In Vitro Cell-Free Assay: Test compounds in aqueous solution were added at a volume of 10 microliters, to a reaction mixture comprising 10 microliters Assay Dilution Buffer (ADB; 20 mM MOPS, pH 7.2, 25 mM beta-glycerolphosphate, 5 mM EGTA, 1 mM sodium orthovanadate and 1 mM dithiothreitol), 10 microliters of substrate peptide (RRRDDDSDDD, dissolved in ADB at a concentration of 1 mM), 10 microliters of recombinant human CK2 (25 ng dissolved in ADB; Upstate). Reactions were initiated by the addition of 10 microliters of ATP Solution (90% 75 mM MgCl2, 75 micromolar ATP dissolved in ADB; 10% [gamma-33P]ATP (stock 1 mCi/100 ul; 3000 Ci/mmol (Perkin Elmer) and maintained for 10 minutes at 30 degrees C. The reactions were quenched with 100 microliters of 0.75% phosphoric acid, then transferred to and filtered through a phosphocellulose filter plate (Millipore). After washing each well 5 times with 0.75% phosphoric acid, the plate was dried under vacuum for 5 min.
Chemiluminescent PARP Assay: PARP assays are conducted using a chemiluminescent PARP assay kit (Trevigen). Briefly, reactions are performed in Histone-coated strip wells, by adding 10 microliters test compound dissolved in 1x PARP Buffer (prepared by mixing 20x PARP buffer diluted with high-purity water) and 15 microliters diluted PARP-HSA enzyme (diluted in 1x PARP buffer, 0.1 unit per well) to 25 microliters PARP cocktail (prepared from 10x PARP cocktail and 10x activated DNA, both 2.5 microliters per well and 20 microliters per well of 1x PARP buffer). The reactions are incubated at ambient temperature for 60 minutes, then the liquid was removed. After washing the wells four times with PBS (200 ul), 50 microliters of STREP-HRP (Horseradish Peroxidase) solution (diluted 500-fold in 1x Strep-Diluent) was added and the reactions were allowed to incubate for 30 minutes at ambient temperature.