Biogenesis of the covalently flavinylated mitochondrial enzyme dimethylglycine dehydrogenase

Remaining challenges in cellular flavin cofactor homeostasis and flavoprotein biogenesis

The primary role of the water-soluble vitamin B₂ (riboflavin) in cell biology is connected with its conversion into FMN and FAD, the cofactors of a large number of dehydrogenases, oxidases and reductases involved in a broad spectrum of biological activities, among which energetic metabolism and chromatin remodeling. Subcellular localisation of FAD synthase (EC 2.7.7.2, FADS), the second enzyme in the FAD forming pathway, is addressed here in HepG2 cells by confocal microscopy, in the frame of its relationships with kinetics of FAD synthesis and delivery to client apo-flavoproteins. FAD synthesis catalyzed by recombinant isoform 2 of FADS occurs via an ordered bi-bi mechanism in which ATP binds prior to FMN, and pyrophosphate is released before FAD. Spectrophotometric continuous assays of the reconstitution rate of apo-D-aminoacid oxidase with its cofactor, allowed us to propose that besides its FAD synthesizing activity, hFADS is able to operate as a FAD “chaperone.” The physical interaction between FAD forming enzyme and its clients was further confirmed by dot blot and immunoprecipitation experiments carried out testing as a client either a nuclear lysine-specific demethylase 1 (LSD1) or a mitochondrial dimethylglycine dehydrogenase (Me₂GlyDH, EC 1.5.8.4). Both enzymes carry out similar reactions of oxidative demethylation, in which tetrahydrofolate is converted into 5,10-methylene-tetrahydrofolate. A direct transfer of the cofactor from hFADS2 to apo-dimethyl glycine dehydrogenase was also demonstrated. Thus, FAD synthesis and delivery to these enzymes are crucial processes for bioenergetics and nutri-epigenetics of liver cells.

Molecular mechanisms of riboflavin responsiveness in patients with ETF-QO variations and multiple acyl-CoA dehydrogenation deficiency

Riboflavin-responsive forms of multiple acyl-CoA dehydrogenation deficiency (RR-MADD) have been known for years, but with presumed defects in the formation of the flavin adenine dinucleotide (FAD) co-factor rather than genetic defects of electron transfer flavoprotein (ETF) or electron transfer flavoprotein-ubiquinone oxidoreductase (ETF-QO). It was only recently established that a number of RR-MADD patients carry genetic defects in ETF-QO and that the well-documented clinical efficacy of riboflavin treatment may be based on a chaperone effect that can compensate for inherited folding defects of ETF-QO. In the present study, we investigate the molecular mechanisms and the genotype-phenotype relationships for the riboflavin responsiveness in MADD, using a human HEK-293 cell expression system. We studied the influence of riboflavin and temperature on the steady-state level and the activity of variant ETF-QO proteins identified in patients with RR-MADD, or non- and partially responsive MADD. Our results showed that variant ETF-QO proteins associated with non- and partially responsive MADD caused severe misfolding of ETF-QO variant proteins when cultured in media with supplemented concentrations of riboflavin. In contrast, variant ETF-QO proteins associated with RR-MADD caused milder folding defects when cultured at the same conditions. Decreased thermal stability of the variants showed that FAD does not completely correct the structural defects induced by the variation. This may cause leakage of electrons and increased reactive oxygen species, as reflected by increased amounts of cellular peroxide production in HEK-293 cells expressing the variant ETF-QO proteins. Finally, we found indications of prolonged association of variant ETF-QO protein with the Hsp60 chaperonin in the mitochondrial matrix, supporting indications of folding defects in the variant ETF-QO proteins.

Mitochondrial localization of human FAD synthetase isoform 1

FAD synthetase or ATP:FMN adenylyl transferase (FADS or FMNAT, EC 2.7.7.2) is a key enzyme in the metabolic pathway that converts riboflavin into the redox cofactor FAD. We face here the still controversial sub-cellular localization of FADS in eukaryotes. First, by western blotting experiments, we confirm the existence in rat liver of different FADS isoforms which are distinct for molecular mass and sub-cellular localization. A cross-reactive band with an apparent molecular mass of 60 kDa on SDS-PAGE is localized in the internal compartments of freshly isolated purified rat liver mitochondria. Recently we have identified two isoforms of FADS in humans, that differ for an extra-sequence of 97 amino acids at the N-terminus, present only in isoform 1 (hFADS1). The first 17 residues of hFADS1 represent a cleavable mitochondrial targeting sequence (by Target-P prediction). The recombinant hFADS1 produced in Escherichia coli showed apparent K(m) and V(max) values for FMN equal to 1.3+/-0.7 microM and 4.4+/-1.3 nmol x min(-1) x mg protein(-1), respectively, and was inhibited by FMN at concentration higher than 1.5 microM. The in vitro synthesized hFADS1, but not hFADS2, is imported into rat liver mitochondria and processed into a lower molecular mass protein product. Immunofluorescence confocal microscopy performed on BHK-21 and Caco-2 cell lines transiently expressing the two human isoforms, definitively confirmed that hFADS1, but not hFADS2, localizes in mitochondria.

Apoenzyme reconstitution as a chemical tool for structural enzymology and biotechnology

Many enzymes contain a nondiffusible organic cofactor, often termed a prosthetic group, which is located in the active site and essential for the catalytic activity of the enzyme. These cofactors can often be extracted from the protein to yield the respective apoenzyme, which can subsequently be reconstituted with an artificial analogue of the native cofactor. Nowadays a large variety of synthetic cofactors can be used for the reconstitution of apoenzymes and, thus, generate novel semisynthetic enzymes. This approach has been refined over the past decades to become a versatile tool of structural enzymology to elucidate structure-function relationships of enzymes. Moreover, the reconstitution of apoenzymes can also be used to generate enzymes possessing enhanced or even entirely new functionality. This Review gives an overview on historical developments and the current state-of-the-art on apoenzyme reconstitution.

Molecular basis of dimethylglycine dehydrogenase deficiency associated with pathogenic variant H109R

Dimethylglycine dehydrogenase (DMGDH) is a mitochondrial matrix flavoprotein that catalyses the demethylation of dimethylglycine to form sarcosine, accompanied by the reduction of the covalently bound FAD cofactor. Electron-transfer flavoprotein reoxidizes the reduced flavin and transfers reducing equivalents to the main mitochondrial respiratory chain through the enzyme ETF-ubiquinone oxidoreductase. DMGDH plays a prominent role in choline and 1-carbon metabolism. We have expressed the mature form of human DMGDH and the H109R variant identified in a DMGDH-deficient patient as N-terminally His(6)-tagged proteins in E. coli. The enzymes were purified to homogeneity by nickel affinity and anion exchange chromatography. The presence of FAD in the wild-type enzyme was confirmed by spectrophotometric analysis. The H109R variant, however, had only 47% of the wild-type level of bound flavin as expressed in E. coli, indicating its reduced affinity for FAD As previously described for rat enzyme studies, the wild-type human enzyme exhibited two K (m) values for N,N-dimethylglycine (K (m1) = 0.039 +/- 0.010 mmol/L and K(m2) = 15.4 +/- 1.2 mmol/L). The addition of 4 micromol/L tetrahydrofolate resulted in a slight decrease in specific activity and a substantial decrease in K (m2) (1.10 +/- 0.55 mmol/L). The flavinated H109R variant protein exhibited a 27-fold decrease in specific activity and a 65-fold increase in K (m), explaining its pathogenicity. Additionally, the current expression system represents a significant improvement over a previously described rat DMGDH expression system and will enhance our ability to further study this important metabolic enzyme.

The purified recombinant precursor of rat mitochondrial dimethylglycine dehydrogenase binds FAD via an autocatalytic reaction

The precursor of the rat mitochondrial flavoenzyme dimethylglycine dehydrogenase (Me(2)GlyDH) has been produced in Escherichia coli as a C-terminally 6-His-tagged fusion protein, purified by one-step affinity chromatography and identified by ESI-MS/MS. It was correctly processed into its mature form upon incubation with solubilized rat liver mitoplasts. The purified precursor was mainly in its apo-form as demonstrated by immunological and fluorimetric detection of covalently bound flavin adenine dinucleotide (FAD). Results described here definitively demonstrate that: (i) covalent attachment of FAD to Me(2)GlyDH apoenzyme can proceed in vitro autocatalytically, without third reactants; (ii) the removal of mitochondrial presequence by mitochondrial processing peptidase is not required for covalent autoflavinylation.

Over-expression in Escherichia coli, functional characterization and refolding of rat dimethylglycine dehydrogenase

Dimethylglycine dehydrogenase (Me(2)GlyDH) is a mitochondrial enzyme that catalyzes the oxidative demethylation of dimethylglycine to sarcosine. The enzyme requires flavin adenine dinucleotide (FAD), which is covalently bound to the apoprotein via a histidyl(N3)-(8alpha)FAD linkage. In the present study, the mature form of rat Me(2)GlyDH has been over-expressed in Escherichia coli as an N-terminally 6-His-tagged fusion protein. The over-expressed protein distributed almost equally between the soluble and insoluble (inclusion bodies) cell fraction. By applying the soluble cell lysate to a nickel-chelating column, two fractions were eluted, both containing a nearly homogeneous protein with a molecular mass of 93 kDa, on SDS-PAGE. The first protein fraction was identified by Western blotting analysis as the covalently flavinylated Me(2)GlyDH. It showed optical properties and specific activity (240 nmol/min/mg protein) similar to those of the native holoenzyme. The second fraction was identified as an underflavinylated (apo-) form of Me(2)GlyDH, with a 70% lower specific activity. The recombinant holoenzyme exhibited optimal activity at pH 8.5, an activation energy of about 80 kJ/mol, and two KM values for N,N-dimethylglycine (KM1 = 0.05 mM and KM2 = 9.4 mM), as described for the native holoenzyme. Starting from the inclusion bodies, the unfolded flavinylated enzyme was solubilized by SDS treatment and refolded by an 80-fold dilution step, with a reactivation yield of 50-60%.

Pyruvate:NADP+ oxidoreductase is stabilized by its cofactor, thiamin pyrophosphate, in mitochondria of Euglena gracilis

Pyruvate:NADP(+) oxidoreductase (PNO) is a thiamin pyrophosphate (TPP)-dependent enzyme that plays a central role in the respiratory metabolism of Euglena gracilis, which requires thiamin for growth. When thiamin was depleted in Euglena cells, PNO protein level was greatly reduced, but its mRNA level was barely changed. In addition, a large part of PNO occurred as an apoenzyme lacking TPP in the deficient cells. The PNO protein level increased rapidly, without changes in the mRNA level, after supplementation of thiamin into its deficient cells. In the deficient cells, in contrast to the sufficient ones, a steep decrease in the PNO protein level was induced when the cells were incubated with cycloheximide. Immunofluorescence microscopy indicated that most of the PNO localized in the mitochondria in either the sufficient or the deficient cells. These findings suggest that PNO is readily degraded when TPP is not provided in mitochondria, and consequently the PNO protein level is greatly reduced by thiamin deficiency in E. gracilis.

Converting bacteria to organelles: evolution of mitochondrial protein sorting

During the evolution of mitochondria from free-living alpha-proteobacteria, many bacterial genes were transferred into the nuclear genome of eukaryotic cells. This required the development of both targeting signals on the respective polypeptides and protein translocation machineries (translocases) in the mitochondrial membranes. Most components of these translocases have no obvious homologies to bacterial proteins or proteins found in other organelles. Membrane integration of many inner membrane proteins, however, apparently occurs via a conserved sorting pathway whose components and characteristics resemble protein translocation in bacteria. Consistent with this, the topogenic signals of these mitochondrial inner membrane proteins mimic those of bacterial proteins. The requirement for post-translational transport to their final destination has placed considerable constraints on the evolution of mitochondrial protein sequences.

Flavinylation of the precursor of mitochondrial dimethylglycine dehydrogenase by intact and solubilised mitochondria

The flavinylation and the presequence processing of the mitochondrial matrix enzyme dimethylglycine dehydrogenase (Me(2)GlyDH) were investigated with the reticulocyte lysate translated precursor (pMe(2)GlyDH) added to solubilised mitoplasts of rat liver mitochondria. The flavinylation of pMe(2)GlyDH was strictly dependent on the addition of mitochondrial protein(s), among which the mitochondrial flavinylation stimulating factor [Brizio C., et al. (2000) Eur. J. Biochem 267, 4346-4354], that actively promotes holo-Me(2)GlyDH formation. The precursor processing, that accompanies the biogenesis of the enzyme, was not required to allow the flavinylation to proceed. The comparison of the time course of the flavinylation and the processing of pMe(2)GlyDH demonstrated that the covalent attachment of the flavin moiety preceded the presequence processing by mitochondrial processing peptidase.

Organization of the genes involved in dimethylglycine and sarcosine degradation in Arthrobacter spp.: implications for glycine betaine catabolism

The nucleotide sequences of two cloned DNA fragments containing the structural genes of heterotetrameric sarcosine oxidase (soxBDAG) and dimethylglycine dehydrogenase (dmg) from Arthrobater spp. 1-IN and Arthrobacter globiformis, respectively, have been determined. Open reading frames were identified in the soxBDAG operon corresponding to the four subunits of heterotetrameric sarcosine oxidase by comparison with the N-terminal amino-acid sequences and the subunit relative molecular masses of the purified enzyme. Alignment of the deduced sarcosine oxidase amino-acid sequence with amino-acid sequences of functionally related proteins indicated that the arthrobacterial enzyme is highly homologous to sarcosine oxidase from Corynebacterium P-1. Deletion and expression analysis, and alignment of the deduced amino-acid sequence of the dmg gene, showed that dmg encodes a novel dimethylglycine oxidase, which is related to eukaryotic dimethylglycine dehydrogenase, and contains nucleotide-binding, flavinylation and folate-binding motifs. The recombinant dimethylglycine oxidase was purified to homogeneity and characterized. The DNA located upstream and downstream of both the soxBDAG and dmg genes is predicted to encode enzymes involved in the tetrahydrofolate-dependent assimilation of methyl groups. Based on the sequence analysis reported herein, pathways are proposed for glycine betaine catabolism in Arthrobacter species, which involve the identified folate-dependent enzymes.

Cloning of dimethylglycine dehydrogenase and a new human inborn error of metabolism, dimethylglycine dehydrogenase deficiency

Dimethylglycine dehydrogenase (DMGDH) (E.C. number 1.5.99.2) is a mitochondrial matrix enzyme involved in the metabolism of choline, converting dimethylglycine to sarcosine. Sarcosine is then transformed to glycine by sarcosine dehydrogenase (E.C. number 1.5.99.1). Both enzymes use flavin adenine dinucleotide and folate in their reaction mechanisms. We have identified a 38-year-old man who has a lifelong condition of fishlike body odor and chronic muscle fatigue, accompanied by elevated levels of the muscle form of creatine kinase in serum. Biochemical analysis of the patient's serum and urine, using (1)H-nuclear magnetic resonance NMR spectroscopy, revealed that his levels of dimethylglycine were much higher than control values. The cDNA and the genomic DNA for human DMGDH (hDMGDH) were then cloned, and a homozygous A-->G substitution (326 A-->G) was identified in both the cDNA and genomic DNA of the patient. This mutation changes a His to an Arg (H109R). Expression analysis of the mutant cDNA indicates that this mutation inactivates the enzyme. We therefore confirm that the patient described here represents the first reported case of a new inborn error of metabolism, DMGDH deficiency.

Effectiveness of N,N-dimethylglycine in autism and pervasive developmental disorder

N,N-dimethylglycine, a dietary supplement, has been reported to be beneficial in children with autism and pervasive developmental disorder. We examined the effectiveness of dimethylglycine in children with autism and pervasive developmental disorder in a double-blind, placebo-controlled study. Thirty-seven children between 3 and 11 years of age with a diagnosis of autism and/or pervasive developmental disorder were gender and age matched and randomly assigned to receive either placebo or dimethylglycine for 4 weeks. All children were assessed before and after treatment on two behavioral measures, the Vineland Maladaptive Behavior Domain and the Aberrant Behavior Checklist. Standardized neurologic examinations before and after treatment on 33 children showed no change. An overall improvement on all behavioral measures was observed for both the placebo and the dimethylglycine groups. However, the improvement among the children who received dimethylglycine was not statistically different from the improvement observed among the children who received the placebo. The children who participated in this study were a heterogeneous group, and their apparent responses to the dimethylglycine varied. Some children appeared to respond positively to the dimethylglycine, and there was a smaller proportion of negative changes in the dimethylglycine group, but the quantitative changes in the dimethylglycine behavioral assessments were not significantly different from what was observed among children who received placebo.

Coxsackievirus B3 infection induces anti-flavoprotein antibodies in mice

Enteroviruses, the most common cause of acute myocarditis, are also supposed aetiological agents of dilated cardiomyopathy. Autoantibodies (anti-M7; Klein & Berg, Clin Exp Immunol 1990; 58:283-92) directed against flavoproteins with covalently bound flavin (alphaFp-Ab; Otto et al., Clin Exp Immunol 1998; 111:541-2) are detected in up to 30% of sera of patients with myocarditis and idiopathic dilated cardiomyopathy (IDCM). Mice inoculated with a myocarditic variant of coxsackievirus B3 (CVB3) were employed to study the occurrence of serum alphaFp-Ab following viral infection. The presence of alphaFp-Ab was analysed by Western blotting with the flavoprotein antigens 6-hydroxy-D-nicotine oxidase (6HDNO) and sarcosine oxidase (SaO). Of 10 sera from CVB3-infected mice, five showed a strong reaction with both antigens. The sera were reactive also to the mitochondrial covalently flavinylated proteins dimethylglycine dehydrogenase and sarcosine dehydrogenase. Sera of non-infected mice did not react with these antigens. A 6HDNO mutant protein with non-covalently bound FAD no longer reacted on Western blots with sera of CVB3-infected mice. Preincubation with FAD abolished or reduced the reaction of the sera with the 6HDNO antigen. At 2 weeks p.i. the alphaFp-Ab were of the IgM and IgG isotypes, at 7 and 9 weeks p.i. of the IgG isotype. The sera of CVB3-infected mice reproduced closely the antigenic specificity of the anti-M7 sera of patients, lending further support to the role of coxsackieviruses in the pathogenesis of IDCM.

Molecular characterization of monoamine oxidases A and B

Monoamine oxidase A and B (MAO A and B) are the major neurotransmitter-degrading enzymes in the central nervous system and in peripheral tissues. MAO A and B cDNAs from human, rat, and bovine species have been cloned and their deduced amino acid sequences compared. Comparison of A and B forms of the enzyme shows approximately 70% sequence identity, whereas comparison of the A or B forms across species reveals a higher sequence identity of 87%. Within these sequences, several functional regions have been identified that contain crucial amino acid residues participating in flavin adenine dinucleotide (FAD) or substrate binding. These include a dinucleotide-binding site, a second FAD-binding site, a fingerprint site, the FAD covalent-binding site, an active site, and the membrane-anchoring site. The specific residues that play a role in FAD or substrate binding were identified by comparing sequences in wild-type and variants of MAO with those in soluble flavoproteins of known structures. The genes that encode MAO A and B are closely aligned on the X chromosome (Xp11.23), and have identical exon-intron organization. Immunocytochemical localization studies of MAO A and B in primate brain showed distribution in distinct neurons with diverse physiological functions. A defective MAO A gene has been reported to associate with abnormal aggressive behavior. A deleterious role played by MAO B is the activation of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), a proneurotoxin that can cause a parkinsonian syndrome in mammals. Deprenyl, an inhibitor of MAO B, has been used for the treatment of early-stage Parkinson's disease and provides protection of neurons from age-related decay.

What fuels polypeptide translocation? An energetical view on mitochondrial protein sorting

Protein sorting into mitochondria is achieved by the concerted action of at least four translocation complexes. Vectorial transport of polypeptide chains by these complexes requires different driving forces. In particular, Deltapsi, matrix adenosine triphosphate and the free energy of the binding to other protein components are used in series to achieve sorting of proteins to the various mitochondrial subcompartments. The processes providing the translocation energy are presented in this review and their impact for protein sorting into and within mitochondria is discussed.

The riboflavin/FAD cycle in rat liver mitochondria

Here we provide evidence that mitochondria isolated from rat liver can synthesize FAD from riboflavin that has been taken up and from endogenous ATP. Riboflavin uptake takes place via a carrier-mediated process, as shown by the inverse relationship between fold accumulation and riboflavin concentration, the saturation kinetics [riboflavin Km and Vmax values were 4.4+/-1.3 microM and 35+/-5 pmol x min(-1) (mg protein)(-1), respectively] and the inhibition shown by the thiol reagent mersalyl, which cannot enter the mitochondria. FAD synthesis is due to the existence of FAD synthetase (EC 2.7.7.2), localized in the matrix, which has as a substrate pair mitochondrial ATP and FMN synthesized from taken up riboflavin via the putative mitochondrial riboflavin kinase. In the light of certain features, including the protein thermal stability and molecular mass, mitochondrial FAD synthetase differs from the cytosolic isoenzyme. Apparent Km and apparent Vmax values for FMN were 5.4+/-0.9 microM and 22.9+/-1.4 pmol x min(-1) x (mg matrix protein)(-1), respectively. Newly synthesized FAD inside the mitochondria can be exported from the mitochondria in a manner sensitive to atractyloside but insensitive to mersalyl. The occurrence of the riboflavin/FAD cycle is proposed to account for riboflavin uptake in mitochondria biogenesis and riboflavin recovery in mitochondrial flavoprotein degradation; both are prerequisites for the synthesis of mitochondrial flavin cofactors.

A protein factor of rat liver mitochondrial matrix involved in flavinylation of dimethylglycine dehydrogenase

The involvement of rat liver mitochondria in the flavinylation of the mitochondrial matrix flavoenzyme dimethylglycine dehydrogenase (Me2GlyDH) has been investigated. Me2GlyDH was synthesized as an apoenzyme in the rabbit reticulocyte lysate (RL) transcription/translation system and its flavinylation was monitored by virtue of the trypsin resistance of the holoenzyme. The rate of holoenzyme formation in the presence of FAD was stimulated with increasing efficiency by the addition of solubilized mitoplasts, mitochondrial matrix and DEAE-purified matrix fraction. Apo-Me2GlyDH was also converted into holoenzyme when the solubilized mitoplasts were supplemented with FMN and ATP. This observation is consistent with the existence of a mitochondrial FAD synthetase generating the FAD needed for holoenzyme formation from its precursors. Holoenzyme formation in the presence of FAD increased linearly with the concentration of matrix protein in the assay, and depended on the amount of externally added Me2GlyDH with saturation characteristics. These findings suggest the presence of a protein factor in the mitochondrial matrix which stimulates Me2GlyDH flavinylation. This factor was different from both mitochondrial heat shock protein (Hsp)70, as shown by immunodepletion experiments, and mitochondrial Hsp60, as demonstrated by the capability of a DEAE-purified matrix fraction devoid of Hsp60 to accelerate flavinylation of both RL translated and purified Me2GlyDH.

Characterization of a highly conserved FAD-binding site in human monoamine oxidase B

Monoamine oxidase B (MAO B) catalyzes the oxidative deamination of biogenic and xenobiotic amines. The oxidative step is coupled to the reduction of an obligatory cofactor, FAD, which is covalently linked to the apoenzyme at Cys397. Our previous studies identified two noncovalent flavin-binding regions in MAO B (residues 6-34 and 39-46) (Kwan, S.-W., Lewis, D. A., Zhou, B. P., and Abell, C. W. (1995) Arch. Biochem. Biophys. 316, 385-391; Zhou, B. P., Lewis, D. A., Kwan, S.-W., Kirksey, T. J., and Abell, C. W. (1995) Biochemistry 34, 9526-9531). In these regions, Glu34 and Tyr44 were found to be required for the initial binding of FAD. By comparing sequences with enzymes in the oxidoreductase family, we now have found an additional FAD-binding site in MAO B (residues 222-227), which is highly conserved across species (human, bovine, and rat). This conserved sequence contains adjacent glycine and aspartate residues (Gly226 and Asp227). Based on the x-ray crystal structures of several oxidoreductases (Eggink, G., Engel, H., Vriend, G., Terpstra, P., and Witholt, B. (1990) J. Mol. Biol. 212, 135-142; Van Driessche, G., Kol, M., Chen, Z.-W., Mathews, F. S., Meyer, T. E., Bartsch, R. G., Cusanovich, M. A., and Van Beeumen, J. J. (1996) Protein Sci. 5, 1753-1764), the Gly residue at the end of a beta-strand facilitates a sharp turn and extends the beta-carbonyl group of Asp to interact with the 3'-hydroxyl group of the ribityl chain of FAD. To assess the hypothesis that Gly226 and Asp227 are involved in FAD binding in MAO B, site-specific mutants that encode substitutions at these positions were prepared and expressed in mammalian COS-7 cells. Our results indicate that Gly226 and the beta-carbonyl group of Asp227 are required for covalent flavinylation and catalytic activity of MAO B, but not for noncovalent binding of FAD. Our studies also reveal that mutagenesis at Glu34 and Tyr44 not only interferes with covalent flavinylation and catalytic activity of MAO B, but also with noncovalent binding of FAD. Based on these collective results, we propose that the coupling of FAD to the MAO B apoenzyme is a multistep process.

Enzymic formation of riboflavin 4',5'-cyclic phosphate from FAD: evidence for a specific low-Km FMN cyclase in rat liver1

An enzyme activity splitting FAD to AMP and riboflavin 4',5'-cyclic phosphate (4',5'-cFMN), with a Km of 6-8 microM, was partially purified from the cytosolic fraction of rat liver homogenates. 4', 5'-cFMN was characterized by enzyme, HPLC, UV-visible and NMR spectroscopic analyses. The data suggest that a novel enzyme, tentatively named FAD-AMP lyase (cyclizing) or FMN cyclase, is involved. Also, 4',5'-cFMN was hydrolysed to 5'-FMN by a rat liver cyclic phosphodiesterase. The results indicate a novel enzymic pathway for flavins in mammals, and support the biological relevance of 4',5'-cFMN, perhaps as a flavocoenzyme or a regulatory signal.

Anti-mitochondrial antibodies in patients with dilated cardiomyopathy (anti-M7) are directed against flavoenzymes with covalently bound FAD

Anti-mitochondrial antibodies (anti-M7) in sera from patients with dilated cardiomyopathy and myocarditis recognize, besides mitochondrial antigens, bacterial sarcosine dehydrogenase. The common target antigen was identified as the covalently bound FAD of mitochondrial and bacterial flavoenzymes. Thus, anti-M7-positive serum reacted on Western blots exclusively with covalently flavinylated enzymes. The antigenic specificity of anti-M7 sera was reproduced by an antiserum raised in rabbits with 6-hydroxy-D-nicotine oxidase. The heart mitochondrial membrane antigen recognized by anti-M7 serum was identified as the flavoprotein subunit of succinate dehydrogenase, the antigens in rat liver mitochondrial matrix as the flavoenzymes dimethylglycine dehydrogenase and sarcosine dehydrogenase. Anti-M7 serum contained a specific anti-flavoenzyme antibody fraction. Nanomolar concentrations of FAD and riboflavin inhibited the immune reaction on Western blots and in ELISA, and incubation with FAD-agarose depleted the anti-M7 activity of the serum. N-terminally deleted dimethylglycine dehydrogenase proteins were only immunoprecipitated by anti-M7 sera when the FAD was covalently incorporated. An affinity constant (KD) of 10(-8) M was established for the anti-flavoenzyme antibodies by competitive ELISA. Of patients with cardiomyopathy and myocarditis, 36% and 25%, respectively, were anti-flavoenzyme-positive by Western blot and ELISA, but only two of 15 patients with other heart diseases and none of 50 healthy controls.

Subcellular localization of vanillyl-alcohol oxidase in Penicillium simplicissimum

Growth of Penicillium simplicissimum on anisyl alcohol, veratryl alcohol or 4-(methoxymethyl)phenol, is associated with the synthesis of relatively large amounts of the hydrogen peroxide producing flavoprotein vanillyl-alcohol oxidase (VAO). Immunocytochemistry revealed that the enzyme has a dual location namely in peroxisomes and in the cytosol. The C-terminus of the primary structure of VAO displays a WKL-COOH sequence which might function as a peroxisomal targeting signal type 1 (PTS1). As VAO activity results in production of hydrogen peroxide also the subcellular location of a recently characterized co-inducible catalase-peroxidase was studied. As VAO, this hydroperoxidase is also distributed throughout the cytosol and peroxisomes.

Covalent attachment of flavin adenine dinucleotide (FAD) and flavin mononucleotide (FMN) to enzymes: the current state of affairs

The first identified covalent flavoprotein, a component of mammalian succinate dehydrogenase, was reported 42 years ago. Since that time, more than 20 covalent flavoenzymes have been described, each possessing one of five modes of FAD or FMN linkage to protein. Despite the early identification of covalent flavoproteins, the mechanisms of covalent bond formation and the roles of the covalent links are only recently being appreciated. The main focus of this review is, therefore, one of mechanism and function, in addition to surveying the types of linkage observed and the methods employed for their identification. Case studies are presented for a variety of covalent flavoenzymes, from which general findings are beginning to emerge.