Location of major antigenic sites of the β-subunit of human chorionic gonadotropin

Abstract

Purified beta subunit of human chorionic gonadotropin (hCG) was partially reduced with beta-mercaptoethanol, carboxymethylated, and digested with chymotrypsin. The peptides were isolated by high-voltage electrophoresis and paper chromatography. Five major disulfide-containing peptides were isolated, and their location in the parent molecule was established by amino acid composition and amino- and carboxy-terminal analyses. All of these peptides inhibited the binding of 125I-labeled hCG by anti-beta hCG serum. The inhibitory effect of these peptides was lost when their disulfide bonds were reduced and alkylated. Synthetic carboxy-terminal peptides were not inhibitory. Based on these data it is concluded that a major antigenic site of hCG resides in the region of residues 21-23 with a disulfide bond connecting cysteine-23 or -26 with the cysteines at positions 72 or 110.

Recent Advances in the Chemistry and Immunochemistry of Human Chorionic Gonadotropin: Impact on Clinical Measurements*

“…0 Polyclonal antibodies are indicated by this footnote. In addition, SB-6 crossreacted with hCG free /3-subunit and /3-core fragment, the latter being present at very high concentrations in pregnancy urine but not in serum (96)(97)(98). 6 Synthetic peptide conjugated to diptheria toxoid.…”

“…All other antibodies in this table are monoclonal. To improve the usefulness of polyclonal antisera for specific measurements of hCG, some authors have used immunoabsorption of antiserum with hLH to remove cross-reactivity (93,98,99). c Natural peptide with sugar moieties intact conjugated to thyroglobulin.…”

Human Chorionic Gonadotropin

“…12 The major antigenic site on the beta hCG molecule resides in the region residues 21-23 with a disulfide bond connecting Cys-23 or Cys-26 with the cysteines at positions 72 or 110. 13 The alpha subunit of hCG contains two oligosaccharides attached in N-glycosidic linkage to asparagines at residues 52 and 78, whereas the beta subunit contains N-asparagine-linked oligosaccharides at residues 13 and 30. In addition, the beta subunit also contains four O-serine-linked oligosaccharide units in the unique carboxy-terminal peptide of the mole- cule at residues 121,127,132, and 138.…”