SHIMIZU, M.*; SWANSON, P.; HARA, A.; DICKHOFF, W.W.: Purification of a serum insulin-like growth factor binding protein from chinook salmon, Oncorhynchus tshawytscha

Circulating insulin-like growth factors (IGFs) are bound to multiple binding proteins. IGF-binding proteins (IGFBPs ) modulate biological activities of IGFs by controlling availability of IGFs to their receptors. In salmon, at least three IGFBPs with molecular masses of 41, 28, and 22 kDa have been detected by ligand blotting. The 41-kDa IGFBP is a major binding protein in growing fish and its concentration is influenced by growth hormone treatment and fasting, suggesting that it is homologous to mammalian IGFBP-3. In order to characterize salmon 41-kDa IGFBP, we purified it from serum. One liter of chinook salmon serum was mixed with SP Sephadex C-25 under acidic conditions to remove endogenous IGFs. After neutralization, the sample was loaded onto an affinity column coupled with recombinant human IGF-I and eluted with 0.5 M acetic acid. IGFBP was further purified by reversed-phase chromatography using a Vydac C4 column. Purified IGFBP was N-glycosylated and appeared as doublet bands around 41 kDa. N-terminal amino acid sequence of the 41-kDa IGFBP was similar to mammalian and fish IGFBPs. Although 14 of the 20 amino acids were identical to zebrafish IGFBP-2, the highly conserved nature of the N-terminus makes it impossible to identify the type of IGFBP from partial sequence data. However, based on physiological responses, molecular weight and type of glycosylation, this IGFBP is presumably salmon IGFBP-3.