A homogeneous assay of kinase activity that detects phosphopeptide using fluorescence polarization and zinc

Enzyme Inhibitor Screening Using a Homogeneous Proximity-Based Immunoassay for Estradiol

The authors have previously reported a homogeneous time-resolved fluorescence proximity immunoassay for estradiol. The assay was based on luminescence resonance energy transfer between a long lifetime fluorescent europium(III) chelate-dyed nanoparticle donor and a short lifetime, near-infrared fluorescent acceptor. The energy transfer prolonged the lifetime of the sensitized acceptor emission, and the fluorescence of the acceptor was measured using a time-resolved detection. The developed immunoassay was employed to screen inhibitors for enzyme 17β-hydroxysteroid dehydrogenase type 1. The enzyme overexpressed in MCF-7 cells catalyzed a reversible conversion of estroneto17β-estradiol. The inhibition efficiency of the tested molecule was obtained by comparing the final concentration of converted estradiol after 60 min of conversion reaction in a sample and in a conversion control not containing an inhibitor. The Zβ factor calculated using the E2 concentrations of the homogeneous assay was 0.64, demonstrating a relatively good performance of the assay. The results from the homogeneous assay were comparable with the results obtained using radioactively labeled estrone as a substrate and high-performance liquid chromatography (HPLC) separation of estrone and converted estradiol after the enzyme reaction. Thus, this homogeneous assay can simplify the primary screening of potential new drug molecules by replacing a tedious radiometric HPLC method.

Homogeneous single-label tyrosine kinase activity assay for high throughput screening

Protein post-translational modifications (PTMs) are regulatory mechanisms carried out by different enzymes in a cell. Kinase catalyzed phosphorylation is one of the most important PTM affecting the protein activity and function. We have developed a single-label quenching resonance energy transfer (QRET) assay to monitor tyrosine phosphorylation in a homogeneous high throughput compatible format. Epidermal growth factor receptor (EGFR) induced phosphorylation was monitored using Eu(3+)-chelate labeled peptide and label-free phosphotyrosine specific antibody in presence of a soluble quencher molecule. In the QRET kinase assay, antibody binding to phosphorylated Eu(3+)-peptide protects the Eu(3+)-chelate from luminescence quenching, monitoring high time-resolved luminescence (TRL) signals. In the presence of specific kinase inhibitor, antibody recognition and Eu(3+)-chelate protection is prevented, allowing an efficient luminescence quenching. The assay functionality was demonstrated with a panel of EGFR inhibitors (AG-1478, compound 56, erlotinib, PD174265, and staurosporine). The monitored IC50 values ranged from 0.08 to 155.3 nM and were comparable to those found in the literature. EGFR activity and inhibition assays were performed using low nanomolar enzyme and antibody concentration in a 384-well plate format, demonstrating its compatibility for high throughput screening (HTS).

A universal competitive fluorescence polarization activity assay for S-adenosylmethionine utilizing methyltransferases

A high-throughput, competitive fluorescence polarization immunoassay has been developed for the detection of methyltransferase activity. The assay was designed to detect S-adenosylhomocysteine (AdoHcy), a product of all S-adenosylmethionine (AdoMet)-utilizing methyltransferase reactions. We employed commercially available anti-AdoHcy antibody and fluorescein-AdoHcy conjugate tracer to measure AdoHcy generated as a result of methyltransferase activity. AdoHcy competes with tracer in the antibody/tracer complex. The release of tracer results in a decrease in fluorescence polarization. Under optimized conditions, AdoHcy and AdoMet titrations demonstrated that the antibody had more than a 150-fold preference for binding AdoHcy relative to AdoMet. Mock methyltransferase reactions using both AdoHcy and AdoMet indicated that the assay tolerated 1 to 3 microM AdoMet. The limit of detection was approximately 5 nM (0.15 pmol) AdoHcy in the presence of 3 muM AdoMet. To validate the assay's ability to quantitate methyltransferase activity, the methyltransferase catechol-O-methyltransferase (COMT) and a known selective inhibitor of COMT activity were used in proof-of-principle experiments. A time- and enzyme concentration-dependent decrease in fluorescence polarization was observed in the COMT assay that was developed. The IC(50) value obtained using a selective COMT inhibitor was consistent with previously published data. Thus, this sensitive and homogeneous assay is amenable for screening compounds for inhibitors of methyltransferase activity.

Development of a peptide inhibitor-based cantilever sensor assay for cyclic adenosine monophosphate-dependent protein kinase

A highly sensitive nanomechanical cantilever sensor assay based on an electrical measurement has been developed for detecting activated cyclic adenosine monophosphate (cyclic AMP)-dependent protein kinase (PKA). Employing a peptide derived from the heat-stable protein kinase inhibitor (PKI), a magnetic bead system was first selected as a vehicle to immobilize the PKI-(5-24) peptide for capturing PKA catalytic subunit and the activity assay was applied for indirectly assessing the binding. Synergistic interactions of adenosine triphosphate (ATP) and the peptide inhibitor with the kinase were then investigated by a solution phase capillary electrophoretic assay, and by surface plasmon resonance technology which involved immobilization of the peptide inhibitor. After systemically evaluated by a homogeneous direct binding assay, the ATP-dependent recognition of the catalytic subunit of PKA by PKI-(5-24) was successfully transferred on to the nanomechanical cantilevers at protein concentrations of 6.6 pM-66 nM, exhibiting much higher sensitivity and wider dynamic range than the conventional activity assay. Thus, direct assessment of activated kinases using the cantilever sensor system functionalized with specific peptide inhibitors holds great promise in analytical applications and clinical medicine.

Microarrays of peptides elevated on the protein layer for efficient protein kinase assay

Peptide microarrays can be used for the high-throughput analysis of protein-peptide interactions. However, current peptide microarrays are rather costly to make and require cumbersome steps of introducing novel polymeric surfaces and/or chemical derivatization of peptides. Here, we report a novel method for manufacturing peptide microarrays by elevating the peptide on the layer of protein by a fusion protein approach. Using two protein kinases and their peptide substrates as examples, we show that elevating peptides on the layer of protein allows sensitive, specific, and efficient detection of peptide-protein interactions without the need for complicated chemical modification of solid supports and peptides. It was found that kinase activity could be detected with as low as 0.09 fmol of kemptide, which is about 1000-fold more sensitive than the 0.1 pmol obtained with other microarray systems. Furthermore, peptides can be produced as fusion proteins by fermentation of recombinant Escherichia coli and thus the expensive peptide synthesis process can be avoided. Therefore, this new strategy will not only be useful in high-throughput and cost-effective screening of kinase substrate peptides but also be generally applicable in studying various protein-peptide interactions.