Evidence for a gonadotropin-releasing hormone binding protein in goldfish (Carassius auratus) serum

The binding of salmon gonadotropin-releasing hormone (sGnRH) and its superactive analog, D-Arg6, Pro9-NEt]-sGnRH, to a macromolecular component in goldfish serum was studied, using 125I-D-Arg6, Pro9-NEt]-sGnRH and 125I-sGnRH as labeled ligands. Bound was separated from free labeled ligand by gel filtration with Sephadex G-50. The binding of labeled ligand to goldfish serum was dose-dependent. The results indicate a single class of binding site having low affinity and high capacity. The existence of a GnRH binding protein in serum may, in part, contribute to the long-lasting pharmacological action of GnRHs in goldfish.