The role of subunit sialic acid in the thyrotropic and gonadotropic activities of human chorionic gonadotropin.

The present studies were undertaken to delineate the role of sialic acid residues in the two subunits of hCG in relation to its hormonal activity. Sialic acid was removed by treatment of the individual subunits or intact hCG with insolubilized neuraminidase. Desialylated variants of hCG were obtained by combining an asialo subunit (as alpha or as beta) with its complementary intact or desialylated subunit. When tested in the hCG receptor assay using a rat testis fraction, none of the hCG variants exhibited any loss of activity compared with that of intact purified hCG. In in vitro bioassays that employed cAMP and testosterone generation in rat Leydig cells as indices of response, intact hCG and as alpha in combination with intact beta (as alpha-beta) were equipotent. In contrast, intact alpha-subunit combined with as beta (alpha-as beta) and desialylated intact hCG (asialo-hCG) showed activity that at the highest concentration of each tested (20 ng/ml) was no more than half of that evoked by intact hCG. In a TSH receptor assay in human thyroid membranes, loss of sialic acid from either one or both hCG subunits resulted in enhancement of binding affinity; the rank order of decreasing potency was asialo-hCG, alpha-as beta, as alpha-beta. However, despite their enhanced binding affinity, and like intact hCG itself, none of the variants elicited a cAMP response in either human thyroid membranes or cultured human thyroid cells. Rather, asialo-hCG and alpha-as beta, but not intact hCG and as alpha-beta, were effective antagonists of the stimulatory response induced by bovine TSH in thyroid cells. These findings indicate that desialylation of hCG enhances its binding affinity for hCG receptors in testis and, much more so, for TSH receptors in human thyroid. Desialylation also changes hCG from a full agonist to a partial agonist in testis and from a nonagonist to an antagonist in human thyroid. In all cases, sialic acid in the beta-subunit of hCG appears to have a predominant role in these effects.