CARBOHYDRATE AND PEPTIDE STRUCTURE OF THE ALPHA-SUBUNITS AND BETA-SUBUNITS OF HUMAN CHORIONIC-GONADOTROPIN FROM NORMAL AND ABERRANT PREGNANCY AND CHORIOCARCINOMA

Human chorionic gonadotropin (hCG), purified from the urine of 14 indi viduals with normal pregnancy, diabetic pregnancy, hydatidiform mole, or choriocarcinoma, plus two hCG standard preparations, was examined f or concurrent peptide-sequence and asparagine (N)- and serine (O)-link ed carbohydrate heterogeneity. Protein-sequence analysis was used to m easure amino-terminal heterogeneity and the ''nicking'' of internal pe ptide bonds. The use of high-pH anion-exchange chromatography coupled with the increased sensitivity of pulsed amperometric detection (HPAE/ PAD) revealed that distinct proportions of both hCG alpha- and beta-su bunits from normal and aberrant pregnancy are hyperglycosylated, and t hat it is the extent of the specific subunit hyperglycosylation that s ignificantly increases in malignant disease. Peptide-bond nicking was restricted to a single linkage (beta 47-48) in normal and diabetic pre gnancy, but occurred at two sites in standard preparations, at three s ites in hydatidiform mole, and at three sites in choriocarcinoma beta- subunit. In the carbohydrate moiety, alpha-subunit from normal pregnan cy hCG contained nonfucosylated, mono- and biantennary N-linked struct ures (49.3 and 36.7%, means); fucosylated biantennary and triantennary oligosaccharides were also identified (7,3 and 6.9%). In choriocarcin oma alpha-subunit, the level of fucosylated biantennary increased, off set by a parallel decrease in the predominant biantennary structure of normal pregnancy (P< 0.0001). The beta-subunit from normal pregnancy hCG contained fucosylated and nonfucosylated biantennary N-linked stru ctures; however, mono- and triantennary oligosaccharides were also ide ntified (4.6 and 13.7%). For O-linked glycans, in beta-subunit from no rmal pregnancy, disaccharide-core structure predominated, whereas tetr asaccharide-core structure was also detected (15.6%). A trend was demo nstrated in beta-subunit: the proportions of the nonpredominating N- a nd O-linked oligosaccharides increased stepwise from normal pregnancy to hydatidiform mole to choriocarcinoma. The increases were: for monoa ntennary oligosaccharide, 4.6 to 6.8 to 11.2 %; for triantennary, 13.7 to 26.7 to 51.5% and, for O-linked tetrasaccharide-core structure, 15 .6 to 23.0 to 74.8%. For hCG from individual diabetic pregnancy, the p rincipal N-linked structure (34.7%) was consistent with a biantennary oligosaccharide previously reported only in carcinoma; and sialylation of both N- and O-linked antennae was significantly decreased compared to that of normal pregnancy. Taken collectively, the distinctive patt erns of subunit-specific, predominant oligosaccharides appear to refle ct the steric effect of local protein structure during glycosylation p rocesses. The evidence of alternative or ''hyperbranched'' glycoforms on both alpha- and beta-subunits, seen at low levels in normal pregnan cy and at increased or even predominant levels in malignant disease, s uggests alternative substrate accessibility for Golgi processing enzym es, alpha 1,6fucosyltransferase and N-acetylglucosaminyltransferase IV , in distinct proportions of subunit molecules.