Immunochemical Determinants Unique to Human Chorionic Gonadotropin: Importance of Sialic Acid for Antisera Generated to the Human Chorionic Gonadotropinβ-Subunit COOH-Terminal Peptide*

Abstract

Antisera generated against the structure of the unique hCGβ COOH-terminal region have generally been highly specific for hCG but of low sensitivity and titer when compared with antisera raised against the entire hCG β-subunit. The latter are usually of high affinity but not completely specific for hCG. In this study we characterized several antisera generated against an asialo tryptic peptide from the hCGβ COOH-terminal region in order to search for factors that enhance the sensitivity of these reactions. We found that antisera generated to the asialo hCGβ COOH-terminal tryptic peptide (residues 123–145) frequently reacted with asialo hCG better than with native hCG. Thus, it appeared that the use of an asialo immunogen frequently elicited antibodies to determinants available in the asialo peptide which were masked by sialic acid in native hCG. It is inferred that the use of peptides with their natural complement of sialic acid intact rather than of asialo or synthetic peptides may result in antisera of higher affinity to the native hormone. However, those antisera which are sensitive to the presence or absence of sialic acid may be useful in immunochemical reactions to discriminate between native hCG and the ectopic forms of hCG which have been reported to lack carbohydrate. In addition we have examined the relatively specific anti-β antiserum, SB-6, using a des-COOH-terminal β-mmunochemical competitor to ascertain if this tail peptide plays a role in the capacity of this antiserum to distinguish hCG from human LH. We found that SB-6 recognized this fragment which was devoid of the COOH-terminal region, and it proved to be as good a competitor as hCGβ in SB-6 RIAs using either hCG or hCGβ as tracer. This indicates that the COOH-terminal peptide plays no role in the discrimination between hCG and hLH by this antiserum, and hence, there are at least two unique immunochemical determinants present in hCG which are absent in human LH. (Endocrinology106: 1659, 1980)