Immunochemical determinants unique to human chorionic gonadotropin: importance of sialic acid for antisera generated to the human chorionic gonadotropin beta-subunit COOH-terminal peptide
Birken, S.; Canfield, R.; Lauer, R.; Agosto, G.; Gabel, M.
Endocrinology 106(6): 1659-1664
1980
ISSN/ISBN: 0013-7227 PMID: 6154568 DOI: 10.1210/endo-106-6-1659
Accession: 068628376
Antisera generated against the structure of the unique HCG[human chorionic gonadotropin].beta. COOH-terminal region were generally highly specific for hCG but of low sensitivity and titer when compared with antisera raised against the entire hCG .beta.-subunit. The latter are usually of high affinity but not completely specific for hCG. Several antisera generated against an asialo tryptic peptide from the hCG.beta. COOH-terminal region were characterized in order to search for factors that enhance the sensitivity of these reactions. Antisera generated to the asialo hCG.beta. 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. 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. Those antisera which are sensitive to the presence or absence of sialic acid may be useful in immunochemical reacions to discriminate between native hCG and the ectopic forms of hCG which were reported to lack carbohydrate. The relatively specific anti-.beta. antiserum, SB-6, was examined using a des-COOH-terminal .beta.-immunochemical competitor to ascertain if this tail peptide plays a role in the capacity of this antiserum to distinguish hCG from human LH [lutropin]. SB-6 recognized this fragment which was devoid of the COOH-terminal region, and it proved to be as good a competitor as hCG.beta. in SB-6 radioimmunoassays using either hCG or hCG.beta. as tracer. The COOH-terminal pepetide apparently plays no role in the discrimination between hCG and hLH by this antiserum, and hence, there are at least 2 unique immunochemical determinants present in hCG which are absent in human LH.