GnRH-receptors of rat pituitary is a glycoprotein: differential effect of neuraminidase and lectins on agonists and antagonists binding.

The interaction of 125I-labeled gonadotropin releasing hormone (GnRH) agonist, [D-Ser-(t-Bu)6,des-Gly10-ethylamide]-GnRH, and antagonist [D-pGlu1,D-Phe2,D-Trp3,6]-GnRH with rat pituitary membranes was studied. Their binding was affected differently by pretreatment of membranes with neuraminidase and by wheat-germ agglutinin. Pretreatment of the membranes with neuraminidase abolished the specific binding of both antagonist and agonist, in a dose-response manner, with the former being less affected. Wheat-germ agglutinin affected antagonist binding slightly (22% inhibition at 40 microgram/ml) but inhibited agonist binding more markedly (50% at 40 microgram/ml). This inhibitory effect was specific since it was readily reversed by N-acetylglucosamine. Other lectins, such as concanavalin A and soybean agglutinin affected the binding of both agonist and antagonist to only a small degree. These results suggest that the GnRH-receptor of rat pituitary is a glycoprotein which contains sialic acid residue and that GnRH agonists and antagonists bind differently to the same receptor.