A thyrotropin-like molecule from the pituitary of an Indian freshwater murrel: comparison of its biological activity with other thyrotropins

Murrel pituitary thyrotropin-like molecule (mTSH) was purified to homogeneity with the help of a convenient and sensitive in vitro assay system where addition of this material to the thyroid follicle incubation stimulated thyroxine (T₄) secretion into the medium. Pituitary extract of a freshwater murrel, Channa punctatus, was solvent extracted to obtain glycoprotein enriched fraction. This was subjected to Sephadex G-100 gel filtration and eluate of void volume (peak I) showed strong TSH activity (as reflected from T₄ secretion) which was further purified by using concanavalin A-Sepharose, FPLC Mono Q and immunoaffinity chromatography. Purified mTSH gave a single band in PAGE, and SDS PAGE revealed two dissimilar subunits, α and β. Addition of increasing concentrations of mTSH, Indian carp TSH (cTSH) and bovine TSH (bTSH) to in vitro murrel thyroid follicle incubations caused a linear increase in thyroxine (T₄) release into the medium, effect was highest with mTSH and lowest with bTSH. However, in in vivo experiments, injections of increasing doses of mTSH to murrel elevated plasma T₄ level in a linear manner while bTSH gave a biphasic response. Addition of mTSH and bTSH to rat or goat thyroid epithelial cell incubations equally stimulated T₄ release into the medium, while cTSH had significantly less effect. Binding affinity (K_a) and receptor occupancy (B_max) of mTSH to murrel thyroid follicular membrane preparation was considerably higher in comparison to cTSH or bTSH whereas both mTSH and bTSH had nearly similar K_a and B_max with rat thyroid epithelial cell membrane preparation. Findings indicate that mTSH is a more potent TSH as compared to carp and bovine TSH in murrel and has equipotent biological activity as bTSH on rat and goat thyroid.