The goal of the project is to increase our understanding of the structure-function relationships of the oligosaccharide moieties on glycoprotein hormones and related molecules. The molecules under investigation are corticosteroid binding globulin (CBG), human chorionic gonadotropin (hCG), and gonadotropin free alpha subunits associated with pregnancy or malignancy. We have shown that thet free alpha subunit purified from pregnancy urine stimulates secretion of prolactin from primary cultures of human decidual cells in a dose-dependent manner. These findings represent the first bioassay for pregnancy free alpha molecules and indicate that free alpha may be a glycoprotein hormone with a function that is independent of hCG. Carbohydrate modifications, resulting in a variety of branched oligosaccharide structures, occur on all glycoproteins prior to secretion. these modifications can effect virtually every aspect of the molecule's behavior, including receptor binding and signal transduction, yet the underlying regulatory mechanisms remain elusive. We have investigated changes in the oligosaccharide moieties as a function of gestational development and species specificity. We had shown that specific types of carbohydrate modifications on free alpha can either prevent or facilitate combination with hCG-beta to form intact hormone. In our investigation of gestational development, we found that glycosylation of free alpha changes dramatically as a function of gestational age. Molecules produced in late pregnancy are more highly branched and are much more extensively fucosylated than those of early pregnancy. We plan to examine how the differences observed in glycosylation affect the bioactivity of these molecules. We also demonstrated species specific differences in the glycosylation of CBG. Rat CBG contained a carbohydrate composition strikingly different from that of human CBG. Such variations in glycosylation may account for differences in receptor-hormone interactions observed between species. These studies are especially relevant to inferences that are obtained using heterologous systems, i.e., examining effects of human hormones in rat receptor assays.
