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 - 60,000) and a much higher molecular weight binding protein (apparent mol wt - 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.