Chicken riboflavin-binding protein. cDNA sequence and homology with milk folate-binding protein

From Metabolism to Vitality: Uncovering Riboflavin's Importance in Poultry Nutrition

Riboflavin, or vitamin B2, is indispensable for poultry, profoundly impacting their metabolic equilibrium, growth, and overall health. In a climate of increasing demand for poultry products and heightened production intensity, grasping the multifaceted roles of riboflavin in domestic fowl nutrition becomes paramount. This essential vitamin serves as a precursor to two vital coenzymes, flavin mononucleotide and flavin adenine dinucleotide, integral players in pivotal redox reactions and energy metabolism. Inadequate riboflavin levels translate into stunted growth, skeletal deformities, and compromised feed conversion efficiency, thereby adversely affecting poultry performance and bottom-line profitability. Riboflavin goes beyond its fundamental role, ameliorating nutrient utilization, facilitating protein synthesis, and augmenting enzyme activity, rightfully earning its epithet as the "growth-promoting vitamin". Poultry's reproductive success intricately hinges on riboflavin levels, dictating egg production and hatchability. It is imperative to note that riboflavin requirements exhibit variations among poultry species and distinct production phases, emphasizing the importance of judicious and balanced supplementation strategies. Aligning dietary recommendations with genetic advancements holds the promise of fostering sustainable growth within the poultry sector. Exploring the multifaceted aspects of riboflavin empowers researchers, nutritionists, and producers to elevate poultry nutrition and overall well-being, harmonizing with the industry's evolving demands.

The complex interplay between ligand binding and conformational structure of the folate binding protein (folate receptor): Biological perspectives

This review analyzes how interplay between folate binding and changes in folate binding protein (FBP) conformation/self-association affects the biological function of FBP. Concentration-dependent, reversible self-association of hydrophobic apo-FBP at pI=7.4 is associated with decreased affinity for folate, probably due to shielding of binding sites between interacting hydrophobic patches. Titration with folate removes apo-monomers, favoring dissociation of self-associated apo-FBP into apo-monomers. Folate anchors to FBP through a network of hydrogen bonds and hydrophobic interactions, and the binding induces a conformational change with formation of hydrophilic and stable holo-FBP. Holo-FBP exhibits a ligand-mediated concentration-dependent self-association into multimers of great thermal and chemical stability due to strong intermolecular forces. Both ligand and FBP are thus protected against biological/physicochemical decomposition. In biological fluids with low FBP concentrations, e.g., saliva, semen and plasma, hydrophobic apo-monomers and hydrophilic holo-monomers associate into stable asymmetrical complexes with aberrant binding kinetics unless detergents, e.g., cholesterol or phospholipids are present.

Folate-linked drugs for the treatment of cancer and inflammatory diseases

Folic acid, also known as vitamin B9 (Fig. 9.1), is an essential co-enzyme in one-carbon metabolism pathways, including the biosynthesis of nucleotides (i.e. purines, thymidine) and several amino acids. In general, two functionally different systems mediate the cellular uptake of folate: (1) the reduced folate carrier (RFC, Kd ∼ 10-6 M), an anion transporter that delivers folates across the plasma membrane in a bidirectional fashion, and (2) the folate receptor (FR, Kd ∼ 10-10 M), which internalizes folate through active receptor-mediated endocytosis. The RFC, a membrane-spanning anion transporter, is present in virtually all tissues and is responsible for the majority of folate transport in and out of cells. In contrast, FR expression is largely restricted to malignant cells, activated macrophages, and the proximal tubule cells of the kidneys. Because a variety of important diseases are caused by the former two cell types, interest in exploiting FR for drug targeting applications has rapidly increased. And achievement of this targeting objective, primarily through conjugation of drugs to folic acid is believed to enable (1) enhanced net drug uptake by pathologic cells, and more importantly (2) reduction in drug deposition into non-pathologic cells, thereby mitigating collateral toxicity to normal tissues.

Downregulation of leaf flavin content induces early flowering and photoperiod gene expression in Arabidopsis

BACKGROUND

Riboflavin is the precursor of flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD), essential cofactors for many metabolic enzymes that catalyze a variety of biochemical reactions. Previously we showed that free flavin (riboflavin, FMN, and FAD) concentrations were decreased in leaves of transgenic Arabidopsis plants expressing a turtle riboflavin-binding protein (RfBP). Here, we report that flavin downregulation by RfBP induces the early flowering phenotype and enhances expression of floral promoting photoperiod genes.

RESULTS

Early flowering was a serendipitous phenomenon and was prudently characterized as a constant phenotype of RfBP-expressing transgenic Arabidopsis plants in both long days and short days. The phenotype was eliminated when leaf free flavins were brought back to the steady-state levels either by the RfBP gene silencing and consequently nullified production of the RfBP protein, or by external riboflavin feeding treatment. RfBP-induced early flowering was correlated with enhanced expression of floral promoting photoperiod genes and the florigen gene FT in leaves but not related to genes assigned to vernalization, autonomous, and gibberellin pathways, which provide flowering regulation mechanisms alternative to the photoperiod. RfBP-induced early flowering was further correlated with increased expression of the FD gene encoding bZIP transcription factor FD essential for flowering time control and the floral meristem identity gene AP1 in the shoot apex. By contrast, the expression of FT and photoperiod genes in leaves and the expression of FD and AP1 in the shoot apex were no longer enhanced when the RfBP gene was silenced, RfBP protein production canceled, and flavin concentrations were elevated to the steady-state levels inside plant leaves.

CONCLUSIONS

Token together, our results provide circumstantial evidence that downregulation of leaf flavin content by RfBP induces early flowering and coincident enhancements of genes that promote flowering through the photoperiod pathway.

Structural and functional insights on folate receptor α (FRα) by homology modeling, ligand docking and molecular dynamics

Folate receptor α (FRα) is a cell surface, glycophosphatidylinositol (GPI)-anchored protein with a high affinity for its ligand partner, which is highly expressed in malignant cells and has been selected as a therapeutic target and marker for the diagnosis of cancer. No direct structural information is available from either X-ray diffraction or NMR on the post-translational structure of this disulfide-rich protein. Three-dimensional models of the FRα structure have been derived with the recent homology modeling packages, using the crystal structure of the riboflavin-binding protein (RfBP) as a template. Molecular dynamics trajectories have been exploited allowing successfully the formation of a full disulfide bridge network, which was expected based on the similarities between FRα and RfBP. After the selection of the best model, a folic acid molecule was docked "in silico" onto the putative binding site and its binding mode was compared with that of vintafolide, a much larger molecule designed as a chemotherapy agent targeting specifically FRα. In both cases, a 40ns molecular dynamics trajectory was calculated, providing suggestions regarding the key structural determinants driving the affinity and specificity of FRα for folic acid with respect to other folate homologues. Moreover, some other crucial experimental results related to the structure of the receptor are discussed, such as the expected location/accessibility of known immune epitopes, the set of N-linked glycosylation sites and the effect of point mutations on the impairment of folate binding. Our results may provide useful insights for studies related to folate-targeted drug delivery or cancer therapies involving folate uptake.

Ligand binding induces a sharp decrease in hydrophobicity of folate binding protein assessed by 1-anilinonaphthalene-8-sulphonate which suppresses self-association of the hydrophobic apo-protein

High affinity folate binding protein (FBP) regulates as a soluble protein and as a cellular receptor intracellular trafficking of folic acid, a vitamin of great importance to cell growth and division. We addressed two issues of potential importance to the biological function of FBP, a possible decrease of the surface hydrophobicity associated with the ligand-induced conformation change of FBP, and protein-inter-protein interactions involved in self-association of hydrophobic apo-FBP. The extrinsic fluorescent apolar dye 1-anilinonaphthalene-8-sulphonate (ANS) exhibited enhanced fluorescence intensity and a blueshift of emission maximum from 510-520 nm to 460-470 nm upon addition of apo-FBP indicating binding to a strongly hydrophobic environment. Neither enhancement of fluorescence nor blueshift of ANS emission maximum occurred when folate-ligated holo-FBP replaced apo-FBP. The drastic decrease in surface hydrophobicity of holo-FBP could have bearings on the biological function of FBP since changes in surface hydrophobicity have critical effects on the biological function of receptors and transport proteins. ANS interacts with exposed hydrophobic surfaces on proteins and may thereby block and prevent aggregation of proteins (chaperone-like effect). Hence, hydrophobic interactions seemed to participate in the concentration-dependent self-association of apo-FBP which was suppressed by high ANS concentrations in light scatter measurements.

Down-regulation of free riboflavin content induces hydrogen peroxide and a pathogen defense in Arabidopsis

Riboflavin mediates many bioprocesses associated with the generation of hydrogen peroxide (H₂O₂), a cellular signal that regulates defense responses in plants. Although plants can synthesize riboflavin, the levels vary widely in different organs and during different stages of development, indicating that changes in riboflavin levels may have physiological effects. Here, we show that changing riboflavin content affects H₂O₂ accumulation and a pathogen defense in Arabidopsis thaliana. Leaf content of free riboflavin was modulated by ectopic expression of the turtle gene encoding riboflavin-binding protein (RfBP). The RfBP-expressing Arabidopsis thaliana (REAT) plants produced the RfBP protein that possessed riboflavin-binding activity. Compared with the wild-type plant, several tested REAT lines had >70% less flavins of free form. This change accompanied an elevation in the level of H₂O₂ and an enhancement of plant resistance to a bacterial pathogen. All the observed REAT characters were eliminated due to RfBP silencing (RfBPi) under REAT background. When an H₂O₂ scavenger was applied, H₂O₂ level declined in all the plants, and REAT no longer exhibited the phenotype of resistance enhancement. However, treatment with an NADPH oxidase inhibitor diminished H₂O₂ content and pathogen defense in wild-type and RfBPi but not in REAT. Our results suggest that the intrinsic down-regulation of free flavins is responsible for NADPH oxidase-independent H₂O₂ accumulation and the pathogen defense.

A chemometric analysis of ligand-induced changes in intrinsic fluorescence of folate binding protein indicates a link between altered conformational structure and physico-chemical characteristics

Ligand binding alters the conformational structure and physico-chemical characteristics of bovine folate binding protein (FBP). For the purpose of achieving further information we analyzed ligand (folate and methotrexate)-induced changes in the fluorescence landscape of FBP. Fluorescence excitation and emission two-dimensional (2D) spectra were recorded over a wide range of wavelengths on a Perkin-Elmer LS 55 spectrofluorometer at varying pH in different buffers, and the resulting three-dimensional data were subjected to a chemometric analysis, parallel factor analysis (PARAFAC). The most important finding was the occurrence of two maximum intensity emission wavelengths of tryptophan, 350 nm (component one) and 330 nm (component two). In contrast to the first component, the score of the short wavelength component increased with increasing ligation of FBP. Since the emission wavelengths of indole groups in tryptophan shorten with increasing distance from the solvent surface of proteins, an increasing number of the 11 tryptophan residues seem to reorientate from the solvent surface to the interior of FBP with increasing ligation. The sharp decrease in hydrophobicity at pI=7-8 following binding of folate accords fairly well with the disappearance of strongly hydrophobic tryptophan residues from the solvent-exposed surface of FBP. The PARAFAC has thus proven useful to establish a hitherto unexplained link between parallel changes in conformational structure and physico-chemical characteristics of FBP induced by folate binding. Parameters for ligand binding derived from PARAFAC analysis of the fluorescence data were qualitatively and quantitatively similar to those obtained from binding of radiofolate to FBP. Herein, methotrexate exhibited a higher affinity for FBP than in competition with radiofolate. This could suggest a rapid and firm complexation of folate to FBP, blocking access of competing ligands.

Biochemical characterization of recombinant chicken Riboflavin carrier protein

Chicken Riboflavin Carrier Protein (cRCP) transports riboflavin from the maternal circulation to the egg yolk for fetal development. The cRCP is a globular protein and structurally very stable due to the presence of nine intra-molecular disulphide bonds. The cRCP comprises of two domains; the larger riboflavin binding, and the smaller, oocyte receptor binding domain. With the objective to study domain folding in cRCP, these two domains of the corresponding gene were amplified, cloned, and expressed in a eukaryotic expression system to obtain soluble product. Our studies on the biochemical characterization of the recombinant proteins indicated that though the ligand binding domain assumed near-native conformation, as determined by immunological methods, it did not bind riboflavin, suggesting the interdependence of the two domains for proper organization of the riboflavin binding pocket.

Dynamin 2 Regulates Riboflavin Endocytosis in Human Placental Trophoblasts

Riboflavin is thoroughly established to be indispensable in a multitude of cellular oxidation-reduction reactions through its conversion to coenzyme forms flavin mononucleotide and flavin adenine dinucleotide. Despite its physiological importance, little is known about specific mechanisms or proteins involved in regulating its cellular entry in humans. Studies involving biochemical modulators and immunological inhibition assays have indirectly revealed that riboflavin internalization and trafficking occurs at least in part through a clathrin-dependent receptor-mediated endocytic process. Here, using a two-tiered strategy involving RNA interference and the overexpression of dominant-negative constructs, we directly show the involvement of this endocytic mechanism through the requirement of the pluripotent endocytic vesicle scission enzyme, dynamin 2 GTPase, in human placental trophoblasts. Similar to the endocytic control ligand, transferrin, riboflavin is shown to exhibit 50% dependence on the functional expression of dynamin 2 for its active cellular entry. Furthermore, this reduced vitamin uptake correlates with >2-fold higher riboflavin association at the cell surface. In addition, fluorescent ligand endocytosis assays showing colocalization between rhodamine-riboflavin and the immunostained caveolar coat protein, caveolin 1, suggest that the active absorption of this important nutrient involves multiple and distinct endocytosis pathways.

Stopped-Flow Kinetic Studies of the Interaction of Bovine Folate Binding Protein (FBP) and Folate

The kinetics of the interaction of bovine folate binding protein and folate at pH 7.4 and 5.0 were followed by measuring the changes of the intrinsic protein fluorescence intensity using the stopped-flow technique, which enables the study of reactions from the millisecond time-range. Our results immediately reject a simple one-step binding model, which requires a linear dependence of the observed rate constant on the concentration of the ligand. Thus, we are able to conclude that at pH 5.0 the interaction occurs in two steps and at pH 7.4 in three steps. Changes of fluorescence spectra at equilibrium were used to estimate the overall binding constants. Comparative studies on the binding of folate to human albumin are also reported.

Application of near-infrared and Fourier transform infrared spectroscopy in the characterization of ligand-induced conformation changes in folate binding protein purified from bovine milk: influence of buffer type and pH

Fourier transform infrared (FT-IR) and near-infrared (NIR) spectroscopy have been applied to detect structural alterations in folate binding protein (FBP) induced by ligation in different buffer types. The amide I region pointed to a beta-sheet to alpha-helix transition upon ligation in acetate and phosphate buffers, and the formation of intermolecular beta-sheet was indicated at pH 5.0, in agreement with a dimerization of FBP taking place at this pH. The ligand-induced changes in the 2100-2300 nm NIR region were significant for FBP in acetate and phosphate buffers of pH 5.0, and the variations were interpreted as secondary structure changes, based on previous assignments of secondary structures to the combination bands in the NIR region. In the case of acetate buffer, variations in the amide combination bands agreed with the amide I analysis, but for the other buffer types some discrepancies were found and explained by side-chain contributions to the NIR, which could reflect the tertiary and quaternary structure differences. NIR spectra of FBP at pH 7.4 and 5.0 revealed contradictory effects on the side chains, reflecting different polymerization events at the two pH values, whereas the amide I region indicated similar changes at the two pH values. Therefore, we suggest that FT-IR and NIR spectroscopy may complement each other, such that the two techniques in combination may give information on all three types of protein conformational changes. While the secondary structure changes are revealed by FT-IR, the tertiary and quaternary structure changes are reflected in the NIR spectra, although the general influence of the latter changes on the NIR spectra remains to be confirmed.

Current perspectives on the cellular uptake and trafficking of riboflavin

The role of riboflavin in cell maintenance and growth, and the mechanism by which it is absorbed into various human tissues and cell lines has been extensively studied over the past decade. Evidence suggests two absorption mechanisms, a saturable-active component that dominates at near physiological vitamin concentrations and a passive component that is revealed at oversupplemented riboflavin conditions. Various transport modulator studies consistently suggest a highly riboflavin specific, temperature-dependent active transport mechanism that is regulated by the Ca2+/calmodulin pathway. The PKA and PKG pathways have also been implicated in absorption regulation. The long-standing model that riboflavin absorption involves a carrier-mediated transporter has recently been challenged through studies suggesting a receptor-mediated endocytic component. The presence of a soluble, human riboflavin binding protein in the transport stratagem has been shown to play an important role in fetal development. The relationship of this binding protein with the riboflavin specific membrane bound protein, though currently not well defined, may involve a protein-protein interaction that plays a primary role in this proposed receptor-mediated component.

Dietary interactions influence the effects of bovine folate-binding protein on the bioavailability of tetrahydrofolates in rats

The newborns of mammals have a high folate demand, yet obtain adequate folate nutrition solely from their mothers' milk despite its low folate content. Milk folate is entirely bound by an excess of folate-binding protein (FBP), prompting speculation that FBP may affect the bioavailability of the limited folate supply. Previous research has shown that FBP-bound folic acid is more gradually absorbed, thereby reducing the peak plasma folate concentration and preventing loss into the urine. Natural folates are reduced derivatives of folic acid, with milk predominantly containing 5-methyltetrahydrofolate, yet little research has been carried out to determine the role of FBP in the bioavailability of reduced folates. We studied the effect of FBP on folate nutrition of rats in both single-dose and 4-wk feeding experiments. The effect of FBP was influenced by the presence of other milk components. FBP increased bioavailability of dietary folate when it was consumed with other whey proteins or with soluble casein. However, in the presence of acid-precipitated casein or a whey preparation enriched in lipids, bioavailability was decreased. These results highlight the difficulties of extrapolating from experimental results obtained using purified diets alone and of studying interactions among dietary components. They suggest that the addition of FBP-rich foods to folate-rich foods could enhance the bioavailability of natural folates, but that the outcome of such a combination would depend on interactions with other components of the diet.

Characterization of chicken riboflavin carrier protein gene structure and promoter regulation by estrogen

The chicken riboflavin carrier protein (RCP) is an estrogen induced egg yolk and white protein. Eggs from hens which have a splice mutation in RCP gene fail to hatch, indicating an absolute requirement of RCP for the transport of riboflavin to the oocyte. In order to understand the mechanism of regulation of this gene by estrogen, the chicken RCP gene including 1 kb of the 5' flanking region has been isolated. Characterization of the gene structure shows that it contains six exons and five introns, including an intron in the 5' untranslated region. Sequence analysis of the 5' flanking region does not show the presence of any classical, palindromic estrogen response element (ERE). However, there are six half site ERE consensus elements. Four deletion constructs of the 5' flanking region with varying number of ERE half sites were made in pGL3 basic vector upstream of the luciferase-coding region. Transient transfection of these RCP promoter deletion constructs into a chicken hepatoma cell line (LMH2A) showed 6-12-fold transcriptional induction by a stable estrogen analogue, moxesterol. This suggests that the RCP gene is induced by estrogen even in the absence of a classical ERE and the half sites of ERE in this promoter may be important for estrogen induction

An unusual photohaemolytic property of riboflavin

Riboflavin (RF) is a known photoreactive and phototoxic molecule. However, unlike other photosensitizers, it does not induce photohaemolysis of erythrocytes by itself. On the other hand, illuminated RF caused haemolysis but in the presence of serum or plasma. The kinetics of photohaemolysis in the presence of serum/plasma has been studied by monitoring the rate of haemolysis spectrophotometrically and morphological changes at erythrocytes membrane by scanning electron microscopy. We found that the extent of mammalian RBC membrane damage was dependent on the concentration of RF or hematoporphyrin (HP) (0-20 microgram/ml) and dose of sunlight (0-20 min). The RBC membrane-damaging potential of illuminated HP was not affected by the presence of plasma in the reaction system. Furthermore, RF showed a protective role against photohaemolysis caused by photoexcited HP if erythrocytes were preincubated with RF in the absence of serum/plasma. For mechanistic studies, biochemical parameters such as acetylcholinesterase activity (AChE) and formation of TBA-reactive substance (TBA-RS) were analysed in RBC and RBC+plasma under a similar set of experimental conditions. We observed about a 25% decrease in AchE activity as a synergistic action of RF or HP (20 microgram/ml) and sunlight (30 min) in both cases (RBC or RBC+plasma). Interestingly, illuminated RF caused about a 125% increase of TBA-RS in a reaction system consisting of RBC+plasma. On the other hand, an increase in the production of TBA-RS by illuminated RF was not observed in the absence of plasma/serum, in the reaction system. These results suggested that photooxidation of RBC membrane lipids by illuminated RF, under the influence of plasma/serum, may be one of the causes of membrane modification leading to RBC lysis.

Identification and characterization of receptors for riboflavin carrier protein in the chicken oocyte. Role of the phosphopeptide in mediating receptor interaction

Riboflavin carrier protein (RCP) is a phosphoglycoprotein found in the egg and the serum of laying birds and other animals. We have investigated the binding of chicken RCP (cRCP) to membranes prepared from the whole chicken oocytes. RCP binding had an absolute requirement for calcium, with an affinity (Kd 10(-8) M) high enough to be physiologically relevant. Ligand blotting experiments using labeled RCP and vitellogenin, with proteins solubilized from oocyte membranes, indicated that RCP and vitellogenin bound specifically to three proteins of Mr 380, 260 and 110 kDa. Vitellogenin also bound to proteins of Mr 515 kDa and 97 kDa, similar in size to those identified by receptor associated protein of RAP. Reduced and carboxyamidated RCP inhibited the binding of 125I-labeled RCP to chicken oocyte membranes, but recombinant RCP expressed in E. coli, and dephosphorylated RCP, failed to interact with the receptors, indicating that post-translational modifications were necessary for ligand-receptor interaction. The purified phosphopeptide, prepared from tryptic digests of egg white RCP, was able to inhibit the binding of RCP to the receptor proteins, with an affinity comparable to native RCP indicating that the phosphopeptide sequence present in RCP serves as the focal point for RCP-receptor interactions.

Refolding of riboflavin carrier protein as probed by biochemical and immunological parameters

The unfolding of the chicken egg white riboflavin carrier protein by disulfide reduction with dithiothreitol led to aggregation with concomitant loss of ligand binding characteristics and the capacity to interact with six monoclonal antibodies directed against surface-exposed discontinuous epitopes. The reduced protein could, however, bind to a monoclonal antibody recognizing sequential epitope. Under optimal conditions of protein refolding, the vitamin carrier protein regained its folded structure with high efficiency with simultaneous complete restoration of hydrophobic flavin binding site as well as the epitopic conformations exposed at the surface in a manner comparable to its native form.

Riboflavin binding proteins and flavin assimilation in insects

Recent studies on developmentally regulated hemolymph proteins in insects have shown that two proteins, a lipoprotein and a member of a hexamerin gene family, bind riboflavin. The biosynthesis, developmental regulation, and properties of these proteins are described and compared with the riboflavin-binding proteins and flavin distributions in vertebrates. The importance of riboflavin-binding proteins in insect development is discussed in relation to existing information and avenues for future research are presented.

Studies on the delineation of the antigenic determinants of chicken riboflavin carrier protein (cRCP). Identification of a determinant in the region 10-24 of the protein

As riboflavin carrier proteins play a critical role in the maintenance of pregnancy, studies on the immunology of these proteins have become very relevant. This paper describes our attempts to identify the sequential antigenic determinants of chicken riboflavin carrier protein (cRCP). Potential surface oriented regions of cRCP were identified by constructing acrophilicity and hydrophilicity profiles of the protein. Of the three regions identified, the pentadecapeptide 10-24 of cRCP forms the subject of the present study. The pentadecapeptide amide was synthesized by solid phase synthesis using Fmoc chemistry. Immunization of rabbits with the peptide-diphtheria toxoid conjugate elicited high titres of the antipeptide antibodies, revealing the high immunogenicity of the peptide. The antipeptide antibodies bound to both cRCP and reduced carboxymethylated RCP (RCM-RCP). Antisera to RCM-RCP showed significant binding to the peptide showing thereby that the region 10-24 of cRCP is perhaps one of the major epitopes of RCM-RCP. Thus, the studies have resulted in the identification of the region 10-24 as an antigenic determinant of cRCP.