Human pituitary thyrotropin and gonadotropins: a comparative study of their in vitro lipolytic activity

Highly purified preparations of human thyrotropin, follitropin, lutropin, and choriogondotropin were evaluated for their ability to stimulate lipolysis in isolated rat adipocytes. Thyrotropin was the most active of all while follitropin showed some activity. Lutropin and choriogonadotropin were essentially inactive. The isolated subunits of thyrotropin were inactive and 60-100% activity was regenerated upon recombination. The results provide direct evidence that lipolytic activity is intrinsic to human thyrotropin. The hybrid of human lutropin alpha + thyrotropin beta was considerably less active than the natural recombinant (thyrotropin alpha + beta), possibly indicating the presence of some differences in the alpha subunits.