Purification and characterization of T cell protein tyrosine phosphatase reveals significant functional homology to protein tyrosine phosphatase-1B

We have developed a protocol for rapid purification of T cell protein tyrosine phosphatase (TCPTP) and the structurally related protein tyrosine phosphatase-1B (PTP-1B) from bacterial cells. The pH profile for TCPTP was bell-shaped with an optimum of 5.5. The catalytic domain and full-length versions of TCPTP bound a potent inhibitor with affinities similar to those of PTP-1B. The Km values for the catalytic domains of TCPTP and PTP-1B increased with increasing ionic strength, whereas the kcat values remained unchanged. Arrhenius plots revealed that TCPTP and PTP-1B possess similar activation energies of 25.3±1.2 and 18.4±3.0 kJ/mol, respectively. Increasing solvent microviscosity (up to 40% (w/v) sucrose) did not affect kcat/Km of either enzyme. However, high sucrose concentrations protected both enzymes from thermal inactivation. These studies show that, although they share a 72% amino acid sequence identity within their catalytic domains, TCPTP and PTP-1B are functionally very similar in vitro.