Enzymatic deglycosylation of the subunits of chorionic gonadotropin. Effects on formation of tertiary structure and biological activity

Deglycosylated ovine lutropin: preparation and characterization by in vitro binding and steroidogenesis

Ovine lutropin (oLH) subunits were deglycosylated in anhydrous hydrofluoric acid. All possible combinations of native and deglycosylated alpha- and beta-subunits were prepared and characterized. The two native plus deglycosylated counterpart preparations showed properties intermediate between the native only and the deglycosylated subunit hybrid. For brevity, only the properties of the latter hybrid were described in detail. Deglycosylation did not alter subunit-subunit recombination, protein conformation (circular dichroism) or the peptide chains (N-termini). Deglycosylation enhanced receptor-binding activity, but the carbohydrate moieties are important for maximal steroidogenesis (Leydig cell testosterone production). The deglycosylated hybrid molecule acts as an antagonist or agonist to native hormone action depending on the test system and conditions. The deglycosylated hormone binds to the receptor much more rapidly than the native hormone and binding plateaued at a higher level in the case of the deglycosylated hormone, although cross-labeling of tracers (either native or deglycosylated) indicated that competition was for the same type of receptors. To eliminate internalization of the hormone as a factor in the binding studies we used broken cell (membrane) preparations of Leydig cells. Differences in the preparations are thus attributable to a greater 'on-rate' for the deglycosylated hormone. The 'off-rate' from the receptor was essentially equal for both native and deglycosylated hormones. For this reason the deglycosylated hormone has only limited antagonistic action for native hormone activity.

Role of the carbohydrate moiety of human choriogonadotropin in its thyrotropic activity

To investigate the role of the carbohydrate moiety of human choriogonadotropin (hCG) in its thyrotropic activity, highly purified hCG and its desialylated subunits were treated with anhydrous HF/anisole (1 h, 0 degree C). The deglycosylated alpha- and beta-subunits were recombined with their native complementary subunits, and the interactions of these hCG congeners with the thyrotropin (TSH) receptor-adenylate cyclase system were investigated using human thyroid membranes. Deglycosylated hCG (dghCG) bound to the high affinity-low capacity TSH-binding sites of thyroid membranes; its equilibrium dissociation constant was lower than that of asialo-hCG (ashCG) (ED50: 2.6 and 6 microM, respectively). Like ashCG, dghCG did not stimulate thyroidal adenylate cyclase, but rather inhibited TSH stimulation of this enzyme in a dose-dependent manner. Thus, dghCG behaved as an antagonist and exhibited an inhibition constant of 0.78 microM while ashCG exhibited a constant of 1.50 microM. As might be predicted from the behavior of dghCG, absence of carbohydrate from either subunit enhanced the ability of the hCG hybrid recombinants to interact with the TSH receptor-adenylate cyclase system. However, only the hybrid recombinant lacking carbohydrate on its alpha-subunit lacked intrinsic thyrotropic activity; the hybrid recombinant lacking carbohydrate on its beta-subunit not only displayed intrinsic thyrotropic activity, but was of even higher potency than intact hCG in stimulating thyroidal adenylate cyclase. These results demonstrate that the carbohydrate moieties of both hCG subunits impede the process of recognition of hCG by the TSH receptor, while the carbohydrate moiety of the alpha-subunit, but not that of the beta-subunit, is essential for the process of hCG activation of thyroidal adenylate cyclase.