Identification of a novel binding protein for insulin-like growth factors in adult rat serum.

Using gel filtration, ligand-affinity chromatography and reversed phase HPLC, four insulin-like growth factor-binding proteins (IGF-BPs) have been purified from adult rat serum. Sodium dodecyl sulfate polyacrylamide gel electrophoretic (SDS-PAGE) analysis revealed that all four proteins migrated as doublets at 45/37 (peak 1 in Fig.4), 36/32 (peaks 2 and 5 in Fig. 4), 35/33 (peak 3 in Fig. 4) and 22/28 kDa (peaks 4a, 4b in Fig. 4), respectively under reducing conditions. N-terminal sequence analysis of the 45/37 kDa doublet showed that it is identical to the N-terminal of the 45 kDa rat IGF-BP whereas the 22/28 kDa doublet is the C-terminal truncated form of the 45 kDa species. The 35/33 kDa doublet has the same N-terminal sequence as that of the rat IGF-BP isolated from a BRL-3A cell line. However, the N-terminal sequence of the 36/32 kDa doublet is unique, although it may be related to the BRL-3A protein. The most abundant IGF-BP in adult rat serum corresponds to the 45 kDa species plus its C-terminal truncated forms, whereas the second most abundant IGF-BP is the novel protein detected in this study, while the least abundant species is the BRL-3A IGF-BP.