Fetal sheep serum contains a high molecular weight insulin-like growth factor (IGF) binding protein that is acid stable and specific for IGF-II

We have used radiolabeled ovine insulin-like growth factor II (oIGF-II) and human IGF-I (hIGF-I) to investigate the nature of the IGF binding proteins in fetal sheep serum. Incubation of fetal sheep serum with [125I]oIGF-II, followed by chromatography on Fractogel TSK HW55(S), revealed the presence of two major binding protein species, a lower molecular weight binding protein (apparent mol wt approximately 60,000) and a much higher molecular weight binding protein (apparent mol wt approximately 500,000). Only the lower molecular weight binding protein complex was seen in serum from adult nonpregnant sheep. The lower molecular weight binding protein in fetal and adult sheep serum bound both [125I]oIGF-II and [125I]hIGF-I, both of which could be displaced by unlabeled oIGF-II or hIGF-I. However, the very high molecular weight binding protein bound only [125I]oIGF-II, and this could only be displaced by unlabeled oIGF-II. The very high molecular weight binding protein appears to bind approximately 40% of the endogenous oIGF-II. We have purified the very high molecular weight binding protein from fetal sheep serum using anion exchange, Concanavalin A Sepharose, and hydrophobic interaction chromatography. The purified binding protein preparation did not contain any lower molecular weight binding protein and did not bind [125I]hIGF-I. In addition, this binding protein was stable at pH 3.2 for 1 h. Thus, fetal sheep serum contains a very high molecular weight IGF binding glycoprotein that is acid stable and specific for IGF-II.