Binding study of riboflavin-binding protein with riboflavin and its analogues by differential scanning calorimetry

Quantitative metabolome analysis of boiled chicken egg yolk

Boiling has important effects on the texture of chicken eggs, but its effects on egg nutrients have not been systematically investigated. In this study, changes in the metabolites of egg yolks boiled under different heating intensities were quantified and compared by metabolomic analysis. A total of 797 metabolites were identified, and the abundance of 162 metabolites changed significantly after boiling. The significant reduction of L-lysine and D-fructose suggested that Maillard reactions occurred in over-boiled egg yolks. Egg yolk endogenous enzymes might induce a partial hydrolysis of proteins and phospholipids during the warm-up period of boiling, as the abundance of dipeptides, lysophospholipids, and free fatty acids was significantly increased in boiled egg yolks. Boiling increased the detectable abundance of fat-soluble vitamins, riboflavin, and biotin, possibly by altering the complex structure of protein-lipid-lipophilic compounds or denaturing vitamin-binding proteins. The results of metabolomic analyses provide important information for understanding the nutritional changes of egg yolk boiled under different heating intensities.

Riboflavin as an internal marker for spoilage and adulteration detection in milk

Milk is a common consumable in daily life due to its nutritional values. Ensuring milk's integrity and authenticity is a prime task for researchers and food industries by providing solutions to prevent spoilage and adulteration. We present a robust and reliable optical method to ensure milk quality through its constituent riboflavin as an internal biomarker. Riboflavin is a widely present constituent in several food matrices. This research demonstrates the characteristic fluorescence of riboflavin for checking spoilage and urea adulteration in real-time. The proposed method can even detect and quantify high urea adulteration levels up to 80 mM (i.e., eight times permissible standard value) with a LOD value of 9.3 mM. The linearity (0-80 mM) and high R2 value (0.98, 0.93) of riboflavin's fluorescence in pure and milk solutions, respectively present this strategy closely associated with fate of milk samples in terms of spoilage and adulteration. Thus, this optical method of riboflavin biosensing in real-time is intuitive and conclusive for determining milk quality.

Visible Light-Curable Hydrogel Systems for Tissue Engineering and Drug Delivery

Visible light-curable hydrogels have been investigated as tissue engineering scaffolds and drug delivery carriers due to their physicochemical and biological properties such as porosity, reservoirs for drugs/growth factors, and similarity to living tissue. The physical properties of hydrogels used in biomedical applications can be controlled by polymer concentration, cross-linking density, and light irradiation time. The aim of this review chapter is to outline the results of previous research on visible light-curable hydrogel systems. In the first section, we will introduce photo-initiators and mechanisms for visible light curing. In the next section, hydrogel applications as drug delivery carriers will be emphasized. Finally, cellular interactions and applications in tissue engineering will be discussed.

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.

Biophysical characterization of a riboflavin-conjugated dendrimer platform for targeted drug delivery

The present study describes the biophysical characterization of generation-five poly(amidoamine) (PAMAM) dendrimers conjugated with riboflavin (RF) as a cancer-targeting platform. Two new series of dendrimers were designed, each presenting the riboflavin ligand attached at a different site (isoalloxazine at N-3 and d-ribose at N-10) and at varying ligand valency. Isothermal titration calorimetry (ITC) and differential scanning calorimetry (DSC) were used to determine the binding activity for riboflavin binding protein (RfBP) in a cell-free solution. The ITC data shows dendrimer conjugates have K(D) values of ≥ 465 nM on a riboflavin basis, an affinity ~93-fold lower than that of free riboflavin. The N-3 series showed greater binding affinity in comparison with the N-10 series. Notably, the affinity is inversely correlated with ligand valency. These findings are also corroborated by DSC, where greater protein-conjugate stability is achieved with the N-3 series and at lower ligand valency.

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.

Structural changes of milk components during acid-induced coagulation kinetics as studied by synchronous fluorescence and mid-infrared spectroscopy

Dynamic oscillatory experiments and front-face synchronous fluorescence spectroscopy and mid-infrared (mid-IR) spectroscopy have been used to investigate structure evolution, at the macroscopic and molecular levels, during milk acidification kinetics. The studies were performed using skim milk, at two different temperatures (30 °C and 40 °C), to which was added glucono-δ-lactone (GDL) to generate different structural changes in casein micelles and gels. Synchronous fluorescence spectra were recorded in the 250-500 nm excitation wavelength range using an offset of 80 nm between the excitation and emission monochromators for each system during the 300 min acidification kinetics. The change in the fluorescence intensity at 281 nm reflects the pH-induced physicochemical changes of casein micelles and, in particular, structural changes in the micelles in the pH range 5.5-5.0. Regarding mid-infrared spectroscopy, the region located between 1700 and 1500 cm(-1), corresponding to the amide I and II bands, and the 1500-900 cm(-1) region, called the fingerprint region, were considered for the characterization of milk coagulation kinetics. Changes in the absorbance at 1063 cm(-1) as a function of pH for kinetics recorded at 30 °C and 40 °C reflected pH-induced phosphate dissolution in the pH range 5.5-5.0. Compared to rheometry, which reveals microstructure changes only in the gel state, spectroscopic methods make it possible to monitor molecular structure changes in micelles throughout the acidification processes.

Effects of added minerals (calcium, phosphate, and citrate) on the molecular structure of skim milk as investigated by mid-infrared and synchronous fluorescence spectroscopies coupled with chemometrics

Minerals play an important role in the structure and stability of casein micelles: minerals and caseins in milk are in dynamic equilibrium. Front-face synchronous fluorescence and mid-infrared spectra in combination with multivariate statistical analysis have been used to investigate, at a molecular level, the effects of added minerals (calcium, phosphate, or citrate) on mineral equilibria and casein micelle structure. Synchronous fluorescence spectra were recorded in the 250-500 nm excitation wavelength range using an offset of 80 nm between the excitation and emission monochromators for skim-milk samples fortified with 0, 3, 6, and 9 mM of calcium, phosphate, or citrate at 30 degrees C and 4 degrees C. Regarding midinfrared spectroscopy, the region located between 1700-1500 cm(-1), corresponding to the amide I and II bands, and the 1500-900 cm(-1) region, called the fingerprint region, were considered for the characterization of the fortified skim-milk samples at the two considered temperatures. Principal component analysis (PCA) was applied to the collections of fluorescence and infrared spectral data of the two systems to optimize their description. The results show that the phenomena induced by the addition of phosphate were different from the ones observed following the addition of calcium or citrate, a calcium-chelating agent. Finally, common components and specific weights analysis was applied to infrared spectra and fluorescence data collected on fortified skim-milk samples. This analysis enabled the relationship between the different data tables to be established.

Electrochemistry of riboflavin-binding protein and its interaction with riboflavin

Riboflavin-binding protein (RBP, a carrier of riboflavin) plays an essential role in embryo development. Electrochemical studies of the riboflavin-RBP interactions have been so far limited to changes in polarographic and voltammetric responses of riboflavin because of lack of methods capable to detect electrochemical changes in the RBP responses. Here we used constant current chronopotentiometric stripping analysis (CPSA) with the hanging mercury drop electrode (HMDE) and square wave voltammetry (SWV) with carbon paste electrode (CPE) to investigate RBP. We found that CPSA of RBP produces electrocatalytic peak H, capable to discriminate between apoprotein and holoprotein forms of RBP. This peak is suitable for studies of RBP-riboflavin interaction at nanomolar concentrations. We observed no sign of a release of riboflavin from holoprotein adsorbed at the HMDE surface. SWV at CPE required higher concentrations of RBP and displayed almost identical oxidation peaks of apoprotein and holoprotein.

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.

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.

Evaluation of riboflavin binding protein domain interaction using differential scanning calorimetry

Riboflavin binding (or carrier) protein (RfBP) is a monomeric, two-domain protein, originally purified from hens' egg white. RfBP contains nine disulfide bridges; as a result, the protein forms a compact structure and undergoes reversible three-state thermal denaturation. This was demonstrated using a differential scanning calorimetry (DSC) method [Wasylewski M. (2000) J. Prot. Chem. 19(6), 523-528]. It has been shown that the RfBP complex with riboflavin denaturates in a three-state process which may be attributed to sequential unfolding of the RfBP domains. In case of apo RfBP, the ligand binding domain denaturates at a lower temperature than the C-terminal domain. Ligand binding greatly enhances the thermostability of the N-terminal domain, whereas the C-terminal domain thermostability is only slightly affected and, in case of the examined holo RfBPs, the denaturation peaks of both domains merge or cross over. The magnitude of the changes depends on ligand structure. A detailed study of protein concentration effects carried out in this work allowed to estimate not only the thermostability of both domains but also the strength of domain interactions. The DeltaCp, of denaturation was found for C-terminus and N-terminus of RfBP-riboflavin complex to amount to 2.5 and -1.9 kcal mol(-1), respectively. The calculated domain interaction free energy, DeltaGCN, was estimated to be approximately -1580 cal mol(-1) at 67.0 degrees C. This value indicates that the interdomain interaction is of medium strength.