH was much lower. Collectively, the results show that FMOs can oxidize peptides containing a free N-terminal methionine. Thus, the role of FMOs in the oxidation of methionine in larger peptides or proteins warrants further investigation.

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.

Developmental Expression of Human Hepatic CYP2C9 and CYP2C19

The CYP2C subfamily is responsible for metabolizing many important drugs and accounts for about 20% of the cytochrome p450 in adult liver. To determine developmental expression patterns, liver microsomal CYP2C9 and -2C19 were measured (n = 237; ages, 8 weeks gestation-18 years) by Western blotting and with diclofenac or mephenytoin, respectively, as probe substrates. CYP2C9-specific content and catalytic activity were consistent with expression at 1 to 2% of mature values (i.e., specific content, 18.3 pmol/mg protein and n = 79; specific activity, 549.5 pmol/mg/min and n = 72) during the first trimester, with progressive increases during the second and third trimesters to levels approximately 30% of mature values. From birth to 5 months, CYP2C9 protein values varied 35-fold and were significantly higher than those observed during the late fetal period, with 51% of samples exhibiting values commensurate with mature levels. Less variable CYP2C9 protein and activity values were observed between 5 months and 18 years. CYP2C19 protein and catalytic activities that were 12 to 15% of mature values (i.e., specific content, 14.6 pmol/mg and n = 20; specific activity, 18.5 pmol/mg/min and n = 19) were observed as early as 8 weeks of gestation and were similar throughout the prenatal period. CYP2C19 expression did not change at birth, increased linearly over the first 5 postnatal months, and varied 21-fold from 5 months to 10 years. Adult CYP2C19 protein and activity values were observed in samples older than 10 years. The ontogeny of CYP2C9 and -2C19 were dissimilar among both fetal and 0- to 5-months postnatal samples, implying different developmental regulatory mechanisms.

Developmental regulation of flavin-containing monooxygenase form 1 in the liver and kidney of fetal and neonatal rabbits

Flavin-containing monooxygenases (FMOs) comprise a multi-gene family and catalyze the oxygenation of soft nucleophilic sulfur, nitrogen, phosphorus, and selenium in xenobiotics. Previous studies have demonstrated that FMO is regulated developmentally and by the administration of certain steroid hormones. This study examined the expression of FMO form 1 in the livers and kidneys of fetal and neonatal rabbits, from day 25 of gestation through 3 weeks of age, by assaying FMO1 mRNA and protein levels, as well as catalytic activity. FMO1 mRNA and protein expression and FMO catalytic activity were present in fetal livers at the earliest time point measured (day 25 of gestation), although at levels approximately 10% of that found in adult livers. Hepatic FMO1 mRNA levels increased during and after gestation; levels were not significantly different from those measured in adult male livers. FMO1 protein content and activity rose rapidly after birth to reach 70-80% of adult levels by 3 weeks of age. The expression of FMO1 in fetal and neonatal kidneys was markedly lower than in liver. FMO1 mRNA levels never averaged more than 3.4% of adult male liver levels, but did not differ from adult kidney levels at any of the points measured. Protein levels and enzyme activity rose significantly after birth to approximately 30% of the level in adult kidneys by 3 weeks of age. The early developmental appearance of FMO1 suggests a possible role in the metabolism of xenobiotics through transplacental or lactational exposures.

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.

A Mutation in the Flavin-containing Monooxygenase 3 Gene and its Effects on Catalytic Activity for N-oxidation of Trimethylamine In Vitro

To clarify the mutation of the flavin-containing monooxygenase (FMO) 3 gene causing fish-odor syndrome, we analyzed the FMO3 gene of a Thai subject who possibly suffered from fish-odor syndrome. A novel mutation, a single-base substitution from G to A at the position of 265 (G265A), was identified in exon 3. The mutation caused an amino acid substitution from valine to isoleucine at residue 58 (V58I). The mutated FMO3 protein with V58I exhibited the reduced trimethylamine N-oxidase activity when it was expressed in E. coli. The V(max)/K(m) value for the activity of the mutant-type FMO3 was about 5 times lower than that for the wild-type FMO3.

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.

Stereoselective sulfoxidation of sulindac sulfide by flavin-containing monooxygenases. Comparison of human liver and kidney microsomes and mammalian enzymes

The stereoselective sulfoxidation of the pharmacologically active metabolite of sulindac, sulindac sulfide, was characterized in human liver, kidney, and cDNA-expressed enzymes. Kinetic parameter estimates (pH = 7.4) for sulindac sulfoxide formation in human liver microsomes (N = 4) for R- and S-sulindac sulfoxide were V(max) = 1.5 +/- 0.50 nmol/min/mg, K(m) = 15 +/- 5.1 microM; and V(max) = 1.1 +/- 0.36 nmol/min/mg, K(m) = 16 +/- 6.1 microM, respectively. Kidney microsomes (N = 3) produced parameter estimates (pH = 7.4) of V(max) = 0.9 +/- 0.29 nmol/min/mg, K(m) = 15 +/- 2.9 microM; V(max) = 0.5 +/- 0.21 nmol/min/mg, K(m) = 22 +/- 1.9 microM for R- and S-sulindac sulfoxide, respectively. In human liver and flavin-containing monooxygenase 3 (FMO3) the V(max) for R-sulindac sulfoxide increased 60-70% at pH = 8.5, but for S-sulindac sulfoxide was unchanged. In fourteen liver microsomal preparations, significant correlations occurred between R-sulindac sulfoxide formation and either immunoquantified FMO or nicotine N-oxidation (r = 0.88 and 0.83; P < 0.01). The R- and S-sulindac sulfoxide formation rate also correlated significantly (r = 0.85 and 0.75; P < 0.01) with immunoquantified FMO in thirteen kidney microsomal samples. Mild heat deactivation of microsomes reduced activity by 30-60%, and a loss in stereoselectivity was observed. Methimazole was a potent and nonstereoselective inhibitor of sulfoxidation in liver and kidney microsomes. n-Octylamine and membrane solubilization with lubrol were potent and selective inhibitors of S-sulindac sulfoxide formation. cDNA-expressed CYPs failed to appreciably sulfoxidate sulindac sulfide, and CYP inhibitors were ineffective in suppressing catalytic activity. Purified mini-pig liver FMO1, rabbit lung FMO2, and human cDNA-expressed FMO3 efficiently oxidized sulindac sulfide with a high degree of stereoselectivity towards the R-isomer, but FMO5 lacked catalytic activity. The biotransformation of the sulfide to the sulfoxide is catalyzed predominately by FMOs and may prove to be useful in characterizing FMO activity.

The effect of arginine-428 mutation on modulation of activity of human liver flavin monooxygenase 3 (FMO3) by imipramine and chlorpromazine

This study was carried out to investigate the molecular basis for modulation of recombinant FMO3-catalyzed activity by the tricyclic antidepressants, imipramine and chlorpromazine. A mutant of human liver FMO3 (T428R) was formed by site-directed mutagenesis and characterized along with the native enzyme in order to elucidate a possible structure-function relationship. Functional properties of native and T428R human FMO3s were studied with methimazole as substrate. Both enzymes catalyzed the S-oxidation of methimazole with the same Km value. Imipramine modulated the activities of the native and T428R human FMO3s differently; the activity of the native FMO3 was increased at all concentrations, whereas the activity of the mutant enzyme was inhibited at concentrations above 300 microM. Chlorpromazine activated the native enzyme at all concentrations of methimazole but activated the mutant enzyme only at high substrate concentrations. The direction (activation or inhibition) and extend of modulation of FMO3 activity is not only dependent on the concentration of the modulator, it is also dependent on the substrate concentration. This study confirms our previous findings with FMO1 that position 428 is important in the interaction of the FMO with modulators.

Modulation of human flavin-containing monooxygenase 3 activity by tricyclic antidepressants and other agents: importance of residue 428

Human flavin-containing monooxygenase 3 (FMO3) is subject to modulation by tricyclic antidepressants and other agents. Imipramine activates FMO3-catalyzed metabolism of methimazole at all substrate concentrations tested. This distinguishes FMO3 from rabbit FMO1 and FMO2, which are activated at high substrate concentration and inhibited at low substrate concentration, and pig FMO1, which is inhibited at all substrate concentrations. The response of FMO3 is also unique in that chlorpromazine is markedly more effective as a modulator than is imipramine. n-Octylamine, MgCl2, and HgCl2 all inhibit FMO3, the first two in a biphasic manner. Substitution of lysine for threonine at position 428 significantly alters the response of FMO3 to modulators without changing the kinetic parameters for the metabolism of the substrate. Activation by imipramine and chlorpromazine is reduced or abolished and inhibition, most obvious at low substrate concentrations, is observed. This is consistent with elimination of self-activation in the metabolism of imipramine. The mutation at 428 also eliminates the biphasic nature of the inhibition by n-octylamine and MgCl2, but does not alter the effect of HgCl2. Our findings show that the activity of FMO3 can be modulated by large drug molecules as well as short-chain amines and metal ions. This modulation can be markedly altered by changing a single amino acid in the enzyme.

P450-catalyzed in-chain desaturation of valproic acid: isoform selectivity and mechanism of formation of Delta 3-valproic acid generated by baculovirus-expressed CYP3A1

The mechanism of formation of the in-chain, unsaturated fatty acid metabolite, Delta3-valproic acid (Delta3-VPA) by rat liver microsomes was examined. Microsomal rates of formation of Delta3-VPA were below quantifiable limits in reactions catalyzed by control female rat liver microsomes, but were induced more than 20-fold following pretreatment with triacetyloleandomycin and pregnenolone-16alpha-carbonitrile. Microsomal incubations conducted with 3-hydroxy-VPA or [2-2H1]VPA demonstrated that Delta3-VPA did not arise by dehydration of preformed alcohol nor was it reversibly isomerized to Delta2-VPA. CYP3A1 expression was optimized in the baculovirus expression vector system, and infected insect cell membranes which were supplemented with P450 reductase catalyzed formation of 3-OH-, 4-OH-, 5-OH-, Delta3-, and Delta4-VPA in ratios of 160:35:6:3:1. Intramolecular deuterium isotope effects on metabolite formation, determined with cDNA-expressed CYP3A1 and either [3,3-2H2]VPA or [4,4-2H2]VPA, yielded kH/kD values for Delta3-VPA of 2.00 +/- 0.06 and 2.36 +/- 0.08, respectively. These values were significantly lower than the isotope effects observed in the same incubations for 3-OH-VPA formation from 3,3-D2-VPA (kH/kD = 6.04 +/- 0.08), or for 4-OH- and Delta4-VPA formation from 4, 4-D2-VPA (kH/kD > 5). Collectively, these data demonstrate the existence of a microsomal P450-dependent in-chain fatty acid desaturase system distinct from the well-documented cytochrome b5-linked CoA desaturases and suggest further that CYP3A1-dependent formation of Delta3-VPA arises via nonselective, initial hydrogen atom abstraction from either the C-3 or the C-4 position.

Molecular cloning, sequencing, and expression in Escherichia coli of mouse flavin-containing monooxygenase 3 (FMO3): comparison with the human isoform

The sequence of mouse flavin-containing monooxygenase 3 (FMO3) was obtained from several clones isolated from a mouse liver cDNA library. The nucleotide sequence of mouse FMO3 was 2020 bases in length containing 37 bases in the 5' flanking region, 1602 in the coding region, and 381 in the 3' flanking region. The derived protein sequence consisted of 534 amino acids including the putative flavin adenine dinucleotide and NADP+ pyrophosphate binding sites (characteristic of mammalian FMOs) starting at positions 9 and 191, respectively. The mouse FMO3 protein sequence was 79 and 82% identical to the human and rabbit FMO3 sequences, respectively. Mouse FMO3 was expressed in Escherichia coli and compared to E. coli expressed human FMO3. The FMO3 proteins migrated with the same mobility ( approximately 58 kDa) as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and immunoblotting. The expressed FMO3 enzymes (mouse and human forms) were sensitive to heat and reacted in a similar manner toward metal ions and detergent. Catalytic activities of mouse and human FMO3 were high toward the substrate methimazole; however, in the presence of trimethylamine and thioacetamide, FMO-dependent methimazole oxidation by both enzymes was reduced by greater than 85%. Other substrates which inhibited methimazole oxidation were thiourea and thiobenzamide and to a lesser degree N,N-dimethylaniline. When probed with mouse FMO3 cDNA, FMO3 transcripts were detected in hepatic mRNA samples from female mice, but not in samples from males. FMO3 was detected in mRNA samples from male and female mouse lung, but FMO3 message was not detected in mouse kidney sample from either gender. Results of immunoblotting confirmed the tissue- and gender-dependent expression of mouse FMO3.

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

Variable amounts of flavin-containing monooxygenase isoforms 3 and 5 (FMO3 and FMO5) are present in microsomal preparations from adult, male, human liver. Quantitation with monospecific antibodies and recombinant isoforms as standards showed levels of FMO3 and of FMO5 that ranged from 12.5 to 117 and 3.5 to 34 pmol/mg microsomal protein, respectively. The concentration of FMO3 was greater than that of FMO5 in all samples, but the ratio of FMO3 to FMO5 varied from 2:1 to 10:1. Human hepatic microsomal samples also showed variable activities for the S-oxidation of methimazole. This activity was associated totally with FMO3; no participation of FMO5 was apparent. This conclusion was supported by several lines of evidence: first, the catalytic efficiency of FMO3 with methimazole was found to be approximately 5000 times greater than that of FMO5; second, the rate of metabolism showed a direct, quantitative relationship with FMO3 content; third, the plot of the relationship between metabolism and FMO3 content extrapolated close to the origin. A second reaction, the N-oxidation of ranitidine, exhibited a much higher Km with recombinant FMO3 than did methimazole (2 mM vs. 35 microM). However, a direct relationship between this reaction and FMO3 content in human hepatic microsomal preparations was also apparent. This result shows that even with a high Km substrate, FMO3-catalyzed metabolism can account for the majority of the product formation with some drugs. Our findings demonstrate that the contribution of FMO isoforms to human hepatic drug metabolism can be assessed quantitatively on the basis of the characteristics of the enzymes expressed in Escherichia coli.

Regulation of mouse liver flavin-containing monooxygenases 1 and 3 by sex steroids

Based on enzyme activity, protein levels, and mRNA levels, we have previously demonstrated the female-predominant, female-specific, and gender-independent expression in mouse liver of FMO forms 1, 3, and 5, respectively. This study investigated the roles of testosterone, 17 beta-estradiol, and progesterone in the regulation of hepatic FMOs. FMO expression was examined in gonadectomized CD-1 mice, normal CD-1 mice receiving hormonal implants, and gonadectomized mice receiving various hormonal treatments. Following castration of males, hepatic FMO activity levels were significantly increased and serum testosterone levels significantly decreased; however, administration of physiological levels of testosterone to castrated animals returned FMO activity and testosterone concentrations to control levels. When sexually intact and ovariectomized female mice were treated with testosterone, their hepatic FMO activity levels were reduced to those of their male counterparts, concomitant with high serum testosterone levels. In males, castration dramatically increased FMO3 and FMO1 expression, and testosterone replacement to castrated males resulted in ablation of FMO3 expression. In addition, testosterone administration to females (sexually intact and gonadectomized animals) reduced FMO1 expression and obviated FMO3 expression. In females, ovariectomy alone slightly reduced FMO activity, indicative of a possible stimulatory role of female sex steroids; however, female FMO isozyme expression was relatively unchanged, and hormone replacement therapy to ovariectomized females had no discernible effect. In males and females, FMO5 levels were unaffected by gonadectomy or hormone administration, thus indicating a sex hormone-independent mechanism of regulation for this isoform. Interestingly, FMO1 protein levels were increased in sexually intact males following treatment with 17 beta-estradiol; however, only a slight increase in FMO3 protein level was observed. No positive hormone effectors of female FMO expression were identified.

Antibodies to a synthetic peptide that react with flavin-containing monooxygenase (HLFMO3) in human hepatic microsomes

Flavin-containing monooxygenases (FMOs) catalyze the oxidation of a diverse array of xenobiotic compounds. The purpose of this investigation was to develop a specific immunological probe to human hepatic flavin-containing monooxygenase (HLFMO3). An oligopetide corresponding to amino acid residues 257-270 of HLFMO3 was coupled to keyhole limpet hemocyanin (KLH) through the sulfhydryl group of a cysteine residue added to the amino-terminus of the peptide. This peptide-KLH conjugate was used to generate a polyclonal antibody. The resulting immunoglobulin showed specific Western blot reactivity with HLFMO3 protein in human hepatic microsomes, the same protein that is recognized by a polyclonal antibody directed against macaque liver FMO. These findings demonstrate that an antibody directed against a synthetic peptide derived from HLFMO3 can be easily produced in large quantities and used in studies for the immunodetection and immunoquantification of HLFMO3. This is also the first antipeptide antibody directed against an FMO of any species.

Pulmonary flavin-containing monooxygenase (FMO) in rhesus macaque: expression of FMO2 protein, mRNA and analysis of the cDNA

Pulmonary microsomes from Rhesus macaque express a flavin-containing monooxygenase (FMO) resembling the FMO2 ortholog from rabbit with respect to immunochemical cross-reactivity and expression in lung, but not liver. A full-length cDNA was cloned following screening of a Rhesus macaque lung cDNA library. The nucleotide sequence contained an open reading frame encoding 535 amino acids with 85 and 84% identity to FMO2 from rabbit and guinea pig, respectively, and an identical location of the putative FAD- and NADP-binding sites. Northern blots of monkey lung mRNA revealed multiple size FMO2 transcripts. These mRNA transcripts are expressed in lung, but not in liver or kidney.

Gender differences in hepatic expression of flavin-containing monooxygenase isoforms (FMO1, FMO3, and FMO5) in mice

Hepatic flavin-containing monooxygenase (FMO) activity of microsomes from adult CD-1, Swiss-Webster, C57BL/6, and DBA/2 mice was found to be significantly higher in females than in males. Based on protein and mRNA levels in CD-1 mice, FMO forms responsible for the gender difference in FMO activity were FMO1 and FMO3. FMO1 expression was two to three times higher in female mice compared with males; FMO3, however, which was expressed at levels equivalent to FMO1 in female mice, was not detected in males. The expression of FMO5 was approximately equal in both sexes. FMO2 and FMO4 transcripts were not evident in hepatic mRNA from mice. Protein and mRNA levels appear to be coregulated with regard to gender-selective or gender-specific expression of FMO1 or FMO3, respectively. FMO5, which demonstrates no gender-selective expression in mice, may be regulated by different mechanisms. Examination of protein levels among Swiss-Webster, C57BL/6, and DBA/2 strains revealed a gender-dependent expression of FMO isozymes identical to the CD-1 strain.

An essential lysyl residue (Lys208) in the substrate-binding site of porcine FAD-containing monooxygenase

The substrate (amine)-binding site of porcine FAD-containing monooxygenase (FMO) (EC 1.14.13.8) was examined using pyridoxal 5'-phosphate (pyridoxal-P) to modify lysyl residues. The enzymic activity of the FMO was inhibited competitively by pyridoxal-P. Upon reduction of pyridoxal-P-treated FMO with NaBH4, a new characteristic absorption peak of substituted pyridoxal-P appeared at 325 nm. The amino acid residue compositions of the native and pyridoxal-P-treated FMOs indicated that the lysyl residues were modified by pyridoxal-P. The about 74% inactivation of the enzymic activity on covalent pyridoxal-P treatment of the FMO was nearly completely prevented in the presence of the substrate, N,N-dimethylaniline. The FMO covalently modified with pyridoxal-P in the presence or absence of N,N-dimethylaniline was digested with trypsin treated with tosylphenylalanylchloromethane and the resultant peptide fragments were separated with a reverse-phase high-performance liquid chromatography system; only one peptide was specifically labeled with pyridoxal-P and was detected at 325 nm in the absence of N,N-dimethylaniline. The modified peptide was analyzed and identified as that comprising the amino acid residues 186-208. These results suggest that Lys208 plays an important role in the substrate (amine)-binding site of FMO.

Prochiral sulfides as in vitro probes for multiple forms of the flavin-containing monooxygenase

A homologous series of alkyl-substituted p-tolyl sulfides have been synthesized and evaluated as in vitro, isozyme-selective substrate probes for the microsomal flavin containing monooxygenases. Straight-chain and branched-chain alkyl homologs were metabolized to the corresponding (R)- and (S)-sulfoxides which were analyzed by chiral phase high-performance liquid chromatography. Initial studies demonstrated that the stereochemical composition of alkyl p-tolyl sulfoxides generated by FMO2, purified from rabbit lung, was a function of the degree of steric crowding about the prochiral center. In contrast, purified rabbit liver FMO1 formed the (R)-sulfoxide from the n-alkyl series of substrates in a highly stereoselective manner (> 90%). Similar results were obtained with these two rabbit cDNAs expressed in E. coli. In contrast to rabbit FMO1 and FMO2, a characteristic feature of catalysis by cDNA-expressed rabbit FMO3 was the lack of stereoselectivity observed for formation of methyl p-tolyl sulfoxide. Collectively, these data demonstrate that the stereochemical composition of sulfoxides generated from the n-alkyl series of sulfides is isozyme-dependent. 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. However, solubilized human and macaque liver preparations catalyzed this reaction in a relatively non-stereoselective manner. Macaque liver FMO was purified and the metabolite profile generated from the n-alkyl p-tolyl sulfides was found to be most similar to rabbit FMO3. Moreover, antibodies directed against macaque liver FMO selectively reacted with rabbit FMO3 and a microsomal protein expressed in adult human, but not fetal human liver, adult human kidney or adult human lung. Therefore, an FMO isoform expressed selectively in adult primate liver has catalytic and immunochemical properties consistent with its classification in the FMO3 family.

Role of hepatic flavin-containing monooxygenase 3 in drug and chemical metabolism in adult humans

In conjunction with asymmetric chemical syntheses and spectral, chiroptical, chromatographic and stereochemical correlation methods, we have developed procedures for the quantification of sulfoxide enantiomers and tertiary amine N-oxide diastereomer metabolites arising from the action of the adult human liver and other flavin-containing monooxygenases (FMOs). The parallel nature of the metabolic in vitro-in vivo studies and the use of chemical model oxidation systems allowed us to identify the FMO isoform involved. We investigated the enantioselective S-monooxygenation of cimetidine and the diastereoselective tertiary amine N-1'-oxygenation of (S)-nicotine as stereoselective functional probes of adult human liver FMO action. In both cases, the majority of evidence points to adult human liver FMO3 as the principal enzyme responsible for cimetidine S-oxygenation and (S)-nicotine N-1'-oxygenation in vitro and in vivo. The excellent agreement between the absolute configuration of the major cimetidine S-oxide and (S)-nicotine N-1'-oxide metabolites isolated from human urine and the major metabolite formed in the presence of adult human liver microsomes suggests that in vitro hepatic preparations may serve as a useful model for the in vivo condition. Further, that adult human liver cDNA-expressed FMO3 in Escherichia coli also gave the same absolute stereoselectivity (i.e. for (S)-nicotine N-1'-oxygenation) confirms the identity of the monoxygenase in vivo. Although we cannot rule out the involvement of minor contributions of cytochrome P-450 monooxygenases in cimetidine and (S)-nicotine oxidation, the majority of the data support the fact that cimetidine S-oxygenation and (S)-nicotine N-1'-oxygenation are stereoselective functional probes of adult human liver FMO3 activity. Finally, because the stereochemistry of the principal metabolite of cimetidine and (S)-nicotine in small experimental animals is distinct from that observed in humans, it is likely that species variation in predominant FMO isoforms exist and this may have important consequences for the choice of experimental animals in human preclinical drug design and development programs.

Asymmetric metabolic N-oxidation of N-ethyl-N-methylaniline by purified flavin-containing monooxygenase

The prochiral tertiary amine N-ethyl-N-methylaniline (EMA) is known to be metabolically N-oxygenated in vitro with microsomal preparations. This biotransformation is thought to be mediated predominantly by the flavin-containing monooxygenase (FMO) enzyme system. Microsomal N-oxygenation of EMA is known to be stereoselective and varies between species. In order to further characterise this metabolic transformation, we have examined the in vitro metabolism of EMA using purified porcine hepatic FMO. Following incubation of EMA with purified FMO, EMA N-oxide, the only metabolic detected, was found to be produced stereoselectively [ratio (-)-(S):(+)-(R), ca. 4:1]. The enantiomeric ratio of the N-oxide product did not change markedly with respect to time, enzyme or substrate concentration. Determination of the kinetics of formation of the N-oxide indicated a single affinity for the prochiral substrate with differential rates of formation of the enantiomers. The extent of EMA N-oxide formation was shown to be affected by activators and inhibitors of FMO and pH, but its stereoselectively was unaltered.