Tyrosine kinase activity of brain insulin and IGF-1 receptors

Lectin-purified rat brain preparations demonstrate specific [¹²⁵I]-insulin and [¹²⁵I]-IGF-1 binding. Insulin-stimulable tyrosine kinase activity as mesured by exogenous substrate phosphorylation was present in brain and liver lectin purified preparations with the Δ kinase activity/B/F of brain -2.5 fold greater than that of liver. Insulin-stimulable tyrosine kinase activity was abolished in liver but decreased by only -50 percent in brain after immunodepletion with antiserum which recognizes insulin but not IGF-1 receptors. Insulin and IGF-1 dose responses for phosphorylation of the immunodepleted brain preparations suggested that the remaining tyrosine kinase activity was IGF-1 receptor mediated. Thus, functional IGF-1 receptors are present in rat brain, and the doses of insulin typically used to evaluate insulin receptor tyrosine kinase activity will stimulate IGF-1 receptor tyrosine kinase activity as well.