O. Lechner et al., FRAGMENTS OF HUMAN CHORIONIC-GONADOTROPIN (HCG) - IMPACT ON PREGNANCYAND TUMOR-DIAGNOSIS, Wiener Klinische Wochenschrift, 107(1), 1995, pp. 15-19
The pregnancy and tumor marker human chrionic gonadotropin (hCG) belon
gs to the family of the glyco-protein hormones. Information on epitope
forming sequences of hCG and its subunits hCGalpha and hCGbeta has si
gnificant impact on the examination of intra- and extracellular metabo
lism and the standardization of diagnostic assay systems. Variants of
hCG appear in biological fluids with variable modifications on differe
nt parts of the molecule. These changes may influence the binding patt
erns of monoclonal antibodies (MCA), thereby causing erroneous results
in hCG immunoassays. The aim of the present work was to investigate t
he influence of peptide bond cleavages and the loss of certain segment
s of the molecule, which were induced by proteases on the expression o
f the seven hCGalpha(alpha1-alpha7), nine hCGbeta- (beta1-beta9) and f
our hCGbeta-core-fragment-epitopes (beta10-beta13), previously identif
ied by us [1-10]. To this end, we digested hCGalpha and hCGbeta with c
hymotrypsin. Hormone fragments were separated by high performance liqu
id chromatography (HPLC) and subsequently immunochemically examined by
direct binding radioimmunoassay (DB-RIA), competitive RIA and immunoe
nzymometric assays (IEMA). Fractions containing hCG-like immunoreactiv
ity were sequenced by Edman and carboxypeptidase-Y degradation. It app
eared that: (I) Amino acids (AA) alpha 41-47 and the peptide bonds bet
ween AA alpha 40/41, alpha 47/48 and alpha 29/30 do not influence the
expression of the 7 alpha-epitopes. (II) The absence of the hCGbeta N-
terminus plays a crucial role for the formation of epitopes beta10 and
beta13. (III) Neither the presence nor the absence of the C-terminal
peptide of hCGbeta (hCGbetaCTP, AA beta 114-145) has any importance fo
r the expression of epitopes beta1-beta7 and beta10-beta13. (IV) Since
epitopes beta11 and beta12 are not detectable on chymotryptically dig
ested hCGbeta, we conclude that AA beta 1-5, beta 115-145 and the inte
rruption of the peptide bond between the AA beta 45/46 contribute neit
her directly nor indirectly to the formation of those epitopes. Thus,
immunometric assay systems based on carefully selected MCA can measure
enzymatically altered and metabolic products of hCGalpha and -beta mo
lecules and are therefore suitable tools for hCG-diagnostic tests whic
h are reliable with respect to both specifity and sensitivity.