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PubMed1987

Electrochemical reduction of methemoglobin either directly or with flavin mononucleotide as a mediator.

Electrochemical reduction of methemoglobin on a platinum electrode is studied by means of thin layer spectroelectrochemistry. For methemoglobin alone in solution, direct reduction is very slow even for potentials close to those of the reduction of the solvent. The reduction of a methemoglobin-oxyhemoglobin mixture with an imposed potential causes the electrochemical reduction of oxygen, the conversion of oxyhemoglobin into deoxyhemoglobin, and a simultaneous transformation of part of the molecules into methemoglobin. When fixed oxygen has disappeared, reduction of methemoglobin takes place. The reduction of methemoglobin and deoxyhemoglobin is catalyzed by the presence of flavin mononucleotide (FMN). For the oxyhemoglobin-methemoglobin mixture, flavin makes a fast deoxygenation of oxyhemoglobin without a change in the oxidation state of the iron. It also allows the rapid reduction of methemoglobin. In each case, the resulting deoxyhemoglobin solutions do not show any electrolysis-induced modification of the equilibrium curves for oxygen binding.

Electrochemistry↗
PubMed1985

Kinetics of reduction of Clostridium pasteurianum rubredoxin by laser photoreduced spinach ferredoxin:NADP+ reductase and free flavins. Electron transfer within a protein-protein complex.

The kinetics of reduction of oxidized Clostridium pasteurianum rubredoxin (Rdox) by free flavin semiquinones generated by the laser flash photolysis technique and by spinach ferredoxin:NADP+ reductase (FNR) semiquinone (also produced by flavin semiquinone reduction) have been investigated under anaerobic conditions. 5-Deazariboflavin semiquinone (5-dRf) rapidly reduces oxidized rubredoxin (Rdox) (k = 3.0 X 10(8) M-1 S-1) and oxidized ferredoxin:NADP+ reductase (FNRox) to the semiquinone level (k = 5.5 X 10(8) M-1 S-1). Lumiflavin semiquinone reduces Rdox more slowly (k = 1.3 X 10(7) M-1 S-1) and is not measurably reactive with FNRox. Absorption difference spectroscopy and difference CD indicate that Rdox and FNRox form a 1:1 complex at low ionic strength (10 mM), which is completely dissociated at higher ionic strength (310 mM). Apparent second order rate constants for reduction of Rdox in its free and complexed state by lumiflavin semiquinone are the same. Reduction of Rdox (both free and complexed) by free FNR semiquinone and intracomplex electron transfer were investigated using 5-dRf as the reductant. At I = 10 mM, a first order rate constant of 2.0 X 10(3) S-1 was obtained, which corresponds to the processes involved in intracomplex electron transfer from FNR semiquinone to Rdox. A second order reaction between free FNR semiquinone and complexed Rdox was also observed to occur (k = 5 X 10(7) M-1 S-1). At I = 310 mM, these reactions are not observed and the reaction of FNR semiquinone with free Rdox is second order (k = 4 X 10(6) M-1 S-1).

Circular Dichroism↗
PubMedDate not supplied

[Change in the morphological structure of the population and in the flavin level in the cells of Candida guilliermondii yeasts under periodic cultivation].

The growth of Candida guilliermondii on media with glucose and acetate is described by statistical functions for cell volume (size) distribution. Special instrument has been used to measure the volume of cells and volume distributions. Changes in the morphological structure of populations (both in the average size of cells and in the extent of the size heterogeneity) were detected throughout the whole period of the cultural growth including the exponential phase. These changes hardly differed during the growth on media with glucose or with acetate. The content of flavins in cells also varied in the course of periodic growth. Their total level in cells, as well as the level of FAD, was higher in the exponential phase of growth than during the subsequent phases. Changes in the intracellular level of flavins were found also in yeast populations synchronized by nitrogen limitation.

Acetates↗
PubMed1982

The room temperature potentiometry of xanthine oxidase. pH-dependent redox behavior of the flavin, molybdenum, and iron-sulfur centers.

A series of potentiometric titrations of xanthine oxidase have been performed at room temperature in the pH range 6.1-9.9. Reduction of the two Fe/S centers was monitored by CD, and that of the FAD and Mo center by EPR. The Fe/S centers behave as centers having a protonable group whose pKa changes with reduction state (E = -344 mV, pKo = 6.4, and pKr = 8.1 for Fe/S I; E = -249 mV, pKo = 6.4, and pKr = 8.0 for Fe/S II). The flavin and the two types of molybdenum centers show varying behavior, but, in all cases, electron addition is accompanied by protonation. The sequence for FAD is reduction, protonation, reduction, protonation with E1 = -398 mV, E2 = -240 mV, pK1 = 9.5, pK2 = 7.4. For "rapid" molybdenum, the sequence is protonation, reduction, protonation, reduction with E1 = -369 mV, E2 = -301 mV, pK1 = 7.9, pK2 = 8.4; and for slow molybdenum, protonation, reduction, protonation with E1 = 320 mV, E2 = -477 mV, pK1 = 7.5, pK2 = 9.5. Comparison to data obtained previously at cryogenic temperatures (Cammack, R., Barber, M. J., and Bray, R. C. (1976) Biochem. J. 157, 469-468 and Barber, M. J., and Seigel, L. M. (1982) in Flavins and Flavoproteins (Massey, V., and Williams, C. H., eds) pp. 796-804, Elsevier/North-Holland, New York) showed the centers to have significant temperature dependence, which calls for a re-evaluation of conclusions reached using cryogenic techniques (e.g. rapid-freeze). The optical absorbance characteristics of the enzyme were also investigated and a possible absorbance for molybdenum was suggested.

Animals↗
PubMed1980

Studies on the reaction mechanism of lactate oxidase. Formation of two covalent flavin-substrate adducts on reaction with glycollate.

L-Lactate oxidase from Mycobacterium smegmatis catalyzes the oxidative decarboxylation of glycollate, with formate, CO2, and H2O as the major products. In addition, some "uncoupling" of the normal reaction occurs, with glyoxylate and H2O adition, some "uncoupling" of the normal reaction occurs, with glyoxylate and H2O2 as products. Glyoxylate is also a substrate (presumably as its hydrate); in this case, the reaction products are oxalate and H2O2. Evidence is presented that the enzyme recognizes glycollate as a prochiral substrate, differentiating between the Re- and Si-faces of the alpha carbon atom. Two highly fluorescent species are formed concomitantly from the reaction with glycollate; they are proposed to be covalent alpha-glycollyl adducts to the reduced flavin position N(5). One of these adducts is labile and in rapid equilibrium with oxidized enzyme and glycollate, and with the complex of reduced enzyme and glyoxylate; this adduct is a catalytically competent intermediate. The other adduct is comparatively stable (t 1/2 for decay = 20 min at 25 degrees C) and does not react with O2. It is formed at a rate approximately 1% that of the catalytic adduct, but because of its lack of reaction with O2 and its stability, it gradually accumulates during catalytic turnover, resulting in catalytically incompetent enzyme. An isotope effect of approximately 4 is found in the reduction of oxidized enzyme flavin and in the formation of the labile fluorescent adduct, when alpha-2H2-glycollate or (R)-glycollate-2-d is used, but not with the (S)-glycollate-2-d enantiomer. It is concluded that the catalytic adduct is formed by hydrogen abstraction from the Re-face of glycollate.

Aerobiosis↗
PubMed1980

Stabilization of lactate oxidase flavin anion radical by complex formation.

The red-colored flavin anion radical of lactate oxidase was formed by phtochemical reducion. the rdical was found to be reoxidized readily by molecular oxygen, with a rate constant of 4 x 10(5) M-1 min-1 at pH 7.0, 25 degrees C. On mixing the radical under anaerobic conditions with pyruvate, a change in spectrum typical of charge transfer interaction was found. The complex was composed of 1 eq of pyruvate per eq of enzyme flavin radical, with a Kd of 1.4 x 10(-5) M. The complex was sufficiently stable at 0 degrees C that it could be isolated free of exces keto acid by gel filtration under aerobic conditions. The isolted complex appears to be quite inert to oxidation by O2; the slow reoxidation which was observed was due to the slow dissociation of the complex and the subsequent fast reaction of O2 with the radical not in complex. The observed rate of oxidation is markedly dependent on temperature, with an activation energy of 25 kcal per mol.

Anaerobiosis↗
PubMed1993

[Hemolysis of isolated erythrocytes by UVA-sensitization of flavins].

Riboflavin (RF), flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD) caused hemolysis of isolated rabbit erythrocytes by UVA-sensitization. Hemolysis induced by UVA-sensitization with RF and FMN was inhibited by 1.0 mM histidine and EDTA, and was promoted by 0.15 mM histidine and 0.01 mM NaN3. Hemolysis induced by irradiation in anaerobic condition was smaller than those in aerobic condition. Hemolysis was proceeded in the dark for 5 hours after the UVA-irradiation with RF and FAD. Erythrocyte treated by UVA-sensitization with RF easily caused hypotonic hemolysis.

Animals↗
PubMed1993

Crystal structure of cholesterol oxidase complexed with a steroid substrate: implications for flavin adenine dinucleotide dependent alcohol oxidases.

Cholesterol oxidase from Brevibacterium sterolicum is a flavin-dependent enzyme that catalyzes the oxidation and isomerization of 3 beta-hydroxy steroids with a double bond at delta 5-delta 6 of the steroid ring backbone. The crystal structure of the free enzyme in the absence of a steroid substrate has previously been determined. In this paper we report the crystal structure of the complex of cholesterol oxidase with the steroid substrate dehydroisoandrosterone, refined at 1.8-A resolution. The final crystallographic R-value is 15.7% for all reflections between 10.0- and 1.8-A resolution. The steroid is buried within the protein in an internal cavity which, in the free enzyme crystal structure, was occupied by a lattice of water molecules. The conformations of a number of side chains lining the active-site cavity have changed in order to accommodate the steroid substrate. A loop region of the structure between residues 70 and 90 differs significantly between the substrate-free and substrate-bound forms of the enzyme, presumably to facilitate binding of the steroid. The hydroxyl group of the steroid substrate is hydrogen-bonded to both the flavin ring system of the FAD cofactor and a bound water molecule. FAD-dependent cholesterol oxidase shares significant structural homology with another flavoenzyme, glucose oxidase, suggesting that it might also be a member of the glucose-methanol-choline (GMC) oxidoreductase family. Although there is only limited sequence homology, a superposition of these two structures reveals a conserved histidine residue within hydrogen-bonding distance of the active-site water molecule.(ABSTRACT TRUNCATED AT 250 WORDS)

Brevibacterium↗
PubMed1993

Functional characterization of flavin-containing monooxygenase 1B1 expressed in Saccharomyces cerevisiae and Escherichia coli and analysis of proposed FAD- and membrane-binding domains.

A cDNA encoding the flavin-containing monooxygenase of rabbit lung (FMO 1B1) was expressed in yeast and Escherichia coli and the recombinant enzymes characterized. A high copy, isopropyl-1-thio-beta-D-galactopyranoside (IPTG)-inducible E. coli expression vector, pKKHC, was used for expression in E. coli strain JM109, and a galactose-inducible vector, YEp53, was used for expression in yeast strain 334. Following transcriptional induction with IPTG or galactose, subcellular fractions were prepared and analyzed immunochemically and catalytically. Antibodies to rabbit FMO 1B1 were used to detect the recombinant proteins in the 100,000 x g pellet prepared from the 10,000 x g supernatant fraction of yeast homogenates and the 2,000 x g supernatant fraction of E. coli homogenates. No FMO 1B1 was detected in cytosol. Mobilities of the recombinant proteins in SDS-polyacrylamide gel electrophoresis appeared identical to that of the native microsomal enzyme. Catalytic similarity to the native FMO 1B1 was demonstrated by the ability of the expressed enzymes to metabolize methimazole, thiourea, dimethylaniline, and cysteamine, but not chlorpromazine or imipramine. In addition, the recombinant enzymes exhibited a number of the unique physical properties associated with FMO 1B1, including stability to elevated temperature and activation by sodium cholate and magnesium chloride. Based on the specific content of FAD, the level of expression was estimated to be approximately 2% of the total protein in the E. coli 100,000 x g particulate fraction and 1% in the fraction from yeast. To demonstrate the utility of the E. coli expression system for studying structure/function relationships of the flavin-containing monooxygenase, two mutant FMOs were expressed and characterized. One mutant, formed by deletion of a putative membrane-anchoring peptide (the 26 carboxyl-terminal amino acids) was tested for membrane association. No difference in the subcellular distribution was found between the truncated and unmodified proteins, suggesting that the 26-residue COOH-terminal peptide is not important in membrane association. Catalytic analysis of the truncated FMO 1B1 established its functional similarity to the full-length protein, indicating that the COOH terminus does not contribute to any of the unique properties of the lung enzyme.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1977

Superoxide-forming enzyme from human neutrophils: evidence for a flavin requirement.

The superoxide-forming activity of 27,000-g particles prepared from homogenates of zymosan-treated human neutrophils is lost if the assay is conducted in the presence of 0.045% Triton X-100. This loss in activity in the presence of detergent is prevented by 40 micron flavin adenine dinucleotide (FAD), but not by flavin mononucleotide, riboflavin, adenosine 5'-diphosphate, or adenosine 5'-monophosphate. With resting particles or particles from zymosan-treated chronic granulomatous disease neutrophils, no superoxide-forming activity is detectable even in the presence of FAD; this is true whether or not detergent is present in the assay. Particles extracted with detergent prior to assay are fully active if assayed in the presence of FAD, but show little activity if FAD is omitted from the assay mixture. These results suggest that the superoxide-forming enzyme from human neutrophils is a FAD-requiring enzyme.

Adenine Nucleotides↗
PubMed1978

Purification and characterization of NADPH-dependent flavin reductase. An enzyme required for the activation of chorismate synthase in Bacillus subtilis.

NADPH-dependent flavin reductase (required for the activation of chorismate synthase) was purified to homogeneity from cell-free extracts of Bacillus subtilis. The enzyme has a molecular weight of 13,000 as determined by sodium dodecyl sulfate-gel electrophoresis, is specific for NADPH, and requires a divalent metal ion and either FMN or FAD for maximal rates of NADPH oxidation. The enzyme is able to reduce 2,6-dichlorophenolindophenol (DCIP) in the presence of NADPH and a divalent metal ion. Both catalytic activities were completely inhibited by EDTA. The Km for FMN is 1.25 X 10(-5) M and for NADPH 7.8 X 10(-5) M with oxygen as the final electron acceptor, and 3.85 X 10(-4) M with DCIP as the final electron acceptor. The enzyme was also isolated in association with chorismate synthase and dehydroquinate synthase. The enzyme associated with the complex has the same catalytic properties as the dissociated enzyme except that it requires both a divalent metal ion and FMN for DCIP reduction. Maximal enzyme activity was observed when the enzyme was preincubated with FMN and the divalent metal ion. The enzyme complex is easily dissociable and the dissociation of the enzyme complex resulted in the failure of NADPH-dependent flavin reductase to adsorb to phosphocellulose.

Bacillus subtilis↗
PubMed1978

Dehydroquinate synthase in Bacillus subtilis. An enzyme associated with chorismate synthase and flavin reductase.

Dehydroquinate synthase, the enzyme which catalyzes the conversion of 3-deoxy-D-arabino-heptulosonic acid 7-phosphate (DAHP) to 5-dehydroquinate, has been purified from Bacillus subtilis in association with chorismate synthase and NADPH-dependent flavin reductase. The enzyme was only active when associated with chorismate synthase, whereas the flavin reductase could be separated from the complex with retention of dehydroquinate synthase activity. The enzyme requires NAD and either Co2+ or Mn2+ for maximal activity. The activity was completely inhibited by EDTA. The Km of the enzyme for DAHP, NAD, and Co2+ were estimated to be 1.3 X 10(-4), 5.5 X 10(-5), and 5.5 X 10(-5) M, respectively. Enzyme activity was completely inhibited by NADH and the inhibition was not reversed by the addition of NAD, NADPH and NADP were not inhibitory. The enzyme was unstable to heat and lost all activity at 55 degrees C. A protein fraction which did not adsorb to phosphocellulose was found to inhibit the enzyme.

Bacillus subtilis↗
PubMed1999

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.

Animals↗
PubMed1994

Oxidation of dimethylselenide to dimethylselenoxide by microsomes from rat liver and lung and by flavin-containing monooxygenase from pig liver.

Oxidation of [75Se]dimethylselenide by rat liver and lung microsomes and by purified flavin-containing monooxygenase from pig liver was demonstrated. Quantitation of the nonvolatile product showed a 1:1 stoichiometry with NADPH oxidation, consistent with selenoxide formation. The apparent Km for dimethylselenide was 0.7 microM with rat liver microsomes and 0.3 microM with purified pig liver enzyme. Facile reversal of dimethylselenide oxidation by reducing agents present in microsomes, and by glutathione, indicates that redox cycling can occur. Unlabeled dimethylselenoxide carrier circumvented reduction of the labeled product, permitting quantitation. This is the first demonstration of a naturally occurring selenium substrate for the microsomal flavin-containing monooxygenase.

Animals↗
PubMed1995

Characterization of flavin-containing monooxygenase 5 (FMO5) cloned from human and guinea pig: evidence that the unique catalytic properties of FMO5 are not confined to the rabbit ortholog.

Several full-length clones encoding the human and guinea pig orthologs of flavin-containing monooxygenase 5 (FMO5) have been isolated from libraries constructed with hepatic mRNA. The clones were detected by hybridization with the cDNA encoding FMO5 expressed in rabbit. The human and guinea pig cDNAs encode for proteins of 533 amino acids that contain putative pyrophosphate binding domains characteristic of mammalian FMOs. The sequences derived for the human and guinea pig FMO5 proteins are 87% identical and are 85 and 82% identical, respectively, to the sequence of rabbit FMO5. As is the case with other FMOs, FMO5 in human and guinea pig is encoded by multiple transcripts. Rabbit FMO5 expressed in Escherichia coli was purified and used to elicit antibodies in goat. These antibodies detected FMO5 in samples from livers of adult humans, rabbits, and guinea pigs and fetal livers of humans. The human and guinea pig forms of FMO5 were expressed in E. coli and characterized. Neither enzyme effectively catalyzed the metabolism of methimazole, a general FMO substrate; however, both were active with n-octylamine. The responses of the human FMO5 and guinea pig FMO5 to detergent, ions and elevated temperature are all similar to the responses described for rabbit FMO5. These results indicate that the unique properties of FMO5 from rabbit are species-independent and that this form of the flavin-containing monooxygenase is not readily classified as a drug-metabolizing enzyme.

Amino Acid Sequence↗
PubMed1996

Molecular cloning and kinetic characterization of a flavin-containing monooxygenase from Saccharomyces cerevisiae.

An open reading frame from yeast coding for a homologue of flavin containing monooxygenase (FMO) has been cloned into several Escherichia coli expression vectors. A His10 peptide attached to the amino terminus produced a high yield of soluble protein when coexpressed with GroEL and GroES. The protein was purified on an affinity column and characterized. The protein binds one mole per mole of flavin but the binding is relatively weak and 50 microM exogenous FAD is used to maintain full occupancy. The yeast enzyme, like mammalian enzymes, exhibits NADPH oxidase activity. The enzyme does not catalyze the oxidation of amines, but thiols, including glutathione, cysteine, and cysteamine, show substrate activity. The Km values for these are 7.0, 9.9, and 1.3 mM, respectively; kcat values are 94, 246, and 94 per min, respectively. The enzyme apparently does not accept xenobiotic compounds but may be involved in maintaining cellular reducing potential, probably through its action on cysteamine. This activity may represent the initial role of the FMO family of enzymes, giving rise to the multigene family of drug metabolizing enzymes seen in modern mammals.

Cloning, Molecular↗
PubMed1997

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.

Amino Acid Sequence↗
PubMed1994

Cloning and nucleotide sequence of a cDNA of the human erythrocyte NADPH-flavin reductase.

A cDNA of human erythrocyte NADPH-flavin reductase was cloned from a lambda gt 11 human reticulocyte cDNA library by polymerase chain reaction using degenerate primers and followed by a plaque hybridization. The nucleotide sequence of the cDNA contains an open reading frame of 621 base pairs which encodes 206 amino acid residues including an initial methionine. The amino acid sequence deduced from the base sequence coincided well with peptide sequence determined for the purified human erythrocyte NADPH-flavin reductase. A homologous sequence to the FMN-binding site of flavodoxins was found at the amino-terminal region.

Amino Acid Sequence↗