Inhibitor Screen Identifies Covalent Inhibitors of the Protein Histidine Phosphatase PHPT1

The protein histidine phosphatase PHPT1 is implicated in a variety of cellular signaling pathways. However, little is known about the precise biological roles of this enzyme and a dearth of chemical tools for studying histidine phosphorylation and dephosphorylation has hampered progress in the field. With the goal of identifying the first inhibitors of PHPT1 activity, we carried out an inhibitor screen using a facile fluorogenic assay for PHPT1 activity recently developed in our laboratory. From a panel of approximately 4000 compounds obtained from the Microsource Spectrum Collection and the NCI Diversity Set IV, we identified several potential hits with significant selectivity for inhibiting PHPT1 activity over other phosphatases. Of these, norstictic acid was the most potent inhibitor of PHPT1 activity with an IC₅₀ value of 7.9 ± 0.8 μM under our assay conditions. Norstictic acid is a time-dependent, covalent inhibitor of PHPT1 activity with K _I = 90 ± 20 μM and k _inact = 1.7 ± 0.1 min^-1.