Separation of the Insulin-Like Growth Factor-Binding Proteins in Plasma and Purification of the Larger Molecular Weight Species*

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Abstract
Methods were developed for purification of the high mol wt (150K) insulin-like growth factor (IGF)-binding protein and its acid stable (70K) component from human plasma. High mol wt IGF-binding protein was highly purified by chromatography of Cohn IV-1 fraction of human plasma on Concanavalin A-Sepharose followed by chromatography on IGFI-Sepharose. The acid-stable component of the high mol wt IGF-binding protein was purified to near homogeneity by chromatography of Cohn IV-1 fraction of human plasma on Con-A Sepharose, followed by chromatography on Sephadex G-50 at pH 2.5 and subsequent chromatography on IGF-I-Sepharose. Both fractions obtained after IGF-I-Sepharose chromatography were capable of binding [125I]IGF-I and gave a single protein band of 79,000 mol wt, when subjected to sodium dodecyl sulfate-polyacrylamide gel electrophoresis (Coomassie blue staining), suggesting that the large mol wt species may be a dimer of identical or iso-mol wt subunits. Three minor contaminants of less than 5% each were detected upon subsequent silver staining. These methods represent important tools that should aid in furthering our understanding of the role of the IGF carrier proteins in the actions of IGF-I and IGF-II.