Flavin-containing monooxygenases: enzymes adapted for multisubstrate specificity

Practical Aspects of Flavin-Containing Monooxygenase-Mediated Metabolism

Hepatic flavin-containing monooxygenase 3 (FMO3) is arguably the most important FMO in humans from the standpoint of drug metabolism. Recently, adult hepatic FMO3 has been linked to several conditions including cardiometabolic diseases, aging, obesity, and atherosclerosis in small animals. Despite the importance of FMO3 in drug and chemical metabolism, relative to cytochrome P-450 (CYP), fewer studies have been published describing drug and chemical metabolism. This may be due to the properties of human hepatic FMO3. For example, FMO3 is thermally labile, and often methods reported in the study of human hepatic FMO3 are not optimal. Herein, I describe some practical aspects for studying human hepatic FMO3 and other FMOs.

Assessment of sub-chronic oral toxicity of Nityanand Rasa: An ayurvedic herbo-metallic formulation

ETHNOPHARMACOLOGICAL RELEVANCE

Nityananda Rasa (NR) is an ayurvedic herbo-metallic formulation used to treat gout, obesity, hypothyroidism, elephantiasis, and other diseases. However, its safety is a concern owing to the use of heavy metals like mercury and arsenic.

AIM OF THE STUDY

To study the sub-chronic oral toxicity of NR on albino wistar rats for safety evaluation.

MATERIALS AND METHODS

The male and female albino wistar rats were administered a daily dose of 30 (low), 300 (medium) and 600 (high) mg/kg BW/day of NR for 90-day period. The body weight and feed consumption were monitored once a week. After 90 days, blood and vital organs were harvested for genotoxicity, hematology, biochemistry, histopathology, gene expression and the biodistribution analysis.

RESULTS

There was no mortality or severe behavioural changes observed in rats. Significant changes in biochemical enzyme levels were seen at medium and high doses of NR i. e. 300 and 600 mg/kg BW/day respectively. No hematological changes were observed. Mild histopathological changes seen at high dose of NR which were found in concurrence with the biochemical alterations in liver and brain. There was mild genotoxicity and no detectable level of mercury but significant arsenic level in blood at high dose. Gene expression was mildly affected.

CONCLUSIONS

NR induced moderate toxic effects at high dose but can be considered safe at therapeutic doses.

The Phylogeny of Class B Flavoprotein Monooxygenases and the Origin of the YUCCA Protein Family

YUCCA (YUCCA flavin-dependent monooxygenase) is one of the two enzymes of the main auxin biosynthesis pathway (tryptophan aminotransferase enzyme (TAA)/YUCCA) in land plants. The evolutionary origin of the YUCCA family is currently controversial: YUCCAs are assumed to have emerged via a horizontal gene transfer (HGT) from bacteria to the most recent common ancestor (MRCA) of land plants or to have inherited it from their ancestor, the charophyte algae. To refine YUCCA origin, we performed a phylogenetic analysis of the class B flavoprotein monooxygenases and comparative analysis of the sequences belonging to different families of this protein class. We distinguished a new protein family, named type IIb flavin-containing monooxygenases (FMOs), which comprises homologs of YUCCA from Rhodophyta, Chlorophyta, and Charophyta, land plant proteins, and FMO-E, -F, and -G of the bacterium Rhodococcus jostii RHA1. The type IIb FMOs differ considerably in the sites and domain composition from the other families of class B flavoprotein monooxygenases, YUCCAs included. The phylogenetic analysis also demonstrated that the type IIb FMO clade is not a sibling clade of YUCCAs. We have also identified the bacterial protein group named YUC-like FMOs as the closest to YUCCA homologs. Our results support the hypothesis of the emergence of YUCCA via HGT from bacteria to MRCA of land plants.

Investigating the Photochemistry of C7 and C8 Functionalized N(5)-Ethyl-flavinium Cation: A Computational Study

Flavins are a diverse set of compounds with a wide variety of biological and nonbiological applications. Applications of flavins receiving attention recently consist of electro- and photocatalytic oxidation of substrates for organic synthesis, bioengineered nanotechnology, and water splitting catalysts, among others. While there is vast knowledge regarding the structure-property relationships of flavins and their electrochemistry, there is much less work elucidating the structure property relationships as they pertain to flavinium photochemistry. Herein, we report the effect of molecular tailoring on the molecular properties of N(5)-ethyl-flavinium cation (Et-Fl⁺), a derivative of the biocatalytic coenzyme riboflavin, by incorporating electron withdrawing and donating groups at the C7 and C8 position of the isoalloxazine ring. The presence of electron withdrawing groups at the C8 position caused a red shift in the absorption spectrum, while the electron donating groups caused a blue shift. Functionalization at the C7 position had the opposite effect on the absorption spectrum. The effects of single substitution were relatively negated with simultaneous functionalization at both the C8 and C7 positions. Difference density plots indicate no change in the nature of the S₁ excited state, which was confirmed by optimization of the excited state geometries. The results presented in this study indicate that functionalization of the isoalloxazine unit affects the photophysical properties of N(5)-ethyl-flavinium cations.

Ancestral-sequence reconstruction unveils the structural basis of function in mammalian FMOs

Flavin-containing monooxygenases (FMOs) are ubiquitous in all domains of life and metabolize a myriad of xenobiotics, including toxins, pesticides and drugs. However, despite their pharmacological importance, structural information remains bereft. To further our understanding behind their biochemistry and diversity, we used ancestral-sequence reconstruction, kinetic and crystallographic techniques to scrutinize three ancient mammalian FMOs: AncFMO2, AncFMO3-6 and AncFMO5. Remarkably, all AncFMOs could be crystallized and were structurally resolved between 2.7- and 3.2-Å resolution. These crystal structures depict the unprecedented topology of mammalian FMOs. Each employs extensive membrane-binding features and intricate substrate-profiling tunnel networks through a conspicuous membrane-adhering insertion. Furthermore, a glutamate-histidine switch is speculated to induce the distinctive Baeyer-Villiger oxidation activity of FMO5. The AncFMOs exhibited catalysis akin to human FMOs and, with sequence identities between 82% and 92%, represent excellent models. Our study demonstrates the power of ancestral-sequence reconstruction as a strategy for the crystallization of proteins.

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

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

Side-Chain Pruning Has Limited Impact on Substrate Preference in a Promiscuous Enzyme

Detoxifying enzymes such as flavin-containing monooxygenases deal with a huge array of highly diverse xenobiotics and toxic compounds. In addition to being of high physiological relevance, these drug-metabolizing enzymes are useful catalysts for synthetic chemistry. Despite the wealth of studies, the molecular basis of their relaxed substrate selectivity remains an open question. Here, we addressed this issue by applying a cumulative alanine mutagenesis approach to cyclohexanone monooxygenase from Thermocrispum municipale, a flavin-dependent Baeyer–Villiger monooxygenase which we chose as a model system because of its pronounced thermostability and substrate promiscuity. Simultaneous removal of up to eight noncatalytic active-site side chains including four phenylalanines had no effect on protein folding, thermostability, and cofactor loading. We observed a linear decrease in activity, rather than a selectivity switch, and attributed this to a less efficient catalytic environment in the enlarged active-site space. Time-resolved kinetic studies confirmed this interpretation. We also determined the crystal structure of the enzyme in complex with a mimic of the reaction intermediate that shows an unaltered overall protein conformation. These findings led us to propose that this cyclohexanone monooxygenase may lack a distinct substrate selection mechanism altogether. We speculate that the main or exclusive function of the protein shell in promiscuous enzymes might be the stabilization and accessibility of their very reactive catalytic intermediates.

Goose FMO3 gene cloning, tissue expression profiling, polymorphism detection and association analysis with trimethylamine level in the egg yolk

Flavin-containing monooxygenase 3 (FMO3) plays a critical role in catalyzing the conversion of trimethylamine (TMA) to trimethylamine-N-oxide (TMAO) in vivo. Despite the well-documented association between FMO3 mutations and a 'fishy' off-flavor eggs in chicken and quail, little information is available regarding the molecular characteristic of goose (Anser cygnoides) FMO3 and its relationship with the yolk TMA content. To fill these gaps, we cloned the full-length cDNA sequence of goose FMO3, which comprised 1851bp encoding 531 amino acids. FMO3 mRNA was dramatically expressed in liver than in other tissues in the geese. Eight single nucleotide polymorphisms (SNPs) were detected in the entire coding region. The CC genotype at the T669C site, GG at the A723G site, and AA at the G734A site of FMO3 were highly significantly associated with elevated TMA content in goose egg yolk (P<0.001). Carriers of the A allele of G734A or C allele of T885C had yolk TMA content that had a high probability of being elevated after feeding with additional choline chloride (P=0.0429, OR=4.1300, 95%CI=1.0390-16.4270, and P=0.0251, OR=4.6060, 95%CI=1.1620-18.2620, respectively). This work lays a foundation for studying the function of FMO3 and yolk TMA content in goose. However, studies using larger sample sizes and more goose breeds are required to determine whether the fishy off-flavor trait exists in goose.

Functional roles of Arabidopsis CKRC2/YUCCA8 gene and the involvement of PIF4 in the regulation of auxin biosynthesis by cytokinin

Auxin and cytokinin (CK) are both important hormones involved in many aspects of plant growth and development. However, the details of auxin biosynthesis and the interaction between auxin and CK are still unclear. Isolation and characterization of an auxin deficient mutant cytokinin induced root curling 2 (ckrc2) in this work reveal that CKRC2 encodes a previously identified member of YUCCA (YUC) flavin monooxygenase-like proteins (YUC8). Our results show that, like other YUCs, CKRC2/YUC8 is a rate-limiting enzyme for catalyzing the conversion of indole-3-pyruvic acid (IPyA) to indole-3-acetic acid (IAA), acting downstream of CKRC1/TAA1 in the IPyA pathway. Here we show that the transcription of both CKRC1/TAA and CKRC2/YUC8 can be induced by CK and that the phytochrome-interacting factor 4 (PIF4) is required for this upregulation. Transcription of PIF4 itself is induced by CK via the AHKs-ARR1/12 signalling pathway. These results indicate that PIF4 plays an essential role in mediating the regulatory effect of CK on the transcriptions of CKRC1 and CKRC2 genes in the IPyA pathway of auxin biosynthesis.

Cytochrome P450 and Non-Cytochrome P450 Oxidative Metabolism: Contributions to the Pharmacokinetics, Safety, and Efficacy of Xenobiotics

The drug-metabolizing enzymes that contribute to the metabolism or bioactivation of a drug play a crucial role in defining the absorption, distribution, metabolism, and excretion properties of that drug. Although the overall effect of the cytochrome P450 (P450) family of drug-metabolizing enzymes in this capacity cannot be understated, advancements in the field of non-P450-mediated metabolism have garnered increasing attention in recent years. This is perhaps a direct result of our ability to systematically avoid P450 liabilities by introducing chemical moieties that are not susceptible to P450 metabolism but, as a result, may introduce key pharmacophores for other drug-metabolizing enzymes. Furthermore, the effects of both P450 and non-P450 metabolism at a drug's site of therapeutic action have also been subject to increased scrutiny. To this end, this Special Section on Emerging Novel Enzyme Pathways in Drug Metabolism will highlight a number of advancements that have recently been reported. The included articles support the important role of non-P450 enzymes in the clearance pathways of U.S. Food and Drug Administration-approved drugs over the past 10 years. Specific examples will detail recent reports of aldehyde oxidase, flavin-containing monooxygenase, and other non-P450 pathways that contribute to the metabolic, pharmacokinetic, or pharmacodynamic properties of xenobiotic compounds. Collectively, this series of articles provides additional support for the role of non-P450-mediated metabolic pathways that contribute to the absorption, distribution, metabolism, and excretion properties of current xenobiotics.

Microarray analysis of di-n-butyl phthalate and 17α ethinyl-oestradiol responses in three-spined stickleback testes reveals novel candidate genes for endocrine disruption

Phthalate esters are plasticizers frequently found in wastewater effluents. Previous studies on phthalates have reported anti-androgenic activity in mammals, causing concerns of their potential effects on the reproduction of aquatic organisms. Another group of environmental endocrine disrupters, steroidal estrogens, are known to inhibit steroid biosynthesis in the gonads, but the effects related to spermatogenesis are not well understood in fish. In this study, three-spined sticklebacks were exposed to di-n-butyl phthalate (DBP) and 17α ethinyl-oestradiol (EE2) at nominal concentrations 35μg/L and 40ng/L, respectively, for four days. The aim of the study was to obtain insight into the acute transcriptional responses putatively associated with endocrine disruption. RNA samples from eight individual male fish per treatment (including controls) were used in microarray analysis, covering the expression of approximately 21,000 genes. In the EE2 treatment the results show transcriptional downregulation of genes associated with steroid biosynthesis pathway and up-regulation of genes involved in pathways related to epidermal growth factor signaling and xenobiotic metabolism. The transcriptional response to DBP was in general weaker than to EE2, but based on enrichment analysis, we suggest adverse effects on retinoid metabolism, creatine kinase activity and cell adhesion. Among the genes showing highest fold changes after DBP treatment compared to control was the teleost fish -specific cytochrome P450 17A2. Overall, this study promotes our understanding on molecular responses to anti-androgens and estrogens in fish testes.

The biochemical mechanism of auxin biosynthesis by an arabidopsis YUCCA flavin-containing monooxygenase

Auxin regulates every aspect of plant growth and development. Previous genetic studies demonstrated that YUCCA (YUC) flavin-containing monooxygenases (FMOs) catalyze a rate-limiting step in auxin biosynthesis and that YUCs are essential for many developmental processes. We proposed that YUCs convert indole-3-pyruvate (IPA) to indole-3-acetate (IAA). However, the exact biochemical mechanism of YUCs has remained elusive. Here we present the biochemical characterization of recombinant Arabidopsis YUC6. Expressed in and purified from Escherichia coli, YUC6 contains FAD as a cofactor, which has peaks at 448 nm and 376 nm in the UV-visible spectrum. We show that YUC6 uses NADPH and oxygen to convert IPA to IAA. The first step of the YUC6-catalyzed reaction is the reduction of the FAD cofactor to FADH(-) by NADPH. Subsequently, FADH(-) reacts with oxygen to form a flavin-C4a-(hydro)peroxy intermediate, which we show has a maximum absorbance at 381 nm in its UV-visible spectrum. The final chemical step is the reaction of the C4a-intermediate with IPA to produce IAA. Although the sequences of the YUC enzymes are related to those of the mammalian FMOs, which oxygenate nucleophilic substrates, YUC6 oxygenates an electrophilic substrate (IPA). Nevertheless, both classes of enzymes form quasi-stable C4a-(hydro)peroxyl FAD intermediates. The YUC6 intermediate has a half-life of ∼20 s whereas that of some FMOs is >30 min. This work reveals the catalytic mechanism of the first known plant flavin monooxygenase and provides a foundation for further investigating how YUC activities are regulated in plants.

Compound lipophilicity as a descriptor to predict binding affinity (1/K(m)) in mammals

In bioaccumulation models, biotransformation is one of the processes decreasing the concentration of chemicals in an organism. In order to be metabolized, a compound needs to bind to an enzyme. In this study, we derived relationships between binding affinity and lipophilicity, expressed as Log (1/K(m)) and Log K(ow), respectively. We focused on oxidations in mammals catalyzed by alcohol dehydrogenase (ADH), aldehyde dehydrogenase (ALDH), flavin-containing monooxygenase (FMO), and cytochrome P450 (CYP) enzymes. For all regressions, 1/K(m) increased with compound K(ow), which can be understood from the tendency to biotransform lipophilic compounds into more polar, thus more easily excretable metabolites. Lipophilicity was relevant to the binding of most of the substrate classes of ADH, ALDH, and CYP. The resulting slopes had 95% Confidence Intervals covering the value of 0.63, typically noted in protein-water distribution (Log K(pw)) and Log K(ow) regressions. A reduced slope (0.2-0.3) was found for FMO: this may be due to a different reaction mechanism involving a nucleophilic attack. The general patterns of metabolism were mechanistically interpreted in terms of partitioning theory. Information on the overall principles determining biotransformation may be helpful in predicting metabolic rates.

Conversion of tryptophan to indole-3-acetic acid by TRYPTOPHAN AMINOTRANSFERASES OF ARABIDOPSIS and YUCCAs in Arabidopsis

Auxin is an essential hormone, but its biosynthetic routes in plants have not been fully defined. In this paper, we show that the TRYPTOPHAN AMINOTRANSFERASE OF ARABIDOPSIS (TAA) family of amino transferases converts tryptophan to indole-3-pyruvate (IPA) and that the YUCCA (YUC) family of flavin monooxygenases participates in converting IPA to indole-3-acetic acid, the main auxin in plants. Both the YUCs and the TAAs have been shown to play essential roles in auxin biosynthesis, but it has been suggested that they participate in two independent pathways. Here, we show that all of the taa mutant phenotypes, including defects in shade avoidance, root resistance to ethylene and N-1-naphthylphthalamic acid (NPA), are phenocopied by inactivating YUC genes. On the other hand, we show that the taa mutants in several known auxin mutant backgrounds, including pid and npy1, mimic all of the well-characterized developmental defects caused by combining yuc mutants with the auxin mutants. Furthermore, we show that overexpression of YUC1 partially suppresses the shade avoidance defects of taa1 and the sterile phenotypes of the weak but not the strong taa mutants. In addition, we discovered that the auxin overproduction phenotypes of YUC overexpression lines are dependent on active TAA genes. Our genetic data show that YUC and TAA work in the same pathway and that YUC is downstream of TAA. The yuc mutants accumulate IPA, and the taa mutants are partially IPA-deficient, indicating that TAAs are responsible for converting tryptophan to IPA, whereas YUCs play an important role in converting IPA to indole-3-acetic acid.

Allelic analyses of the Arabidopsis YUC1 locus reveal residues and domains essential for the functions of YUC family of flavin monooxygenases

Flavin monooxygenases (FMOs) play critical roles in plant growth and development by synthesizing auxin and other signaling molecules. However, the structure and function relationship within plant FMOs is not understood. Here we defined the important residues and domains of the Arabidopsis YUC1 FMO, a key enzyme in auxin biosynthesis. We previously showed that simultaneous inactivation of YUC1 and its homologue YUC4 caused severe defects in vascular and floral development. We mutagenized the yuc4 mutant and screened for mutants with phenotypes similar to those of yuc1 yuc4 double mutants. Among the isolated mutants, five of them contained mutations in the YUC1 gene. Interestingly, the mutations identified in the new yuc1 alleles were concentrated in the two GXGXXG motifs that are highly conserved among the plant FMOs. One such motif presumably binds to flavin adenine dinucleotide (FAD) cofactor and the other binds to nicotinamide adenine dinucleotide phosphate (NADPH). We also identified the Ser(139) to Phe conversion in yuc1, a mutation that is located between the two nucleotide-binding sites. By analyzing a series of yuc1 mutants, we identified key residues and motifs essential for the functions of YUC1 FMO.

Characterization of enzymes responsible for biotransformation of the new antileukotrienic drug quinlukast in rat liver microsomes and in primary cultures of rat hepatocytes

The promising new drug quinlukast, 4-(4-(quinoline-2′-yl-methoxy)phenylsulphanyl)benzoic acid (VÚFB 19363), is under investigation for its anti-inflammatory and anti-asthmatic effects. The main metabolite of quinlukast identified in incubations of rat microsomal fraction, and in primary culture of rat hepatocytes, is quinlukast sulfoxide (M2). Also, several other metabolites of quinlukast were found: two dihydrodiol derivatives (M3, M5) and quinlukast sulfone (M4). This study was conducted to characterize the enzymes involved in quinlukast biotransformation in rat in-vitro. Primary cultures of rat hepatocytes were treated with inducers of different cytochrome P450s (CYPs) for 48 h. Quinlukast (100 μm) was incubated for 24 h in a primary culture of induced or control hepatocytes. The effects of CYP inhibitors, ketoconazole, methylpyrazole, metyrapone and α-naphthoflavone (2, 10, 50 μm), on quinlukast metabolism were tested in induced and control hepatocytes. Significant induction of M2 (6 times), M5 (twice) and M3 (by 50%) formation by dexamethasone and strong concentration-dependent inhibition by ketoconazole indicated that CYP3A participates in formation of these metabolites. CYP1A catalyses formation of metabolite M3 mainly, as β-naphthoflavone induced (10 times) production of M3 and a strong inhibitory effect of α-naphthoflavone on its formation was observed. A significant inhibitory effect of quinlukast (2, 10, 50 μm) on ethoxyresorufin, methoxyresorufin and benzyloxyresorufin O-dealkylase activity was observed as well.

Effect of hyperosmotic conditions on flavin-containing monooxygenase activity, protein and mRNA expression in rat kidney

Flavin-containing monooxigenases (FMOs) are a polymorphic family of drug and pesticide metabolizing enzymes, found in the smooth endoplasmatic reticulum that catalyze the oxidation of soft nucleophilic heteroatom substances to their respective oxides. Previous studies in euryhaline fishes have indicated induction of FMO expression and activity in vivo under hyperosmotic conditions. In this study we evaluated the effect of hypersaline conditions in rat kidney. Male Sprague-Dawley rats were injected intraperitoneal with 3.5M NaCl at a doses ranging from 0.3cm(3)/100g to 0.6cm(3)/100g in two separate treatments. Three hours after injection, FMO activities and FMO1 protein was examined in the first experiment, and the expression of FMO1 mRNA was measured in the second experiment from kidneys after treatment with NaCl. A positive significant correlation was found between FMO1 protein expression and plasma osmolarity (p<0.05, r=0.6193). Methyl-p-tolyl sulfide oxidase showed a statistically significant increase in FMO activity, and a positive correlation was observed between plasma osmolarity and production of FMO1-derived (R)-methyl-p-tolyl sulfoxide (p<0.05, r=0.6736). Expression of FMO1 mRNA was also positively correlated with plasma osmolality (p<0.05, r=0.8428). Similar to studies in fish, these results suggest that expression and activities of FMOs may be influenced by hyperosmotic conditions in the kidney of rats.

Steroid binding sites in liver membranes: interplay between glucocorticoids, sex steroids, and pituitary hormones

Steroid hormones activate target cells through specific receptors that discriminate among ligands based upon recognition of distinct structural features. For most known steroids, membrane and nuclear receptors co-exist in many target cells. However, while the structure of the nuclear receptors and their function as transcriptional activators of specific target genes is generally well understood, the identity of the membrane receptors remains elusive. Using pharmacological and biochemical approaches, we are beginning to characterize receptors for glucocorticoids and anabolic-androgenic steroids in male rat liver membranes. Male rat liver endoplasmic reticulum contains two steroid binding sites which are functionally related and associated with a 90-134 kDa oligomeric protein: (1) the low-affinity glucocorticoid binding site (LAGS), composed at least in part of two peptides (37 and 53 kDa) that bind glucocorticoids and (2) the stanozolol binding protein (STBP), composed at least in part of three peptides (22, 31, and 55 kDa) that bind the synthetic androgen stanozolol. These steroid binding proteins have many properties different from those of classical nuclear receptors, with the salient differences being a failure to recognize "classical" ligands for nuclear receptors together with marked differences in biochemical properties and physiological regulation. The mechanism of interaction of glucocorticoids with the LAGS can be clearly distinguished from that with STBP. Moreover, STBP shows an extremely narrow pharmacological profile, being selective for ST and its analog, danazol, among more than 100 steroids and non-steroidal compounds that were assayed, including those that are able to displace glucocorticoids from the LAGS. The level of LAGS activity undergoes dramatic variations following changes from the physiological serum levels of thyroid hormones, glucocorticoids, GH, vitamin A, and E2. However, neither thyroid hormones nor GH have a critical role on STBP activity. The STBP is functionally related to LAGS. We have suggested a novel mechanism for STBP whereby membrane-associated glucocorticoid binding activity is targeted by stanozolol (and 16beta-hydroxylated stanozolol): stanozolol modulates glucocorticoid activity in the liver through negative allosteric modulation of the LAGS resulting in an effective increase in classical GR-signaling by increasing glucocorticoid availability to the cytosolic GR.

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.

An integrated metabonomic approach to describe temporal metabolic disregulation induced in the rat by the model hepatotoxin allyl formate

The time-related metabolic events in rat liver, plasma, and urine following hepatotoxic insult with allyl formate (75 mg/kg) were studied using a combination of high-resolution liquid state and magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopic methods together with pattern recognition analysis. The metabonomics results were compared with the results of conventional plasma chemistry and histopathological assessments of liver damage. Various degrees of liver damage were observed in different animals, and this variation was reflected in all of the analyses. Furthermore, each analysis revealed a high degree of functional and structural recovery by the end of the study. The allyl formate-induced changes included hepatocellular necrosis, hepatic lipidosis, decreased liver glycogen and glucose, decreased plasma lipids, increased plasma creatine and tyrosine, increased urinary taurine and creatine, and decreased urinary TCA cycle intermediates. The observed reductions in hepatic glycogen and glucose suggest increased glucose utilization and are consistent with the expected depletion of hepatic ATP following mitochondrial impairment, assuming that there is a consequent increase in energy production from glycolysis. The increase in plasma tyrosine is consistent with impaired protein synthesis, a known consequence of ATP depletion. Partial least squares-based cross-correlation of the variation in the liver and plasma NMR profiles indicated that the allyl formate-induced increase in liver lipids correlated with the decrease in plasma lipids. This suggests disruption in lipid transport from the liver to plasma, which could arise through impaired apolipoprotein synthesis, as with ethionine.

Metabolic profiling reveals a contribution of gut microbiota to fatty liver phenotype in insulin-resistant mice

Here, we study the intricate relationship between gut microbiota and host cometabolic phenotypes associated with dietary-induced impaired glucose homeostasis and nonalcoholic fatty liver disease (NAFLD) in a mouse strain (129S6) known to be susceptible to these disease traits, using plasma and urine metabotyping, achieved by (1)H NMR spectroscopy. Multivariate statistical modeling of the spectra shows that the genetic predisposition of the 129S6 mouse to impaired glucose homeostasis and NAFLD is associated with disruptions of choline metabolism, i.e., low circulating levels of plasma phosphatidylcholine and high urinary excretion of methylamines (dimethylamine, trimethylamine, and trimethylamine-N-oxide), coprocessed by symbiotic gut microbiota and mammalian enzyme systems. Conversion of choline into methylamines by microbiota in strain 129S6 on a high-fat diet reduces the bioavailability of choline and mimics the effect of choline-deficient diets, causing NAFLD. These data also indicate that gut microbiota may play an active role in the development of insulin resistance.

Determination of the sulphoxides and sulphones of three simple sulphides in rat urine: effects of phenobarbitone,?-naphtho?avone and methimazole

In this investigation, the measurement and identification of the S-oxidation products of three simple sulphides-ethyl methyl sulphide (EMS), 4-chlorophenyl methyl sulphide (CPMS) and diphenyl sulphide (DPS)-in rat urine were carried out and a study of the effects of phenobarbitone (PB), beta-naphtho flavone (betaNF) and methimazole on the urinary levels of their metabolites was conducted. Male Wistar rats (n = 4) were pretreated with PB (80 mg/kg/day in saline, i.p.), betaNF (100 mg/kg/day in corn oil, i.p.), methimazole (50 mg/kg/day in saline, i.p.) or the vehicles alone (1 mL/kg) for three consecutive days. After pretreatment, EMS, CPMS or DPS (50 mg/kg in corn oil, 500 microL) was administered orally to the appropriate groups of rats. The animals were placed in metabolic cages and urine samples collected at 24 h intervals over 96 h. Chromatographic and spectroscopic techniques were used for the measurement and identification of the sulphoxides and sulphones of EMS, CPMS and DPS in rat urine. Although only a trace of ethyl methyl sulphoxide (EMSO) was present in rat urine after administration of EMS, ethyl methyl sulphone (EMSO(2)) accounted for about 16% of the administered dose in the urine of male rats given EMS. In addition, pretreatment of rats with methimazole significantly decreased the S-oxidation of EMS. 4-Chlorophenyl methyl sulphone (CPMSO(2)) was the main metabolite recovered in the urine of male rats treated with CPMS, accounting for about 10% of the dose. Pretreatment of rats with PB before administration of CPMS significantly increased the levels of CPMSO(2) excreted in the urine. Additionally, pretreatment of rats with methimazole significantly decreased the S-oxidation of CPMS in vivo. About 2.5% of diphenyl sulphoxide (DPSO) and 4% of diphenyl sulphone (DPSO(2)) were recovered in the urine of male rats given DPS. Pretreatment of rats with PB, betaNF or methimazole before administration of DPS decreased the levels of DPSO and DPSO(2) excreted in the urine, although this was not statistically significant. These results indicate that microsomal monooxygenases mediate the S-oxidation of EMS, CPMS and DPS to their corresponding sulphones via a transient sulphoxide in rats.

Genetic Polymorphisms of Human Flavin Monooxygenase 3 in Sulindac-Mediated Primary Chemoprevention of Familial Adenomatous Polyposis

PURPOSE

Sulindac is a nonsteroidal anti-inflammatory drug (NSAID) effective in regressing adenomas in patients with familial adenomatous polyposis (FAP). However, a recent randomized trial showed that sulindac, when compared with placebo, failed to prevent the development of adenomatous polyps in genotypically positive but phenotypically negative FAP patients. The present study determined whether polymorphisms in the gene encoding flavin monooxygenase 3 (FMO3), a hepatic microsomal enzyme that inactivates sulindac, played a role in determining the efficacy of sulindac in preventing polyposis in this cohort of FAP patients.

EXPERIMENTAL DESIGN

Genotyping was performed on seven established FMO3 polymorphisms previously shown to have functional relevance-M66I, P153L, E158K, V257M, E305X, E308G, and R492W-in 21 and 20 FAP patients, who received sulindac and placebo, respectively.

RESULTS

None of the 41 patients exhibited heterozygous or homozygous M66I and R492W variant alleles, or homozygous P153L, V257M, and E305X variant alleles. Among sulindac-treated patients who did not develop adenomas ("responders"), 4 (33%) were homozygous for E158K and 2 (17%) were homozygous for E308G variant alleles. In contrast, none of the patients on sulindac who developed adenomas ("nonresponders") exhibited homozygosity for either of the two variant alleles. In addition, polymorphisms in the E158K or E308G allele were associated with a significant reduction in mucosal prostanoid levels in patients treated with sulindac.

CONCLUSIONS

Polymorphisms in FMO3, particularly at the E158K and E308G loci, may reduce activity in catabolizing sulindac and result in an increased efficacy to prevent polyposis in FAP.

EFFECT OF TOTAL PARENTERAL NUTRITION AND CHOLINE ON HEPATIC FLAVIN-CONTAINING AND CYTOCHROME P-450 MONOOXYGENASE ACTIVITY IN RATS

Total parenteral nutrition provides nutrition by infusion into the systemic circulation. Bypassing the intestine and processes associated with absorption can cause additional pathophysiological changes to occur. For example, in rats, normal gut and pancreatic cell function may change, absorptive capacity may be altered, and enzyme functional activity including drug metabolism may be affected. The objective of this study was to examine the effects of a control diet or a diet of total parenteral nutrition in the presence or absence of choline on urinary biomarkers and hepatic microsome functional activity from rats. Selective functional markers of cytochrome P-4502E1 (CYP2E1) and flavin-containing monooxygenase (FMO) were examined in vitro. The N-oxygenation of trimethylamine was used as an in vivo selective functional marker for FMO. After the administration of total parenteral nutrition plus choline for 5 days, the urinary excretion of trimethylamine and trimethylamine N-oxide declined approximately 7- and 3-fold, respectively, compared with rats treated with control diet. The concentration of urinary biogenic amines was also significantly affected by total parenteral nutrition. Compared with control animals, rats administered total parenteral nutrition plus choline for 5 days showed a decrease of approximately 5- and 2-fold in urinary dopamine and norepinephrine concentration, respectively. To examine a molecular basis for the influence of total parenteral nutrition +/- choline on monooxygenase regulation, hepatic microsomal activity of the FMO and CYP2E1 was examined. Compared with animals treated with a control diet, total parenteral nutrition plus choline in rats caused a 3-fold increase in hepatic microsomal FMO and a 2-fold increase in hepatic cytochrome CYP2E1 functional activity, respectively. Although the data did not reach statistical significance, selective immunoblot studies using hepatic microsomes from rats treated with total parenteral nutrition + choline showed that compared with controls, FMO1 protein was decreased 1.4-fold and FMO3 increased 1.3-fold, respectively. In hepatic microsomes from rats treated with total parenteral nutrition + choline, compared with control animals, FMO4 immunoreactivity was increased 1.6-fold. The data suggest that total parenteral nutrition has a detectable effect on modulating rat FMO3, FMO4, and CYP2E1 monooxygenase functional activity. The clinical relevance of these results is unknown but may be of significance for individuals receiving total parenteral nutrition and those afflicted with trimethylaminuria.

Photoaffinity labeling identification of thyroid hormone-regulated glucocorticoid-binding peptides in rat liver endoplasmic reticulum: an oligomeric protein with high affinity for 16beta-hydroxylated stanozolol

Steroid-binding proteins unrelated to the classical nuclear receptors have been proposed to play a role in non-genomic actions of the17alpha-alkylated testosterone derivative (17alpha-AA) stanozolol (ST). We have previously reported that male rat liver endoplasmic reticulum contains two steroid-binding sites associated with high molecular mass oligomeric proteins: (1) the ST-binding protein (STBP); and (2) the low-affinity glucocorticoid-binding protein (LAGS). To further explore the role of LAGS on the mechanism of action of ST, we have now studied: (1) the interaction of ST and its hydroxylated metabolites with solubilized LAGS and the cytosolic glucocorticoid receptor (GR); and (2) the effects of hormones on the capability of STBP to bind ST. We found that, unlike 17alpha-methyltestosterone, neither ST nor its hydroxylated metabolites bind to GR. However, the 16beta-hydroxylation of ST significantly increases the capability of LAGS to bind ST. Interestingly, 3'-hydroxylation of ST abrogates the capability of LAGS to bind ST. ST (k(i)=30 nM) and 16beta-hydroxystanozolol (k(i)=13 nM) bind with high affinity to LAGS, and are capable of accelerating the rate of dissociation of previously bound dexamethasone from the LAGS. STBP and LAGS are strongly induced by ethinylestradiol. However, unlike STBP, LAGS is regulated by thyroid hormones and growth hormone, which proves that these steroid-binding activities are associated with different binding sites. These findings seem to suggest a novel mechanism for ST whereby membrane-associated glucocorticoid-binding activity is targeted by the 16beta-hydroxylated metabolite of ST. ST and its 16beta-hydroxylated metabolite modulate glucocorticoid activity in the liver through negative allosteric modulation of LAGS, with the result of this interaction an effective increase in classical GR-signaling by increasing glucocorticoid availability to the cytosolic GR.

Expression and characterization of functional dog flavin-containing monooxygenase 1

A full-length dog (beagle) flavin-containing monooxygenase 1 (FMO1) cDNA (dFMO1) was obtained from liver by reverse transcription-polymerase chain reaction. The amino acid sequence of dFMO1 was 89% homologous to human FMO1. Using a baculovirus expression system in Sf-9 insect cells, dFMO1 was expressed to protein levels of 0.4 nmol/mg, as determined by immunoquantitation. The flavin content of the expressed enzyme was consistent with immunodetectable dFMO1 protein levels. Expressed dFMO1 catalyzed NADPH-dependent methyl p-tolyl sulfide oxidation, with K(m) and V(max) values of 98.6 microM and 63.8 nmol of S-oxide formed/min/mg of protein, respectively. By comparison, human FMO1 showed similar values of 87.1 microM (K(m)) and 51.0 nmol/min/mg (V(max)). Activity for dFMO1 showed characteristic pH dependence, with a 4.5-fold increase in S-oxidase activity as the incubation pH increased from 7.6 to 9.0. Human FMO1 also showed an increase in reaction rate with pH but a somewhat lower optimum of 8.0 to 8.4. dFMO1 also catalyzed imipramine N-oxidation, with a K(m) of 4.7 microM and a V(max) of 82.1 nmol/min/mg of protein. This enzyme displayed other characteristics of FMO enzymes, with rapid depletion of enzyme activity upon heating in the absence of NADPH. Protein levels of 74 pmol of dFMO1/mg of microsomal protein were determined for a pooled liver microsome sample, suggesting that this enzyme is a major canine hepatic monooxygenase. In conclusion, the expression and characterization of catalytically active dFMO1 will allow the role of this enzyme in the metabolism of xenobiotics to be determined.

Solubilization and photoaffinity labeling identification of glucocorticoid binding peptides in endoplasmic reticulum from rat liver

Steroid-binding proteins unrelated to the classical nuclear receptors have been proposed to play a role in non-genomic effects of steroid hormones. We have previously described that the low-affinity glucocorticoid binding protein (LAGS), present in the endoplasmic reticulum of the male rat liver, has pharmacological and biochemical properties different from those of nuclear receptors. The LAGS is under multihormonal regulation and binds glucocorticoids, progestins, and synthetic steroids but is unable to bind either estradiol, testosterone, or triamcinolone acetonide. In this study, we have solubilized the LAGS and investigated their pharmacological and hydrodynamic properties and their peptide composition. We found that LAGS is an integral protein bound to the endoplasmic reticulum. CHAPS provided its optimal solubilization without changes in its pharmacological properties. Hydrodynamic properties of LAGS showed that it has a molecular mass of at least 135 kDa. SDS-PAGE of covalently-labeled LAGS showed that [3H]dexamethasone binds two peptides of 53 and 37 kDa, respectively. Thus, the LAGS appears as an oligomeric protein under multihormonal regulation. The availability of solubilized LAGS and the fact that it can be induced in vivo represent major steps toward purification and understanding the functional significance of this unique steroid-binding protein.

Substrate specificity and enantioselectivity of 4-hydroxyacetophenone monooxygenase

The 4-hydroxyacetophenone monooxygenase (HAPMO) from Pseudomonas fluorescens ACB catalyzes NADPH- and oxygen-dependent Baeyer-Villiger oxidation of 4-hydroxyacetophenone to the corresponding acetate ester. Using the purified enzyme from recombinant Escherichia coli, we found that a broad range of carbonylic compounds that are structurally more or less similar to 4-hydroxyacetophenone are also substrates for this flavin-containing monooxygenase. On the other hand, several carbonyl compounds that are substrates for other Baeyer-Villiger monooxygenases (BVMOs) are not converted by HAPMO. In addition to performing Baeyer-Villiger reactions with aromatic ketones and aldehydes, the enzyme was also able to catalyze sulfoxidation reactions by using aromatic sulfides. Furthermore, several heterocyclic and aliphatic carbonyl compounds were also readily converted by this BVMO. To probe the enantioselectivity of HAPMO, the conversion of bicyclohept-2-en-6-one and two aryl alkyl sulfides was studied. The monooxygenase preferably converted (1R,5S)-bicyclohept-2-en-6-one, with an enantiomeric ratio (E) of 20, thus enabling kinetic resolution to obtain the (1S,5R) enantiomer. Complete conversion of both enantiomers resulted in the accumulation of two regioisomeric lactones with moderate enantiomeric excess (ee) for the two lactones obtained [77% ee for (1S,5R)-2 and 34% ee for (1R,5S)-3]. Using methyl 4-tolyl sulfide and methylphenyl sulfide, we found that HAPMO is efficient and highly selective in the asymmetric formation of the corresponding (S)-sulfoxides (ee > 99%). The biocatalytic properties of HAPMO described here show the potential of this enzyme for biotechnological applications.

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.

An overview of the mechanism, substrate specificities, and structure of FMOs

Kinetic studies carried out over the past three decades, primarily with purified pig liver flavin-containing monooxygenase (FMO1), demonstrated that the mechanism of this flavoenzyme was distinctly different from other widely studied flavin-dependent monooxygenases in that reduction of O2 by nicotinamide-adenine-dinucleotide-phosphate reduced (NADPH) occurred before the addition of the xenobiotic substrate. Compounds bearing a soft nucleophilic heteroatom show substrate activity provided they could contact the enzyme-bound 4a-hydroperoxy flavin. Structure-activity studies suggest that in addition to nucleophilicity, size and charge of potential substrates are important parameters limiting access to the enzyme-bound hydroxylating intermediate form of the enzyme. The mechanism of FMO 1, 2, 3, and 4 are similar and differences in the substrate specificities of these isoforms can be attributed almost entirely to differences in the dimensions of the cleft or channel limiting access to the 4a-hydroperoxy flavin. While this model provides a satisfactory mechanism for the FMO catalyzed oxidation of very soft nucleophiles, it does not address another very important element of the catalytic cycle. The amine nitrogen atom is not an especially soft nucleophile readily hydroxylated by peroxides or peracids. How the enzymes convert an amine substrate to a form readily attacked by the hydroperoxy flavin is presently unknown. A complete resolution of this problem will only be possible after the tertiary structures of these enzymes are solved.

Human and plant flavin-containing monooxygenase N-oxygenation of amines: detoxication vs. bioactivation

Generally, the flavin-containing monooxygenase of mammalian systems has been considered a detoxication enzyme converting nucleophilic heteroatom-containing chemicals into polar, more readily excreted metabolites. The beneficial effects of this process are thought to be participation in the detoxication of foodstuffs and other xenobiotics that might otherwise be bioactivated by other enzyme systems. The physiological role of mammalian FMO is unknown although it has been observed that many heteroatom-containing chemicals in plants are efficiently oxygenated by FMO. Recently, a plant FMO-like enzyme has been characterized and it appears to possess many of the same functional properties of mammalian FMO. The advantage of studying the FMO-like genes and enzymes in Arabidopsis as a model system allows for the investigation of the role of FMO-like enzymes in plant growth and development. The Arabidopsis FMO-like gene product, while resembling the mammalian counterpart in many respects, is quite distinct. However, study of the Arabidopsis FMO-like enzyme may provide considerable insight into the possible role of mammalian FMOs in biogenic amine metabolism.

Molecular cloning and characterization of a full-length flavin-dependent monooxygenase from yeast

Eucaryotes contain a class of enzymes called flavin-dependent monooxygenases (FMOs). Unlike mammals, yeast have only a single isoform-yFMO. Deletion mutants suggested that yFMO may play a role in folding proteins which contain disulfide bonds. Recently we detected two nucleotide errors in the GenBank sequences attributed to the yFMO gene. This previously led us to express and characterize a 373-residue catalytically active protein instead of the correct 432-residue enzyme. Here we report the sequencing, expression, and enzyme characterization of the full-length form of yFMO. Comparison of the two forms of yFMO showed similar pH profiles and K(m), K(cat), and V(max) values using glutathione as a substrate. These results indicate that the full-length yeast FMO has biochemical and catalytic properties similar to those of the truncated protein. Therefore, it is likely that the hypotheses concerning the enzyme's function proposed earlier are still valid.

Identification of a Baeyer-Villiger monooxygenase sequence motif

Baeyer-Villiger monooxygenases (BVMOs) form a distinct class of flavoproteins that catalyze the insertion of an oxygen atom in a C-C bond using dioxygen and NAD(P)H. Using newly characterized BVMO sequences, we have uncovered a BVMO-identifying sequence motif: FXGXXXHXXXW(P/D). Studies with site-directed mutants of 4-hydroxyacetophenone monooxygenase from Pseudomonas fluorescens ACB suggest that this fingerprint sequence is critically involved in catalysis. Further sequence analysis showed that the BVMOs belong to a novel superfamily that comprises three known classes of FAD-dependent monooxygenases: the so-called flavin-containing monooxygenases (FMOs), the N-hydroxylating monooxygenases (NMOs), and the BVMOs. Interestingly, FMOs contain an almost identical sequence motif when compared to the BVMO sequences: FXGXXXHXXX(Y/F). Using these novel amino acid sequence fingerprints, BVMOs and FMOs can be readily identified in the protein sequence databank.

Evolutionary recruitment of a flavin-dependent monooxygenase for the detoxification of host plant-acquired pyrrolizidine alkaloids in the alkaloid-defended arctiid moth Tyria jacobaeae

Larvae of Tyria jacobaeae feed solely upon the pyrrolizidine alkaloid-containing plant Senecio jacobaea. Ingested pyrrolizidine alkaloids (PAs), which are toxic to unspecialized insects and vertebrates, are efficiently N-oxidized in the hemolymph of T. jacobaeae by senecionine N-oxygenase (SNO), a flavin-dependent monooxygenase (FMO) with a high substrate specificity for PAs. Peptide microsequences obtained from purified T. jacobaeae SNO were used to clone the corresponding cDNA, which was expressed in active form in Escherichia coli. T. jacobaeae SNO possesses a signal peptide characteristic of extracellular proteins, and it belongs to a large family of mainly FMO-like sequences of mostly unknown function, including two predicted Drosophila melanogaster gene products. The data indicate that the gene for T. jacobaeae SNO, highly specific for toxic pyrrolizidine alkaloids, was recruited from a preexisting insect-specific FMO gene family of hitherto unknown function. The enzyme allows the larvae to feed on PA-containing plants and to accumulate predation-deterrent PAs in the hemolymph.

Physiological factors affecting the expression of FMO1 and FMO3 in the rat liver and kidney

FMO1 and FMO3, the main FMOs described in the rat, are highly expressed in the liver and the kidney. The age, from 3 to 11 weeks, and gender-dependent expression of FMO1 and FMO3 in the rat liver and kidney were investigated. Based on the enzyme activities, protein levels and mRNA levels, this study demonstrates an important increase in the expression of the FMO3 in the liver of male rats during a period that corresponds to the acquisition of the sexual maturity. Rat liver FMO1 remains unchanged during this period of observation. The evolutions of both isoforms in the kidney of the male rat are similar to those observed in the liver. On the contrary, the important decrease in the total flavin-containing monooxygenase (FMO) activity observed in the liver of female rat is linked to a considerable decrease in the FMO1-dependent activity, FMO1 protein and FMO1 mRNA levels as a function of age. The expression of the FMO3 in the liver does not seem to be affected by the age of the female rat. Inversely, the expression of FMO1 in the female rat kidneys does not seem to be modified as a function of age while the expression of FMO3 is strongly increased.

Extrathyroidal actions of antithyroid thionamides

Some compounds having thionamide structure inhibit thyroid functions. Such antithyroid thionamides include mercaptomethylimidazole (methimazole), thiourea and propylthiouracil, of which mercaptomethylimidazole is widely used to treat hyperthyroidism. Undesirable side effects develop from these drugs due to extrathyroidal actions. Antithyroid thionamides inhibit lactoperoxidase which contributes to the antibacterial activities of a number of mammalian exocrine gland secretions that protect a variety of mucosal surfaces. These drugs stimulate both gastric acid and pepsinogen secretions, thereby augmenting the severity of gastric ulcers and preventing wound healing. Increased gastric acid secretion is partially due to the H2 receptor activation, and also through the stimulation of the parietal cell by intracellular generation of H2O2 following inactivation of the gastric peroxidase-catalase system. Severe abnormalities may develop in blood cells and the immune system after thionamide therapy. It causes agranulocytosis, aplastic anemia, and purpura along with immune suppression. Olfactory and auditory systems are also affected by these drugs. Thionamide affects the sense of smell and taste and also causes loss of hearing. It binds to the Bowman's glands in the olfactory mucosa and causes extensive lesion in the olfactory mucosa. Thionamides also affect gene expression and modulate the functions of some cell types. A brief account of the chemistry and metabolism of antithyroid thionamides, along with their biological actions are presented.

Cloning, sequencing, and tissue-dependent expression of flavin-containing monooxygenase (FMO) 1 and FMO3 in the dog

The expression of flavin-containing monooxygenases (FMOs) in dog liver microsomes was suggested by a high methimazole S-oxidase activity. When the reaction was catalyzed by dog liver microsomes, apparent V(max) and K(m) values were 6.3 nmol/min/mg and 14 microM, respectively. This reaction was highly inhibited (73%) in the presence of imipramine, but it was also weakly affected by trimethylamine, suggesting the involvement of different isoforms. The sequences of dog FMO1 and FMO3 were obtained by reverse transcription-polymerase chain reaction and 5'/3' terminal extension. The cDNAs of dog FMO1 and dog FMO3 encode proteins of 532 amino acids, which contain the NADPH- and FAD-binding sites. The dog FMO1 amino acid sequence is 88, 86, and 89% identical to sequences of human, rabbit, and pig FMO1, respectively. The dog FMO3 amino acid sequence is 83, 84, and 82% identical to sequences of human, rabbit, and rat FMO3, respectively. Dog FMO1 and dog FMO3 exhibited only 56% identities. The FMO1 and FMO3 recombinant proteins and the FMO1 and FMO3 microsomal proteins migrated with the same mobility (56 kDa), as determined in SDS-polyacrylamide gel electrophoresis and immunoblotting. By Western blotting, dog FMO1 and dog FMO3 were detected in microsomes from liver and lung but not in kidney microsomes. By Northern blotting, the probe for FMO1 specifically hybridized a 2.6-kilobase (kb) transcript in liver and lung samples only. The probe for FMO3 hybridized two transcripts of approximately 3 and 4.2 kb in the liver and lung samples.

Glycoprotein folding in the endoplasmic reticulum

Our understanding of eukaryotic protein folding in the endoplasmic reticulum has increased enormously over the last 5 years. In this review, we summarize some of the major research themes that have captivated researchers in this field during the last years of the 20th century. We follow the path of a typical protein as it emerges from the ribosome and enters the reticular environment. While many of these events are shared between different polypeptide chains, we highlight some of the numerous differences between proteins, between cell types, and between the chaperones utilized by different ER glycoproteins. Finally, we consider the likely advances in this field as the new century unfolds and we address the prospect of a unified understanding of how protein folding, degradation, and translation are coordinated within a cell.

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.

A novel mutation in the flavin-containing monooxygenase 3 gene, FM03, that causes fish-odour syndrome: activity of the mutant enzyme assessed by proton NMR spectroscopy

We have previously shown that primary trimethylaminuria, or fish-odour syndrome, is caused by an inherited defect in the flavin-containing monooxygenase 3 (FMO3) catalysed N-oxidation of the dietary-derived malodorous amine, trimethylamine (TMA). We now report a novel causative mutation for the disorder identified in a young girl diagnosed by proton nuclear magnetic resonance (NMR) spectroscopy of her urine. Sequence analysis of genomic DNA amplified from the patient revealed that she was homozygous for a T to C missense mutation in exon 3 of the FMO3 gene. The mutation changes an ATG triplet, encoding methionine, at codon 82 to an ACG triplet, encoding threonine. A polymerase chain reaction/restriction enzyme-based assay was devised to genotype individuals for the FMO3Thr82 allele. Wild-type and mutant FMO3, heterologously expressed in a baculovirus-insect cell system, were assayed by ultraviolet spectrophotometry and NMR spectroscopy for their ability to catalyse the N-oxidation of TMA. The latter technique has the advantage of enabling the simultaneous, direct and semi-continuous measurement of both of the products, TMA N-oxide and NADP, and of one of the reactants, NADPH. Results obtained from both techniques demonstrate that the Met82Thr mutation abolishes the catalytic activity of the enzyme and thus represents the genetic basis of the disorder in this individual. The combination of NMR spectroscopy with gene sequence and expression technology provides a powerful means of determining genotype-phenotype relationships in trimethylaminuria.

Relative contribution of cytochromes P-450 and flavin-containing monoxygenases to the metabolism of albendazole by human liver microsomes

AIMS

Albendazole (ABZ; methyl 5-propylthio-1H-benzimidazol-2-yl carbamate) is a broad spectrum anthelmintic whose activity resides both in the parent compound and its sulphoxide metabolite (ABS). There are numerous reports of ABZ metabolism in animals but relatively few in humans. We have investigated the sulphoxidation of ABZ in human liver microsomes and recombinant systems.

METHODS

The specific enzymes involved in the sulphoxidation of ABZ were determined by a combination of approaches; inhibition with an antiserum directed against cytochrome P450 reductase, the effect of selective chemical inhibitors on ABZ sulphoxidation in human liver microsomes, the capability of expressed CYP and FMO to mediate the formation of ABS, regression analysis of the rate of metabolism of ABZ to ABS in human liver microsomes against selective P450 substrates and regression analysis of the rate of ABS sulphoxidation against CYP expression measured by Western blotting.

RESULTS

Comparison of Vmax values obtained following heat inactivation (3min at 45 degrees C) of flavin monoxygenases (FMO), chemical inhibition of FMO with methimazole and addition of an antiserum directed against cytochrome P450 reductase indicate that FMO and CYP contribute approximately 30% and 70%, respectively, to ABS production in vitro. Comparison of CLint values suggests CYP is a major contributor in vivo. A significant reduction in ABZ sulphoxidation (n = 3) was seen with ketoconazole (CYP3 A4; 32-37%), ritonavir (CYP3 A4: 34-42%), methimazole (FMO: 28-49%) and thioacetamide (FMO; 32-35%). Additive inhibition with ketoconazole and methimazole was 69 +/- 8% (n = 3). ABS production in heat - treated microsomes (3 min at 45 degrees C) correlated significantly with testosterone 6beta-hydroxylation (CYP3A4; P < 0.05) and band intensities on Western blots probed with an antibody selective for 3A4 (P < 0.05). Recombinant human CYP3 A4, CYP1A2 and FMO3 produced ABS in greater quantities than control microsomes, with those expressing CYP3A4 producing threefold more ABS than those expressing CYP1A2. Kinetic studies showed the Km values obtained with both CYP3A4 and FMO3 were similar.

CONCLUSIONS

We conclude that the production of ABS in human liver is mediated via both FMO and CYP, principally CYP3A4, with the CYP component being the major contributor.

Phenotypes of flavin-containing monooxygenase activity determined by ranitidine N-oxidation are positively correlated with genotypes of linked FM03 gene mutations in a Korean population

A non-invasive urine analysis method to determine the in-vivo flavin-containing mono-oxygenase (FMO) activity catalysing N-oxidation of ranitidine (RA) was developed and used to phenotype a Korean population. FMO activity was assessed by the molar concentration ratio of RA and RANO in the bulked 8 h urine. This method was used to determine the FMO phenotypes of 210 Korean volunteers (173 men and 37 women, 110 nonsmokers and 100 smokers). Urinary RA/RANO ratio, representing the metabolic ratio and the reciprocal index of FMO activity, ranged from 5.67-27.20 (4.8-fold difference) and was not different between men and women (P = 0.76) or between smokers and nonsmokers (P = 0.50). The frequencies of RA/RANO ratios were distributed in a trimodal fashion. Among the 210 Korean subjects, 93 (44.3%) were fast metabolizers, 104 (49.5%) were intermediate metabolizers and 13 (6.2%) were slow metabolizers. Subsequently, the relationship between the ranitidine N-oxidation phenotypes and FMO3 genotypes, determined by the presence of two previously identified mutant alleles (Glu158Lys: FMO3/Lys158 and Glu308Gly: FMO3/Gly308 alleles) commonly found in our Korean population was examined. The results showed that subjects who were homozygous and heterozygous for either one or both of the FMO3/Lys158 and FMO3/Gly308 mutant alleles had significantly lower in-vivo FMO activities than those with homozygous wild-type alleles (FMO3/Glu158 and FMO3/Glu308) (P < 0.001, Mann-Whitney U-test). Furthermore, the FMO activities of subjects with either FMO3/Lys158 or FMO3/Gly308 mutant alleles were almost identical to those having both FMO3 mutant alleles (FMO3/Lys158 and FMO3/Gly308). These two mutant alleles located, respectively, at exons 4 and 7 in the FMO3 gene appeared to be strongly linked by cis-configuration in Koreans. Therefore, we concluded that presence of FMO3/Lys158 and FMO3/Gly308 mutant alleles in FMO3 gene is responsible for the low ranitidine N-oxidation (FMO3 activity) in our Korean population.