Immunoreactive and receptor-active insulin-like growth factor-I (IGF-I) and IGF-binding protein in blood plasma from the freshwater fish Macquaria ambigua (golden perch)

ABSTRACT The presence of insulin-like growth factor-I (IGF-I)-related molecules and IGF-binding factors in blood from golden perch, Macquaria ambigua, an Australian native freshwater fish, was investigated. Serum was acidified to dissociate IGF and IGF-binding protein complexes that might be present, and fractionated by size-exclusion high-performance liquid chromatography at pH 2·8. Fractions were neutralized and their activities assessed by (i) an immunoassay for mammalian IGF-I which also detects chicken IGF-I but in which all known forms of IGF-II react very poorly, (ii) a receptor assay for IGF-II in which all known forms of IGF-I react poorly, and (iii) a type-I IGF receptor assay in which mammalian IGF-I and IGF-II polypeptides are almost equivalent. No IGF-II-like activity was detected. Three peaks of IGF-I-like activity were detected by IGF-I immunoassay and type-I IGF receptor assay. The major peak of activity was similar in molecular size to human IGF-binding protein-3, 45–55 kDa ('large IGF'), and a minor peak of activity which was similar in size to mammalian IGFs, 7·5 kDa. A third peak of activity was observed eluting at a time which indicates that it is a smaller molecule than any previously described IGF. The large IGF was temperature-sensitive, but was not a binding protein for 125I-labelled human IGF-I (hIGF-I). This material therefore was able to bind to anti-hIGF-I antibodies and to human type-I IGF receptors, and may represent the fish equivalent of mammalian prepro-IGFs. The two smallest forms of IGF activity identified by IGF-I radioimmunoassay and type-I radioreceptor assay following acidic size-exclusion chromatography were able to stimulate protein synthesis by L-6 myoblasts in culture, although large IGF did not. When fresh (but not frozen and thawed) golden perch serum was incubated with 125I-labelled hIGF-I and then fractionated by size-exclusion liquid chromatography at pH 7·4 through Sephadex G-100, the radioactivity became associated with a complex, intermediate in size between free IGF-I and the major IGF-binding protein in human serum. The association of 125I-labelled hIGF-I with the complex was inhibited by the presence of unlabelled hIGF-I in the incubation. These studies show that receptor-active, immunoreactive and bioactive IGF-I-like activity is present in golden perch serum, and demonstrate the presence of an IGF-I-binding factor in this species. Journal of Endocrinology (1993) 136, 191–198