Production and characterization of a single-chain variable fragment linked alkaline phosphatase fusion protein for detection of O,O-diethyl organophosphorus pesticides in a one-step enzyme-linked immunosorbent assay

Construction of a Highly Selective Enrichment, Ionization, and Detection Platform Based on a Broad-Spectrum Antibody

Ambient mass spectrometry (AMS) allows direct analysis of various raw food samples with minimal or no sample pretreatment, but the trace analytes in complex food samples still have problems with limitations. In this work, we developed a platform based on coated stainless steel sheet spray mass spectrometry for fast, in situ, high-throughput, and high selectivity multiresidue analysis of fluoroquinolone drugs (FQs). The sensitivity of the platform was enhanced via coupling broad-spectrum antibodies against FQs to graphene oxide coated blade spray (CBS)-MS through a streptavidin-biotin (SA-biotin) interaction. The prepared platform had sufficient loading capacity for SA (1.37 mg/piece) and the antibody (84.8 μg/piece), which is greater than that of physical mixing and the EDC/NHS covalent coupling strategy. With simplified sample pretreatment, this platform demonstrated comparable sensitivity to high performance liquid chromatography-mass spectrometry (HPLC-MS/MS) (0.08-0.16 ng/mL in phosphate-buffered saline and 0.21-0.32 ng/mL in diluted milk). Meanwhile, compared with HPLC-MS/MS, the method is rapid (enrichment: 10 min, detection: <1 min) and acceptable recoveries (81.94-102.08%) can be obtained. The presence of analytes can be monitored by MS/MS spectra, and multiple analytes can be measured simultaneously in a single assay. This study is expected to provide a powerful and portable tool for rapid laboratory analysis and reliable screening in the field.

Development of alkaline phosphatase-scFv and its use for one-step enzyme-linked immunosorbent assay for His-tagged protein detection

A histidine (His)-tag is composed of six His residues and typically exerts little influence on the structure and solubility of expressed recombinant fusion proteins. Purification methods for recombinant proteins containing His-tags are relatively well-established, thus His-tags are widely used in protein recombination technology. We established a one-step enzyme-linked immunosorbent assay (ELISA) for His-tagged recombinant proteins. We analyzed variable heavy and light chains of the anti-His-tag monoclonal antibody 4C9 and used BLAST analyses to determine variable zones in light (VL) and heavy chains (VH). VH, VL, and alkaline phosphatase (ALP) regions were connected via a linker sequence and ligated into the pGEX-4T-1 expression vector. Different recombinant proteins with His tags were used to evaluate and detect ALP-scFv activity. Antigen and anti-His-scFv-ALP concentrations for direct ELISA were optimized using the checkerboard method. ZIKV-NS1, CHIKV-E2, SCRV-N, and other His-tag fusion proteins demonstrated specific reactions with anti-His-scFv-ALP, which were accurate and reproducible when the antigen concentration was 50 µg mL−1 and the antibody concentration was 6.25 µg mL−1. For competitive ELISA, we observed a good linear relationship when coating concentrations of recombinant human anti-Müllerian hormone (hAMH) were between 0.78 and 12.5 µg mL−1. Our direct ELISA method is simple, rapid, and accurate. The scFv antibody can be purified using a prokaryotic expression system, which provides uniform product quality and reduces variations between batches.

Single-chain fragment variable produced by phage display technology: Construction, selection, mutation, expression, and recent applications in food safety

Immunoassays are reliable, efficient, and accurate methods for the analysis of small-molecule harmful substances (such as pesticides, veterinary drugs, and biological toxins) that may be present in food. However, traditional polyclonal and monoclonal antibodies are limited by animal hosts and hinder further development of immunoassays. With the gradual application of phage display technology as an efficient in vitro selection technology, the single-chain fragment variable (scFv) now provides an exciting alternative to traditional antibodies. Efficiently constructed scFv source libraries and specifically designed biopanning schemes can now yield scFvs possessing specific recognition capabilities. A rational mutation strategy further enhances the affinity of scFv, and allows it to reach a level that cannot be achieved by immunization. Finally, appropriate prokaryotic expression measures ensure stable and efficient production of scFv. Therefore, when developing excellent scFvs, it is necessary to focus on three key aspects of this process that include screening, mutation, and expression. In this review, we analyze in detail the preparation and affinity improvement process for scFv and provide insights into the research progress and development trend of scFv-based immunoassay methods for monitoring small-molecule harmful substances.

Recognition elements based on the molecular biological techniques for detecting pesticides in food: A review

Excessive use of pesticides can cause contamination of the environment and agricultural products that are directly threatening human life and health. Therefore, in the process of food safety supervision, it is crucial to conduct sensitive and rapid detection of pesticide residues. The recognition element is the vital component of sensors and methods for fast testing pesticide residues in food. Improper recognition elements may lead to defects of testing methods, such as poor stability, low sensitivity, high economic costs, and waste of time. We can use the molecular biological technique to address these challenges as a good strategy for recognition element production and modification. Herein, we review the molecular biological methods of five specific recognition elements, including aptamers, genetic engineering antibodies, DNAzymes, genetically engineered enzymes, and whole-cell-based biosensors. In addition, the application of these identification elements combined with biosensor and immunoassay methods in actual detection was also discussed. The purpose of this review was to provide a valuable reference for further development of rapid detection methods for pesticide residues.

Dual-Selective Catalysis in Dephosphorylation Tuned by Hf6-Containing Metal-Organic Frameworks Mimicking Phosphatase

Selective dephosphorylation is full of great challenges in the field of biomimetic catalysis. To mimic the active sites of protein phosphatase, Hf-OH-Hf motif-containing metal-organic frameworks (MOFs) were obtained and structurally characterized, which are assembled from [Hf48Ni6] cubic nanocages and exhibit good stability in various solvents and acid/base solutions. Catalytic investigations suggest as-synthesized Hf-Ni and Hf-Ni-NH 2 display accurate type-selectivity (selectively catalyzed P-O rather than S-O or C-O bonds) and position-selectivity (selectively catalyzed phosphomonoesters over phosphodiesters) for the hydrolysis of phosphoesters. Reaction kinetic studies further revealed the high activity of the catalytic sites in these catalysts, and the unique catalytic selectivity and high activity are comparable to phosphatase. Additionally, these MOF catalysts possess good recursivity and hypotoxicity. Control experiments (including Hf- and Zr-based isomorphous MOFs) and theoretical calculations indicate that both triplet nickel and Hf6 clusters play significant roles in the unique binding site and favorable binding energy. To our knowledge, this is the first example of selective dephosphorylation through MOF catalysts as mimic enzymes, which paves a potential way for the development of specific therapeutic MOFs.

Preparation of a Broad-Spectrum Heterocyclic Aromatic Amines (HAAs) Antibody and Its Application in Detection of Eight HAAs in Heat Processed Meat

Heterocyclic aromatic amines (HAAs) are potential human mutagens and carcinogens mainly generated in heat-treated meat. In this work, a broad-spectrum HAAs antibody was prepared and used to develop an indirect competitive ELISA (ic-ELISA) for simultaneous determination of eight HAAs, including 2-amino-3-methylimidazo[4,5-f]quinoline (IQ), 2-amino-3,4-dimethylimidazo[4,5-f] quinoline (MeIQ), 2-amino-3-methylimidazo[4,5-f]quinoxaline (IQx), 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline (MeIQx), 2-amino-3,4,8-trimethylimidazo[4,5-f]quinoxaline (4,8-DiMeIQx), 2-amino-3,7,8-trimethylimidazo[4,5-f]quinoxaline (7,8-DiMeIQx), 2-amino-3,4,7,8-tetramethylimidazo[4,5-f]quinoxaline (4,7,8-TriMeIQx), and 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) in grilled and fried meat samples. The limit of detection (LOD, calculated as IC₁₀) and 50% inhibition concentration (IC₅₀) of ic-ELISA were 5.29 μg/L and 99.08 μg/L, respectively. The detection results of this ic-ELISA were in good agreement with the detection results of UPLC-MS/MS in real samples, which indicated that this ic-ELISA can be applied to detect the total content of eight HAAs in heat processed meat. Use of a broad-spectrum antibody is an efficient strategy in developing immunoassay for simultaneous measuring food risk factors with similar structure.

Toxicant substitutes in immunological assays for mycotoxins detection: A mini review

Recurring mycotoxins contamination has posedaseriousthreatto food safety worldwide. Competitive immunoassays are widely used techniques for high-throughput mycotoxins detection in agricultural products and foods. However, the inevitable introduction of mycotoxin conjugates produced by chemical conjugation usually results in complicated by-products, large batch errors and threats to operators and environment. Biologically derived surrogates of mycotoxin conjugates or mycotoxin standards are renewable immunoreagents. They can serve the same function as the responding counterparts in the immunoassays. The substitute-based immunoassays exhibit satisfactory sensitivity, pose less health threats to operators and environment, and contribute to the standardization of immunoassays for mycotoxins. This review focuses on the current applications of substitute-based immunoassays, clarifies their underlying mechanisms and provides a careful comparison. Challenges and future prospects are discussed.

Application of phage-display developed antibody and antigen substitutes in immunoassays for small molecule contaminants analysis: A mini-review

Toxic small molecule contaminants (SMCs) residues in food threaten human health. Immunoassays are popular and simple techniques for SMCs analysis. However, immunoassays based on polyclonal antibodies, monoclonal antibodies and chemosynthetic antigens have some defects, such as complicated preparation of antibodies, risk of toxic haptens using for antigen chemosynthesis and so on. Phage-display technique has been proven to be an attractive alternative approach to producing antibody and antigen substitutes of SMCs, and opened up new realms for developing immunoassays of SMCs. These substitutes contain five types, including anti-idiotypic recombinant antibody (AIdA), anti-immune complex peptide (AIcP), anti-immune complex recombinant antibody (AIcA) and anti-SMC recombinant antibody (anti-SMC RAb). In this review, the principle of immunoassays based on the five types of substitutes, as well as their application and advantages are summarized and discussed.

A novel and effective natural product-based immunodetection tool for TNT-like compounds

This study aimed to develop a fast and reliable protocol for Trinitrophenol-Tris(hydroxymethyl)aminomethane (TNP-Tris) detection applying a β-lactamase-fusion protein of choice, the natural product-based immunoreagent tool of competitive sensitivity developed herein for the first time. Since the fusion protein 11B3-scFv-β-lactamase is constructed from a scFv-antibody (11B3) linked to an enzyme (β-lactamase), the step related to the use of secondary antibody in the enzyme-linked immunosorbent assay (ELISA) is completely omitted. Indeed, this fusion protein itself serves both as binding mean of the antigen model and detecting agent, due to the presence of the naturally occurring enzyme. In such a way, it actually affords the one-step TNP-Tris detection reaching a promising LOD value of 45 ± 2 fmol or 157 ± 6 pg/mL. Taken all together, the current protocol does represent much cheaper and significantly less-time consuming alternative compared both to the recombinant antibodies and recombinant phages, previously designed means in our labs for the same purpose.

Rapid Multi-Residue Detection Methods for Pesticides and Veterinary Drugs

The excessive use or abuse of pesticides and veterinary drugs leads to residues in food, which can threaten human health. Therefore, there is an extremely urgent need for multi-analyte analysis techniques for the detection of pesticide and veterinary drug residues, which can be applied as screening techniques for food safety monitoring and detection. Recent developments related to rapid multi-residue detection methods for pesticide and veterinary drug residues are reviewed herein. Methods based on different recognition elements or the inherent characteristics of pesticides and veterinary drugs are described in detail. The preparation and application of three broadly specific recognition elements-antibodies, aptamers, and molecular imprinted polymers-are summarized. Furthermore, enzymatic inhibition-based sensors, near-infrared spectroscopy, and SERS spectroscopy based on the inherent characteristics are also discussed. The aim of this review is to provide a useful reference for the further development of rapid multi-analyte analysis of pesticide and veterinary drug residues.

Screening Antibody Libraries with Colony Assay Using scFv-Alkaline Phosphatase Fusion Proteins

Screening antibody libraries is an important step in establishing recombinant monoclonal antibodies. The colony assay can identify positive clones without almost any false-positives; however, its antibody library is smaller than those used in other recombinant screening methods such as phage display. Thus, to improve the efficiency of colony assays, it is necessary to increase library size per screening. Here, we report developing a colony assay with single-chain variable fragment (scFv) fused to the N-terminus of bacterial alkaline phosphatase (scFv-PhoA). The scFv-PhoA library was constructed in an expression vector specifically designed for this study. Use of this library allowed the successful and direct detection of positive clones exhibiting PhoA activity, without the need for a secondary antibody. Colony assay screening with scFv-PhoA is simple, rapid, offers a higher success rate than previous methods based on scFv libraries, and—most importantly—it enables high-throughput procedures.

One-Step Ultrasensitive Bioluminescent Enzyme Immunoassay Based on Nanobody/Nanoluciferase Fusion for Detection of Aflatoxin B1 in Cereal

Nanoluciferase (Nluc), the smallest luciferase known, was used as the fusion partner with a nanobody against aflatoxin B₁ to develop a bioluminescent enzyme immunoassay (BLEIA) for detection of the aflatoxin B₁ in cereal. Nanobody (clone G8) against aflatoxin B₁ was fused with nanoluciferase and cloned into a pET22b expression vector, and then transformed into Escherichia coli. The nanobody fusion gene contained a hexahistidine tag for purification by immobilized metal affinity chromatography, yielding a biologically active fusion protein. The fusion protein G8-Nluc retained binding properties of the original nanobody. Concentration of the coelenterazine substrate and buffer composition were also optimized to provide high intensity and long half-life of the luminescent signal. The G8-Nluc was used as a detection antibody to establish a competitive bioluminescent ELISA for the detection of aflatoxin B₁ in cereals successfully. Compared to classical ELISA, this novel assay showed more than 20-fold improvement in detection sensitivity, with an IC₅₀ value of 0.41 ng/mL and linear range from 0.10 to 1.64 ng/mL. In addition, the entire BLEIA detection procedure can be completed in one step within 2 h, from sample preparation to data analysis. These results suggest that nanobody fragments fused with nanoluciferase might serve as useful and highly sensitive dual functional reagents for the development of rapid and highly sensitive immunoanalytical methods.

Production and characterization of a monoclonal antibody for Pefloxacin and mechanism study of antibody recognition

In this report, an artificial antigen (PFLX–BSA: Pefloxacin connected bovine serum albumin) was successfully prepared. The monoclonal antibody against pefloxacin was produced and characterized using a direct competitive ELISA. The linear range of detection was 0.115–6.564 µg/L. The limit of detection defined as IC15 was 0.170 ± 0.05 µg/L and the IC50 was 0.902 ± 0.03 µg/L. The antibody variable region genes were amplified, assembled, and sequenced. A three–dimensional structural model of the variable region was constructed to study the mechanism of antibody recognition using molecular docking analysis. Three predicted essential amino acids, Thr53, Arg97 of heavy chain and Thr52 of light chain, were mutated to verify the theoretical model. Three mutants lost binding activity significantly against pefloxacin as predicted. These may provide useful insights for studying antigen–antibody interaction mechanisms to improve antibody affinity maturation in vitro.

Isolation of Bactrian Camel Single Domain Antibody for Parathion and Development of One-Step dc-FEIA Method Using VHH-Alkaline Phosphatase Fusion Protein

A heavy chain variable fragment of heavy chain only antibodies derived from camelids termed VHH shows beneficial characteristics for immunoassay in terms of high sensitivity, outstanding stability and ease in expression. In the present study, we isolated six VHHs from phage display library against parathion, which is a widely used organophosphorus pesticide with high toxicity and persistence. One of six selected VHHs named VHH9, showed highest specificity and superior thermo-stability. A VHH9-alkaline phosphatase (AP) fusion was constructed and used to establish a one-step direct competitive fluorescence enzyme immunoassay (dc-FEIA) with a half maximal inhibitory concentration (IC50) of 1.6 ng/mL and a limit of detection of 0.2 ng/mL which was 4-fold or 3-fold higher sensitivity than direct competitive enzyme-linked immunoassay (dc-ELISA) and indirect competitive enzyme-linked immunoassay (ic-ELISA) for parathion. Furthermore, our assay indicated a 50% reduction on operation time compared with the ic-ELISA method. The presented immunoassay was validated with spiked Chinese cabbage, cucumber, and lettuce samples, and confirmed by UPLC-MS/MS. The results indicated that the VHH-AP-based dc-FEIA is a reproducible detection assay for parathion residues in vegetable samples.

Development of a Highly Sensitive Direct Competitive Fluorescence Enzyme Immunoassay Based on a Nanobody-Alkaline Phosphatase Fusion Protein for Detection of 3-Phenoxybenzoic Acid in Urine

3-Phenoxybenzoic acid (3-PBA) is a human urinary metabolite of many pyrethroid insecticides and can be used as a biomarker to monitor human exposure to these pesticides. A rapid and sensitive direct competitive fluorescence enzyme immunoassay (dc-FEIA) for detecting 3-PBA on the basis of a nanobody (Nb)-alkaline phosphatase (AP) fusion protein was developed. The anti-3-PBA Nb-AP fusion protein was expressed and purified. The 50% inhibitory concentration (IC₅₀) and linear range of dc-FEIA were 0.082 and 0.015-0.447 ng/mL, respectively, with a detection limit of 0.011 ng/mL. The IC₅₀ of dc-FEIA was improved by nearly ten times compared with those of one-step and three-step direct competitive enzyme-linked immunosorbent assay (dc-ELISA). Spiked urine samples were detected by both dc-FEIA and liquid chromatography-mass spectrometry (LC-MS), and the results showed good consistency between the two analysis methods, indicating the reliability of dc-FEIA based on the Nb-AP fusion protein for detecting 3-PBA in urine.

Identification and characterisation of the proteins bound by specific phage-displayed recombinant antibodies (scFv) obtained against Brazil nut and almond extracts

BACKGROUND

Almonds and Brazil nuts are widely consumed allergenic nuts whose presence must be declared according to food labelling regulations. Their detection in food products has been recently achieved by ELISA methods with recombinant antibodies (scFv) isolated against complete Brazil nut and almond protein extracts. The screening of phage-scFv libraries against complete protein extracts confers a series of advantages over the use of purified proteins, as recombinant proteins might alter their native folding. However, using this strategy, the nature of the target detected by phage-displayed antibodies remains unknown, and requires further research to identify whether they are nut allergens or other molecules present in the extract, but not related to their allergenic potential.

RESULTS

Electrophoretic, chromatographic, immunological and spectrometric techniques revealed that the Brazil nut (BE95) and almond (PD1F6 and PD2C9) specific phage-scFvs detected conformational epitopes of the Brazil nut and almond 11S globulins, recognised by WHO/IUIS as Ber e 2 and Pru du 6 major allergens. Circular dichroism data indicated that severe heat treatment would entail loss of epitope structure, disabling scFv for target detection.

CONCLUSIONS

The presence of important Brazil nut and almond allergens (Ber e 2 and Pru du 6) in foodstuffs can be determined by using phage-display antibodies BE95, PD1F6 and PD2C9 as affinity probes in ELISA. © 2017 Society of Chemical Industry.

Cloning of scFv from hybridomas using a rational strategy: Application as a receptor to sensitive detection microcystin-LR in water

Single chain variable fragment (scFv), containing of heavy and light chains (VH and VL) joined by a short peptide linker, has been used widely for immunodetection. Nevertheless, cloning functional variable genes is still a bottle neck for the scFv generation technology. Here, a rational strategy for cloning and selecting variable region genes from an anti-microcystin-LR hybridoma was devised, then the functional VH and VL genes were recloned and assembled to scFv using splicing overlap extension PCR. The resulting scFv gene was recombinantly expressed as a soluble scFv-alkaline phosphatase fusion protein (scFv-AP) by vector PLIP6/GN. Then an indirect competitive chemiluminescent enzyme immunoassay (ic-CLEIA) for detection of microcystin-LR was developed. The half-maximum inhibition concentrations (IC50) and limits of detection (LODs, IC15) were 0.81 ± 0.04 μgL(-1) and 0.13 ± 0.03 μgL(-1), respectively. With the mean coefficient of variation lowing 8%, the mean recovery in intra-assay and inter-assay were 100.06% and 96.46%, The proposed strategy should be useful for generation scFv in a rapid and simple way.

Expression and Functional Properties of an Anti-Triazophos High-Affinity Single-Chain Variable Fragment Antibody with Specific Lambda Light Chain

Triazophos is a widely used organophosphorous insecticide that has potentially adverse effects to organisms. In the present study, a high-affinity single-chain variable fragment (scFv) antibody with specific lambda light chain was developed for residue monitoring. First, the specific variable regions were correctly amplified from a hybridoma cell line 8C10 that secreted monoclonal antibody (mAb) against triazophos. The regions were then assembled as scFv via splicing by overlap extension polymerase chain reaction. Subsequently, the recombinant anti-triazophos scFv-8C10 was successfully expressed in Escherichia coli strain HB2151 in soluble form, purified through immobilized metal ion affinity chromatography, and verified via Western blot and peptide mass fingerprinting analyses. Afterward, an indirect competitive enzyme-linked immunosorbent assay was established based on the purified anti-triazophos scFv-8C10 antibody. The assay exhibited properties similar to those based on the parent mAb, with a high sensitivity (IC50 of 1.73 ng/mL) to triazophos and no cross reaction for other organophosphorus pesticides; it was reliable in detecting triazophos residues in spiked water samples. Moreover, kinetic measurement using a surface plasmon resonance biosensor indicated that the purified scFv-8C10 antibody had a high affinity of 1.8 × 10(-10) M and exhibited good binding stability. Results indicated that the recombinant high-affinity scFv-8C10 antibody was an effective detection material that would be promising for monitoring triazophos residues in environment samples.

Development of an Immunoassay for Chloramphenicol Based on the Preparation of a Specific Single-Chain Variable Fragment Antibody

Specific antibodies are essential for the immune detection of small molecule contaminants. In the present study, the heavy and light variable regions (V(H )and V(L)) of the immunoglobulin genes from a hybridoma secreting a chloramphenicol (CAP)-specific monoclonal antibody (mAb) were cloned and sequenced. In addition, the light and heavy chains obtained from the monoclonal antibody were separated using SDS-PAGE and analyzed using Orbitrap mass spectrometry. The results of DNA sequencing and mass spectrometry analysis were compared, and the V(H) and V(L) chains specific for CAP were determined and used to construct a single-chain variable fragment (scFv). This fragment was recombinantly expressed as a soluble scFv-alkaline phosphatase fusion protein and used to develop a direct competitive ELISA. Compared with the parent mAb, scFv exhibits lower sensitivity but better food matrix resistance. This work highlights the application of engineered antibodies for CAP detection.

Production of a soluble single-chain variable fragment antibody against okadaic acid and exploration of its specific binding

Okadaic acid is a lipophilic marine algal toxin commonly responsible for diarrhetic shellfish poisoning (DSP). Outbreaks of DSP have been increasing and are of worldwide public health concern; therefore, there is a growing demand for more rapid, reliable, and economical analytical methods for the detection of this toxin. In this study, anti-okadaic acid single-chain variable fragment (scFv) genes were prepared by cloning heavy and light chain genes from hybridoma cells, followed by fusion of the chains via a linker peptide. An scFv-pLIP6/GN recombinant plasmid was constructed and transformed into Escherichia coli for expression, and the target scFv was identified with IC-CLEIA (chemiluminescent enzyme immunoassay). The IC15 was 0.012 ± 0.02 μg/L, and the IC50 was 0.25 ± 0.03 μg/L. The three-dimensional structure of the scFv was simulated with computer modeling, and okadaic acid was docked to the scFv model to obtain a putative structure of the binding complex. Two predicted critical amino acids, Ser32 and Thr187, were then mutated to verify this theoretical model. Both mutants exhibited significant loss of binding activity. These results help us to understand this specific scFv-antigen binding mechanism and provide guidance for affinity maturation of the antibody in vitro. The high-affinity scFv developed here also has potential for okadaic acid toxin detection.

Development of a nanobody-alkaline phosphatase fusion protein and its application in a highly sensitive direct competitive fluorescence enzyme immunoassay for detection of ochratoxin A in cereal

A rapid and sensitive direct competitive fluorescence enzyme immunoassay (dc-FEIA) for ochratoxin A (OTA) based on a nanobody (Nb)-alkaline phosphatase (AP) fusion protein was developed. The VHH (variable domain of heavy chain antibody) gene of Nb28 was subcloned into the expression vector pecan45 containing the AP double-mutant gene. The Nb28-AP construct was transformed into Escherichia coli BL21(DE3)plysS, and soluble expression in bacteria was confirmed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and Western blot. Both the Nb properties and AP enzymatic activity were validated by colorimetric and fluorometric analysis. The 50% inhibitory concentration and the detection limit of the dc-FEIA were 0.13 and 0.04 ng/mL, respectively, with a linear range of 0.06-0.43 ng/mL. This assay was compared with LC-MS/MS, and the results indicated the reliability of Nb-AP fusion protein-based dc-FEIA for monitoring OTA contamination in cereal.

Molecular modeling application on hapten epitope prediction: an enantioselective immunoassay for ofloxacin optical isomers

To deepen our understanding of the physiochemical principles that govern hapten-antibody recognition, ofloxacin enantiomers were chosen as a model for epitope prediction of small molecules. In this study, two monoclonal antibodies (mAbs) mAb-WR1 and mAb-MS1 were raised against R-ofloxacin and S-ofloxacin, respectively. The enantioselective mAbs have a high sensitivity and specificity, and the enantioselectivity is not affected by heterologous coating format reactions. The epitopes of the ofloxacin isomers were predicted using the hologram quantitative structure-activity relationship (HQSAR) and comparative molecular field analysis (CoMFA) approaches. The results consistently show that the epitope of the chiral hapten should be primarily composed of the oxazine ring and the piperazinyl ring and mAbs recognize the hapten from the side of this moiety. The enantioselectivity of mAbs is most likely due to the steric hindrance caused by the stereogenic center of the epitope. Modeling of chiral hapten-protein mimics reveals that ofloxacin isomers remain upright on the surface of the carrier protein. Suggestions to improve the enantioselectivity of antibodies against ofloxacin isomers were also proposed. This study provided a simple, efficient, and general method for predicting the epitopes of small molecules via molecular modeling. The epitope predictions for small molecules may create a theoretical guide for hapten design.