Development of a time-resolved fluoroimmunoassay for salmonid insulin-like growth factor binding protein-2b

Insulin-like growth factor (IGF)-1 promotes the growth of vertebrates, and its binding proteins (IGFBPs) regulate the activity of circulating IGF-1. Three IGFBPs, IGFBP-2b, -1a, and -1b, were consistently detected in the circulatory system of salmonids. IGFBP-2b is thought to be the main carrier of IGFs and promoter of IGF-1-mediated growth in salmonids. Currently, there are no immunoassays for detecting IGFBP-2b. In this study, we developed a time-resolved fluoroimmunoassay (TR-FIA) for IGFBP-2b detection in salmonid fishes. To establish TR-FIA, we produced two recombinant trout (rt) IGFBP-2bs expressed, one with thioredoxin (Trx) and a histidine (His) tag, and the other with His-tag only. We labeled both recombinant proteins with europium (Eu). Only Eu-Trx.His.rtIGFBP-2b cross-reacted with anti-IGFBP-2b, and the addition of increasing amounts of Trx.His.rtIGFBP-2b replaced the binding, indicating its utility as a tracer and assay standard. The addition of unlabeled salmon IGF-1 did not affect the binding of the standard or sample. Serial dilution curves of sera from rainbow trout, Chinook salmon, and chum salmon were parallel to those of the standard. The assay range (ED₈₀-ED₂₀) of the TR-FIA was 60.4 to 251.3 ng/ml, and its minimum detection limit of this assay was 21 ng/ml. The intra- and inter-assay coefficients of variation were 5.68% and 5.65%, respectively. Circulating IGFBP-2b levels in fed rainbow trout were higher than those in fasted fish and were correlated with individual growth rates. This TR-FIA is useful for further exploring the physiological responses of circulating IGFBP-2b and evaluating the growth status of salmonids.