Solid phase enzyme-linked competitive binding assay for riboflavin

Immunoassays for riboflavin and flavin mononucleotide using antibodies specific to d-ribitol and d-ribitol-5-phosphate

Riboflavin (vitamin B₂), a water-soluble vitamin, plays a key role in maintaining human health. Though, numerous methods have been reported for the determination of total riboflavin (TRF) content in foods and biological samples, very few methods are reported for quantifying riboflavin and its coenzymes [flavin mononucleotide (FMN); flavin adenine dinucleotide (FAD)] individually. Recently, we have demonstrated that antibodies specific to d-ribitol and d-ribitol-5-phosphate also recognize riboflavin and FMN, respectively, and not vice-versa. In this study, we have evaluated these two antibodies for the analysis of riboflavin and FMN by indirect competitive ELISA (icELISA) in selected foods and pharmaceuticals. Under the optimal assay conditions, 50% inhibition concentration (IC₅₀) and limit of detection (LOD, IC₁₀) were 3.41ng/mL and 0.02ng/mL for riboflavin, and 7.84ng/mL and 0.24ng/mL for FMN, respectively, with detectable concentration range between 0.1 and 100ng of analytes and <0.1% cross-reactivity with other water-soluble vitamins. The amounts of TRF in food samples, as analyzed by icELISA using ribitol antibody, were 90-95% of the reported values in the literature or label values. Quantification of individual flavins (riboflavin and FMN) from the same food samples showed variation in their values compared to TRF, and were in good agreement with values obtained from HPLC and AOAC methods. Further, spiking and recovery analysis of food samples and pharmaceuticals showed no significant matrix effects. The immunoassays were validated in terms of accuracy and precision using inter- and intra-assays. The immunoassays developed in this study are sensitive and appears feasible for screening a large number of samples in the quantification of riboflavin and FMN in various biological samples, pharmaceuticals and natural/processed foods.

Advantageous sensitivity in the DNA homolog of the RNA dopamine aptamer

A competitive enzyme-linked aptamer assay for DA was performed by using two aptamers, individually; one is a 57 mer-RNA aptamer and the other is its homolog DNA aptamer. The difference between the RNA aptamer and the DNA aptamer are based on their particular nucleotides. It is known that the lack of a hydroxyl group in the 2' position of DNA is related with its chemical and biological stability. Thus, the use of the DNA homolog of the RNA aptamer could improve the affinity toward DA due to stability and finally, lower the detection limit. In this paper, we report advantageous sensitivity and specificity of its homolog DNA aptamer assay as compared to the RNA aptamer assay. Both aptamer assays were performed with 0.01 µg mL⁻¹ of each aptamer and 1.205 × 10⁻⁸ M DA-HRP conjugate using the optimized method. A dose-response curve was constructed, and the limit of detection for the DA was determined as 6.3 × 10⁻⁸ M for RNA aptamer assay, and 3.2 × 10⁻¹² M for the homolog DNA aptamer assay, respectively. These results demonstrated that the assay sensitivity was more than 10⁴ times improved with the DNA homolog of the RNA aptamer compared to its original RNA aptamer obtained through SELEX process. Also these results confirmed that the DNA homolog of the RNA aptamer can maintained the binding site and retained a function in both structure. Thus, the switching to the DNA version of RNA aptamer is possible to bind more stably and still able to bind to dopamine.

Recognition of riboflavin and the capsular polysaccharide of Haemophilus influenzae type b by antibodies generated to the haptenic epitope D-ribitol

D-Ribitol, a five-carbon sugar alcohol, is an important metabolite in the pentose phosphate pathway; it is an integral part of riboflavin (vitamin B2) and cell wall polysaccharides in most Gram-positive and a few Gram-negative bacteria. Antibodies specific to D-ribitol were generated in New Zealand white rabbits by using reductively aminated D-ribose-BSA conjugate as the immunogen. MALDI-TOF and amino group analyses of ribitol-BSA conjugate following 120 h reaction showed ~27-30 mol of ribitol conjugated per mole BSA. The presence of sugar alcohol in the conjugates was also confirmed by an increase in molecular mass and a positive periodic acid-Schiff staining in SDS-PAGE. Caprylic acid precipitation of rabbit serum followed by hapten affinity chromatography on ribitol-KLH-Sepharose CL-6B resulted in pure ribitol-specific antibodies (~45-50 μg/mL). The affinity constant of ribitol antibodies was found to be 2.9 × 10(7) M(-1) by non-competitive ELISA. Ribitol antibodies showed 100% specificity towards ribitol, ~800% cross-reactivity towards riboflavin, 10-15% cross-reactivity with sorbitol, xylitol and mannitol, and 5-7% cross-reactivity with L-arabinitol and meso-erythritol. The specificity of antibody to ribitol was further confirmed by its low cross-reactivity (0.4%) with lumichrome. Antibodies to D-ribitol recognized the purified capsular polysaccharide of Haemophilus influenzae type b, which could be specifically inhibited by ribitol. In conclusion, antibodies specific to D-ribitol have been generated and characterized, which have potential applications in the detection of free riboflavin and ribitol in biological samples, as well as identification of cell-surface macromolecules containing ribitol.

Microtitre-plate enzyme-linked ligand-sorbent assay of riboflavin (vitamin B2) in human plasma and urine

Enzyme-linked ligand-sorbent assay (ELLSA) of riboflavin was performed in standard, multi-well microtitre plates. 3-Carboxymethylriboflavin was carbodiimide-coupled to bovine serum albumin and the conjugate was adsorbed on the well surface. Riboflavin-binding protein from egg-white was biotinylated with biotinamidocaproate N-hydroxysuccinimide ester. The assay was based on competition of riboflavin analyte with the immobilized flavin for the biotinylated binder. Secondary adsorption of the biotinylated riboflavin-binding protein was measured by using avidin-bearing horseradish peroxidase label. The optimized method had a detection limit of 0.8 pmol of riboflavin and was expected to work within a riboflavin concentration range of 2 X 10(-8)(-4) X 10(-6) mol l-1. Preliminary trials suggested that ELLSA was suitable for determining riboflavin in human urine and the sum of riboflavin and flavin nucleotides in human plasma. The analytical performance of ELLSA for those materials was characterized by good consistency of the results with those obtained by conventional, fluorimetric methods, a mean recovery of riboflavin supplement of over 90% and a within-plate relative standard deviation below 20%. Some unique samples of both urine and plasma were assayed with between-plate relative standard deviations higher than 30%, implicating further modification of this version of ELLSA. The method intended for routine control determinations of vitamin B2 status in human subjects and is addressed to laboratories that routinely perform automated, microplate-based enzyme-linked assays.