[37] Purification of protein-tyrosine phosphatases from human placenta

This chapter discusses the purification of protein-tyrosine phosphatases from human placenta. Phosphorylation of proteins on tyrosyl residues is an essential element in the control of normal and neoplastic cell growth. The phosphorylation state of a protein reflects the relative activities of the kinase that phosphorylates it and the phosphatase that removes the phosphate. A major technical difficulty associated with the study of protein phosphatases is the requirement for suitably purified phosphorylated substrates. Characterization of the protein-tyrosine phosphatases (PTPases) provides a necessary complementary perspective for an overall understanding of the control of cellular function by tyrosine phosphorylation. This chapter describes procedures for the preparation of substrates and assay of PTPases. It also discusses the purification of a major low-molecular-weight PTPase from human placenta.