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The carbohydrate moiety of bovine thyrotropin is essential for full bioactivity but not for receptor recognition
Abstract:
TSH is a glycoprotein hormone whose carbohydrate content varies among different species. Although recent studies suggest that variants of TSH deficient in carbohydrate occur naturally, the significance of the carbohydrate moiety of TSH in respect to its thyrotropic function is unclear. The present studies were undertaken, therefore, to examine this question. A highly purified preparation of bovine TSH (bTSH) was deglycosylated by treatment with anhydrous hydrogen fluoride. Amino acid and carbohydrate analyses of the original and deglycosylated preparations indicated that approximately 85% of the carbohydrate originally present had been removed and that the protein moiety was unaltered. As judged from TSH radioreceptor assays, bTSH and deglycosylated bTSH (dg-bTSH) bound to human thyroid membranes with equal affinity, since both caused a half-maximal inhibition of [125I]bTSH binding at approximately equal concentrations. Nonetheless, dg-bTSH at optimal concentration displayed only about one third the activity of intact TSH in stimulating adenylate cyclase activity in human thyroid membranes. dg-bTSH also antagonized the adenylate cyclase-stimulating activity of intact bTSH in this system, but only weakly, since abolition of the bTSH effect required an approximately 40-fold higher concentration of dg-bTSH. In cultures of FRTL5 cells, a cloned line of follicular cells derived from normal rat thyroid, both intact and dg-bTSH enhanced cell growth, as measured by [3H]thymidine incorporation and stimulated cAMP release in the medium, but the response elicited by dg-bTSH was much less than that caused by equal concentrations of the intact hormone. In accord with the findings in the in vitro assays, dg-bTSH evoked a much smaller response than bTSH did in the in vivo mouse assay. It is concluded that although not required for receptor recognition, the carbohydrate moiety of bTSH is essential for the full expression of its biological activity.