Role of the carbohydrate residues of human chorionic gonadotropin in binding and stimulation of adenosine 3',5'-monophosphate accumulation by porcine granulosa cells.

Removal of the sugar residues internal to sialic acid from the hCG molecule markedly diminished the ability of hCG to stimulate cAMP accumulation by porcine granulosa cells during a 30-min incubation period. At the same time, removal of sugars internal to sialic acid enabled the resulting hCG derivatives to become competitive inhibitors of hCG stimulation of cAMP accumulation. This contrasts with the finding that removal of sugars internal to sialic acid residues has a lesser effect on the ability of hCG to inhibit the binding of human [125I]iodo-CG to granulosa cells, provided the hCG derivatives were added before the tracer. It would, therefore, seem that hCG with sugars internal to sialic acid removed is able to bind to the receptor, but that, once bound, it is unable to activate adenylate cyclase. Under these conditions, it can also act as a competitive inhibitor of hCG stimulation of cAMP accumulation.