The thyroxine-stimulating activity is only present in the glycoprotein and not in the protein fraction of a carp hypophyseal homogenate.

The in vivo release of thyroxine (T4) in the carp (Cyprinus carpio) and the in vitro release of T4 in the frog (Rana ridibunda) from isolated thyroids were used as a bioassay to study the thyrotropic activity of carp hypophyses. The addition of up to 1 hypophyseal equivalent to the medium did not increase the in vitro release of T4 from isolated frog thyroids in November whereas 3 mU of bovine thyrotropin (b-TSH) was effective. However, when this experiment was repeated at the end of February, the responsiveness of frog thyroids was increased since the addition of 0.5 carp hypophyseal equivalent did stimulate the release of T4. When Winter carp alone were used in an in vivo bioassay, an intracardial injection of up to a dilution of 1/64 hypophyseal equivalent increased plasma concentrations of T4 after 2 and 4 hr. However, 3 mU of b-TSH was ineffective in this regard. Affinity chromatography of b-TSH indicated that all thyrotropic activity as tested in the frog bioassay was retained on a concanavalin (Con A)-Sepharose column. Following Con A separation of a carp hypophyseal homogenate it was finally demonstrated that the unadsorbed protein fractions did not increase plasma concentrations of T4, whereas the adsorbed glycoprotein fraction did. No influence on circulating triiodothyronine (T3) levels was noted with both fractions. These results indicate that the thyrotropic activity of a carp hypophysis is of a glycoprotein nature.