Assay Method Information

Assay Name:  Enzyme Assay
Description:  AXL enzyme inhibition (% inhibition, Kiapp and Ki values) by small molecule inhibitors was evaluated using a fluorescence-based microfluidic mobility shift assay. AXL catalyzes the production of ADP from ATP that accompanies the phosphoryl transfer to the substrate peptide FL-Peptide-30 (5-FAM-KKKKEEIYFFF-CONH2, CPC Scientific, Sunnyvale, Calif.). The mobility shift assay electrophoretically separates the fluorescently labeled peptides (substrate and phosphorylated product) following the kinase reaction. Both substrate and product are measured and the ratio of these values is used to generate % conversion of substrate to product by the LabChip EZ Reader. Human wild-type receptor tyrosine kinase protein Axl comprising residues 505-811 was produced in-house using the baculoviral expression vector system that incorporated a hexahistidine affinity tag into the protein (LJIC-1916B1.1). The enzyme was preactivated by auto-phosphorylation of 34 uM non-activated enzyme in the presence of 2 mM ATP, 4 mM MgCl2, 50 mM NaCl and 1 mM TCEP in 20 mM HEPES, pH 7.3 at 4° C. for 30 minutes. Typical reaction solutions (50 μL final reaction volume) contained 2% DMSO (±inhibitor), 10 mM MgCl2, 1 mM DTT, 120 μM ATP (ATP Km=70.4 μM), 0.01% Tween-20, 3 μM FL-Peptide-30, and 0.5 nM phosphorylated AXL enzyme in 100 mM HEPES buffer at pH 7.3. The assay was initiated with the addition of ATP, following a fifteen minutes pre-incubation of enzyme and inhibitor at room temperature in the reaction mixture. The reaction was stopped after 30 minutes at 25° C. by the addition of 50 μL of 200 mM EDTA, pH 7.5. The Ki values were determined from the fit of the data to the Morrison tight-binding competitive inhibition equation with the enzyme concentration as a variable. (See, Morrison, J. F. (1969) Kinetics of the reversible inhibition of enzyme-catalysed reactions by tight-binding inhibitors, Biochimica et biophysica acta 185, 269-286; and Murphy, D. J. (2004) Determination of accurate KI values for tight-binding enzyme inhibitors: an in silico study of experimental error and assay design, Analytical biochemistry 327, 61-67.)
Affinity data for this assay

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