SELDI-TOF-MS analysis of GSK-3β target peptides. (A) GSK-3β target peptide substrates and representative SELDI-TOF-MS analysis spectra. SELDI-TOF-MS representative spectra of peptides 2B-Sp, 2B-S, and 2B-A reveal a single m/z peak corresponding to their natural molecular weights (2063.2 Da., 1983.2 Da., and 1967.2 Da., respectively). (B) SELDI-TOF-MS spectra of target peptides subjected to a kinase assay with recombinant GSK-3β. Each target peptide substrate was incubated with 6.25 ng of recombinant GSK-3β enzyme, a magnesium/ATP kinase reaction mixture, and incubated for 20 minutes at 37°C. GSK-3β kinase activity was detected as an 80 Da. shift in peptide m/z ratio.