The Role of Carbohydrate in the Biological Function of Human Chorionic Gonadotropin

Abstract

The binding sites or the so-called receptors for polypeptide and glycoprotein hormones such as insulin (1), glucagon (2), adrenocorticotropin (3), human chorionic gonadotropin (4–6), luteinizing hormone (7–9), and follicle stimulating hormone (10, 11) are located on the plasma membranes of the target cells. The binding is characterized by the high degree of affinity and hormonal specificity, which suggests that the binding sites must have highly specific chemical structures. It is not clearly understood whether all of the sites are equivalent. Moyle and Ramachandran have suggested the presence of more than one type of receptor on Leydig cells for LH with different affinities for the hormone; the sites which induce steroidogenesis have higher affinity than those which stimulate the accumulation of cyclic AMP (12). That the receptors are protein in nature is indicated by their properties after solubilization from the corpora lutea or interstitial tissue from testes with nonionic detergents (13). It is not known, however, whether the receptors are composed of one or several proteins or if any prosthetic groups such as lipids or polysaccharides are also present. The precise chemical elucidation of the receptors must await until all of its components are identified and purified.