SHIMIZU, M.*; HARA, A.; DICKHOFF, W.W.; Nat. Mar. Fish. Ser. and Univ. of Washington, Seattle; Hokkaido Univ., Hakodate, Japan; Nat. Mar. Fish. Ser. and Univ. of Washington, Seattle: Development of a radioimmunoassay for salmon 41-kDa insulin-like growth factor binding protein

Insulin-like growth factor binding proteins (IGFBPs) are important modulators of IGF actions controlling IGF availability to IGF receptors in the target tissues. Salmon plasma contains at least three IGFBPs with molecular masses of 41, 28 and 22 kDa. The 41-kDa IGFBP is similar to mammalian IGFBP-3 in size, type of glycosylation and physiological responses. In this study, we developed a radioimmunoassay (RIA) for the 41-kDa IGFBP. The 41-kDa IGFBP purified from serum was used for antibody production and assay standard. Among three different preparations of tracer, ¹²⁵I-41-kDa IGFBP, ¹²⁵I-41-kDa IGFBP cross-linked with IGF-I (¹²⁵I-41-kDa IGFBP/IGF-I) and 41-kDa IGFBP/¹²⁵I-IGF-I, examined, only 41-kDa IGFBP/¹²⁵I-IGF-I did not show binding interference by IGF, and therefore was used for the RIA tracer. Plasma 41-kDa IGFBP levels measured by RIA were increased by growth hormone treatment and decreased after fasting. The molarities of plasma 41-kDa IGFBP and IGF-I were comparable, and they were positively correlated, suggesting that 41-kDa IGFBP is a main carrier of circulating IGF-I as is mammalian IGFBP-3. During the parr-smolt transformation (smoltification) of coho salmon, plasma 41-kDa IGFBP levels showed a transient peak in March and stayed relatively constant thereafter, whereas IGF-I showed peaks in March and April. Differences in the molar ratio between 41-kDa IGFBP and IGF-I may influence availability of IGF-I from the circulation during smoltification.