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

The FAD synthetase from the human pathogen Streptococcus pneumoniae: a bifunctional enzyme exhibiting activity-dependent redox requirements

Prokaryotic bifunctional FAD synthetases (FADSs) catalyze the biosynthesis of FMN and FAD, whereas in eukaryotes two enzymes are required for the same purpose. FMN and FAD are key cofactors to maintain the flavoproteome homeostasis in all type of organisms. Here we shed light to the properties of the hitherto unstudied bacterial FADS from the human pathogen Streptococcus pneumoniae (SpnFADS). As other members of the family, SpnFADS catalyzes the three typical activities of prokaryotic FADSs: riboflavin kinase (RFK), ATP:FMN:adenylyltransferase (FMNAT), and FAD pyrophosphorylase (FADpp). However, several SpnFADS biophysical properties differ from those of other family members. In particular; i) the RFK activity is not inhibited by the riboflavin (RF) substrate, ii) the FMNAT and FADSpp activities require flavin substrates in the reduced state, iii) binding of adenine nucleotide ligands is required for the binding of flavinic substrates/products and iv) the monomer is the preferred state. Collectively, our results add interesting mechanistic differences among the few prokaryotic bifunctional FADSs already characterized, which might reflect the adaptation of the enzyme to relatively different environments. In a health point of view, differences among FADS family members provide us with a framework to design selective compounds targeting these enzymes for the treatment of diverse infectious diseases.

Riboflavin transport and metabolism in humans

Recent studies elucidated how riboflavin transporters and FAD forming enzymes work in humans and create a coordinated flavin network ensuring the maintenance of cellular flavoproteome. Alteration of this network may be causative of severe metabolic disorders such as multiple acyl-CoA dehydrogenase deficiency (MADD) or Brown-Vialetto-van Laere syndrome. A crucial step in the maintenance of FAD homeostasis is riboflavin uptake by plasma and mitochondrial membranes. Therefore, studies on recently identified human plasma membrane riboflavin transporters are presented, together with those in which still unidentified mitochondrial riboflavin transporter(s) have been described. A main goal of future research is to fill the gaps still existing as for some transcriptional, functional and structural details of human FAD synthases (FADS) encoded by FLAD1 gene, a novel "redox sensing" enzyme. In the frame of the hypothesis that FADS, acting as a "FAD chaperone", could play a crucial role in the biogenesis of mitochondrial flavo-proteome, several basic functional aspects of flavin cofactor delivery to cognate apo-flavoenzyme are also briefly dealt with. The establishment of model organisms performing altered FAD homeostasis will improve the molecular description of human pathologies. The molecular and functional studies of transporters and enzymes herereported, provide guidelines for improving therapies which may have beneficial effects on the altered metabolism.

LAT1 is the transport competent unit of the LAT1/CD98 heterodimeric amino acid transporter

LAT1 (SLC7A5) and CD98 (SLC3A2) constitute a heterodimeric transmembrane protein complex that catalyzes amino acid transport. Whether one or both subunits are competent for transport is still unclear. The present work aims to solve this question using different experimental strategies. Firstly, LAT1 and CD98 were immuno-detected in protein extracts from SiHa cells. Under oxidizing conditions, i.e., without addition of SH (thiol) reducing agent DTE, both proteins were revealed as a 120kDa major band. Upon DTE treatment separated bands, corresponding to LAT1(35kDa) or CD98(80kDa), were detected. LAT1 function was evaluated in intact cells as BCH sensitive [(3)H]His transport inhibited by hydrophobic amino acids. Antiport of [(3)H]His was measured in proteoliposomes reconstituted with SiHa cell extract in presence of internal His. Transport was increased by DTE. Hydrophobic amino acids were best inhibitors in addition to hydrophilic Tyr, Gln, Asn and Lys. Cys, Tyr and Gln, included in the intraliposomal space, were transported in antiport with external [(3)H]His. Similar experiments were performed in proteoliposomes reconstituted with the recombinant purified hLAT1. Results overlapping those obtained with native protein were achieved. Lower transport of [(3)H]Leu and [(3)H]Gln with respect to [(3)H]His was detected. Kinetic asymmetry was found with external Km for His lower than internal one. No transport was detected in proteoliposomes reconstituted with recombinant hCD98. The experimental data demonstrate that LAT1 is the sole transport competent subunit of the heterodimer. This conclusion has important outcome for following studies on functional characterization and identification of specific inhibitors with potential application in human therapy.

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.

FAD synthesis and degradation in the nucleus create a local flavin cofactor pool

FAD is a redox cofactor ensuring the activity of many flavoenzymes mainly located in mitochondria but also relevant for nuclear redox activities. The last enzyme in the metabolic pathway producing FAD is FAD synthase (EC 2.7.7.2), a protein known to be localized both in cytosol and in mitochondria. FAD degradation to riboflavin occurs via still poorly characterized enzymes, possibly belonging to the NUDIX hydrolase family. By confocal microscopy and immunoblotting experiments, we demonstrate here the existence of FAD synthase in the nucleus of different experimental rat models. HPLC experiments demonstrated that isolated rat liver nuclei contain ∼300 pmol of FAD·mg(-1) protein, which was mainly protein-bound FAD. A mean FAD synthesis rate of 18.1 pmol·min(-1)·mg(-1) protein was estimated by both HPLC and continuous coupled enzymatic spectrophotometric assays. Rat liver nuclei were also shown to be endowed with a FAD pyrophosphatase that hydrolyzes FAD with an optimum at alkaline pH and is significantly inhibited by adenylate-containing nucleotides. The coordinate activity of these FAD forming and degrading enzymes provides a potential mechanism by which a dynamic pool of flavin cofactor is created in the nucleus. These data, which significantly add to the biochemical comprehension of flavin metabolism and its subcellular compartmentation, may also provide the basis for a more detailed comprehension of the role of flavin homeostasis in biologically and clinically relevant epigenetic events.

Emerging concepts in the flavinylation of succinate dehydrogenase

The Succinate Dehydrogenase (SDH) heterotetrameric complex catalyzes the oxidation of succinate to fumarate in the tricarboxylic acid (TCA) cycle and in the aerobic respiratory chains of eukaryotes and bacteria. Essential in this catalysis is the covalently-linked cofactor flavin adenine dinucleotide (FAD) in subunit1 (Sdh1) of the SDH enzyme complex. The mechanism of FAD insertion and covalent attachment to Sdh1 is unknown. Our working concept of this flavinylation process has relied mostly on foundational works from the 1990s and by applying the principles learned from other enzymes containing a similarly linked FAD. The discovery of the flavinylation factor Sdh5, however, has provided new insight into the possible mechanism associated with Sdh1 flavinylation. This review focuses on encapsulating prior and recent advances towards understanding the mechanism associated with flavinylation of Sdh1 and how this flavinylation process affects the overall assembly of SDH. This article is part of a Special Issue entitled: Respiratory complex II: Role in cellular physiology and disease.

Human FAD synthase (isoform 2): a component of the machinery that delivers FAD to apo-flavoproteins

A soluble form of human FAD synthase (isoform 2; hFADS2) was produced and purified to homogeneity as a recombinant His-tagged protein. The enzyme binds 1 mole of the FAD product very tightly, although noncovalently. Complete release of FAD from the 'as isolated' protein requires extensive denaturation. A 75 : 25 mixture of apo/holoprotein could be prepared by treatment with mild chaotropes, allowing estimatation of the contribution made by bound FAD to the protein stability and evaluatation of whether structural rearrangements may be required for FAD release. Under turnover conditions, the enzyme catalyzes FAD assembly from ATP and FMN and, at a much lower rate, the pyrophosphorolytic hydrolysis of FAD. Several mechanistic features of both reactions were investigated in detail, along with their dependence on environmental conditions (pH, temperature, dependence on metals). Our data indicate that FAD release may represent the rate-limiting step of the whole catalytic cycle and that the process leading to FAD synthesis, and delivery to client apoproteins may be tightly controlled.

Evidence for the presence of a FAD pyrophosphatase and a FMN phosphohydrolase in yeast mitochondria: a possible role in flavin homeostasis

Despite the crucial roles of flavin cofactors in metabolism, we know little about the enzymes responsible for the turnover of flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD) and their subcellular localization. The mechanism by which mitochondria obtain their own flavin cofactors is an interesting point of investigation, because FMN and FAD are mainly located in mitochondria, where they act as redox cofactors of a number of dehydrogenases and oxidases that play a crucial function in both bioenergetics and cellular regulation. In this context, the capability of yeast mitochondria to metabolize externally added and endogenous FAD and FMN was investigated and use was made of purified and bioenergetically active mitochondria prepared starting from the Saccharomyces cerevisiae cell. To determine whether flavin metabolism can occur, the amounts of flavins in aliquots of neutralized perchloric extracts of both spheroplasts and mitochondria were measured by HPLC, and the competence of S. cerevisiae mitochondria to metabolize FAD and FMN was investigated both spectroscopically and via HPLC. FAD deadenylation and FMN dephosphorylation were studied with respect to dependence on substrate concentration, pH profile and inhibitor sensitivity. The existence of two novel mitochondrial FAD pyrophosphatase (diphosphatase) (EC 3.6.1.18) and FMN phosphohydrolase (EC 3.1.3.2) activities, which catalyse the reactions FAD + H₂O → FMN + AMP and FMN + H₂O → riboflavin + Pi respectively, is here shown by fractionation studies. Considering cytosolic riboflavin, FMN and FAD concentrations, as calculated by measuring both spheroplast and mitochondrial contents via HPLC, probably mitochondria play a major role in regulating the flavin pool in yeast and in relation to flavin homeostasis.

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.

The occurrence of riboflavin kinase and FAD synthetase ensures FAD synthesis in tobacco mitochondria and maintenance of cellular redox status

Intact mitochondria isolated from Nicotiana tabacum cv. Bright Yellow 2 (TBY-2) cells can take up riboflavin via carrier-mediated systems that operate at different concentration ranges and have different uptake efficiencies. Once inside mitochondria, riboflavin is converted into catalytically active cofactors, FMN and FAD, due to the existence of a mitochondrial riboflavin kinase (EC 2.7.1.26) and an FAD synthetase (EC 2.7.7.2). Newly synthesized FAD can be exported from intact mitochondria via a putative FAD exporter. The dependence of FMN synthesis rate on riboflavin concentration shows saturation kinetics with a sigmoidal shape (S(0.5), V(max) and Hill coefficient values 0.32+/-0.12 microm, 1.4 nmol x min(-1) x mg(-1) protein and 3.1, respectively). The FAD-forming enzymes are both activated by MgCl(2), and reside in two distinct monofunctional enzymes, which can be physically separated in mitochondrial soluble and membrane-enriched fractions, respectively.

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.

Coordinated and reversible reduction of enzymes involved in terminal oxidative metabolism in skeletal muscle mitochondria from a riboflavin-responsive, multiple acyl-CoA dehydrogenase deficiency patient

In this case report we studied alterations in mitochondrial proteins in a patient suffering from recurrent profound muscle weakness, associated with ethylmalonic-adipic aciduria, who had benefited from high dose of riboflavin treatment. Morphological and biochemical alterations included muscle lipid accumulation, low muscle carnitine content, reduction in fatty acid beta-oxidation and reduced activity of complexes I and II of the respiratory chain. Riboflavin therapy partially or totally reversed these symptoms and increased the level of muscle flavin adenine dinucleotide, suggesting that aberrant flavin cofactor metabolism accounted for the disease. Proteomic investigation of muscle mitochondria revealed decrease or absence of several flavoenzymes, enzymes related to flavin cofactor-dependent mitochondrial pathways and mitochondrial or mitochondria-associated calcium-binding proteins. All these deficiencies were completely rescued after riboflavin treatment. This study indicates for the first time a profound involvement of riboflavin/flavin cofactors in modulating the level of a number of functionally coordinated polypeptides involved in fatty acyl-CoA and amino acid metabolism, extending the number of enzymatic pathways altered in riboflavin-responsive multiple acyl-CoA dehydrogenase deficiency.

Over-expression in Escherichia coli and characterization of two recombinant isoforms of human FAD synthetase

FAD synthetase (FADS) (EC 2.7.7.2) is a key enzyme in the metabolic pathway that converts riboflavin into the redox cofactor FAD. Two hypothetical human FADSs, which are the products of FLAD1 gene, were over-expressed in Escherichia coli and identified by ESI-MS/MS. Isoform 1 was over-expressed as a T7-tagged protein which had a molecular mass of 63kDa on SDS-PAGE. Isoform 2 was over-expressed as a 6-His-tagged fusion protein, carrying an extra 84 amino acids at the N-terminal with an apparent molecular mass of 60kDa on SDS-PAGE. It was purified near to homogeneity from the soluble cell fraction by one-step affinity chromatography. Both isoforms possessed FADS activity and had a strict requirement for MgCl(2), as demonstrated using both spectrophotometric and chromatographic methods. The purified recombinant isoform 2 showed a specific activity of 6.8+/-1.3nmol of FAD synthesized/min/mg protein and exhibited a K(M) value for FMN of 1.5+/-0.3microM. This is the first report on characterization of human FADS, and the first cloning and over-expression of FADS from an organism higher than yeast.

Mitochondrial Abeta: a potential focal point for neuronal metabolic dysfunction in Alzheimer's disease

Although amyloid-beta peptide (Abeta) is the neurotoxic species implicated in the pathogenesis of Alzheimer's disease (AD), mechanisms through which intracellular Abeta impairs cellular properties, resulting in neuronal dysfunction, remain to be clarified. Here we demonstrate that intracellular Abeta is present in mitochondria from brains of transgenic mice with targeted neuronal overexpression of mutant human amyloid precursor protein and AD patients. Abeta progressively accumulates in mitochondria and is associated with diminished enzymatic activity of respiratory chain complexes (III and IV) and a reduction in the rate of oxygen consumption. Importantly, mitochondria-associated Abeta, principally Abeta42, was detected as early as 4 months, before extensive extracellular Abeta deposits. Our studies delineate a new means through which Abeta potentially impairs neuronal energetics, contributing to cellular dysfunction in AD.

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%.

Riboflavin uptake and FAD synthesis in Saccharomyces cerevisiae mitochondria: involvement of the Flx1p carrier in FAD export

We have studied the functional steps by which Saccharomyces cerevisiae mitochondria can synthesize FAD from cytosolic riboflavin (Rf). Riboflavin uptake into mitochondria took place via a mechanism that is consistent with the existence of (at least two) carrier systems. FAD was synthesized inside mitochondria by a mitochondrial FAD synthetase (EC 2.7.7.2), and it was exported into the cytosol via an export system that was inhibited by lumiflavin, and which was different from the riboflavin uptake system. To understand the role of the putative mitochondrial FAD carrier, Flx1p, in this pathway, an flx1Delta mutant strain was constructed. Coupled mitochondria isolated from flx1Delta mutant cells were compared with wild-type mitochondria with respect to the capability to take up Rf, to synthesize FAD from it, and to export FAD into the extramitochondrial phase. Mitochondria isolated from flx1Delta mutant cells specifically lost the ability to export FAD, but did not lose the ability to take up Rf, FAD, or FMN and to synthesize FAD from Rf. Hence, Flx1p is proposed to be the mitochondrial FAD export carrier. Moreover, deletion of the FLX1 gene resulted in a specific reduction of the activities of mitochondrial lipoamide dehydrogenase and succinate dehydrogenase, which are FAD-binding enzymes. For the flavoprotein subunit of succinate dehydrogenase we could demonstrate that this was not due to a changed level of mitochondrial FAD or to a change in the degree of flavinylation of the protein. Instead, the amount of the flavoprotein subunit of succinate dehydrogenase was strongly reduced, indicating an additional regulatory role for Flx1p in protein synthesis or degradation.

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.

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.