Validation of positive compounds in the glutathione S-transferase (GST) pull-down assay. GST–GTP cyclohydrolase feedback regulatory protein (GFRP) was bound to glutathione agarose beads, and then His-tagged GTP cyclohydrolase 1 (GTPCH-1) was added. These experiments were performed in the presence of various concentrations of compounds identified by initial high-throughput screening (HTS) of the Library of Pharmacologically Active Compounds (LOPAC) library. The Western blot in the lower panel shows the amount of GFRP used as “bait,” whereas the “trapped” GTPCH-1 is shown in the upper blot. The disappearance of a band in the upper panel indicates disruption of GTPCH/GFRP binding. Panels A, B, and C show the representative GST pull-down assay testing the potential positive compounds. Panels D, E, and F show the representative dose-response curves determined from the densitometry of the GST pull-down assays.