Fluorescence quenching in riboflavin-binding protein and its complex with riboflavin

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.

Riboflavin mobilization from eleocyte stores in the earthworm Dendrodrilus rubidus inhabiting aerially-contaminated Ni smelter soil

A 6-week reciprocal transfer laboratory exposure experiment was conducted with two populations of the epigeic earthworm Dendrodrilus rubidus; one population inhabited a site approx. 200 m downwind of an active Ni smelter co-contaminated with Ni and Cu (3648 and 977 microg g(-1)d.w., respectively), the other inhabited uncontaminated soil. Worms transferred from unpolluted to Ni/Cu-polluted soil lost body mass (62%); they also had reduced (70%) total coelomocyte number, including autofluorescent eleocytes, and had significantly decreased (92%) riboflavin-derived fluorescence emission measured at 525 nm. Coelomocyte counts were low, and 525 nm emission was negligible in worms maintained on their native Ni/Cu soil. Earthworms and their coelomocytes were unaffected when transferred from Ni/Cu-polluted soil to unpolluted soil. In conclusion, exposing worms to stress-inducing factors, including metal pollution, alters the riboflavin status within the immune-competent cells of D. rubidus, but it requires further in vivo studies to establish whether the reduction in the fluorescence signal is predominantly due to depletion of riboflavin-containing eleocytes, or to riboflavin quenching, or to enzymatic conversion (and thus depletion) of stored riboflavin into its functional immune-potentiating flavin derivatives, FMN and FAD. The flavin budget of D. rubidus coelomocytes recovered by a reproducible extrusion procedure is a potentially useful biomarker for assessing sublethal stress in this early colonizer of disturbed soils.

Biosensor-Based Determination of Riboflavin in Milk Samples

An assay for quantification of riboflavin (Rf) in milk-based products has been developed using the principle of surface plasmon resonance with on-chip measurement. The quantification was done indirectly by measuring excess of Rf binding protein (RBP) that remains free after complexation with Rf molecules originally present in the sample solution. The chip was modified with covalently immobilized Rf in order to bind the RBP in excess. A chemical modification was performed to introduce a reactive ester group at the N-3 position of the natural Rf to bind amino groups present on the chip surface. Calibration solutions were prepared by mixing a range of Rf standard solutions with an optimized concentration of RBP. The Rf content in the milk-based products was then measured by comparison of the response against the calibration. Results obtained were very close to those from an official HPLC-fluorescence procedure. The limit of quantification was determined to 234 microg/L and the limit of detection to 70 microg/L by an injection volume of 160 microL.

Dynamics of the Lens culinaris agglutinin-lactotransferrin and serotransferrin complexes, followed by fluorescence intensity quenching of fluorescein (FITC) with iodide and temperature

Dynamics of the fluorescent Lens culinaris agglutinin-fluorescein complex (LCA-FITC) are studied in absence and in presence of two glycoproteins, lactotransferrin (LTF) and serotransferrin (STF). Glycans of the serotransferrin are not fucosylated, while those of the lactotransferrin have an alpha-1,6-fucose bound to the N-acetylglucosamine residue linked to the peptide chain, and an alpha-1,3-fucose bound to the N-acetyllactosamine residues. Interaction between the lectin and the two glycoproteins occurs via the carbohydrate residues. Affinity between LCA and LTF is 50 times higher than that between LCA and STF, as a result of the alpha-1, 6-fucose-LCA linkage. In the present work, we studied the effect of the tight bond between the alpha-1,6-fucose and LCA on the dynamics of the amino acids of the lectin, by fluorescence intensity quenching with iodide and by thermal intensity quenching. Fluorescence intensity quenching with iodide indicates that the bimolecular diffusion constant of iodide is 2.402+/-0.068x109 and 1. 160+/-0.090x109 M-1 s-1, when the interaction occurs with free fluorescein and with fluorescein bound to LCA, respectively. Binding of STF or LTF to the LCA-FITC complex yields a bimolecular diffusion constant of 1.675+/-0.06x109 and 1.155+/-0.087x109 M-1 s-1, respectively. Thermal intensity quenching does not occur for fluorescein free in solution while it is linear with temperature with a relative change of 0.656%, 0.889% and 0.488% for FITC-LCA, FITC-LCA-LTF and FITC-LCA-STF complexes, respectively. Fluorescence intensity quenching with iodide and thermal quenching experiments indicate that the dynamics of LCA increase as the result of the flexibility of the carbohydrate residues (case of STF-LCA complex), and the presence of the alpha-1,6-fucose inhibits the effect of the other carbohydrate residues as the result of the tight bond that exists between the fucose and the lectin (case of LTF-LCA complex).