Use of L-lysine fluorescence derivatives as tracers to enhance the performance of polarization fluoroimmunoassays. A study using two herbicides as model antigens

Heterologous strategy enhancing the sensitivity of the fluorescence polarization immunoassay of clinafloxacin in goat milk

BACKGROUND

Clinafloxacin is used for the treatment of disease in food-producing animals, e.g. Brucella melitensis, which often occurs in goats; however, the clinafloxacin residue in goat milk may harm human health and result in the development of drug-resistant bacterial strains or allergies. Despite this, there is not a rapid, sensitive and accurate analytical method in goat milk for rapid screening or monitoring purposes.

RESULTS

One homologous and five heterologous tracers were designed and compared for fluorescence polarization immunoassay (FPIA) optimization. Based on the combination of a heterologous tracer (PAZ-FITC, synthesized with pazufloxacin and FITC) and the antibody against clinafloxacin, a highly sensitive FPIA was established for the detection of clinafloxacin residue in goat milk for the first time. The IC50 value was 29.3 µg L(-1) for clinafloxacin in the heterologous format - six times lower than that of the combination of the homologous tracers and the antibody. The recoveries ranged from 86.8% to 104.5%, with the relative standard deviation ranging from 4.1% to 7.2%. Validation by high-performance liquid chromatography (HPLC) confirmed that the results obtained from the proposed FPIA were in agreement with those of HPLC.

CONCLUSION

This proposed heterologous strategy for enhanced FPIA is sensitive and rapid enough for the high-throughput detection of clinafloxacin residue in goat milk.

Novel mutations and mutation combinations of ryanodine receptor in a chlorantraniliprole resistant population of Plutella xylostella (L.)

A previous study documented a glycine to glutamic acid mutation (G4946E) in ryanodine receptor (RyR) was highly correlated to diamide insecticide resistance in field populations of Plutella xylostella (Lepidoptera: Plutellidae). In this study, a field population collected in Yunnan province, China, exhibited a 2128-fold resistance to chlorantraniliprole. Sequence comparison between resistant and susceptible P. xylostella revealed three novel mutations including a glutamic acid to valine substitution (E1338D), a glutamine to leucine substitution (Q4594L) and an isoleucine to methionine substitution (I4790M) in highly conserved regions of RyR. Frequency analysis of all four mutations in this field population showed that the three new mutations showed a high frequency of 100%, while the G4946E had a frequency of 20%. Furthermore, the florescent ligand binding assay revealed that the RyR containing multiple mutations displayed a significantly lower affinity to the chlorantraniliprole. The combined results suggested that the co-existence of different combinations of the four mutations was involved in the chlorantraniliprole resistance. An allele-specific PCR based method was developed for the diagnosis of the four mutations in the field populations of P. xylostella.

Functional analysis of a point mutation in the ryanodine receptor of Plutella xylostella (L.) associated with resistance to chlorantraniliprole

BACKGROUND

The diamondback moth, Plutella xylostella (L.) has developed extremely high resistance to chlorantraniliprole and other diamide insecticides in the field. A glycine to glutamic acid substitution (G4946E) in the P. xylostella ryanodine receptor (PxRyR) has been found in two resistant populations collected in Thailand and Philippines and was considered associated with the diamide insecticides resistance but no experimental evidence was provided. The present study aimed to clarify the function of the reported mutation in chlorantraniliprole resistance in P. xylostella.

RESULTS

We identified the same mutation (G4946E) in PxRyR from four field collected chlorantraniliprole resistant populations of Plutella xylostella in China. Most importantly, we found that the frequency of the G4946E mutation is significantly correlated to the chlorantraniliprole resistance ratios in P. xylostella (R(2)  = 0.82, P = 0.0003). Ligand binding assays showed that the binding affinities of the PxRyR to the chlorantraniliprole in three field resistant populations were 2.41-, 2.54- and 2.60-times lower than that in the susceptible one.

CONCLUSION

For the first time we experimentally proved that the G4946E mutation in PxRyR confers resistance to chlorantraniliprole in Plutella xylostella. These findings pave the way for the complete understanding of the mechanisms of diamide insecticides resistance in insects.

Pushing antibody-based labeling systems to higher sensitivity by linker-assisted affinity enhancement

The sensitivity of antibody/hapten-based labeling systems is limited by the natural affinity ceiling of immunoglobulins. Breaking this limit by antibody engineering is difficult. We thus attempted a different approach and investigated if the so-called bridge effect, a corecognition of the linker present between hapten and carrier protein during antibody generation, can be utilized to improve the affinity of such labeling systems. The well-known haptens 2,4-dinitrophenol (2,4-DNP) and 2,4-dichlorophenoxyacetic acid (2,4-D) were equipped with various linkers, and the resulting affinity change of their cognate antibodies was analyzed by ELISA. Anti-2,4-DNP antibodies exhibited the best affinity to their hapten when it was combined with aminobutanoic acid or aminohexanoic acid. The affinity of anti-2,4-D antibodies could be enhanced even further with longer aliphatic spacers connected to the hapten. The affinity toward aminoundecanoic acid-2,4-D derivatives, for instance, was improved about 100-fold compared to 2,4-D alone and yielded detection limits as low as 100 amoles of analyte. As the effect occurred for all antibodies and haptens tested, it may be sensible to implement the bridge effect in future antibody/hapten-labeling systems in order to achieve the highest sensitivity possible.

Hapten synthesis and monoclonal antibody-based immunoassay development for detection of the fungicide trifloxystrobin

High-affinity and selective monoclonal antibodies have been produced against the strobilurin fungicide trifloxystrobin. A battery of functionalized haptens has been synthesized, and conjugate-coated enzyme-linked immunosorbent assays following different procedures have been developed. On the one hand, a two-step conjugate-coated immunoassay was optimized using extended or short incubation times, with limits of detection of 0.10 ng/mL for the extended assay and 0.17 ng/mL for the rapid assay. On the other hand, an immunoassay in the conjugate-coated format was optimized following a procedure consisting of just one incubation step. This one-step assay had a limit of detection of 0.21 ng/mL. All of these assays showed detection limits for trifloxystrobin in the low parts per billion range, well below the common maximum residue limits for this pesticide in foodstuffs (50 microg/kg).

Hapten synthesis and monoclonal antibody-based immunoassay development for the detection of the fungicide kresoxim-methyl

Strobilurin fungicides have been increasingly used for fungus pest control since they were introduced in 1996. For pesticide residue detection, immunoassays constitute nowadays a valuable approach. This paper describes the synthesis of functionalized haptens of kresoxim-methyl, the production of monoclonal antibodies, and the development of enzyme-linked immunosorbent assays. On the one hand, a two-step conjugate-coated immunoassay was optimized using extended or short incubation times, with limits of detection of 0.4 ng/mL for the extended assay and 0.3 ng/mL for the rapid assay. On the other hand, an immunoassay was optimized following a procedure consisting of just one incubation step. This one-step assay had a limit of detection of 0.4 ng/mL. All of these assays showed a similar performance, with sensitivities well below common maximum residue limits for this pesticide (50 microg/kg) and lower than the detection limits of the usual chromatographic detection methods.

Fluorescence polarization immunoassays and related methods for simple, high-throughput screening of small molecules

Fluorescence polarization immunoassay (FPIA) is a homogeneous (without separation) competitive immunoassay method based on the increase in fluorescence polarization (FP) of fluorescent-labeled small antigens when bound by specific antibody. The minimum detectable quantity of FPIAs with fluorescein label (about 0.1 ng analyte) is comparable with chromatography and ELISA methods, although this may be limited by sample matrix interference. Because of its simplicity and speed, FPIA is readily automated and therefore suitable for high-throughput screening (HTS) in a variety of application areas. Systems that involve binding of ligands to receptor proteins are also susceptible to analysis by analogous FP methods employing fluorescent-labeled ligand and HTS applications have been developed, notably for use in candidate drug screening.

Monoclonal antibody-based fluorescence polarization immunoassay for sulfamethoxypyridazine and sulfachloropyridazine

In this paper, a new monoclonal antibody (Mab) against sulfamethoxypyridazine (SMP) was produced, and a fluorescence polarization immunoassay (FPIA) based on the produced Mab was developed and optimized for the qualitative screening analysis of SMP. The Mab was raised from mice immunized with SMP linked to bovine serum albumin (BSA) by carbodiimide activated ester formation, using a succinic anhydride spacer molecule between SMP and BSA. Fluorescein labeled sulfachloropyridazine (SCP) and SMP (tracer) were synthesized and purified by thin layer chromatography (TLC). The developed screening FPIA method can tolerate up to 20% methanol, and satisfactory assay sensitivity can be obtained between pH 4 and pH 8 and at lower salt concentration. The anti-SMP Mab exhibited a high cross-reactivity with SCP. The effect of the tracer structure on the analytical characteristic of the determination and on antigen-antibody binding constants was studied. The limits of detection (LOD) were 0.7 ng/mL for SMP and 0.25 ng/mL for SCP in buffer, respectively, whereas negligible cross-reactivities were exhibited by related sulfonamides. Analysis of SMP and SCP-fortified milk samples by the FPIA showed average recoveries from 60 to 145%.

A fluorescence polarisation molecular imprint sorbent assay for 2,4-D: a non-separation pseudo-immunoassay

The first pseudo-immunoassay which employs a molecularly imprinted receptor and a fluorescent probe, and quantifies the bound analyte directly using the fluorescence anisotropy of the polymer-probe-analyte suspension, is described.

Microfluidic enzyme immunosensors with immobilised protein A and G using chemiluminescence detection

Affinity proteins were covalently immobilised on silicon microchips with overall dimensions of 13.1 x 3.2 mm, comprising 42 porous flow channels of 235 microm depth and 25 microm width, and used to develop microfluidic immunosensors based on horseradish peroxidase (HRP), catalysing the chemiluminescent oxidation of luminol/p-iodophenol (PIP). Different hydrophilic polymers with long flexible chains (polyethylenimine (PEI), dextran (DEX), polyvinyl alcohol, aminodextran) and 3-aminopropyltriethoxysilane (APTS) were employed for modification of the silica surfaces followed by attachment of protein A or G. The resulting immunosensors were compared in an affinity capture assay format, where the competition between the labelled antigen and the analyte for antibody-binding sites took place in the bulk of the solution. The formed immunocomplexes were then trapped by the microchip affinity capture support and the amount of bound tracer was monitored by injection of luminol, PIP and H2O2. All immunosensors were capable of detecting atrazine at the sub-microg l(-1) level. The most sensitive assays were obtained with PEI and DEX polymer modified supports and immobilised protein G, with limits of detection of 0.006 and 0.010 microg l(-1), and IC50 values of 0.096 and 0.130 microg l(-1), respectively. The protein G based immunosensors were regenerated with 0.4 M glycine-HCl buffer pH 2.2, with no loss of activity observed for a storage and operating period of over 8 months. To estimate the applicability of the immunosensors to the analysis of real samples, PEI and DEX based protein G microchips were used to detect atrazine in surface water and fruit juice, spiked with known amounts of the atrazine, giving recovery values of 87-102 and 88-124% at atrazine fortification levels of 0.5-3 and 80-240 microg l(-1), respectively.