Interactions of a monoclonal antibody to the insulin receptor with receptors for insulin-like growth factors

A monoclonal antibody to the human insulin receptor was tested for its ability to inhibit the binding of ¹²⁵I-insulin-like growth factor I (IGF-I) and ¹²⁵I-insulin-like growth factor II (IGF-II) to their receptors in human placenta membranes and cultured human IM-9 lymphocytes. In both placenta membranes and IM-9 cells, the antibody progressively inhibited the binding of ¹²⁵I-IGF-I to its receptor with a potency that was 300-fold less than its ability to inhibit the binding of ¹²⁵I-insulin to its own receptor. In contrast, in human placenta membranes, this antibody inhibited the binding of ¹²⁵I-IGF-II to its receptor only slightly. These studies indicate, therefore, that this monoclonal antibody binds preferentially to the insulin receptor but also crossreacts to a lesser extent with the IGF-I receptor.