Identification of distinct hepatic and pulmonary forms of microsomal flavin-containing monooxygenase in the mouse and rabbit

Flavin Containing Monooxygenases and Metabolism of Xenobiotics

This review summarizes recent information concerning the pharmacological and toxicological significance of the flavin-containing monooxygenases (FMOs). FMOs are a family of microsomal enzymes involving in the oxygenation of certain xenobiotics and drugs containing nucleophilic heteroatoms. The activities of FMOs in drug metabolism and their relationships with diseases are the areas of research requiring further exploration. Future studies on FMOs may provide considerable information about the pathophysiology of diseases and the information related to this enzyme family may be important for drug designs in future.

Benzydamine as a useful substrate of hepatic flavin-containing monooxygenase activity in veterinary species

Benzydamine (BZ), a weak base and an indazole derivative with analgesic and antipyretic properties used in human and veterinary medicine, is metabolized in human, rat, cattle and rabbit to a wide range of metabolites. One of the main metabolites, BZ N-oxide (BZ-NO), is produced in the liver and brain by flavin-containing monooxygenases (FMOs), by liver and brain enzymes. To evaluate the suitability of BZ as an FMO probe in veterinary species, BZ metabolism was studied in vitro using liver microsomes from bovine, rabbit and swine. Kinetic parameters, K(m) and V(max), of BZ-NO production, were evaluated to corroborate the pivotal role of FMOs. Inhibition studies were carried out by heat inactivation and by specific FMO chemical inhibitors: trimethylamine and methimazole. The results confirmed the presence of FMO activity in the liver and the role of BZ as a suitable marker of FMO enzyme activities for the veterinary species considered.

Characterization of mouse flavin-containing monooxygenase transcript levels in lung and liver, and activity of expressed isoforms

The significance of active versus inactive flavin-containing monooxygenase 2 (FMO2) for human drug and xenobiotic metabolism and sensitivity is unknown, but the underlying ethnic polymorphism is well documented. We used quantitative real-time PCR to measure message levels of Fmo1, Fmo2, Fmo3 and Fmo5 in lung and liver from eight strains of 8 week old female mice to determine if a strain could be identified that predominately expressed Fmo2 in lung, recapitulating the human FMO expression profile and being the ideal strain for Fmo2 knockout studies. We also characterized enzyme activity of baculovirus expressed mouse Fmo1, Fmo2 and Fmo3 to identify a substrate or incubation conditions capable of discriminating Fmo2 from Fmo mixtures. Fmo transcript expression patterns were similar for all strains. In lung, 59% of total FMO message was Fmo2, but Fmo1 levels were also high, averaging 34%, whereas Fmo3 and Fmo5 levels were 2 and 5%, respectively. In liver, Fmo1, Fmo2, Fmo3 and Fmo5 contributed 16, 1, 7 and 76% respectively, of detected message. Peak activity varied by isoform and was pH- and substrate-dependent. Fmo3 oxidation of methyl p-tolyl sulfide was negligible at pH 9.5, but Fmo3 oxidation of methimazole was comparable to Fmo1 and Fmo2. Heating microsomes at 50 degrees C for 10min eliminated most Fmo1 and Fmo3 activity, while 94% of Fmo2 activity remained. Measurement of activity in heated and unheated lung and liver microsomes verified relative transcript abundance. Our results show that dual Fmo1/2 knockouts will be required to model the human lung FMO profile.

Human flavin-containing monooxygenases

This review summarizes recent information concerning the pharmacological and toxicological significance of the human flavin-containing monooxygenase (FMO, EC 1.14.13.8). The human FMO oxygenates nucleophilic heteroatom-containing chemicals and drugs and generally converts them into harmless, polar, readily excreted metabolites. Sometimes, however, FMO bioactivates chemicals into reactive materials that can cause toxicity. Most of the interindividual differences of FMO are due to genetic variability and allelic variation, and splicing variants may contribute to interindividual and interethnic variability observed for FMO-mediated metabolism. In contrast to cytochrome P450 (CYP), FMO is not easily induced nor readily inhibited, and potential adverse drug-drug interactions are minimized for drugs prominently metabolized by FMO. These properties may provide advantages in drug design and discovery, and by incorporating FMO detoxication pathways into drug candidates, more drug-like materials may be forthcoming. Although exhaustive examples are not available, physiological factors can influence FMO function, and this may have implications for the clinical significance of FMO and a role in human disease.

C-Terminal truncation of rabbit flavin-containing monooxygenase isoform 2 enhances solubility

Flavin-containing monooxygenases (FMO) are membrane-associated enzymes contributing to oxidative metabolism of drugs and other chemicals. There are no known structures similar enough to FMO to provide accurate insights into the structural basis for differences in metabolism observed among FMOs. To develop an FMO amenable to crystallization, we introduced mutations into rabbit FMO2 (rF2) to increase solubility, decrease aggregation, and simplify isolation. Alterations included removal of 26 AA (Delta26) from the carboxyl-terminus, His(6)-fusion to the amino-terminus and a double Ser substitution designed to reduce local hydrophobicity. Only Delta26 FMO variants retained normal activity, increased the yield of cytosolic rF2 and decreased protein aggregation. Delta26 constructs increased rF2 in cytosol in low (from 2 to 13%), and high salt (from 24 to 62%) conditions. His-fusion proteins, while active and useful for purification, did not affect solubility. Delta26 variants should prove useful for identifying conditions suitable for production of an FMO crystal.

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

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

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.

Identification and characterization of the FMO2 gene in Rattus norvegicus: a good model to study metabolic and toxicological consequences of the FMO2 polymorphism

BACKGROUND

In lung of many animal species flavin-containing monooxygenase 2 (FMO2) is a 535-amino acid residues drug-metabolizing enzyme. In humans FMO2 exhibits a genetic polymorphism. The major allele encodes a truncated FMO2, the minor allele a full-length FMO2. In laboratory rats we previously reported a FMO2 gene encoding a truncated FMO2 (432-AA residues). In these strains, a double deletion leads to the appearance of a premature stop codon. All laboratory rat strains were derived from the same wild ancestor, Rattus norvegicus.

METHODS

A PCR-based method able to specifically recognize either the wild-type or the mutant allele was developed to investigate a putative FMO2 polymorphism in a population of wild rats. The FMO2 gene was analyzed in 42 wild rats.

RESULTS

A genetic FMO2 polymorphism similar to that described in humans was found in R. norvegicus. We observed three different genotypes: homozygotes for the wild-type FMO2 (33.3%), homozygotes for the mutant FMO2 (38.1%) and heterozygotes (28.6%). Comparative FMO2 mRNA and protein expressions in lungs were studied by reverse transcription-PCR and western blotting. FMO2 mRNA expression was identical between the three groups. In contrast, major differences in the expression of FMO2 protein were detected. FMO2 was strongly expressed in lungs of homozygotes for the wild-type FMO2, faintly expressed in lungs of heterozygotes and non-expressed in lungs of homozygotes for the mutant FMO2. Comparative catalytic properties of lung microsomes were studied by the determination of the oxygenation of methimazole. FMO2 genetic polymorphism was associated with major differences in the S-oxidative metabolism.

S-Oxygenation of the thioether organophosphate insecticides phorate and disulfoton by human lung flavin-containing monooxygenase 2

Phorate and disulfoton are organophosphate insecticides containing three oxidizable sulfurs, including a thioether. Previous studies have shown that only the thioether is oxygenated by flavin-containing monooxygenase (FMO) and the sole product is the sulfoxide with no oxygenation to the sulfone. The major FMO in lung of most mammals, including non-human primates, is FMO2. The FMO2*2 allele, found in all Caucasians and Asians genotyped to date, codes for a truncated, non-functional, protein (FMO2.2A). Twenty-six percent of individuals of African descent and 5% of Hispanics have the FMO2*1 allele, coding for full-length, functional protein (FMO2.1). We have here demonstrated that the thioether-containing organophosphate insecticides, phorate and disulfoton, are substrates for expressed human FMO2.1 with Km of 57 and 32 microM, respectively. LC/MS confirmed the addition of oxygen and formation of a single polar metabolite for each chemical. MS/MS analysis confirmed the metabolites to be the respective sulfoxides. Co-incubations with glutathione did not reduce yield, suggesting they are not highly electrophilic. As the sulfoxide of phorate is a markedly less effective acetylcholinesterase inhibitor than the cytochrome P450 metabolites (oxon, oxon sulfoxide or oxon sulfone), humans possessing the FMO2*1 allele may be more resistant to organophosphate-mediated toxicity when pulmonary metabolism is an important route of exposure or disposition.

Measurements of flavin-containing monooxygenase (FMO) activities

Measurement of Flavin-Containing Monooxygenase (FMO) Activities (Randy L. Rose, North Carolina State University, Raleigh, North Carolina). This unit describes methods used for measuring the presence of flavin-containing monooxygenases using NADPH oxygenation and methamizole oxidation. Methods are also provided to determine the relative contributions of FMO versus cytochrome P450 from microsomes.

Genetic polymorphisms of flavin-containing monooxygenase (FMO)

Mammalian flavin-containing monooxygenase (FMO) exists as six gene families and metabolizes a plethora of drugs and xenobiotics. The major FMO in adult human liver, FMO3, is responsible for trimethylamine (TMA) N-oxygenation. A number of FMO3 mutant alleles have been described and associated with a disease termed trimethylaminuria (TMAU). The TMAU patient excretes large amounts of TMA in urine and sweat. A more recent ethnically related polymorphism in expression of the major FMO in lung, FMO2, has been described. All Caucasians and Asians genotyped to date are homozygous for a CAG --> TAG amber mutation resulting in a premature stop codon and a nonfunctional protein truncated at AA 472 (wildtype FMO2 is 535 AA). This allele has been designated hFMO2*2A. Twenty-six percent of individuals of African descent and 5% of Hispanics genotyped to date carry at least one allele coding for full-length FMO2 (hFMO2*1 allele). Preliminary evidence indicates that FMO2.1 is very active toward the S-oxygenation of low MW thioureas, including the lung toxicant ethylene thiourea. Polymorphic expression of functional FMO2 in the individuals of African and Hispanic descent may markedly influence drug metabolism and/or xenobiotic toxicity in the lung.

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.

Sequencing, expression, and characterization of cDNA expressed flavin-containing monooxygenase 2 from mouse

The cDNA clone of mouse flavin-containing monooxygenase 2 (FMO2) was obtained as an expressed sequence tag (EST) isolated from a female mouse kidney cDNA library from the I.M.A.G.E. consortium (I.M.A.G.E. CloneID 1432164). Complete sequencing of the EST derived a nucleotide sequence for mouse FMO2, which contains 112 bases of 5' flanking region, 1607 bases of coding region, and 309 bases of 3' flanking region. This FMO2 sequence encodes a protein of 535 amino acids including two putative pyrophosphate binding sequences (GxGxxG/A) beginning at positions 9 and 191. Additionally, this mouse FMO protein sequence shows 87 and 86% homology to rabbit and human FMO2 respectively. The mouse FMO2 sequence was subcloned into the expression vector pJL-2, a derivative of pKK233-2 and used to transform XL1-Blue Escherichia coli. FMO activity in particulate fractions isolated from isopropyl-beta-D-thiogalactopyanoside (IPTG) induced cells was heat stable (45 degrees C for 5 min) and demonstrated optimal activity at a relatively high pH of 10.5. The expressed FMO2 enzyme showed catalytic activity towards the FMO substrate methimazole and further analysis of E. coli fractions utilizing NADPH oxidation demonstrated that the mouse FMO2 enzyme also exhibits catalytic activity towards thiourea, trimethylamine, and the insecticide phorate.

The FMO2 gene of laboratory rats, as in most humans, encodes a truncated protein

We describe the isolation and characterization of cDNAs for FMO2 from the laboratory rat. In contrast to FMO2 in other animals, each of which contain 535 amino acid residues, analysis of the sequence of the cDNAs and of a section of the corresponding gene revealed that the ORF of the laboratory rat FMO2 encodes a polypeptide of only 432 residues. This truncated protein is due to the presence of a double deletion corresponding to 1263 and 1264 nucleotides of the orthologous FMO2 cDNAs. This double deletion provokes a frame-shift, with the appearance of a premature stop codon in position 1297-1299. By Northern blotting, the probe for FMO2 hybridized a 2.5-kb transcript in lung and kidney samples only. Heterologous expression of the cDNA revealed that the truncated protein was catalytically inactive. By Western blotting, FMO2 was faintly detected at approximately 50 kDa in laboratory rat lung.

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.

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.

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

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

Flavin-containing monooxygenase isoform 2: developmental expression in fetal and neonatal rabbit lung

Mammalian flavin-containing monooxygenase functions in the oxygenation of numerous xenobiotics containing a soft nucleophile, usually a nitrogen or sulfur. A total of five distinct flavin monooxygenase (FMO) isoforms are expressed in mammals. Individual isoforms are expressed in a sex-, age-, and tissue-specific fashion. In this study, we document the early developmental appearance of the major isoform in rabbit lung, FMO2. FMO2 catalytic activity as well as protein and mRNA are not only present in fetal and neonatal lung but, in some instances, approach levels found in the adult. The expression pattern of FMO2 is similar to that of the two major constitutive cytochromes P450 found in rabbit lung, 2B4 and 4B1. The early developmental appearance of these monooxygenases indicate an important role in the protection of the fetus and neonate against toxic insult from foreign chemicals.

Effect of inhibitors on the biotransformation of tamoxifen by female rat and mouse liver slices and homogenates

The metabolism of tamoxifen was studied in female Sprague-Dawley rat and mouse liver slices and homogenates, and the three principal tamoxifen metabolites, 4-hydroxytamoxifen, N-desmethyl-tamoxifen and tamoxifen N-oxide, were identified by HPLC using authentic standards. It was not possible to identify any of the minor metabolites such as the epoxides using this technique. The N-oxide metabolite only appeared when NADPH was added to the system; this is because the production of tamoxifen N-oxide is primarily mediated by microsomal flavin monooxygenase (FMO) which is NADPH dependent. However, this metabolite did appear in incubations with mouse liver slices only, because they are rich in flavin monooxygenases (FMOs). It did not appear in female rat or mouse liver homogenates, because the NADPH present is destroyed during homogenisation, therefore it was necessary to add NADPH to the system to produce the N-oxide metabolite. The purpose of this study was to investigate the effect of inhibitors on the biotransformation of tamoxifen by female rat and mouse liver slices and homogenates. Female rat liver slices and homogenates were incubated with the following inhibitors (1 mM): cimetidine, ascorbate, sodium azide and reduced glutathione. Cimetidine, a general P-450 inhibitor, inhibited the production of the N-desmethyl metabolite by about 80%; this is in agreement with the action of the other inhibitors. Reduced glutathione, ascorbate and sodium azide are mainly peroxidase inhibitors, so therefore from these novel and interesting results it was possible to suggest that peroxidases play a role in the metabolism of tamoxifen. This observation was also strengthened when the production of the N-desmethyl metabolite increased when horseradish peroxidase was added to the incubate. The production of 4-hydroxytamoxifen was reduced and the N-oxide metabolite was completely inhibited in the presence of peroxidase inhibitors. When rat liver homogenates was incubated with superoxide dismutase (SOD) and catalase, it was observed that the N-desmethyl metabolite disappeared completely at 60 min and the N-oxide and 4-hydroxy metabolites were completely inhibited. However, this phenomenon was only observed when SOD and catalase were preincubated for 30 min with the rat liver homogenate at 37 degrees C; without preincubation the production of these metabolites was unaffected. Finally, the effect of long incubation periods (300 min) on the production of metabolites was examined. It was found that there was a reduction in the concentration of metabolite produced after 60 min which was due to enzyme and co-factor degradation.

The flavin-containing monooxygenase 2 gene (FMO2) of humans, but not of other primates, encodes a truncated, nonfunctional protein

Flavin-containing monooxygenases (FMOs) are NADPH-dependent flavoenzymes that catalyze the oxidation of heteroatom centers in numerous drugs and xenobiotics. FMO2, or "pulmonary" FMO, one of five forms of the enzyme identified in mammals, is expressed predominantly in lung and differs from other FMOs in that it can catalyze the N-oxidation of certain primary alkylamines. We describe here the isolation and characterization of cDNAs for human FMO2. Analysis of the sequence of the cDNAs and of a section of the corresponding gene revealed that the major FMO2 allele of humans encodes a polypeptide that, compared with the orthologous protein of other mammals, lacks 64 amino acid residues from its C terminus. Heterologous expression of the cDNA revealed that the truncated polypeptide was catalytically inactive. The nonsense mutation that gave rise to the truncated polypeptide, a C --> T transition in codon 472, is not present in the FMO2 gene of closely related primates, including gorilla and chimpanzee, and must therefore have arisen in the human lineage after the divergence of the Homo and Pan clades. Possible mechanisms for the fixation of the mutation in the human population and the potential significance of the loss of functional FMO2 in humans are discussed.

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.

Determination of FAD-binding domain in flavin-containing monooxygenase 1 (FMO1)

The flavin-containing monooxygenases (FMOs) are a family of flavoenzymes and contain one molecule of FAD per monomer. In order to demonstrate where FMO interacts with FAD, four mutants for the rat liver FMO1 protein were expressed in yeast and characterized. All four mutants were immunochemically similar to the unmodified form, although the contents of FAD in all four mutants were much lower than that in the unmodified form. Interestingly, the mutant generated by changing the first glycine of the proposed FAD-binding domain (GxGxxG) to alanine revealed catalytic activities, but was lower than those seen with the unmodified form. The conversion of the first glycine to alanine markedly increased and decreased the Km and Vmax values for imipramine N-oxidation, respectively. The other three mutants (RFMOm2, RFMOm3, and RFMOm4) were catalytically inactive. Our results suggest that three glycines, especially the second and third glycines, in the proposed FAD-binding domain were necessary for FMO to show catalytic activities. Using RFMOm1 and the unmodified form, the effects of n-octylamine on the activity of FMO1 were investigated. The activities of both wild-type and RFMOm1 enzymes for all of the compounds examined were enhanced by n-octylamine. The Km and Vmax values of both RFMOm1 and the unmodified form for imipramine N-oxidation were lowered and raised by n-octylamine, respectively.

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.

Expression and characterization of a modified flavin-containing monooxygenase 4 from humans

The inability to obtain flavin-containing monooxygenase 4 (FMO4) in heterologous systems has hampered efforts to characterize this isoform of the FMO gene family. Neither the human nor the rabbit ortholog of FMO4, each of which has been cloned and sequenced, has been expressed. Attempts to achieve expression of FMO4 have been made with Escherichia coli, baculovirus, yeast, and COS systems. The cDNAs encoding FMO4 have extended coding regions compared with those encoding other FMO isoforms. The derived amino acid sequences of FMO1, -2, -3, and -5 from all species examined contain about the same number of residues (531-535 residues), whereas the derived sequences of human and rabbit FMO4 contain 558 and 555 residues, respectively. We have investigated whether the elongation of the FMO4 coding region is related to the inability to achieve expression. The cDNA encoding human FMO4 has been modified by a single base change that introduces a stop codon at the consensus position. This modification allows for expression in E. coli. Lack of expression of intact FMO4 is caused by a problem that occurs following transcription, a problem that is overcome completely by relocation of the stop codon 81 bases to 5' of its normal position. Truncated FMO4 is expressed as an active enzyme with characteristics typical of an FMO isoform. Possible functional changes resulting from altering the 3'-end of an FMO were investigated with human FMO3. Elongation of the coding region of the FMO3 cDNA to the next available stop codon (FMO3*) resulted in the expression of an enzyme with properties very similar to those of unmodified FMO3. Elongation of FMO3 lowered the level of expression in E. coli but did not eliminate it. As with FMO4, the difference in expression levels between FMO3 and elongated FMO3 (FMO3*) appears to be related to translation rather than transcription. The functional characteristics of FMO3 and FMO3* are not significantly different.

Further characterization of rat brain flavin-containing monooxygenase. Metabolism of imipramine to its N-oxide

Flavin-containing monooxygenase (FMO) activity was compared in rat liver and brain microsomes by estimating the actual amount of imipramine N-oxide relative to the corresponding activity, measured using substrate-stimulated rates of NADPH oxidation. The activities measured as NADPH oxidation rates were significantly higher than those estimated from the N-oxide formed. The brain FMO activity was detectable only in the presence of detergents (sodium cholate or Lubrol PX) or in microsomes that were freeze-thawed several times. The antibody to rabbit pulmonary FMO selectively inhibited imipramine N-oxidation. The antiserum to the rat liver NADPH cytochrome P-450 reductase had no effect on imipramine N-oxidation, indicating the noninvolvement of cytochrome P-450 in the above metabolic pathway. A flavin-containing monooxygenase was partially purified from the rat brain microsomes using sequential chromatography on n-octylamino-Sepharose 4B, DEAE-Sephacel and 2',5'-ADP agarose. The purified FMO was resolved by SDS-PAGE into two bands (approximately 57 and 61 KDa, respectively) both of which cross-reacted with antibody to rabbit pulmonary FMO. The purified enzyme metabolized imipramine and the model substrate methimazole to their respective N-oxide and S-oxides.

Differential developmental and tissue-specific regulation of expression of the genes encoding three members of the flavin-containing monooxygenase family of man, FMO1, FMO3 and FM04

We have previously described the isolation and sequencing of cDNA clones encoding flavin-containing monooxygenases (FMOs) 1 and 4 of man [Dolphin, C., Shephard, E. A., Povey, S., Palmer, C. N. A., Ziegler, D. M., Ayesh, R., Smith, R. L. & Phillips, I. R. (1991) J. Biol. Chem. 266, 12379-12385; Dolphin, C., Shephard E. A., Povey, S., Smith, R. L. & Phillips, I. R. (1992) Biochem. J. 287, 261-267]. We present here the isolation of a cDNA for FM03 of man. The sequence of this CDNA and the amino acid sequence deduced from it differ substantially from those previously reported for this member of the FMO family of man. In addition, we have investigated, by quantitative RNase protection assays, the expression in several foetal and adult human tissues of genes encoding FMO1, FMO3 and FMO4, Our results demonstrate that, in the adult, FMO1 is expressed in kidney but not in liver, whereas in the foetus it is expressed in both organs. The lack of expression of FMO1 in adult human liver is in marked contrast to the situation in other mammals, such as pig and rabbit, in which FMO1 constitutes a major form of the enzyme in the liver of the adult animal. The mRNA encoding FMO3 is abundant in adult liver and is also present, in low abundance, in some foetal tissues. Thus, FMO1 and FMO3 are both subject to developmental and tissue-specific regulation, with a developmental switch in the expression of the genes taking place in the liver. FMO4 mRNA is present in low abundance in several foetal and adult tissues and thus the corresponding gene appears to be expressed constitutively.

The assessment of flavin-containing monooxygenase activity in intact animals

A large number of drug metabolising enzymes with different substrate specificities and induction and inhibition characteristics have been described, suggesting that specific test drugs, i.e. probes, should be used for assessing the activity of distinct metabolising enzymes. The flavin-containing monooxygenase (FMO) and cytochrome P-450 (P-450) are the two main microsomal enzyme systems involved in the oxidation of xenobiotics. FMO is present in liver and other tissues of most vertebrates. It catalyses the oxidation of a wide range of xenobiotics, especially soft nucleophiles bearing nitrogen and sulphur centres. There is substantial information on both in vitro and in vivo probes for cytochrome P-450. For example antipyrine has been widely used for assessing the activity of P-450 in vivo by utilising pharmacokinetic parameters as indices of enzyme activity. In more recent years, isozyme specific probes have also been developed for some of the P-450s. Whereas a number of substrates are available for measuring FMO activity in vitro (e.g. N,N-dimethylaniline), probes for assessing FMO activity in vivo are limited. In this review a background to the use of in vitro and in vivo probes for assessing the activity of FMO is presented, and approaches and criteria for development of potential pharmacokinetic probes for FMO are described. Preliminary data on the development of ethyl methyl sulphide (EMS) and trimethylamine (TMA) as potential pharmacokinetic probes for assessing FMO activity in rats are discussed in detail. Clinical implications of modulation of FMO activity are discussed, and arguments presented as to why the development of FMO probes for use in man will be useful additions to the range of other compounds available for assessment of liver metabolic function.

S-oxidation of (+)-cis-3,5-dimethyl-2-(3-pyridyl)-thiazolidin-4-one hydrochloride by rat hepatic flavin-containing monooxygenase 1 expressed in yeast

1. Rat hepatic flavin-containing monooxygenase 1 (FMO1) expressed in yeast catalyzed the S-oxidation of (+)-cis-3,5-dimethyl-2-(3-pyridyl)thiazolidin-4-one hydrochloride (SM-12502) in vitro. 2. S-oxidation was inhibited by 1-(1-naphthyl)-2-thiourea and thiobenzamide, known inhibitors of FMO, but was not enhanced by n-octylamine, a known enhancer of FMO. 3. The rate of S-oxide formation from SM-12502 was about four-fold lower than that from (+/-)-trans-3,5-dimethyl-2-(3-pyridyl)thiazolidin-4-one hydrochloride (SM-9979) and enantioselectivity and diastereoselectivity of the S-oxidation reaction were observed. 4. The ability of the recombinant yeast to produce the S-oxide from SM-12502 was maintained for long periods and exemplifies the recombinant yeast as a bioreactor to produce a large amount of the S-oxide.

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.

Developmental regulation of flavin-containing monooxygenase (FMO) isoforms 1 and 2 in pregnant rabbit

Mammalian flavin-containing monooxygenase (FMO, EC 1.14.13.8) metabolizes a vast number of structurally diverse xenobiotics containing a soft-nucleophile, typically a nitrogen or sulfur. FMO is not inducible by the classical cytochrome P450 (CYP) inducers, such as phenobarbital, polycyclic aromatic hydrocarbons, ethanol or macrolide antibiotics. Evidence does exist from a number of laboratories, however, for developmental and hormonal regulation of FMO. Our laboratory has confirmed previous observations of enhanced FMO activity during mid- and late-gestation in maternal rabbit lung and have demonstrated that this response is due to increased protein and catalytic activity associated with FMO2. The time course of expression of FMO2 during mid- and late-gestation correlates to plasma peaks of progesterone or cortisol. FMO2 also peaks at parturition in maternal kidney, coincident with plasma cortisol levels. FMO2 is induced by s.c. administration of either progesterone or dexamethasone in lung, or by dexamethasone in kidney. Correlation of plasma progesterone or cortisol levels during gestation and postpartum support a role for progesterone, but not cortisol in regulation of FMO2 in maternal rabbit lung. The levels of FMO1 also appear to be increased during mid- and late-gestation in liver. FMO1 in liver may also be regulated during gestation by progesterone or glucocorticoids as administration of these steroids enhanced FMO1 mRNA levels 4-fold.

Rat liver flavin-containing monooxygenase (FMO): cDNA cloning and expression in yeast

A rat liver cDNA clone, RFMO1, coding for a flavin-containing monooxygenase (FMO) was isolated. This cDNA clone encoded a protein of 532 amino acids. The deduced amino acid sequence was 84, 82 and 82% identical to those of the pig, human (Form 1) and rabbit (Form 1) liver FMOs, while it was only 52, 50, 54, 56 and 54% identical to the human (Form II), human (Form 2) and rabbit liver FMOs (Form 2) and rabbit and guinea pig lung FMOs. RNA blot analysis showed that rat liver FMO was also expressed in lung and kidney and to a lesser extent in the heart and brain. An expression plasmid, pAMFMO, was constructed and the FMO protein expressed in yeast (AH22). This FMO protein catalyzed thiobenzamide S-oxidation, and NADPH oxidation associated with the S- or N-oxidation of thiourea, N,N-dimethylaniline, trimethylamine, imipramine, chlorpromazine, N,N-dimethylhydrazine, thioacetamide as substrates. The S-oxidation activities of thiobenzamide and thiourea were enhanced by n-octylamine, a known enhancer of FMO, and inhibited by alpha-naphthylthiourea, a known inhibitor of FMO. This is the first report in which FMO with catalytic activities was stably expressed.

Catalytic activity and immunohistochemical localization of flavin-containing monooxygenase in rat kidney

The presence of flavin-containing monooxygenase activity was examined in rat kidney microsomes using N,N-dimethylaniline and methimazole as substrates. Western immunoblot analysis using antisera to porcine liver and rabbit lung flavin-containing monooxygenase indicated immunological cross-reactivity between rat kidney, porcine liver and rabbit lung flavin-containing monooxygenase. Immunohistochemical studies using antisera to rabbit lung flavin-containing monooxygenase demonstrated localization of this enzyme in the proximal and distal convoluted tubules of the renal cortex, the collecting ducts in the renal medulla, but not the glomeruli. This observation indicates the colocalization of flavin-containing monooxygenase and cytochrome P-450 in the metabolically active and absorptive compartment of the renal parenchyma.

Use of thiocarbamides as selective substrate probes for isoforms of flavin-containing monooxygenases

The oxidation of thiourea, phenylthiourea, 1,3-diphenylthiourea, 1,3-bis-(3,4-dichlorophenyl)-2-thiourea and 1,1-dibenzyl-3-phenyl-2-thiourea was measured in reactions catalyzed by purified pig liver flavin-containing monooxygenase (FMO-1) and by microsomal fractions isolated from pig, guinea pig, chicken, rat and rabbit tissues. The reactions, followed by measuring substrate-dependent thiocholine oxidation [Guo and Ziegler, Anal Biochem 198: 143-148, 1991], were carried out in the presence of 2 mM 1-benzylimidazole to minimize potential interference from reactions other than those catalyzed by isoforms of the flavin-containing monooxygenase (FMO). While at saturating substrate concentrations the Vmax for purified FMO-1 catalyzed oxidation of all five thiocarbamides was essentially constant, velocities for the microsomal catalyzed reactions varied not only with tissue and species but also with the van der Waals' surface area of the thiocarbamide. Rat liver, rat kidney and rabbit liver microsomes failed to catalyze detectable oxidation of thiocarbamides larger than 1,3-diphenylthiourea and lung microsomes from a female rabbit only accepted substrates smaller than 1,3-diphenylthiourea. On the other hand, liver microsomes from chickens, pigs and guinea pigs catalyzed the oxidation of larger thiocarbamides, but the rates decreased with increasing substrate size and chicken liver microsomes showed no detectable activity with the largest thiocarbamide tested. To define more precisely the parameters affecting thiocarbamide substrate specificity of microsomal preparations, activities present in detergent extracts of guinea pig liver microsomes were separated into three distinct fractions. The substrate specificities of these partially purified fractions were different and consistent with the difference observed with microsomal catalyzed reactions. This strongly suggests that thiocarbamides that differ in size may be useful probes for measuring the number of activities of FMO isoforms in crude tissue preparations.

The role of the flavin-containing monooxygenase (EC 1.14.13.8) in the metabolism and mode of action of agricultural chemicals

1. The flavin-containing monooxygenase (FMO) (EC 1.14.13.8) is a versatile enzyme that catalyses the monooxygenation of a large number of xenobiotic soft nucleophiles ranging from inorganic ions to organic compounds with nitrogen, sulphur, phosphorus or selenium heteroatoms. 2. The substrate specificity relative to agricultural chemicals is discussed and compared with that of the cytochrome P-450-dependent monooxygenase system. The relative activity of these two enzymes towards common substrates varies from substrate to substrate and from tissue to tissue as is shown in the case of the insecticide, phorate and the hepatotoxicant, thiobenzamide. 3. The products of FMO action may be chemically different (e.g. nicotine) to those from P-450, or the two enzymes may produce different isomers of the same product (e.g. phorate). 4. Recent studies have demonstrated that, in the rabbit, the FMOs from liver and lung are different gene products which differ not only in primary sequence but also in physical, catalytic and immunochemical properties. These studies are being extended to include other tissues such as skin and brain. 5. Immunocytochemical localization of FMO in lung and skin correlates well with measurements of the oxidation of methimazole, a specific FMO substrate.

Molecular cloning of the flavin-containing monooxygenase (form II) cDNA from adult human liver

Complementary DNA (cDNA) clones encoding the adult human liver flavin-containing monooxygenase (FMO; dimethylaniline N-oxidase, EC 1.14.13.8) were isolated from lambda gt10 and lambda gt11 libraries. The cDNA libraries were screened with three synthetic 36-mer oligonucleotide probes derived from the nucleic acid sequence of the pig liver FMO cDNA. The deduced amino acid sequence for the adult human liver FMO was quite distinct from the pig liver FMO, and adult human liver FMO was designated form II (HLFMO II). The full-length cDNA sequence of HLFMO II [2119 base pairs (bp)] had an open reading frame of 1599 nucleotides, which encoded a 533-amino acid protein of Mr 59,179, a 5'-noncoding region of 136 nucleotides and a 3'-noncoding region of 369 nucleotides excluding the poly(A) tail. The deduced amino acid sequence of HLFMO II had 80% similarity with the rabbit liver FMO II but only a 52%, 55%, and 53% amino acid similarity with the rabbit liver (form I), the pig liver (form I), and fetal human liver (form I) FMOs, respectively. RNA analysis of adult human liver RNA showed that there was one HLFMO II mRNA species. Analysis of genomic DNA indicated that HLFMO II was the product of a single gene. These results indicated that the deduced amino acid sequence for HLFMO II contained highly conserved residues and suggested that FMO enzymes were closely related and, undoubtedly, derived from the same ancestral gene.

Cellular localization of flavin-containing monooxygenase in rabbit lung

A specific form of flavin monooxygenase has been identified in the lungs of a number of species. Distribution of the pulmonary flavin-containing monooxygenase (FMOp) is of interest because it oxidatively metabolizes a wide variety of nitrogen-, sulfur-, and phosphorous-containing xenobiotics, some of which form highly toxic reactive intermediates. We have identified the nonciliated bronchiolar epithelial (Clara) cell as the predominant location for this enzyme in rabbit lung. In addition, protein in ciliated, endothelial, type I, and type II cells and in tracheal lining layer reacted with antibodies to FMOp. In all these cell types antigen was found associated with cytoplasmic organelles, and in the Clara cell antigen was most concentrated in areas rich in smooth endoplasmic reticulum. Staining of ciliated surfaces was also observed at both the light and electron microscopy levels. Extracellular antigen was also apparent in tracheal lining layer smeared onto glass slides. We compared the location of the FMOp with that of two enzymes of the cytochrome P-450 monooxygenase system (studied here and elsewhere), cytochrome P450 IIB (P450 IIB), and NADPH cytochrome P450 reductase (reductase), and concluded that (1) FMOp is detected in all cells where P450 IIB and reductase are both present (Clara, type II, and ciliated); (2) FMOp and P450 IIB, but not reductase, are detected in endothelial cells; (3) P450 IIB alone is detected in the plasma membrane, cilia, and microvillae of ciliated cells and plasma membrane of endothelial cells; and (4) FMOp alone is detected in type I cells.

The flavin-containing monooxygenase in mouse lung: evidence for expression of multiple forms

The flavin-containing monooxygenase (FMO) was purified from mouse lung microsomes. On SDS-PAGE, the purified enzyme separated as two bands, a major band of 58,000 daltons and a minor band of 59,000 daltons. Antibodies to mouse liver FMO cross-reacted with both bands in the purified preparations, whereas antibodies to rabbit lung FMO cross-reacted only with the major band. In microsomal preparations the major band was recognized by both antibodies, but neither antibody detected the minor band in microsomes. A cDNA encoding the pig liver FMO hybridized with mRNA isolated from mouse liver, kidney, and lung, whereas cDNA encoding the rabbit lung FMO hybridized only with mouse lung and kidney mRNA. Thermal stability studies showed that the FMO preparation purified from mouse lung consisted of a heat-stable and a heat-labile component. The heat-labile component of lung FMO was inhibited competitively by imipramine, whereas the heat-stable component was insensitive to the presence of imipramine. Immunoprecipitation of purified mouse lung FMO with anti-rabbit lung FMO completely removed the protein band reactive to anti-rabbit lung FMO while leaving reactivity to anti-liver FMO. The catalytic and immunochemical differences seen between FMO from rabbit lung and mouse lung appear to result from the expression of at least two forms of FMO in the mouse lung, one similar to the rabbit pulmonary form and one similar to the major mouse liver form of FMO.

Evidence for complex formation between rabbit lung flavin-containing monooxygenase and calreticulin

Rabbit lung flavin-containing monooxygenase (FMO, EC 1.14.13.8) was denatured, reduced, carboxymethylated, digested with endoproteinase Glu-C or trypsin, and subjected to mass spectrometric analysis. The amino acid sequences of selected peptides were determined by tandem mass spectrometry. Over 90% of rabbit lung FMO was mapped by liquid secondary ion mass spectrometry (LSIMS). The FMO N-terminal amino acid was found to be N-acetylated, and the N-terminal 23 amino acid peptide contained an FAD binding domain consisting of Gly-X-Gly-X-X-Gly. Another peptide was found to contain a NADP+ binding domain consisting of Gly-X-Gly-X-X-Ala. The mapped and/or sequenced peptides were found to be completely consistent with the peptide sequence deduced from the cDNA data and the previously published gas-phase sequencing data. Further mass spectrometry and protein analytical work unambiguously showed that rabbit lung FMO existed in tight association with a calcium-binding protein, calreticulin. Over 68% of rabbit lung calreticulin was mapped by LSIMS. Tandem mass spectrometric and gas-phase sequencing studies provided direct evidence for the identification of the N-terminal and other rabbit lung calreticulin-derived peptide sequences that were identical to other previously reported calreticulins. The complexation of calreticulin to rabbit lung FMO could account for some of the unusual physical properties of this FMO enzyme form.

Multiplicity of liver microsomal flavin-containing monooxygenase in the guinea pig: its purification and characterization

Two distinct forms (FMO-I and FMO-II) of flavin-containing monooxygenase were purified from the liver microsomes of guinea pig. The minimum molecular weights estimated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis were 54,000 for FMO-I and 56,000 for FMO-II, respectively. Tryptic digestion of these enzymes gave different electrophoretic patterns, suggesting that FMO-I and -II have distinct amino acid sequences. The amino terminal sequence of FMO-II could not be estimated probably due to its blocking while that of FMO-I was determined to be highly homologous to the rabbit liver flavin-containing monooxygenase (J. Ozols, 1989, Biochem. Biophys. Res. Commun. 163, 49-55). Absorption maxima of FMO-I and -II were recorded at 368 and 440 nm and 381 and 456 nm, respectively. Molar ratios of FAD to both of these apoenzymes were shown to be one to one. Substrate specificity of FMO-I and -II was determined using 15 compounds as the substrate. The results showed two enzymes that exhibited overlapped but different specificity toward these substrates although FMO-I had lower activity than did FMO-II with all compounds except thiobenzamide. Of particular interest, only FMO-II showed considerably high activities for primary amines, n-octylamine, and n-decylamine. Immunoglobulin G raised against FMO-II could recognize FMO-I as well as FMO-II, but the reactivity of FMO-I toward the antibody was obviously lower than that of FMO-II. Electrophoresis followed by immunostaining revealed that microsomes of lung, kidney, urinary bladder, testis, and spleen contain the same protein as FMO-II and/or FMO-I. Only lung was shown to have an additional isozyme of FAD-monooxygenase with a molecular weight apparently higher than those of FMO-I and -II. These results strongly suggest that at least two forms of flavin-containing monooxygenases distinct from the lung-type isozyme are expressed in liver of guinea pigs.

Flavin-containing monooxygenases: enzymes adapted for multisubstrate specificity

Unlike all other oxidases, microsomal flavin-containing monooxygenases (FMO) discriminate between essential and foreign compounds by excluding the former rather than selectively binding the latter. As Daniel Ziegler describes here, xenobiotics that readily cross cell membranes can enter the catalytic cavity, whereas charged groups on essential metabolites that prevent their passive diffusion out of the cell also block their access to FMO. FMO appears to be ideally adapted to catalyse the detoxification of structurally diverse soft nucleophiles (e.g. alkaloids with basic side-chains and organic sulfur xenobiotics) so abundant in food derived from plants.

N-terminus determination: FAD and NADP binding domain mapping of hog liver flavin-containing monooxygenase by tandem mass spectrometry

Highly purified hog liver flavin-containing monooxygenase was sequentially denatured, reduced, carboxymethylated, and digested with endoproteinase Glu-C. The purified peptides were subjected to mass spectrometric analysis and the amino acid sequence of selected fragments was determined by tandem mass spectrometry. The amino acid sequence of the first 12 residues of the N-terminus was: Ac-Ala-Lys-Arg-Val-Ala-Ile-Val-Gly-Ala-Gly-Val-Ser-Gly. The amino acid sequence determined for another peptide was: Lys-Ser-Val-Leu-Val-Val-Gly-Met-Gly-Asn-Ser-Gly-Thr-Asp-Ile-Ala-Val-Glu. The results provide direct evidence for the structure of the N-terminal modification of the protein and for the existence of the FAD and NADP binding domains of Gly-X-Gly-X-X-Gly.