Development of fluorescence-based selective assays for serine/threonine and tyrosine phosphatases

A number of aromatic substrates were evaluated for their ability to detect tyrosine phosphatase and serine/threonine phosphatase activity. Results demonstrated that the fluorinated coumarin DiFMUP is the most sensitive substrate for detecting LAR and PP-2A activity. Using this substrate, selective high-throughput screening assays for serine/threonine and tyrosine phosphatases were developed. Specific inhibitor cocktails were added to each assay to limit the activity of other phosphatases. LAR, CD-45, and PTP-1B all rapidly hydrolyze DiFMUP in the tyrosine phosphatase assay. The activity of non-tyrosine phosphatases is less than 6% of the LAR activity. PP-1 and PP-2A are highly active in the serine/threonine phosphatase assay. Inhibition of LAR and PP-2A in these assays is demonstrated using known inhibitors. Both of these assays are sensitive, robust, kinetic assays that can be used to quantify enzyme activity.