Gonadotropin α subunit. Differential processing of free and combined forms in human trophoblast and transfected mouse cells

The gonadotropins luteinizing hormone, follicle-stimulating hormone, and human chorionic gonadotropin are composed of two noncovalently linked subunits, α and β. The α subunit, identical in all three hormones, is produced in excess over the unique β subunits by pituitary and placenta, and is secreted as uncombined, or free subunit. Free α subunit from both tissues has a larger molecular weight than the dimer form. In bovine pituitary an extra O-linked oligosaccharide is added to free α subunit, and this modification has recently been detected at an analogous position (threonine 39) on human α subunit secreted by choriocarcinoma cells. To assess the contribution of N-linked and O-linked oligosaccharides to the heterogeneity of human free α subunit, we have compared free α with human chorionic gonadotropin α secreted by explants and cultured cytotrophoblasts of human first trimester placenta. We have also examined the free and combined forms of human α subunit expressed in transfected C-127 mouse mammary tumor cells. Processing of the α subunit in placental and C-127 cells was similar. Tryptic mapping of placental-derived and transfected α subunits indicated that O-glycosylation at threonine 39 was not a major modification. In the presence of the oligosaccharide processing inhibitor swainsonine the difference in size between the free and combined forms of α was eliminated in both placental and C-127 cells, indicating that the two forms of α differed in their N-linked oligosaccharides. Furthermore, the oligosaccharides of free α subunits from placental and transfected cells were resistant to endoglycosidase H, but the combined forms of α were partially sensitive to the enzyme. Thus, in human first trimester placenta and mouse C-127 cells, combination of α with human chorionic gonadotropin β alters the processing of N-linked oligosaccharides on α subunit.