Characterization of the protein which binds insulin-like growth factor in human serum

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.