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-α-asialo-β, asialo-α-intact-β, 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 μg) proved to be a thyroid stimulator of considerable potency, exceeding the response induced by 0.2 mlU bovine TSH (bTSH), as measured by ¹²⁵I release into the blood after 2- and 8-h intervals. Remarkably, both asialo-α-intact-β and ashCG significantly stimulated the mouse thyroid in this assay, though to a lesser degree than hCG itself. However, in the same assay intact-α-asialo-β 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. The two hCG variants, desialylated in one subunit or the other, were intermediate in this respect. Of these, asialo-α-intact-β exhibited a relatively longer half-life than intact-α-asialo-β. The rate of survival of hCG variants in the circulation corresponded with their abilities to inhibit the binding of [^l25I]ashCG to mouse liver membranes. The results presented suggest that hCG retains a significant part of its in vivo thyrotropic activity after partial or complete desialylation. The removal of sialic acid from either subunit of hCG exerts seemingly opposite effects on the in vivo thyrotropic activity. Although it greatly enhances the receptor-binding affinity of the hormone, it also sharply reduces its plasma half-life. The magnitude of the thyrotropic activity exhibited might be determined by the effect that predominates. The β-subunit sialic acid seems to be more critical than the α-subunit sialic acid in preventing hepatic binding, prolonging plasma half-life, and consequently preserving the thyrotropic activity of hCG in vivo.