A Preferential Binding Site for Insulin-Like Growth Factor II in Human andRat Placental Membranes*

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Abstract
We have compared the binding of insulin-like growth factors I and II (IGF-I and IGF-II) by partially purified human and rat placental membranes. When [125I]IGF-I wasbound by human placental membranes, IGF-I was more effective than IGF-II in displacing the tracer. Scatchard analysis of the binding of IGF-I provided evidence of a high affinity, low capacity binding system and a second lower affinity binding system. When [125I]IGF-II was bound by human placental membranes, it was displaced much more readily by IGF-II than by IGF-I. Scatchard analysis of the binding of IGF-II indicated that the high affinity component of binding was reduced, and most of the binding of IGF-II was due to the lower affinity and higher capacity system. In comparison to human placenta, rat placental membranes bound [125I]IGF-I relatively weakly, the IGF-II was more effective than IGF-I and rat somatomedin in displacing this label. In contrast, the binding of [125I]IGF-II by rat placental membranes was greater than that of [125I]IGF-I. [125I]IGF-II was displaced preferentially by IGF-II; IGF-I was less than 1% as effective. Of the rat somatomedins, multiplication-stimulation activity displaced bound [125I]IGF-II from rat placental membranes about one fifth as well as IGF-II, but rat somatomedin peptide was ineffective. Scatchard analysis of IGF-II binding by rat placental membranes provided evidence of only a single binding system. We conclude that human placental membranes possess two binding systems: one binding IGF-I more avidly than IGF-II, and the second binding IGF-II more avidly than IGF-I. Rat placental membranes bind IGF-II preferentially, and no preferential site for IGF-I could be recognized. IGF-I and rat somatomedin had little or no ability to displace [125I]IGF-II from its binding site. A preparation of multiplication-stimulating activity had about one-fifth the potency of IGF-II in such displacement.