Characterization of human chorionic gonadotropin peptide variants with a radio-receptor assay using recombinant human luteinizing hormone/chorionic gonadotropin receptors

There are potential interactions between various human chorionic gonadotropin (hCG) isoforms at the level of luteinizing hormone/chorionic gonadotropin (LH/CG) receptor. The objective of this study was to characterize the receptor-binding activities of the primary peptide variants of hCG including intact hCG, free beta subunit, beta-core fragment and nicked hCG, and to test the effects of these hCG variants on the binding of intact hCG. A radio-receptor assay based on cell membranes expressing recombinant human LH/CG receptors was validated and used in this study to avoid species differences in the receptor-binding specificity. The results showed that none of the hCG variants that we studied had sufficient binding affinity to compete with binding of intact hCG, nor were they able to antagonize the binding of intact hCG. These results suggest that hCG variants with either abbreviated polypeptide structures or incomplete peptide linkage are products or metabolites which do not have the tropic biological activity of the whole hormone, the intact heterodimeric hCG.