STUDIES ON STANNIOCALCIN - CHARACTERIZATION OF BIOACTIVE AND ANTIGENIC DOMAINS OF THE HORMONE

Stanniocalcin (STC) decreases branchial Ca2+-uptake in fish. In order to determine its bioactive domain, synthetic fragments (U amino acids (aa) 1-20; V aa 103-136; W aa 202-231) of eel STC were tested for thei r effect on Ca2+ uptake in tilapia (Oreochromis mossambicus). Ca2+ upt ake was inhibited by an N-terminal fragment but not by a midfragment n or a C-terminal fragment of the mature hormone. We provide theoretical and experimental evidence that a midportion of STC, which is included in the synthetic fragment V, is the most antigenic site of the molecu le. Polyclonal antibodies.against stanniocalcin are directed against t his midportion although this region of STC appears not to be essential for signal transduction. These results suggest that the currently ava ilable antibodies will recognize inactive STC fragments in the circula tion. We conclude that the bioactive portion of STC does not correspon d with the major antigenic portion of the hormone. The results imply t hat studies on plasma STC levels employing a polyclonal antiserum agai nst STC should be interpreted with care.