Synthesis of Chorionic Gonadotropin Subunits in Human Choriocarcinoma Clonal Cell Line JEG-3: Carbohydrate Differences in Glycopeptides from Free and Combinedα-Subunits

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Abstract
The glycoprotein hormone hCG [human chorionic gonadotropin] and its free .alpha.-subunit are secreted by the clonal choriocarcinoma cell line JEG-3. Free hCG.alpha. has a larger apparent MW (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 [3G]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, that both hCG.alpha. forms contained 2 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. A higher content of carbohydrate in the larger free .alpha.-subunit form is evidenced.

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