Biological Properties of hCG after Removal of Terminal Sialic Acid and Galactose Residues

Abstract

The effects of removal of the terminal sialic acid and galactose residues of hCG on the immunological and biological properties of the hormone were studied in order to further clarify the role of the carbohydrate moieties in the action of glycoprotein hormones on their target tissues. hCG treated sequentially with neuraminidase and β–galactosidase was evaluated for biological in vivoactivity, for binding activity by radioimmunoassay and radioligand—receptor assay, and for in vitro biological activity by assay of testosterone production by the isolated rat testis. Metabolism of the asialo— and asialo—agalacto—hCG preparations labeled with 125I was compared with that of the intact hormone after intravenous injection. The asialo—agalacto—hCG preparation possessed biological activity in vivo of 2.3 IU/mg, comparable to that of the desialylated hormone (0.7 IU/mg) and considerably below that of the stored intact hormone (7400 IU/mg). By contrast, the ability of the modified hormones to compete with intact hCG for binding to antibody and testis receptor sites was found to be unaffected by desialylation and removal of galactose, indicating that these residues play little or no part in specific binding functions. The metabolic clearance of both modified hormones was markedly increased, the plasma half—life of asialo—hCG being significantly shorter (20 seconds) than that of the asialo—agalacto—preparation (30 seconds). Both hCG derivatives displayed considerable in vitro activity in terms of their ability to stimulate steroidogenesis in the isolated rat testis, the asialohCG and asialo—agalacto—hCG preparations showing 46% and 15%, respectively, of the potency of the intact hormone. These results confirm the extreme importance of terminal carbohydrate residues in the metabolic disposal of hCG in vivo, and emphasize their lack of involvement in binding to tissue receptor sites. The residual biological activity of the modified hormone preparations revealed by stimulation of steroid in vitroproduction was considerably higher than that measured by conventional bioassay methods, indicating that their markedly reduced biological activity in vivo is predominantly attributable to increased metabolic clearance. In addition, the partial loss of biological activity in vitro suggests that the presence of carbohydrate groups may influence the ability of hCG to induce optimal activation of target cells. (Endocrinology91: 296, 1972)