Competitive Assay: The PI3 Kinase Activity/Inhibitor Assay is a competitive assay used for the fast and sensitive quantitation of activity of the four class I PI3 kinases (p110 α, β, γ, δ). The PI3 Kinase Activity/Inhibitor Assay works on the principle that PI3 Kinase phosphorylates P1(3,4)P2 (PIP2), converting it to PI(3,4,5)P3 (PIP3). The PH domain of the protein GRP-1 binds PIP3 with high affinity and specificity. The kit includes this recombinant protein that is used as the capture protein. This protein binds to the glutathione plate and captures either the PIP3 generated as part of the kinase reaction or the biotinylated-PIP3 tracer included in the kit. The captured biotinylated-PIP3 is detected using streptavidin-HRP conjugate and a colorimetric read out (OD 450). The lower the signal, the higher the PI3 Kinase activity.
Enzyme Assay: The test uses a lucifeurerin/luciferase system to measure the concentration of ATP and its consumption during the enzymatic reaction. The test is performed in 96-well format (Corning/Costar 96 black flat-bottomed half-wells plate, ref. 3694) in a total volume of 30 μl. To 1 μl of inhibitor in 100% DMSO are added (final concentrations) 50 μM of the substrate PIP2 ((L-α-phosphatidyl-D-myoinositol 4,5-bisphosphate, Echelon 117P-4516-0500), 2 μM of ATP and 1.7 μg/ml of PI3Kα, (p110α/p85α, Invitrogen PV4788) in a buffer of Tris/HCl 50 mM pH 7.5, EGTA 1 mM, MgCl2 10 mM, Chaps 0.03%, 1 mM DTT). After 90 minutes, the reaction is quenched by adding 20 μl/well of KinaseGlo reagent (Promega V6713). After 10 minutes in the dark, the luminescence is read on the PHERAStar microplate reader (reading at 0.8 sec/well).
Inhibition Assay: PI3 kinases catalyse the phosphorylation of phosphatidylinositol 4,5-biphosphate (PIP2) to phosphatidylinositol 3,4,5-triphosphate (PIP3) in the presence of ATP and Mg2+ ions. The PIP3 product can be detected by displacement of biotin-PIP3 from energy transfer complexes consisting of europium labelled anti-GST monoclonal antibody, a GST-tagged Pleckstrin homology (PH) domain, biotinylated PIP3 and streptavidin-allophycocyanin (APC) by the time-resolved fluorescence resonance energy transfer (TR-FRET) (HTRF® PI3K enzyme assay, Millipore). Excitation (330 nm) of europium in the complex results in an energy transfer to the APC and a fluorescent emission at 665 nm although europium itself emits at its characteristic 620 nm. The PIP3 product formed by PI3K activity displaces biotin-PIP3 from the complex and results in a loss of energy transfer (decreasing signal). The compound to be tested was added, at the desired final concentrations, to a mixture of PIP2 substrate and recombinant PI3 kinase α, δ or γ enzymes (Millipore), and the mixture incubated for 2 hr at RT. Following this incubation period, ATP (20 μM) was added to the enzyme/compound/PIP2 substrate mixture and the resulting mixture was incubated for 30 min at RT. A stopping solution containing biotinylated PIP3 and the detection mix containing the GST tagged GRP1 pleckstrin homology (PH) domain and fluorophores were then added and the mixture was incubated at RT for 15-18 hr, prior to detection in a fluorescence microplate reader (Varioskan® Flash, ThermoFisher Scientific).