title: Insulin-like growth factor II binding to the type I somatomedin receptor. Evidence for two high affinity binding sites.
abstract: We have previously shown that the antireceptor antibody alpha IR-3 inhibits binding of 125I-somatomedin-C/insulin-like growth factor I (Sm-C/IGF-I) to the 130-kDa alpha subunit of the type I receptor in human placental membranes, but does not block 125I-insulin-like growth factor II (IGF-II) binding to a similar 130-kDa complex in these membranes. To determine whether the 130-kDa 125I-IGF-II binding complex represents a homologous receptor or whether 125I-IGF-II binds to the type I receptor at a site that is not blocked by alpha IR-3, type I receptors were purified by affinity chromatography on Sepharose linked alpha IR-3. The purified receptors bound both 125I-Sm-C/IGF-I and 125I-IGF-II avidly (KD = 2.0 X 10(-10) M and 3.0 X 10(-10) M, respectively). The maximal inhibition of 125I-Sm-C/IGF-I binding by the antibody, however, was 62% while only 15% of 125I-IGF-II binding was inhibited by alpha IR-3. In the presence of 500 nM alpha IR-3, Sm-C/IGF-I bound with lower affinity (KD = 6.5 X 10(-10) M) than IGF-II (KD = 4.5 X 10(-10) M) and IGF-II was the more potent inhibitor of 125I-Sm-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 alpha IR-3. Site IB has higher affinity for IGF-II than for Sm-C/IGF-I and is not blocked by alpha IR-3.
mesh_terms: D000906:Antibodies; D001665:Binding Sites; D001667:Binding, Competitive; D002462:Cell Membrane / Q000378:metabolism; D005260:Female; D006801:Humans; D007158:Immunologic Techniques; D007328:Insulin / Q000378:metabolism; D007334:Insulin-Like Growth Factor I / Q000378:metabolism; D007335:Insulin-Like Growth Factor II / Q000378:metabolism*; D008970:Molecular Weight; D010920:Placenta / Q000378:metabolism; D011247:Pregnancy; D011956:Receptors, Cell Surface / Q000276:immunology / Q000378:metabolism*; D017451:Receptors, Somatomedin; D013002:Somatomedins / Q000378:metabolism*
chemical_list: D000906:Antibodies; D007328:Insulin; D011956:Receptors, Cell Surface; D017451:Receptors, Somatomedin; D013002:Somatomedins; D007334:Insulin-Like Growth Factor I; D007335:Insulin-Like Growth Factor II