Role of subunit sialic acid in hepatic binding, plasma survival rate, and in vivo thyrotropic activity of human chorionic gonadotropin.

Previous studies have shown that desialylation of human chorionic gonadotropin (hCG) results in a sharp enhancement of its affinity for thyroid thyroid-stimulating hormone (TSH) receptors, transforming it from a weak to a potent antagonist of adenylate cyclase activity in vitro. Because most of the information on the structure-function relation of hCG as a thyroid stimulator has been derived from in vitro experiments, the present studies were undertaken to assess the role of its sialic acid residues in the expression of its thyrotropic activity in vivo. hCG and its various desialylated forms, viz., intact-alpha-asialo-beta, asialo-alpha-intact-beta, and asialo-hCG (ashCG), were initially characterized in terms of their immunoreactivities and receptor-binding abilities as assessed in the rat testis assay. In neither assay did hCG or its variants exhibit a major discordance in activity. In the mouse bioassay, intact hCG (150 micrograms) proved to be a thyroid stimulator of considerable potency, exceeding the response induced by 0.2 mIU bovine TSH (bTSH), as measured by 125I release into the blood after 2- and 8-h intervals. Remarkably, both asialo-alpha-intact-beta and ashCG significantly stimulated the mouse thyroid in this assay, though to a lesser degree than hCG itself. However, in the same assay intact-alpha-asialo-beta was inactive. Studies of the survival of hCG and its variants in the circulation of the mouse, as assessed by radioimmunoassay (RIA) in multiple serum samples drawn over 30 min, showed hCG to have a long half-life, whereas ashCG was cleared very rapidly.(ABSTRACT TRUNCATED AT 250 WORDS)