Immunological Detection of Phosphotyrosine-Containing Proteins in Rat Livers After Insulin Injection

Tyrosine-phosphorylated proteins in Triton X-100–solubilized fractions of rat livers were examined by immunoblotting with anti-phosphotyrosine antibodies. After 2 min of insulin injection via the portal vein into livers, three major bands of 170,000, 140,000, and 95,000 M_r were stimulated. Because the incubation of nitrocellulose membrane with anti-phosphotyrosine antibodies in the presence of 40 mM phosphotyrosine completely abolished these bands, the anti-phosphotyrosine antibodies appear to recognize the phosphotyrosine residues of these proteins. Insulin injection (2–2000 μg) very quickly stimulated the tyrosine phosphorylation of these proteins in a dose-dependent fashion. In contrast, insulinlike growth factor I or epidermal growth factor injection had little effect in stimulating the tyrosine phosphorylation of these proteins. Because anti-insulin-receptor antibodies immunoprecipitated a tyrosine-phosphorylated 95,000-M_r protein, this protein must be the β-subunit of the insulin receptor; i.e., the β-subunit of the insulin receptor and two other proteins were phosphorylated at tyrosine residues in vivo by insulin injection. These data suggest that the tyrosine phosphorylation and tyrosine kinase activity of the insulin receptor may have important roles in in vivo insulin action.