Identification of both human and salmon calcitonin-like molecules in birds suggesting the existence of two calcitonin genes.

To investigate whether human calcitonin (hCT) is preserved during the evolution of vertebrates, we studied extracts of avian (pigeon and chicken) thyroid and ultimobranchial glands (UBG) which have previously been reported to contain salmon calcitonin (sCT)- like molecules. A sensitive and specific radioimmunoassay for hCT, employing two antisera reacting with different regions of the molecule, was used in combination with high performance liquid chromatography (HPLC). We found that extracts of thyroid and UBG from pigeons and chickens contain, in addition to an immunoreactive sCT-like molecule (which is the major immunoreactive form), an hCT-like molecule comprising from 0.4 to 3.75% of the calcitonin content. The extracts produced full displacement of 125I-labelled hCT from both antisera and gave a parallel displacement curve. With HPLC, we found an immunoreactive hCT peak which was 5 ml earlier than the insulin marker and an immunoreactive sCT peak which was 12 ml later than the insulin marker. These results demonstrate the presence of two calcitonins in birds, and suggest the existence of two genes with different degrees of expression.