title: Carbohydrate and peptide structure of the alpha- and beta-subunits of human chorionic gonadotropin from normal and aberrant pregnancy and choriocarcinoma.
abstract: Human chorionic gonadotropin (hCG), purified from the urine of 14 individuals with normal pregnancy, diabetic pregnancy, hydatidiform mole, or choriocarcinoma, plus two hCG standard preparations, was examined for concurrent peptide-sequence and asparagine (N)- and serine (O)-linked carbohydrate heterogeneity. Protein-sequence analysis was used to measure amino-terminal heterogeneity and the "nicking" of internal peptide 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-subunits from normal and aberrant pregnancy are hyperglycosylated, and that it is the extent of the specific subunit hyperglycosylation that significantly increases in malignant disease. Peptide-bond nicking was restricted to a single linkage (beta 47-48) in normal and diabetic pregnancy, but occurred at two sites in standard preparations, at three sites in hydatidiform mole, and at three sites in choriocarcinoma beta-subunit. In the carbohydrate moiety, alpha-subunit from normal pregnancy hCG contained nonfucosylated, mono- and biantennary N-linked structures (49.3 and 36.7%, means); fucosylated biantennary and triantennary oligosaccharides were also identified (7.3 and 6.9%). In choriocarcinoma alpha-subunit, the level of fucosylated biantennary increased, offset 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 structures; however, mono- and triantennary oligosaccharides were also identified (4.6 and 13.7%). For O-linked glycans, in beta-subunit from normal pregnancy, disaccharide-core structure predominated, whereas tetrasaccharide-core structure was also detected (15.6%). A trend was demonstrated in beta-subunit: the proportions of the nonpredominating N- and O-linked oligosaccharides increased stepwise from normal pregnancy to hydatidiform mole to choriocarcinoma. The increases were: for monoantennary 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 principal 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 patterns of subunit-specific, predominant oligosaccharides appear to reflect the steric effect of local protein structure during glycosylation processes. The evidence of alternative or "hyperbranched" glycoforms on both alpha- and beta-subunits, seen at low levels in normal pregnancy and at increased or even predominant levels in malignant disease, suggests alternative substrate accessibility for Golgi processing enzymes, alpha 1,6 fucosyltransferase and N-acetylglucosaminyltransferase IV, in distinct proportions of subunit molecules.
authors.affiliation: Department of Obstetrics and Gynecology, Yale University School of Medicine, New Haven, CT 06510, USA.
authors.forename: M M
authors.initials: MM
authors.lastname: Elliott
authors.forename: A
authors.initials: A
authors.lastname: Kardana
authors.forename: J W
authors.initials: JW
authors.lastname: Lustbader
authors.forename: L A
authors.initials: LA
authors.lastname: Cole
mesh_terms: D000595:Amino Acid Sequence; D002240:Carbohydrate Sequence; D002241:Carbohydrates / Q000737:chemistry*; D002822:Choriocarcinoma / Q000737:chemistry / Q000652:urine*; D018997:Chorionic Gonadotropin, beta Subunit, Human / Q000302:isolation & purification / Q000652:urine*; D005260:Female; D015292:Glycoprotein Hormones, alpha Subunit / Q000302:isolation & purification / Q000652:urine*; D006801:Humans; D006828:Hydatidiform Mole / Q000737:chemistry / Q000652:urine; D006860:Hydrogen Bonding; D006868:Hydrolysis; D008969:Molecular Sequence Data; D009844:Oligosaccharides / Q000737:chemistry; D010455:Peptides / Q000737:chemistry*; D011247:Pregnancy / Q000652:urine*; D011252:Pregnancy Complications, Neoplastic / Q000652:urine*; D011254:Pregnancy in Diabetics / Q000652:urine; D014594:Uterine Neoplasms / Q000737:chemistry / Q000652:urine*
chemical_list: D002241:Carbohydrates; D018997:Chorionic Gonadotropin, beta Subunit, Human; D015292:Glycoprotein Hormones, alpha Subunit; D009844:Oligosaccharides; D010455:Peptides