Presence of insulin‐like growth factor binding protein (IGF‐BP) in rainbow trout (Oncorhynchus mykiss) serum

Recombinant human IGF‐I (hIGF‐I) binding to rainbow trout serum fractions was studied in vitro. Binding protein preparations, from human and trout serum (hBP and tBP), which were obtained using size exclusion chromatography (AcA 54) under acid conditions, were used to characterize IGF binding protein. Mammalian protein binding assay, using hIGF‐I as labeled hormone, was optimized (pH, Tris concentration, bound‐free 125I‐hIGF‐I separation conditions by charcoal) for trout serum. The association kinetics (T = 4°C) of specific binding presented a plateau from 20–30 h, and nonspecific binding remained stable for the same time. Percentage of 125I‐hIGF‐I which was able to bind to tBP was at least 55%. In the same incubation conditions, hIGF‐I binding to trout BP had a higher affinity (Ka = 2.8 1010 M–1) than those obtained with human BP preparation (Ka = 0.17 1010 M–1) which contains four molecular forms of BPs. The binding was saturable but its capacity was fourfold lower in trout (Bmax = 410 ng/ml of serum) than in human (Bmax = 1868 ng/ml) serum. Binding to trout BP is very specific for human IGF‐I and for trout IGF serum extract. hIGF‐II crossreacts partially but no binding was observed with porcine insulin. When trout serum was chromatographed on an analytic AcA 54 column (1 × 100 cm), the maximum hIGF‐I activity was found in a peak which corresponded to 55 kDa. Two other peaks (34 and 23 kDa) also bound hIGF‐I, but with a much lower activity. In Western‐ligand‐blot from SDS‐PAGE, binding of 125I‐hIGF‐I to trout serum presented a major band (MW = 41.5 kDa) similar to the biggest form of IGF‐BP‐3 from human serum. These results suggest that IGF binding protein(s) exists in trout serum but the majority of IGF binding activity is carried only by one form (41.5 kDa), in contrast to mammals which have four forms of BP with a similar magnitude of serum concentration.