High resolution screening of plant natural product extracts for estrogen receptor alpha and beta binding activity using an online HPLC-MS biochemical detection system

Comparison of analytical techniques for the identification of bioactive compounds from natural products

Covering: 2000 to 2016Natural product extracts are a rich source of bioactive compounds. As a result, the screening of natural products for the identification of novel biologically active metabolites has been an essential part of several drug discovery programs. It is estimated that more than 70% of all drugs approved from 1981 and 2006, were either derived from or structurally similar to nature based compounds indicating the necessity for the development of a rapid method for the identification of novel compounds from plant extracts. The screening of biological matrices for the identification of novel modulators is nevertheless still challenging. In this review we discuss current techniques in phytochemical analysis and the identification of biologically active components.

Development of Plate Reader and On-Line Microfluidic Screening to Identify Ligands of the 5-Hydroxytryptamine Binding Protein in Venoms

The 5-HT3 receptor is a ligand-gated ion channel, which is expressed in the nervous system. Its antagonists are used clinically for treatment of postoperative- and radiotherapy-induced emesis and irritable bowel syndrome. In order to better understand the structure and function of the 5-HT3 receptor, and to allow for compound screening at this receptor, recently a serotonin binding protein (5HTBP) was engineered with the Acetylcholine Binding Protein as template. In this study, a fluorescence enhancement assay for 5HTBP ligands was developed in plate-reader format and subsequently used in an on-line microfluidic format. Both assay types were validated using an existing radioligand binding assay. The on-line microfluidic assay was coupled to HPLC via a post-column split which allowed parallel coupling to a mass spectrometer to collect MS data. This high-resolution screening (HRS) system is well suitable for compound mixture analysis. As a proof of principle, the venoms of Dendroapsis polylepis, Pseudonaja affinis and Pseudonaja inframacula snakes were screened and the accurate masses of the found bioactives were established. To demonstrate the subsequent workflow towards structural identification of bioactive proteins and peptides, the partial amino acid sequence of one of the bioactives from the Pseudonaja affinis venom was determined using a bottom-up proteomics approach.

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.

Screening of tobacco smoke condensate for nicotinic acetylcholine receptor ligands using cellular membrane affinity chromatography columns and missing peak chromatography

This manuscript reports an approach to the screening of natural product extracts for compounds which are active at membrane-bound receptors, ion channels and transporters. The technique is based upon cellular membrane affinity chromatography (CMAC) columns created through the immobilization of cellular membrane fragments on liquid chromatography stationary phases. In this study a CMAC(nAChR(+)) column was created out of membranes from a transfected cell line expressing the alpha3beta4 neuronal nicotinic acetylcholine receptor (nAChR) and the column was used to screen tobacco smoke condensates. A strategy involving parallel screening with a CMAC column created from a non-transfected form of the same cell line, CMAC(nAChR(-)) was adopted. The condensate was chromatographed on both columns, timed fractions collected and concentrated. Each fraction was analyzed on a C18 column in order to establish a chromatographic fingerprint of each fraction and a differential elution profile of each compound. Comparison of the elution profiles from the CMAC(nAChR(+)) and CMAC(nAChR(-)) columns identified patterns that could be associated with high affinity ligands and with low-affinity/non-binding compounds. Known strong ligands ((S)-nicotine, (R,S)-anatabine, N'-nitrosonornicotine), weak ligands ((R,S)-nornicotine, anabasine) as well as known non-ligands (N-methyl-gamma-oxo-3-pyridinebutanamide, (1'S,2'S)-nicotine 1'-oxide) have been identified in the complex extract. The results demonstrate that CMAC-based screens can be used in the identification of compounds within natural product extracts that bind to membrane-based targets.

Reversed-phase liquid chromatography coupled on-line to estrogen receptor bioaffinity detection based on fluorescence polarization

We describe the development and validation of a high-resolution screening (HRS) platform which couples gradient reversed-phase high-performance liquid chromatography (RP-HPLC) on-line to estrogen receptor alpha (ERalpha) affinity detection using fluorescence polarization (FP). FP, which allows detection at high wavelengths, limits the occurrence of interference from the autofluorescence of test compounds in the bioassay. A fluorescein-labeled estradiol derivative (E2-F) was synthesized and a binding assay was optimized in platereader format. After subsequent optimization in flow-injection analysis (FIA) mode, the optimized parameters were translated to the on-line HRS bioassay. Proof of principle was demonstrated by separating a mixture of five compounds known to be estrogenic (17beta-estradiol, 17alpha-ethinylestradiol and the phytoestrogens coumestrol, coumarol and zearalenone), followed by post-column bioaffinity screening of the individual affinities for ERalpha. Using the HRS-based FP setup, we were able to screen affinities of off-line-generated metabolites of zearalenone for ERalpha. It is concluded that the on-line FP-based bioassay can be used to screen for the affinity of compounds without the disturbing occurrence of autofluorescence.

An on-line post-column detection system for the detection of reactive-oxygen-species-producing compounds and antioxidants in mixtures

Reactive oxygen species (ROS) can damage proteins, cause lipid peroxidation, and react with DNA, ultimately resulting in harmful effects. Antioxidants constitute one of the defense systems used to neutralize pro-oxidants. Since pro-oxidants and antioxidants are found ubiquitously in nature, pro-and antioxidant effects of individual compounds and of mixtures receive much attention in scientific research. A major bottleneck in these studies, however, is the identification of the individual pro-oxidants and antioxidants in mixtures. Here, we describe the development and validation of an on-line post-column biochemical detection system for ROS-producing compounds and antioxidants in mixtures. Inclusion of cytochrome P450s and cytochrome P450 reductase also permitted the screening of compounds that need bioactivation to exert their ROS-producing properties. This pro-oxidant and antioxidant detection system was integrated on-line with gradient HPLC. The resulting high-resolution screening technology was able to separate mixtures of ROS-producing compounds and antioxidants, allowing each species to be characterized rapidly and sensitively.

Development of three parallel cytochrome P450 enzyme affinity detection systems coupled on-line to gradient high-performance liquid chromatography

A high resolution screening (HRS) technology is described, in which gradient high-performance liquid chromatography (HPLC) is connected on-line to three parallel placed bioaffinity detection systems containing mammalian cytochromes P450 (P450s). The three so-called enzyme affinity detection (EAD) systems contained, respectively, liver microsomes from rats induced by beta-naphthoflavone (CYP1A activity), phenobarbital (CYP2B activity), and dexamethasone (CYP3A activity). Each P450-EAD system was optimized for enzyme, substrate, and organic modifier (isopropyl alcohol, methanol, and acetonitrile) in flow injection analysis mode. Characteristic P450 ligands were used to validate the P450-EAD systems. IC(50) values of the ligands were measured and found to be similar to those obtained with conventional microtiter plate reader assays. Detection limits (n = 3; signal-to-noise ratio = 3) of potent inhibitors ranged from 1 to 3 pmol for CYP1A activity, 4 to 17 pmol for CYP2B activity, and 4 to 15 pmol for CYP3A activity. The three optimized P450-EAD systems were subsequently coupled to gradient HPLC and used to screen compound mixtures for individual ligands. Finally, to increase analysis efficiency, a HRS system was constructed in which all three P450-EAD systems were coupled on-line and in parallel to gradient HPLC. The triple parallelized P450-EAD system was shown to enable rapid profiling of individual components in complex mixtures for inhibitory activity to three different P450s.

On-line Formation, Separation, and Estrogen Receptor Affinity Screening of Cytochrome P450-Derived Metabolites of Selective Estrogen Receptor Modulators

We have developed a fully automated bioreactor coupled to an on-line receptor affinity detection system. This analytical system provides detailed information on pharmacologically active metabolites of selective estrogen receptor modulators (SERMs) generated by cytochromes P450 (P450s). We demonstrated this novel concept by investigating the metabolic activation of tamoxifen and raloxifene by P450-containing pig and rat liver microsomes. The high resolution screening (HRS) system is based on the coupling of a P450-bioreactor to an HPLC-based estrogen receptor alpha (ERalpha) affinity assay. P450-derived metabolites of the SERMs were generated in the bioreactor, subsequently trapped on-line with solid phase extraction, and finally separated with gradient HPLC. Upon elution, the metabolites were screened on affinity for ERalpha with an on-line HRS assay. With this HRS system, we were able to follow, in a time-dependent manner, the formation of ERalpha-binding metabolites of tamoxifen and raloxifene. By analyzing the bioaffinity chromatograms with liquid chromatography-tandem mass spectrometry, structural information of the pharmacologically active metabolites was obtained as well. For tamoxifen, 15 active and 6 nonactive metabolites were observed, of which 5 were of primary, 10 of secondary, and 6 of an as yet unknown order of metabolism. Raloxifene was biotransformed in three primary and three secondary metabolites. MS/MS analysis revealed that three of the observed active metabolites of raloxifene were not described before. The present automated on-line HRS system coupled to a P450-containing bioreactor and an ERalpha-affinity detector proved very efficient, sensitive, and selective in metabolic profiling of SERMs.

Screening of natural products extracts for the presence of phosphodiesterase inhibitors using liquid chromatography coupled online to parallel biochemical detection and chemical characterization

The ability to rapidly identify active compounds in a complex mixture (e.g., natural products extract) is still one of the major problems in natural products screening programs. An elegant way to overcome this problem is to separate the complex mixture by gradient liquid chromatography followed by online biochemical detection parallel with chemical characterization, referred to as high-resolution screening (HRS). To find and identify phosphodiesterase (PDE) inhibitors in natural products extracts using the HRS technology, the authors developed a continuous-flow PDE enzymatic assay. The suitability of the continuous-flow PDE enzymatic assay for natural products screening was demonstrated. After optimization of the continuous-flow PDE assay, the limit of detection for 3-isobutyl-1-methyl-xanthine (IBMX) was 1 muM, with a dynamic range from 1 to 100 muM IBMX. The applicability of the HRS technology for the detection of PDE inhibitors in natural products extracts was demonstrated by the analysis of a plant extract spiked with 2 naturally occurring PDE inhibitors. The plant extract was analyzed with 2 assay lines in parallel, enabling background fluorescence correction of the sample. The simultaneous quantification of the active compounds using evaporative light-scattering detection allowed the estimation of the IC(50) value of the active compounds directly in the crude extract.

Kinetic analysis of estrogen receptor/ligand interactions

Surface plasmon resonance biosensor technology was used to directly measure the binding interactions of small molecules to the ligand-binding domain of human estrogen receptor. In a screening mode, specific ligands of the receptor were easily discerned from nonligands. In a high-resolution mode, the association and dissociation phase binding responses were shown to be reproducible and could be fit globally to a simple interaction model to extract reaction rate constants. On average, antagonist ligands (such as tamoxifen and nafoxidine) were observed to bind to the receptor with association rates that were 500-fold slower than agonists (such as estriol and beta-estradiol). This finding is consistent with these antagonists binding to an altered conformation of the receptor. The biosensor assay also could identify subtle differences in how the same ligand interacted with two different isoforms of the receptor (alpha and beta). The biosensor's ability to determine kinetic rate constants for small molecule/protein interactions provides unique opportunities to understand the mechanisms associated with complex formation as well as new information to drive the optimization of drug candidates.