Evidence for a conformational change in deglycosylated glycoprotein hormones.

Removal of long-chain asparagine-linked carbohydrates leads to loss of receptor-effector coupling in gonadotropin target tissue. This implies a direct interaction between carbohydrate and cell membrane components. To examine other mechanisms by which carbohydrate could activate post-receptor events, we have used sequence-specific and conformation-specific alpha-subunit radioimmunoassays as probes for conformational changes in deglycosylated choriogonadotropin and follitropin. Immunoreactivity of either hormone was enhanced 8-13-fold after removal of carbohydrate by anhydrous hydrogen fluoride. Removal of sialic acid alone had little effect on reactivity. Based on the specificity of the antisera, the effect could be localized to a region in the amino-terminus remote in the linear sequence from actual sites of carbohydrate attachment. The results suggest that a conformational change in the alpha-subunit could account, at least in part, for the observed effects of deglycosylation on glycoprotein hormone action.