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Assay Method Information

Assay Name:  Bub1 Kinase Assay
Description:  Bub1-inhibitory activities of compounds described in the present invention were quantified using a time-resolved fluorescence energy transfer (TR-FRET) kinase assay which measures phosphorylation of the synthetic peptide Blotin-AhxVLLPKKSFAEPG (C-terminus in amide form), purchased from e.g. Biosyntan (Berlin, Germany) by the (recombinant) catalytic domain of human Bub1 (amino acids 704-1085), expressed in HI5 insect cells with an N-terminal HIs6-tag and purified by affinity-(Ni-NTA) and size exclusion chromatography.In a typical assay 11 different concentrations of each compound (0.1 nM, 0.33 nM, 1.1 nM, 3.8 nM, 13 nM, 44 nM, 0.15 μM, 0.51 μM, 1.7 μM, 5.9 μM and 20 μM) were tested in duplicate within the same microtiter plate. To this end, 100-fold concentrated compound solutions (in DMSO) were previously prepared by serial dilution (1:3.4) of 2 mM stocks in a clear low volume 384-well source microtiter plate (Greiner Bio-One, Frickenhausen, Germany), from which 50 nl of compounds were transferred into a black low volume test microtiter plate from the same supplier. Subsequently, 2 μl of Bub1 (the final concentration of Bub1 was adjusted depending on the activity of the enzyme lot in order to be within the linear dynamic range of the assay: typically 200 μg/ml were used) in aqueous assay buffer [50 mM Tris/HCl pH 7.5, 10 mM magnesium chloride (MgCl2), 200 mM potassium chloride (KCl), 1.0 mM dithiothreitol (DTT), 0.1 mM sodium ortho-vanadate, 1% (v/v) glycerol, 0.01% (w/v) bovine serum albumine (BSA), 0.005% (v/v) Trition X-100 (Sigma), 1 Complete EDTA-free protease inhibitor mixture (Roche)] were added to the compounds in the test plate and the mixture was incubated for 15 min at 229 to allow pre-equilibration of the putative enzyme-inhibitor complexes before the start of the kinase reaction, which was initiated by the addition of 3 μl 1.67-fold concentrated solution (in assay buffer) of adenosine-tri-phosphate (ATP, 10 μM final concentration) and peptide substrate (1 μM final concentration). The resulting mixture (5 μl final volume) was incubated at 22 C. during 60 min., and the reaction was stopped by the addition of 5 μl of an aqueous EDTA-solution (50 mM EDTA, in 100 mM HEPES pH 7.5 and 0.2% (w/v) bovine serum albumin) which also contained the TR-FRET detection reagents (0.2 μM streptavidin-XL665 [Cisbio Bioassays, Codolet, France] and 1 nM anti-phosho-Serine antibody [Merck Millipore, cat. #35-001] and 0.4 nM LANCE EU-W1024 labeled anti-mouse IgG antibody [Perkin-Elmer, product no. AD0077, alternatively a Terbium-cryptate-labeled anti-mouse IgG antibody from Cisbio Bioassays can be used]). The stopped reaction mixture was further incubated 1 h at 22 C. in order to allow the formation of complexes between peptides and detection reagents. Subsequently, the amount of product was evaluated by measurement of the resonance energy transfer from the Eu-chelate-antlbody complex recognizing the Phosphoserlne residue to the streptavidin-XL665 bound to the biotin moiety of the peptide. To this end, the fluorescence emissions at 620 nm and 665 nm after excitation at 330-350 nm were measured in a TR-FRET plate reader, e.g. a Rubystar or Pherastar (both from BMG Labtechnologies, Offenburg, Germany) or a Viewlux (Perkin-Elmer) and the ratio of the emissions (665 nm/622 nm) was taken as indicator for the amount of phosphorylated substrate. The data were normalised using two sets of (typically 32-) control wells for high-(=enzyme reaction without Inhibitor=0%=Minimum inhibition) and low-(=all assay components without enzyme=100%=Maximum inhibition) Bub1 activity. IC50 values were calculated by fitting the normalized inhibition data to a 4-parameter logistic equation (Minimum, Maximum, IC50, Hill; Y=Max+(Min−Max)/(1+(X/IC50)Hill)) using a Bayer-proprietary analysis software.
Affinity data for this assay
 

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Last update November 1, 2007
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