Insulin-like Growth Factor I1 Binding to the Type I Somatomedin Receptor

We have previously shown that the antireceptor antibody aIR-3 inhibits binding of '2SI-somatomedin-C/ insulin-like growth factor I (Sm-C/IGF-I) to the 130kDa a subunit of the type I receptor in human placental membranes, but does not block 121-insulin-like growth factor I1 (IGF-11) binding to a similar 130-kDa complex in these membranes. To determine whether the 130kDa '281-IGF-II binding complex represents a homologous receptor or whether '261-IGF-II binds to the type I receptor at a site that is not blocked by aIR-3, type I receptors were purified by affinity chromatography on Sepharose linked aIR-3. The purified receptors bound both '261-Sm-C/IGF-I and '261-IGF-II avidly (KO = 2.0 X 10"' M and 3.0 X 10"' M, respectively). The maximal inhibition of '2SI-Sm-C/IGF-I binding by the antibody, however, was 62% while only 15% of '261-IGFI1 binding was inhibited by aIR-3. In the presence of 500 nM aIR-3, Sm-C/IGF-I bound with lower affinity (KD = 6.5 X 10"' M) than IGF-I1 (KD = 4.5 X 10"' M) and IGF-I1 was the more potent inhibitor of l2'I-Srn-C/ IGF-I binding. These findings suggest that the type I receptor contains two different binding sites. The site designated IA has highest affinity for Sm-C/IGF-I and is blocked by aIR-3. Site IB has higher affinity for IGF-I1 than for Sm-C/IGF-I and is not blocked by aIR3.