Novel caged fluorescein diphosphates as photoactivatable substrates for protein tyrosine phosphatases

We have characterized some novel caged fluorescein diphosphates as photoactivatable, cell-permeable substrates for protein tyrosine phosphatases and explored their usefulness in identifying inhibitors of tyrosine phosphatases. 1-(2-Nitrophenyl)ethyl protected fluorescein diphosphate (NPE-FDP) undergoes rapid photolysis to release FDP upon irradiation with a 450-W UV immersion lamp and its by-product does not inactivate protein tyrosine phosphatase 1B (PTP1B) or alters the viability of cells. The generated FDP from photolysis of NPE-FDP was shown to have exactly the same properties as FDP, which can be used as a PTP substrate in pure enzyme assays. We have also demonstrated that the PTP activity can be measured using NPE-FDP in small droplets. Its advantage as an inert substrate before photolysis allows the possibility of applying nanospray technology in screening and optimizing PTP inhibitors through a large chemical library. Like other caged bioeffectors such as nucleotide and inositol trisphosphate, NPE-FDP is cell-permeable. The NPE-FDP can be photolyzed to generate FDP inside cells, and then can be hydrolyzed by phosphatases to produce fluorescein monophosphate and subsequently to fluorescein. Although Jurkat cells contain high concentrations of CD45, it has not been possible to use FDP as a substrate to measure CD45 activity in the intact cell. This is due to the hydrolysis of FDP by several other cellular phosphatases. However, NPE-FDP can be useful as a cell-permeable substrate for overexpressed phosphatases such as alkaline phosphatase.