Synthesis of chorionic gonadotropin subunits in human choriocarcinoma clonal cell line JEG-3: carbohydrate differences in glycopeptides from free and combined alpha-subunits.

The glycoprotein hormone hCG and its free alpha-subunit are secreted by the clonal choriocarcinoma cell line JEG-3. Free hCG alpha has a larger apparent mol wt (22,000-24,000) than the combined hCG alpha (18,000-19,000) obtained by dissociation of the hCG secreted by these cells. Techniques developed for the specific isolation and purification of the free and combined hCG alpha forms and for the preparation of glycopeptides from these subunits have permitted detection of the incorporation of D-[3H]glucosamine [( 3H]GlcN) and L-[3H]fucose into both alpha-subunit forms. Relative to their [35S]methionine content, 2.3-fold more [3H]GlcN and 6-fold more L-[3H]fucose were incorporated into free hCG alpha than into combined hCG alpha. Analyses of [3H]GlcN glycopeptides prepared from free and combined hCG alpha indicate that the 22,000- to 24,000-dalton subunit form contained more [3H]GlcN and 27% more of the GlcN metabolite N-acetylneuraminic acid than the 18,000- to 19,000-dalton hCG alpha-subunit, than both hCG alpha forms contained two major N-linked oligosaccharide chains differing primarily in their NeuAc content, and that most of the [3H]GlcN was incorporated as GlcN or metabolites of GlcN other than N-acetylneuraminic acid. These studies provide direct chemical evidence of a higher content of carbohydrate in the larger free alpha-subunit form.