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The measurement of insulin-like growth factors (IGFs) in plasma is complicated by the presence of high-affinity IGF-binding proteins (IGFBPs). Consequently, the IGFBPs need to be removed or their IGF-binding effects need to be neutralized prior to assaying samples for IGFs. It was observed that IGFs but not IGFBPs from sheep plasma bind to the size-exclusion gel Sephacryl S-100 HR at a low pH and that the IGFs can subsequently be eluted off at a neutral pH. From this observation a convenient method was developed for the extraction from plasma of ovine (o) IGF-I and -II free from detectable IGFBPs with close to 100% recovery. When assayed in homologous sheep IGF-I and -II radioimmunoassays the Sephacryl-extracted plasma samples gave dose–response curves which were parallel to purified oIGFs. Furthermore, the results obtained by Sephacryl extraction were highly correlated with those found by the established Sephadex G-75 extraction procedure.
Journal of Endocrinology (1993) 137, 239–245
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