Suikkari A M, Baxter R C
Department of Endocrinology, Royal Prince Alfred Hospital, Camperdown, NSW, Australia.
J Clin Endocrinol Metab. 1991 Dec;73(6):1377-9. doi: 10.1210/jcem-73-6-1377.
Although serum immunoreactive insulin-like growth factor binding protein-3 (IGFBP-3) increases during pregnancy, radioligand binding methods such as ligand blotting with iodinated IGFs fail to detect the protein in pregnancy serum. Since IGFBP-3 must bind IGF-I or IGF-II to form a complex with the acid-labile subunit (alpha-subunit), we have used ternary complex formation from [125I]alpha-subunit as a measure of IGF binding to serum IGFBP-3. High-pressure liquid chromatography fractions containing IGFBP-3 from pregnancy serum did not bind [125I]IGF-I, although the equivalent fractions from nonpregnancy serum showed dose-dependent binding. In contrast, IGFBP-3 fractions from nonpregnancy and pregnancy sera both bound [125I]alpha-subunit in the presence of either exogenous IGF-I or endogenous serum IGFs, implying that non-iodinated IGFs could bind to the IGFBP-3. Substitution of nonradioactive iodo-IGF-I for native IGF-I in the complex formation assay confirmed that the pregnancy-induced alteration in IGFBP-3, probably resulting from proteolysis, prevents it from binding iodo-IGF-I while having little effect on its binding of the native peptide. This provides an explanation for the failure to detect IGFBP-3 in pregnancy by radioligand binding methods, and raises the question of the significance of proteolysis of IGFBP-3.
尽管血清免疫反应性胰岛素样生长因子结合蛋白-3（IGFBP-3）在孕期会增加，但诸如用碘化胰岛素样生长因子（IGF）进行配体印迹等放射性配体结合方法却无法检测出孕期血清中的该蛋白。由于IGFBP-3必须与IGF-I或IGF-II结合才能与酸不稳定亚基（α亚基）形成复合物，我们采用了以[125I]α亚基形成三元复合物的方法来衡量IGF与血清IGFBP-3的结合情况。来自孕期血清的含有IGFBP-3的高压液相色谱馏分不结合[125I]IGF-I，而来自非孕期血清的同等馏分则呈现出剂量依赖性结合。相反，在存在外源性IGF-I或内源性血清IGF的情况下，来自非孕期和孕期血清的IGFBP-3馏分均能结合[125I]α亚基，这意味着未碘化的IGF能够与IGFBP-3结合。在复合物形成试验中，用非放射性碘代IGF-I替代天然IGF-I证实，孕期诱导的IGFBP-3改变（可能是蛋白水解所致）使其无法结合碘代IGF-I，而对其结合天然肽的能力影响不大。这就解释了为何放射性配体结合方法无法检测出孕期的IGFBP-3，并提出了IGFBP-3蛋白水解的意义这一问题。
