title: Characterization of the protein which binds insulin-like growth factor in human serum.
abstract: The binding of the 125I-labelled insulin-like growth factors I and II (125I-IGF I and 125I-IGF II) to the high-molecular-mass binding protein of human serum was characterized. With diluted human serum both growth factors showed optimal specific binding at 4 degrees C and pH 5-6. When 0.1% Triton X-100 was present in the incubation buffer an increase in the affinity of the IGF-binding protein was induced, which produced an enhanced binding of IGF I and IGF II. Competition experiments with various peptide hormones revealed that the native IGF-binding protein complex binds both the IGF I and IGF II with high specificity. Analysis of binding data according to the method of Scatchard resulted in linear plots for IGF I and IGF II respectively, indicating that in human serum only a single class of non-interacting binding sites is present. At optimal binding conditions the dissociation constants were determined to be 0.28 x 10(-9) M for IGF I binding and 0.66 x 10(-9) M for IGF II. Human serum was gel-filtered on Sepharose CL-6B at neutral pH and the eluate was assayed for binding activity with both IGF I and IGF II. One peak with an apparent molecular mass of 175 kDa and a Stokes radius of 4.8 nm was determined for both growth factors. Thus, our data suggest that human serum contains one class of high-molecular-mass binding protein with comparable binding characteristics for IGF I and IGF II.
mesh_terms: D001665:Binding Sites / Q000187:drug effects; D001667:Binding, Competitive; D002352:Carrier Proteins / Q000302:isolation & purification*; D002845:Chromatography; D002850:Chromatography, Gel; D006801:Humans; D018969:Insulin-Like Growth Factor Binding Proteins; D007334:Insulin-Like Growth Factor I / Q000097:blood; D007335:Insulin-Like Growth Factor II / Q000097:blood; D017830:Octoxynol; D011092:Polyethylene Glycols / Q000494:pharmacology; D013002:Somatomedins / Q000097:blood*; D013696:Temperature; D013997:Time Factors
chemical_list: D002352:Carrier Proteins; D018969:Insulin-Like Growth Factor Binding Proteins; D013002:Somatomedins; D011092:Polyethylene Glycols; D007334:Insulin-Like Growth Factor I; D007335:Insulin-Like Growth Factor II; D017830:Octoxynol