Theoretical concepts of on-line liquid chromatographic-biochemical detection systems. II. Detection systems based on labelled affinity proteins

Fast Detection of 2,4,6-Trinitrotoluene (TNT) at ppt Level by a Laser-Induced Immunofluorometric Biosensor

The illegal use of explosives by terrorists and other criminals is an increasing issue in public spaces, such as airports, railway stations, highways, sports venues, theaters, and other large buildings. Security in these environments can be achieved by different means, including the installation of scanners and other analytical devices to detect ultra-small traces of explosives in a very short time-frame to be able to take action as early as possible to prevent the detonation of such devices. Unfortunately, an ideal explosive detection system still does not exist, which means that a compromise is needed in practice. Most detection devices lack the extreme analytical sensitivity, which is nevertheless necessary due to the low vapor pressure of nearly all explosives. In addition, the rate of false positives needs to be virtually zero, which is also very difficult to achieve. Here we present an immunosensor system based on kinetic competition, which is known to be very fast and may even overcome affinity limitation, which impairs the performance of many traditional competitive assays. This immunosensor consists of a monolithic glass column with a vast excess of immobilized hapten, which traps the fluorescently labeled antibody as long as no explosive is present. In the case of the explosive 2,4,6-trinitrotoluene (TNT), some binding sites of the antibody will be blocked, which leads to an immediate breakthrough of the labeled protein, detectable by highly sensitive laser-induced fluorescence with the help of a Peltier-cooled complementary metal-oxide-semiconductor (CMOS) camera. Liquid handling is performed with high-precision syringe pumps and chip-based mixing-devices and flow-cells. The system achieved limits of detection of 1 pM (1 ppt) of the fluorescent label and around 100 pM (20 ppt) of TNT. The total assay time is less than 8 min. A cross-reactivity test with 5000 pM solutions showed no signal by pentaerythritol tetranitrate (PETN), 1,3,5-trinitroperhydro-1,3,5-triazine (RDX), and octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX). This immunosensor belongs to the most sensitive and fastest detectors for TNT with no significant cross-reactivity by non-related compounds. The consumption of the labeled antibody is surprisingly low: 1 mg of the reagent would be sufficient for more than one year of continuous biosensor operation.

Simultaneous separation, identification and activity evaluation of three butyrylcholinesterase inhibitors from Plumula nelumbinis using on-line HPLC-UV coupled with ESI-IT-TOF-MS and BChE biochemical detection

We have firstly established a method of high-performance liquid chromatography-ultraviolet analysis coupled with electrospray ionization-ion trap-time-of-flight mass spectrometry and butyrylcholinesterase biochemical detection (HPLC-UV-ESI-IT-TOF-MS-BChEBCD). Applying this on-line method to the identification of BChE inhibitors in a Plumula nelumbinis sample, three alkaloids, namely liensinine, isoliensinine, and neferine, have been detected as having a strong BChE inhibition activity for the first time; in addition, norisoliensinine and 6-hydroxynorisoliensinine were proposed as two new compounds identified by their UV and MS data. The HPLC fingerprint, the MS fragments of the components, and the BChE activity profile could be simultaneously recorded during real-time analysis of complex samples using this on-line approach. Tacrine, a BChE inhibitor, was used as a positive reference compound, and its detection limit in the biochemical detection system was 1 nmol. The BChE activity of 1g of P. nelumbinis sample was equal to that of 127.88 μmol tacrine. The proposed on-line method has been validated as having good precision and reproducibility, and could be used to rapidly identify BChE inhibitors and to screen potential drugs for the treatment of Alzheimer's disease in complicated samples.

A unifying review of bioassay-guided fractionation, effect-directed analysis and related techniques

The success of modern methods in analytical chemistry sometimes obscures the problem that the ever increasing amount of analytical data does not necessarily give more insight of practical relevance. As alternative approaches, toxicity- and bioactivity-based assays can deliver valuable information about biological effects of complex materials in humans, other species or even ecosystems. However, the observed effects often cannot be clearly assigned to specific chemical compounds. In these cases, the establishment of an unambiguous cause-effect relationship is not possible. Effect-directed analysis tries to interconnect instrumental analytical techniques with a biological/biochemical entity, which identifies or isolates substances of biological relevance. Successful application has been demonstrated in many fields, either as proof-of-principle studies or even for complex samples. This review discusses the different approaches, advantages and limitations and finally shows some practical examples. The broad emergence of effect-directed analytical concepts might lead to a true paradigm shift in analytical chemistry, away from ever growing lists of chemical compounds. The connection of biological effects with the identification and quantification of molecular entities leads to relevant answers to many real life questions.

Development of on-line high performance liquid chromatography (HPLC)-biochemical detection methods as tools in the identification of bioactives

Biochemical detection (BCD) methods are commonly used to screen plant extracts for specific biological activities in batch assays. Traditionally, bioactives in the most active extracts were identified through time-consuming bio-assay guided fractionation until single active compounds could be isolated. Not only are isolation procedures often tedious, but they could also lead to artifact formation. On-line coupling of BCD assays to high performance liquid chromatography (HPLC) is gaining ground as a high resolution screening technique to overcome problems associated with pre-isolation by measuring the effects of compounds post-column directly after separation. To date, several on-line HPLC-BCD assays, applied to whole plant extracts and mixtures, have been published. In this review the focus will fall on enzyme-based, receptor-based and antioxidant assays.

Advances in mass spectrometry-based post-column bioaffinity profiling of mixtures

In the screening of complex mixtures, for example combinatorial libraries, natural extracts, and metabolic incubations, different approaches are used for integrated bioaffinity screening. Four major strategies can be used for screening of bioactive mixtures for protein targets-pre-column and post-column off-line, at-line, and on-line strategies. The focus of this review is on recent developments in post-column on-line screening, and the role of mass spectrometry (MS) in these systems. On-line screening systems integrate separation sciences, mass spectrometry, and biochemical methodology, enabling screening for active compounds in complex mixtures. There are three main variants of on-line MS based bioassays: the mass spectrometer is used for ligand identification only; the mass spectrometer is used for both ligand identification and bioassay readout; or MS detection is conducted in parallel with at-line microfractionation with off-line bioaffinity analysis. On the basis of the different fields of application of on-line screening, the principles are explained and their usefulness in the different fields of drug research is critically evaluated. Furthermore, off-line screening is discussed briefly with the on-line and at-line approaches.

Receptor–ligand binding assays: Technologies and Applications

Receptor-ligand interactions play a crucial role in biological systems and their measurement forms an important part of modern pharmaceutical development. Numerous assay formats are available that can be used to screen and quantify receptor ligands. In this review, we give an overview over both radioactive and non-radioactive assay technologies with emphasis on the latter. While radioreceptor assays are fast, easy to use and reproducible, their major disadvantage is that they are hazardous to human health, produce radioactive waste, require special laboratory conditions and are thus rather expensive on a large scale. This has led to the development of non-radioactive assays based on optical methods like fluorescence polarization, fluorescence resonance energy transfer or surface plasmon resonance. In light of their application in high-throughput screening environments, there has been an emphasis on so called "mix-and-measure" assays that do not require separation of bound from free ligand. The advent of recombinant production of receptors has contributed to the increased availability of specific assays and some aspects of the expression of recombinant receptors will be reviewed. Applications of receptor-ligand binding assays described in this review will relate to screening and the quantification of pharmaceuticals in biological matrices.

Enzyme amplification as detection tool in continuous-flow systems. I. Development of an enzyme-amplified biochemical detection system coupled on-line to flow-injection analysis

The on-line coupling of flow-injection analysis (FIA) to an enzyme-amplified biochemical detection (EA-BCD) system is described. The aim of this study is the development of a detection system able to detect biotin-containing compounds at low concentration levels. The detection system is based on the interaction of biotin with enzyme-labeled affinity proteins. Biotin possesses a high affinity to both streptavidin and anti-biotin Fab fragments, which are both tested. Several biotin derivatives are available with different reactive probes, which can be used to label analytes of interest. Therefore, biotin acts as a universal probe for the enzyme-amplified biochemical detection. Alkaline phosphatase (AP) was used as enzyme label. Several parameters, such as substrate type and concentration, concentration of enzyme-labeled affinity protein, reaction time and reaction temperature were examined. Biotin aminocaproic acid was used as a model compound. In addition to biotin aminocaproic hydrazide, other biotinylation reagents were also examined. With fluorescence detection of the enzyme-generated product, a mass detection limit of 1 fmol was achieved.

Theoretical concepts of on-line liquid chromatographic-biochemical detection systems. II. Detection systems based on labelled affinity proteins

A theoretical concept for on-line liquid chromatography-biochemical detection (LC-BCD) using labelled affinity proteins as reporter molecules is presented. The BCD system is based on the post-column addition of labelled affinity proteins such as fluorescein-labelled streptavidin to the LC effluent. After a short reaction time, free and analyte-bound label are separated during passage through a column packed with an immobilised-ligand support. The bound fraction passes the column unretained and is measured downstream by means of a conventional HPLC detector. The theoretical model presented here relates the detector response to the most important instrumental and biochemical parameters such as dispersion, reaction time, concentration and affinity of the affinity protein and the number of binding sites. The theoretical concept is validated using fluorescein-labelled streptavidin and biotin as model system.