A rat liver lysosomal membrane flavin-adenine dinucleotide phosphohydrolase: purification and characterization

Continuous and Discontinuous Approaches to Study FAD Synthesis and Degradation Catalyzed by Purified Recombinant FAD Synthase or Cellular Fractions

Riboflavin, or vitamin B2, is the precursor of flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD), essential redox (and sometimes non-redox) cofactors of a large number of flavoenzymes involved in energetic metabolism, protein folding, apoptosis, chromatin remodeling, and a number of other cell regulatory processes.The cellular and subcellular steady-state concentrations of flavin cofactors, which are available for flavoprotein biogenesis and assembly, depend on carrier-mediated transport processes and on coordinated synthesizing/destroying enzymatic activities, catalyzed by enzymes whose catalytic and structural properties are still matter of investigation.Alteration of flavin homeostasis has been recently correlated to human pathological conditions, such as neuromuscular disorders and cancer, and therefore we propose here protocols useful to detect metabolic processes involved in FAD forming and destroying.Our protocols exploit the chemical-structural differences between riboflavin, FMN , and FAD , which are responsible for differences in the spectroscopic properties (mainly fluorescence) of the two cofactors (FMN and FAD); therefore, in our opinion, when applicable measurements of fluorescence changes in continuo represent the elective techniques to follow FAD synthesis and degradation. Thus, after procedures able to calibrate flavin concentrations (Subheading 3.1), we describe simple continuous and rapid procedures, based on the peculiar optical properties of free flavins, useful to determine the rate of cofactor metabolism catalyzed by either recombinant enzymes or natural enzymes present in cellular lysates/subfractions (Subheading 3.2).Fluorescence properties of free flavins can also be useful in analytical determinations of the three molecular flavin forms, based on HPLC separation, with a quite high sensitivity. Assaying at different incubation times the molecular composition of the reaction mixture is a discontinuous experimental approach to measure the rate of FAD synthesis/degradation catalyzed by cell lysates or recombinant FAD synthase (Subheading 3.3). Continuous and discontinuous approaches can, when necessary, be performed in parallel.

Characterization of a non-nudix pyrophosphatase points to interplay between flavin and NAD(H) homeostasis in Saccharomyces cerevisiae

The flavin cofactors FMN and FAD are required for a wide variety of biological processes, however, little is known about their metabolism. Here, we report the cloning and biochemical characterization of the Saccharomyces cerevisiae pyrophosphatase Fpy1p. Genetic and functional studies suggest that Fpy1p may play a key role in flavin metabolism and is the first-reported non-Nudix superfamily enzyme to display FAD pyrophosphatase activity. Characterization of mutant yeast strains found that deletion of fpy1 counteracts the adverse effects that are caused by deletion of flx1, a known mitochondrial FAD transporter. We show that Fpy1p is capable of hydrolyzing FAD, NAD(H), and ADP-ribose. The enzymatic activity of Fpy1p is dependent upon the presence of K+ and divalent metal cations, with similar kinetic parameters to those that have been reported for Nudix FAD pyrophosphatases. In addition, we report that the deletion of fpy1 intensifies the FMN-dependence of null mutants of the riboflavin kinase Fmn1p, demonstrate that fpy1 mutation abolishes the decreased fitness resulting from the deletion of the flx1 ORF, and offer a possible mechanism for the genetic interplay between fpy1, flx1 and fmn1.

Spectrum- and time-resolved endogenous multiphoton signals reveal quantitative differentiation of premalignant and malignant gastric mucosa

Early identification of premalignant and malignant gastric mucosa is crucial to decrease the incidence and mortality of stomach cancer. Spectrum- and time-resolved multiphoton microscopy are capable of providing not only structural but also biochemical information at the subcellular level. Based on this multidimensional imaging technique, we performed a systematic investigation on fresh human tissue specimens at the typical stages of gastric carcinogenesis, including normal, chronic gastritis with erosion, chronic gastritis with intestinal metaplasia, and intestinal-type adenocarcinoma. The results demonstrate that this technique is available to characterize the three-dimensional subcellular morphological and biochemical properties of gastric mucosa and further provide quantitative indicators of different gastric disorders, by using endogenous contrast. With advances in multiphoton endoscopy, it has the potential to allow noninvasive, label-free, real-time histological and functional diagnosis of premalignant and malignant lesions of stomach in the future.

A regulatory role of NAD redox status on flavin cofactor homeostasis in S. cerevisiae mitochondria

Flavin adenine dinucleotide (FAD) and nicotinamide adenine dinucleotide (NAD) are two redox cofactors of pivotal importance for mitochondrial functionality and cellular redox balance. Despite their relevance, the mechanism by which intramitochondrial NAD(H) and FAD levels are maintained remains quite unclear in Saccharomyces cerevisiae. We investigated here the ability of isolated mitochondria to degrade externally added FAD and NAD (in both its reduced and oxidized forms). A set of kinetic experiments demonstrated that mitochondrial FAD and NAD(H) destroying enzymes are different from each other and from the already characterized NUDIX hydrolases. We studied here, in some detail, FAD pyrophosphatase (EC 3.6.1.18), which is inhibited by NAD(+) and NADH according to a noncompetitive inhibition, with Ki values that differ from each other by an order of magnitude. These findings, together with the ability of mitochondrial FAD pyrophosphatase to metabolize endogenous FAD, presumably deriving from mitochondrial holoflavoproteins destined to degradation, allow for proposing a novel possible role of mitochondrial NAD redox status in regulating FAD homeostasis and/or flavoprotein degradation in S. cerevisiae.

An Arabidopsis FAD pyrophosphohydrolase, AtNUDX23, is involved in flavin homeostasis

Although flavins, riboflavin (RF), FMN and FAD, are essential for primary and secondary metabolism in plants, the metabolic regulation of flavins is still largely unknown. Recently, we found that an Arabidopsis Nudix hydrolase, AtNUDX23, has FAD pyrophosphohydrolase activity and is distributed in plastids. Levels of RF and FAD but not FMN in Arabidopsis leaves significantly increased under continuous light and decreased in the dark. The transcript levels of AtNUDX23 as well as genes involved in flavin metabolism (AtFADS, AtRibF1, AtRibF2, AtFMN/FHy, LS and AtRibA) significantly increased under continuous light. The pyrophosphohydrolase activity toward FAD was enhanced in AtNUDX23-overexpressing (OX-NUDX23) plants and reduced in AtNUDX23-suppressed (KD-nudx23) plants, compared with the control plants. Interestingly intracellular levels of RF, FMN and FAD significantly decreased in not only OX-NUDX23 but also KD-nudx23 plants. The transcript levels of the flavin metabolic genes also decreased in both plants. Similarly, the increase in intracellular levels on treatment with flavins caused a reduction in the transcript levels of genes involved in flavin metabolism. These results suggest that negative feedback regulation of the metabolism of flavins through the hydrolysis of FAD by AtNUDX23 in plastids is involved in flavin homeostasis in plant cells.

Complex morphology and functional dynamics of vital murine intestinal mucosa revealed by autofluorescence 2-photon microscopy

The mucosa of the gastrointestinal tract is a dynamic tissue composed of numerous cell types with complex cellular functions. Study of the vital intestinal mucosa has been hampered by lack of suitable model systems. We here present a novel animal model that enables highly resolved three-dimensional imaging of the vital murine intestine in anaesthetized mice. Using intravital autofluorescence 2-photon (A2P) microscopy we studied the choreographed interactions of enterocytes, goblet cells, enteroendocrine cells and brush cells with other cellular constituents of the small intestinal mucosa over several hours at a subcellular resolution and in three dimensions. Vigorously moving lymphoid cells and their interaction with constituent parts of the lamina propria were examined and quantitatively analyzed. Nuclear and lectin staining permitted simultaneous characterization of autofluorescence and admitted dyes and yielded additional spectral information that is crucial to the interpretation of the complex intestinal mucosa. This novel intravital approach provides detailed insights into the physiology of the small intestine and especially opens a new window for investigating cellular dynamics under nearly physiological conditions.

An FMN hydrolase of the haloacid dehalogenase superfamily is active in plant chloroplasts

FMN hydrolases catalyze dephosphorylation of FMN to riboflavin. Although these enzymes have been described in many organisms, few had their corresponding genes cloned and their recombinant proteins biochemically characterized, and none had their physiological roles determined. We found previously that FMN hydrolase activity in pea chloroplasts is Mg(2+)-dependent, suggesting an enzyme of the haloacid dehalogenase (HAD) superfamily. In this study, a new FMN hydrolase was purified by multistep chromatography after ammonium sulfate precipitation. The molecular weight of the native protein was estimated at ∼59,400, a dimer of about twice the predicted molecular weight of most HAD superfamily phosphatases. After SDS-PAGE of the partially purified material, two separate protein bands within 25-30 kDa were extracted from the gel and analyzed by nanoLC-MS/MS. Peptide sequence matching to the protein samples suggested the presence of three HAD-like hydrolases. cDNAs for sequence homologs from Arabidopsis thaliana of these proteins were expressed in Escherichia coli. Activity screening of the encoded proteins showed that the At1g79790 gene encodes an FMN hydrolase (AtcpFHy1). Plastid localization of AtcpFHy1 was confirmed using fluorescence microscopy of A. thaliana protoplasts transiently expressing the N-terminal fusion of AtcpFHy1 to enhanced green fluorescent protein. Phosphatase activity of AtcpFHy1 is FMN-specific, as assayed with 19 potential substrates. Kinetic parameters and pH and temperature optima for AtcpFHy1 were determined. A phylogenetic analysis of putative phosphatases of the HAD superfamily suggested distinct evolutionary origins for the plastid AtcpFHy1 and the cytosolic FMN hydrolase characterized previously.

In vivo spectral imaging of different cell types in the small intestine by two-photon excited autofluorescence

Spectrally resolved two-photon excited autofluorescence imaging is used to distinguish different cell types and functional areas during dynamic processes in the living gut. Excitation and emission spectra of mucosal tissue and tissue components are correlated to spectra of endogenous chromophores. We show that selective excitation with only two different wavelengths within the tuning range of a Ti:sapphire femtosecond laser system yields excellent discrimination between enterocytes, antigen presenting cells and lysosomes based on the excitation and emission properties of their autofluorescence. The method is employed for time-lapse microscopy over up to 8 h. Changes of the spectral signature with the onset of photodamage are demonstrated, and their origin is discussed.

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.

Flavin nucleotide metabolism in plants: monofunctional enzymes synthesize fad in plastids

FAD synthetases (EC 2.7.7.2) catalyze biosynthesis of FAD from FMN and ATP. Monofunctional FAD synthetases are known to exist in mammals and yeast; bifunctional enzymes also catalyzing phosphorylation of riboflavin to FMN are known to exist in bacteria. Previously known eukaryotic enzymes with FAD synthetase activity have no sequence similarity to prokaryotic enzymes with riboflavin kinase and FAD synthetase activities. Proteins homologous to bacterial bifunctional FAD synthetases, yet shorter and lacking amino acid motifs at the C terminus, were found by bioinformatic analyses in vascular plant genomes, suggesting that plants contain a type of FAD synthetase previously known to exist only in prokaryotes. The Arabidopsis thaliana genome encodes two of such proteins. Both proteins, which we named AtRibF1 and AtRibF2, carry N-terminal extensions with characteristics of organellar targeting peptides. AtRibF1 and AtRibF2 cDNAs were cloned by reverse transcription-PCR. Only FAD synthetase activity was detected in the recombinant enzymes produced in Escherichia coli. FMN and ATP inhibited both enzymes. Kinetic parameters of AtRibF1 and AtRibF2 for the two substrates were similar. Confocal microscopy of protoplasts transformed with enhanced green fluorescence protein-fused proteins showed that AtRibF1 and AtRibF2 are targeted to plastids. In agreement with subcellular localization to plastids, Percoll-isolated chloroplasts from pea (Pisum sativum) synthesized FAD from imported riboflavin. Riboflavin kinase, FMN hydrolase, and FAD pyrophosphatase activities were detected in Percoll-isolated chloroplasts and mitochondria from pea. We propose from these new findings a model for subcellular distribution of enzymes that synthesize and hydrolyze flavin nucleotides in plants.

An FMN hydrolase is fused to a riboflavin kinase homolog in plants

Riboflavin kinases catalyze synthesis of FMN from riboflavin and ATP. These enzymes have to date been cloned from bacteria, yeast, and mammals, but not from plants. Bioinformatic approaches suggested that diverse plant species, including many angiosperms, two gymnosperms, a moss (Physcomitrella patens), and a unicellular green alga (Chlamydomonas reinhardtii), encode proteins that are homologous to riboflavin kinases of yeast and mammals, but contain an N-terminal domain that belongs to the haloacid dehalogenase superfamily of enzymes. The Arabidopsis homolog of these proteins was cloned by RT-PCR, and was shown to have riboflavin kinase and FMN hydrolase activities by characterizing the recombinant enzyme produced in Escherichia coli. Both activities of the purified recombinant Arabidopsis enzyme (AtFMN/FHy) increased when the enzyme assays contained 0.02% Tween 20. The FMN hydrolase activity of AtFMN/FHy greatly decreased when EDTA replaced Mg(2+) in the assays, as expected for a member of the Mg(2+)-dependent haloacid dehalogenase family. The functional overexpression of the individual domains in E. coli establishes that the riboflavin kinase and FMN hydrolase activities reside, respectively, in the C-terminal (AtFMN) and N-terminal (AtFHy) domains of AtFMN/FHy. Biochemical characterization of AtFMN/FHy, AtFMN, and AtFHy shows that the riboflavin kinase and FMN hydrolase domains of AtFMN/FHy can be physically separated, with little change in their kinetic properties.

Purification and properties of a nucleotide pyrophosphatase from lentil seedlings

A nucleotide pyrophosphatase (EC 3.6.1.9) was purified to homogeneity from lentil seedlings. The enzyme is a single polypeptide chain of 75 +/- 2 kDa that exhibits hydrolytic activities toward pyrophosphate linkages of several substrates. Reduced and oxidized forms of NAD(P) were shown to be hydrolyzed to nicotinamide mononucleotide and AMP. Other dinucleotides such as FAD and dinucleoside oligophosphates were hydrolyzed as well, but with lower efficiency. Pyrophosphatase activity was increased in the presence of divalent cations such as Ca2+, Mg2+, and Mn2+, whereas Cu2+, Zn2+, and Ni2+ ions inhibited this activity. The active site in the enzyme was not defined, but histidine residue(s) seemed to be crucial for the enzymatic activity.

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.

Flavin adenine dinucleotide and flavin mononucleotide metabolism in rat liver--the occurrence of FAD pyrophosphatase and FMN phosphohydrolase in isolated mitochondria

In order to gain some insight into mitochondrial flavin biochemistry, rat liver mitochondria essentially free of lysosomal and microsomal contamination were prepared and their capability to metabolise externally added and endogenous FAD and FMN tested both spectroscopically and via HPLC. The existence of two novel mitochondrial enzymes, namely FAD pyrophosphatase (EC 3.6.1.18) and FMN phosphohydrolase (EC 3.1.3.2), which catalyse FAD-->FMN and FMN-->riboflavin conversion, respectively, is shown. They differ from each other and from extramitochondrial enzymes, as judged by their pH profile and inhibitor sensitivity, and can be separated in a partial FAD pyrophosphatase purification. Digitonin titration and subfractionation experiments show that FAD pyrophosphatase is located in the outer mitochondrial membrane and FMN phosphohydrolase in the intermembrane space. Since these enzymes can metabolise endogenous FAD and FMN, which are made available by using both Triton X-100 and the effector oxaloacetate, a proposal is made that FAD pyrophosphatase and FMN phosphohydrolase play a major role in mitochondrial flavoprotein turnover.