The insulin receptor of human placenta even after extensive purification is phosphorylated in the presence of [γ-32P]ATP and NaF, and is dephosphorylated again on incubation in NaF-free medium. Insulin stimulates phosphate incorporation into the M r95 000 subunit probably by activation of the phosphorylation step. Our data suggest that the insulin receptor contains both kinase and phosphatase activities that may control the phosphorylation state of the receptor.
