GSK-3β activity in immunoprecipitated NIH 3T3 cell lysates determined by SELDI. GSK-3β activity was determined as described in the text from immunoprecipitated NIH 3T3 cell lysates. SELDI-TOF-MS representative spectra displaying the mass/charge ratio (m/z) and signal intensities for the 2B-Sp, 2B-S and 2B-A peptides are shown. The mass change from 2063.2 Da. to 2143.3 Da. in the 2B-Sp chip is consistent with the addition of one phosphate group (80 Da.). This experiment was replicated three times and the area under the 3143.3 Da. peak was quantitated relative to the area under the 2063.2 Da. peak indicating 23.0% +/- 3.5% phosphorylation of the 2B-Sp peptide.